draft-ietf-smime-msg-07.txt   rfc2633.txt 
Internet Draft Editor: Blake Ramsdell,
draft-ietf-smime-msg-07.txt Worldtalk
March 31, 1999
Expires September 30, 1999
S/MIME Version 3 Message Specification Network Working Group B. Ramsdell, Editor
Request for Comments: 2633 Worldtalk
Status of this memo Category: Standards Track June 1999
This document is an Internet-Draft and is in full conformance with all S/MIME Version 3 Message Specification
provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Task Status of this Memo
Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months This document specifies an Internet standards track protocol for the
and may be updated, replaced, or obsoleted by other documents at any Internet community, and requests discussion and suggestions for
time. It is inappropriate to use Internet-Drafts as reference improvements. Please refer to the current edition of the "Internet
material or to cite them other than as "work in progress." Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
The list of current Internet-Drafts can be accessed at Copyright Notice
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at Copyright (C) The Internet Society (1999). All Rights Reserved.
http://www.ietf.org/shadow.html.
1. Introduction 1. Introduction
S/MIME (Secure/Multipurpose Internet Mail Extensions) provides a S/MIME (Secure/Multipurpose Internet Mail Extensions) provides a
consistent way to send and receive secure MIME data. Based on the consistent way to send and receive secure MIME data. Based on the
popular Internet MIME standard, S/MIME provides the following popular Internet MIME standard, S/MIME provides the following
cryptographic security services for electronic messaging applications: cryptographic security services for electronic messaging
authentication, message integrity and non-repudiation of origin (using applications: authentication, message integrity and non-repudiation
digital signatures) and privacy and data security (using encryption). of origin (using digital signatures) and privacy and data security
(using encryption).
S/MIME can be used by traditional mail user agents (MUAs) to add S/MIME can be used by traditional mail user agents (MUAs) to add
cryptographic security services to mail that is sent, and to interpret cryptographic security services to mail that is sent, and to
cryptographic security services in mail that is received. However, interpret cryptographic security services in mail that is received.
S/MIME is not restricted to mail; it can be used with any transport However, S/MIME is not restricted to mail; it can be used with any
mechanism that transports MIME data, such as HTTP. As such, S/MIME transport mechanism that transports MIME data, such as HTTP. As such,
takes advantage of the object-based features of MIME and allows secure S/MIME takes advantage of the object-based features of MIME and
messages to be exchanged in mixed-transport systems. allows secure messages to be exchanged in mixed-transport systems.
Further, S/MIME can be used in automated message transfer agents that Further, S/MIME can be used in automated message transfer agents that
use cryptographic security services that do not require any human use cryptographic security services that do not require any human
intervention, such as the signing of software-generated documents and intervention, such as the signing of software-generated documents and
the encryption of FAX messages sent over the Internet. the encryption of FAX messages sent over the Internet.
1.1 Specification Overview 1.1 Specification Overview
This document describes a protocol for adding cryptographic signature This document describes a protocol for adding cryptographic signature
and encryption services to MIME data. The MIME standard [MIME-SPEC] and encryption services to MIME data. The MIME standard [MIME-SPEC]
provides a general structure for the content type of Internet messages provides a general structure for the content type of Internet
and allows extensions for new content type applications. messages and allows extensions for new content type applications.
This draft defines how to create a MIME body part that has been This memo defines how to create a MIME body part that has been
cryptographically enhanced according to CMS [CMS], which is derived cryptographically enhanced according to CMS [CMS], which is derived
from PKCS #7 [PKCS-7]. This draft also defines the application/pkcs7- from PKCS #7 [PKCS-7]. This memo also defines the application/pkcs7-
mime MIME type that can be used to transport those body parts. mime MIME type that can be used to transport those body parts.
This draft also discusses how to use the multipart/signed MIME type This memo also discusses how to use the multipart/signed MIME type
defined in [MIME-SECURE] to transport S/MIME signed messages. This defined in [MIME-SECURE] to transport S/MIME signed messages. This
draft also defines the application/pkcs7-signature MIME type, which is memo also defines the application/pkcs7-signature MIME type, which is
also used to transport S/MIME signed messages. also used to transport S/MIME signed messages.
In order to create S/MIME messages, an S/MIME agent has to follow In order to create S/MIME messages, an S/MIME agent has to follow
specifications in this draft, as well as the specifications listed in specifications in this memo, as well as the specifications listed in
the Cryptographic Message Syntax [CMS]. the Cryptographic Message Syntax [CMS].
Throughout this draft, there are requirements and recommendations made Throughout this memo, there are requirements and recommendations made
for how receiving agents handle incoming messages. There are separate for how receiving agents handle incoming messages. There are separate
requirements and recommendations for how sending agents create requirements and recommendations for how sending agents create
outgoing messages. In general, the best strategy is to "be liberal in outgoing messages. In general, the best strategy is to "be liberal in
what you receive and conservative in what you send". Most of the what you receive and conservative in what you send". Most of the
requirements are placed on the handling of incoming messages while the requirements are placed on the handling of incoming messages while
recommendations are mostly on the creation of outgoing messages. the recommendations are mostly on the creation of outgoing messages.
The separation for requirements on receiving agents and sending agents The separation for requirements on receiving agents and sending
also derives from the likelihood that there will be S/MIME systems agents also derives from the likelihood that there will be S/MIME
that involve software other than traditional Internet mail clients. systems that involve software other than traditional Internet mail
S/MIME can be used with any system that transports MIME data. An clients. S/MIME can be used with any system that transports MIME
automated process that sends an encrypted message might not be able to data. An automated process that sends an encrypted message might not
receive an encrypted message at all, for example. Thus, the be able to receive an encrypted message at all, for example. Thus,
requirements and recommendations for the two types of agents are the requirements and recommendations for the two types of agents are
listed separately when appropriate. listed separately when appropriate.
1.2 Terminology 1.2 Terminology
Throughout this draft, the terms MUST, MUST NOT, SHOULD, and SHOULD The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
NOT are used in capital letters. This conforms to the definitions in "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
[MUSTSHOULD]. [MUSTSHOULD] defines the use of these key words to help document are to be interpreted as described in [MUSTSHOULD].
make the intent of standards track documents as clear as possible. The
same key words are used in this document to help implementors achieve
interoperability.
1.3 Definitions 1.3 Definitions
For the purposes of this draft, the following definitions apply. For the purposes of this memo, the following definitions apply.
ASN.1: Abstract Syntax Notation One, as defined in CCITT X.208. ASN.1: Abstract Syntax Notation One, as defined in CCITT X.208.
BER: Basic Encoding Rules for ASN.1, as defined in CCITT X.209. BER: Basic Encoding Rules for ASN.1, as defined in CCITT X.209.
Certificate: A type that binds an entity's distinguished name to a Certificate: A type that binds an entity's distinguished name to a
public key with a digital signature. public key with a digital signature.
DER: Distinguished Encoding Rules for ASN.1, as defined in CCITT DER: Distinguished Encoding Rules for ASN.1, as defined in CCITT
X.509. X.509.
7-bit data: Text data with lines less than 998 characters long, where 7-bit data: Text data with lines less than 998 characters long, where
none of the characters have the 8th bit set, and there are no NULL none of the characters have the 8th bit set, and there are no NULL
characters. <CR> and <LF> occur only as part of a <CR><LF> end of line characters. <CR> and <LF> occur only as part of a <CR><LF> end of
delimiter. line delimiter.
8-bit data: Text data with lines less than 998 characters, and where 8-bit data: Text data with lines less than 998 characters, and where
none of the characters are NULL characters. <CR> and <LF> occur only none of the characters are NULL characters. <CR> and <LF> occur only
as part of a <CR><LF> end of line delimiter. as part of a <CR><LF> end of line delimiter.
Binary data: Arbitrary data. Binary data: Arbitrary data.
Transfer Encoding: A reversible transformation made on data so 8-bit Transfer Encoding: A reversible transformation made on data so 8-bit
or binary data may be sent via a channel that only transmits 7-bit or binary data may be sent via a channel that only transmits 7-bit
data. data.
Receiving agent: software that interprets and processes S/MIME CMS Receiving agent: software that interprets and processes S/MIME CMS
objects, MIME body parts that contain CMS objects, or both. objects, MIME body parts that contain CMS objects, or both.
Sending agent: software that creates S/MIME CMS objects, MIME body Sending agent: software that creates S/MIME CMS objects, MIME body
parts that contain CMS objects, or both. parts that contain CMS objects, or both.
S/MIME agent: user software that is a receiving agent, a sending S/MIME agent: user software that is a receiving agent, a sending
agent, or both. agent, or both.
1.4 Compatibility with Prior Practice of S/MIME 1.4 Compatibility with Prior Practice of S/MIME
S/MIME version 3 agents should attempt to have the greatest S/MIME version 3 agents should attempt to have the greatest
interoperability possible with S/MIME version 2 agents. S/MIME version interoperability possible with S/MIME version 2 agents. S/MIME
2 is described in RFC 2311 through RFC 2315, inclusive. RFC 2311 also version 2 is described in RFC 2311 through RFC 2315, inclusive. RFC
has historical information about the development of S/MIME. 2311 also has historical information about the development of S/MIME.
1.5 Discussion of This Draft
This draft is being discussed on the "ietf-smime" mailing list. To
subscribe, send a message to:
ietf-smime-request@imc.org
with the single word
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in the body of the message. There is a Web site for the mailing list
at <http://www.imc.org/ietf-smime/>.
2. CMS Options 2. CMS Options
CMS allows for a wide variety of options in content and algorithm CMS allows for a wide variety of options in content and algorithm
support. This section puts forth a number of support requirements and support. This section puts forth a number of support requirements and
recommendations in order to achieve a base level of interoperability recommendations in order to achieve a base level of interoperability
among all S/MIME implementations. [CMS] provides additional details among all S/MIME implementations. [CMS] provides additional details
regarding the use of the cryptographic algorithms. regarding the use of the cryptographic algorithms.
2.1 DigestAlgorithmIdentifier 2.1 DigestAlgorithmIdentifier
Sending and receiving agents MUST support SHA-1 [SHA1]. Receiving Sending and receiving agents MUST support SHA-1 [SHA1]. Receiving
agents SHOULD support MD5 [MD5] for the purpose of providing backward agents SHOULD support MD5 [MD5] for the purpose of providing backward
compatibility with MD5-digested S/MIME v2 SignedData objects. compatibility with MD5-digested S/MIME v2 SignedData objects.
2.2 SignatureAlgorithmIdentifier 2.2 SignatureAlgorithmIdentifier
Sending and receiving agents MUST support id-dsa defined in [DSS]. Sending and receiving agents MUST support id-dsa defined in [DSS].
The algorithm parameters MUST be absent (not encoded as NULL). The algorithm parameters MUST be absent (not encoded as NULL).
Receiving agents SHOULD support rsaEncryption, defined in [PKCS-1]. Receiving agents SHOULD support rsaEncryption, defined in [PKCS-1].
Sending agents SHOULD support rsaEncryption. Outgoing messages are Sending agents SHOULD support rsaEncryption. Outgoing messages are
signed with a user's private key. The size of the private key is signed with a user's private key. The size of the private key is
determined during key generation. determined during key generation.
Note that S/MIME v2 clients are only capable of verifying digital Note that S/MIME v2 clients are only capable of verifying digital
signatures using the rsaEncryption algorithm. signatures using the rsaEncryption algorithm.
2.3 KeyEncryptionAlgorithmIdentifier 2.3 KeyEncryptionAlgorithmIdentifier
Sending and receiving agents MUST support Diffie-Hellman defined in Sending and receiving agents MUST support Diffie-Hellman defined in
[DH]. [DH].
Receiving agents SHOULD support rsaEncryption. Incoming encrypted Receiving agents SHOULD support rsaEncryption. Incoming encrypted
messages contain symmetric keys which are to be decrypted with a messages contain symmetric keys which are to be decrypted with a
user's private key. The size of the private key is determined during user's private key. The size of the private key is determined during
key generation. key generation.
Sending agents SHOULD support rsaEncryption. Sending agents SHOULD support rsaEncryption.
Note that S/MIME v2 clients are only capable of decrypting content Note that S/MIME v2 clients are only capable of decrypting content
encryption keys using the rsaEncryption algorithm. encryption keys using the rsaEncryption algorithm.
2.4 General Syntax 2.4 General Syntax
CMS defines multiple content types. Of these, only the Data, CMS defines multiple content types. Of these, only the Data,
SignedData, and EnvelopedData content types are currently used for SignedData, and EnvelopedData content types are currently used for
S/MIME. S/MIME.
2.4.1 Data Content Type 2.4.1 Data Content Type
Sending agents MUST use the id-data content type identifier to Sending agents MUST use the id-data content type identifier to
indicate the message content which has had security services applied indicate the message content which has had security services applied
to it. For example, when applying a digital signature to MIME data, to it. For example, when applying a digital signature to MIME data,
the CMS signedData encapContentInfo eContentType MUST include the id- the CMS signedData encapContentInfo eContentType MUST include the
data object identifier and the MIME content MUST be stored in the id-data object identifier and the MIME content MUST be stored in the
SignedData encapContentInfo eContent OCTET STRING (unless the sending SignedData encapContentInfo eContent OCTET STRING (unless the sending
agent is using multipart/signed, in which case the eContent is absent, agent is using multipart/signed, in which case the eContent is
per section 3.4.3 of this document). As another example, when absent, per section 3.4.3 of this document). As another example,
applying encryption to MIME data, the CMS EnvelopedData when applying encryption to MIME data, the CMS EnvelopedData
encryptedContentInfo ContentType MUST include the id-data object encryptedContentInfo ContentType MUST include the id-data object
identifier and the encrypted MIME content MUST be stored in the identifier and the encrypted MIME content MUST be stored in the
envelopedData encryptedContentInfo encryptedContent OCTET STRING. envelopedData encryptedContentInfo encryptedContent OCTET STRING.
2.4.2 SignedData Content Type 2.4.2 SignedData Content Type
Sending agents MUST use the signedData content type to apply a digital Sending agents MUST use the signedData content type to apply a
signature to a message or, in a degenerate case where there is no digital signature to a message or, in a degenerate case where there
signature information, to convey certificates. is no signature information, to convey certificates.
2.4.3 EnvelopedData Content Type 2.4.3 EnvelopedData Content Type
This content type is used to apply privacy protection to a message. A This content type is used to apply privacy protection to a message. A
sender needs to have access to a public key for each sender needs to have access to a public key for each intended message
intended message recipient to use this service. This content type does recipient to use this service. This content type does not provide
not provide authentication. authentication.
2.5 Attribute SignerInfo Type 2.5 Attribute SignerInfo Type
The SignerInfo type allows the inclusion of unsigned and signed The SignerInfo type allows the inclusion of unsigned and signed
attributes to be included along with a signature. attributes to be included along with a signature.
Receiving agents MUST be able to handle zero or one instance of each Receiving agents MUST be able to handle zero or one instance of each
of the signed attributes listed here. Sending agents SHOULD generate of the signed attributes listed here. Sending agents SHOULD generate
one instance of each of the following signed attributes in each S/MIME one instance of each of the following signed attributes in each
message: S/MIME message:
- signingTime (section 2.5.1 in this document)
- sMIMECapabilities (section 2.5.2 in this document)
- sMIMEEncryptionKeyPreference (section 2.5.3 in this document)
Further, receiving agents SHOULD be able to handle zero or one - signingTime (section 2.5.1 in this document)
instance in the signed attributes of the signingCertificate attribute - sMIMECapabilities (section 2.5.2 in this document)
(section 5 in [ESS]). - sMIMEEncryptionKeyPreference (section 2.5.3 in this document)
Sending agents SHOULD generate one instance of the signingCertificate Further, receiving agents SHOULD be able to handle zero or one
signed attribute in each S/MIME message. instance in the signed attributes of the signingCertificate attribute
(section 5 in [ESS]).
Additional attributes and values for these attributes may be defined Sending agents SHOULD generate one instance of the signingCertificate
in the future. Receiving agents SHOULD handle attributes or values signed attribute in each S/MIME message.
that it does not recognize in a graceful manner.
Sending agents that include signed attributes that are not listed here Additional attributes and values for these attributes may be defined
SHOULD display those attributes to the user, so that the user is aware in the future. Receiving agents SHOULD handle attributes or values
of all of the data being signed. that it does not recognize in a graceful manner.
Sending agents that include signed attributes that are not listed
here SHOULD display those attributes to the user, so that the user is
aware of all of the data being signed.
2.5.1 Signing-Time Attribute 2.5.1 Signing-Time Attribute
The signing-time attribute is used to convey the time that a message The signing-time attribute is used to convey the time that a message
was signed. Until there are trusted timestamping services, the time of was signed. Until there are trusted timestamping services, the time
signing will most likely be created by a message originator and of signing will most likely be created by a message originator and
therefore is only as trustworthy as the originator. therefore is only as trustworthy as the originator.
Sending agents MUST encode signing time through the year 2049 as Sending agents MUST encode signing time through the year 2049 as
UTCTime; signing times in 2050 or later MUST be encoded as UTCTime; signing times in 2050 or later MUST be encoded as
GeneralizedTime. When the UTCTime CHOICE is used, S/MIME agents MUST GeneralizedTime. When the UTCTime CHOICE is used, S/MIME agents MUST
interpret the year field (YY) as follows: interpret the year field (YY) as follows:
if YY is greater than or equal to 50, the year is interpreted as 19YY; if YY is greater than or equal to 50, the year is interpreted as
if YY is less than 50, the year is interpreted as 20YY. 19YY; if YY is less than 50, the year is interpreted as 20YY.
2.5.2 SMIMECapabilities Attribute 2.5.2 SMIMECapabilities Attribute
The SMIMECapabilities attribute includes signature algorithms (such as The SMIMECapabilities attribute includes signature algorithms (such
"sha1WithRSAEncryption"), symmetric algorithms (such as "DES-EDE3- as "sha1WithRSAEncryption"), symmetric algorithms (such as "DES-
CBC"), and key encipherment algorithms (such as "rsaEncryption"). It EDE3-CBC"), and key encipherment algorithms (such as
also includes a non-algorithm capability which is the preference for "rsaEncryption"). It also includes a non-algorithm capability which
signedData. The SMIMECapabilities were designed to be flexible and is the preference for signedData. The SMIMECapabilities were designed
extensible so that, in the future, a means of identifying other to be flexible and extensible so that, in the future, a means of
capabilities and preferences such as certificates can be added in a identifying other capabilities and preferences such as certificates
way that will not cause current clients to break. can be added in a way that will not cause current clients to break.
If present, the SMIMECapabilities attribute MUST be a SignedAttribute;
it MUST NOT be an UnsignedAttribute. CMS defines SignedAttributes as a
SET OF Attribute. The SignedAttributes in a signerInfo MUST NOT
include multiple instances of the SMIMECapabilities attribute. CMS
defines the ASN.1 syntax for Attribute to include attrValues SET OF
AttributeValue. A SMIMECapabilities attribute MUST only include a
single instance of AttributeValue. There MUST NOT be zero or multiple
instances of AttributeValue present in the attrValues SET OF
AttributeValue.
The semantics of the SMIMECapabilites attribute specify a partial list If present, the SMIMECapabilities attribute MUST be a
as to what the client announcing the SMIMECapabilites can support. A SignedAttribute; it MUST NOT be an UnsignedAttribute. CMS defines
client does not have to list every capability it supports, and SignedAttributes as a SET OF Attribute. The SignedAttributes in a
probably should not list all its capabilities so that the capabilities signerInfo MUST NOT include multiple instances of the
list doesn't get too long. In an SMIMECapabilities attribute, the OIDs SMIMECapabilities attribute. CMS defines the ASN.1 syntax for
are listed in order of their preference, but SHOULD be logically Attribute to include attrValues SET OF AttributeValue. A
separated along the lines of their categories (signature algorithms, SMIMECapabilities attribute MUST only include a single instance of
symmetric algorithms, key encipherment algorithms, etc.) AttributeValue. There MUST NOT be zero or multiple instances of
AttributeValue present in the attrValues SET OF AttributeValue.
The structure of the SMIMECapabilities attribute is to facilitate The semantics of the SMIMECapabilites attribute specify a partial
simple table lookups and binary comparisons in order to determine list as to what the client announcing the SMIMECapabilites can
matches. For instance, the DER-encoding for the SMIMECapability for support. A client does not have to list every capability it supports,
DES EDE3 CBC MUST be identically encoded regardless of the and probably should not list all its capabilities so that the
implementation. capabilities list doesn't get too long. In an SMIMECapabilities
attribute, the OIDs are listed in order of their preference, but
SHOULD be logically separated along the lines of their categories
(signature algorithms, symmetric algorithms, key encipherment
algorithms, etc.)
The structure of the SMIMECapabilities attribute is to facilitate
simple table lookups and binary comparisons in order to determine
matches. For instance, the DER-encoding for the SMIMECapability for
DES EDE3 CBC MUST be identically encoded regardless of the
implementation.
In the case of symmetric algorithms, the associated parameters for the In the case of symmetric algorithms, the associated parameters for
OID MUST specify all of the parameters necessary to differentiate the OID MUST specify all of the parameters necessary to differentiate
between two instances of the same algorithm. For instance, the number between two instances of the same algorithm. For instance, the number
of rounds and block size for RC5 must be specified in addition to the of rounds and block size for RC5 must be specified in addition to the
key length. key length.
There is a list of OIDs (OIDs Used with S/MIME) that is centrally There is a list of OIDs (OIDs Used with S/MIME) that is centrally
maintained and is separate from this draft. The list of OIDs is maintained and is separate from this memo. The list of OIDs is
maintained by the Internet Mail Consortium at <http://www.imc.org/ietf- maintained by the Internet Mail Consortium at
smime/oids.html>. Note that all OIDs associated with the MUST and <http://www.imc.org/ietf-smime/oids.html>. Note that all OIDs
SHOULD implement algorithms are included in section A of this associated with the MUST and SHOULD implement algorithms are included
document. in section A of this document.
The OIDs that correspond to algorithms SHOULD use the same OID as the The OIDs that correspond to algorithms SHOULD use the same OID as the
actual algorithm, except in the case where the algorithm usage is actual algorithm, except in the case where the algorithm usage is
ambiguous from the OID. For instance, in an earlier draft, ambiguous from the OID. For instance, in an earlier draft,
rsaEncryption was ambiguous because it could refer to either a rsaEncryption was ambiguous because it could refer to either a
signature algorithm or a key encipherment algorithm. In the event that signature algorithm or a key encipherment algorithm. In the event
an OID is ambiguous, it needs to be arbitrated by the maintainer of that an OID is ambiguous, it needs to be arbitrated by the maintainer
the registered SMIMECapabilities list as to which type of algorithm of the registered SMIMECapabilities list as to which type of
will use the OID, and a new OID MUST be allocated under the algorithm will use the OID, and a new OID MUST be allocated under the
smimeCapabilities OID to satisfy the other use of the OID. smimeCapabilities OID to satisfy the other use of the OID.
The registered SMIMECapabilities list specifies the parameters for The registered SMIMECapabilities list specifies the parameters for
OIDs that need them, most notably key lengths in the case of variable- OIDs that need them, most notably key lengths in the case of
length symmetric ciphers. In the event that there are no variable-length symmetric ciphers. In the event that there are no
differentiating parameters for a particular OID, the parameters MUST differentiating parameters for a particular OID, the parameters MUST
be omitted, and MUST NOT be encoded as NULL. be omitted, and MUST NOT be encoded as NULL.
Additional values for the SMIMECapabilities attribute may be defined Additional values for the SMIMECapabilities attribute may be defined
in the future. Receiving agents MUST handle a SMIMECapabilities object in the future. Receiving agents MUST handle a SMIMECapabilities
that has values that it does not recognize in a graceful manner. object that has values that it does not recognize in a graceful
manner.
2.5.3 Encryption Key Preference Attribute 2.5.3 Encryption Key Preference Attribute
The encryption key preference attribute allows the signer to The encryption key preference attribute allows the signer to
unambiguously describe which of the signer's certificates has the unambiguously describe which of the signer's certificates has the
signer's preferred encryption key. This attribute is designed to signer's preferred encryption key. This attribute is designed to
enhance behavior for interoperating with those clients which use enhance behavior for interoperating with those clients which use
separate keys for encryption and signing. This attribute is used to separate keys for encryption and signing. This attribute is used to
convey to anyone viewing the attribute which of the listed convey to anyone viewing the attribute which of the listed
certificates should be used for encrypting a session key for future certificates should be used for encrypting a session key for future
encrypted messages. encrypted messages.
If present, the SMIMEEncryptionKeyPreference attribute MUST be a If present, the SMIMEEncryptionKeyPreference attribute MUST be a
SignedAttribute; it MUST NOT be an UnsignedAttribute. CMS defines SignedAttribute; it MUST NOT be an UnsignedAttribute. CMS defines
SignedAttributes as a SET OF Attribute. The SignedAttributes in a SignedAttributes as a SET OF Attribute. The SignedAttributes in a
signerInfo MUST NOT include multiple instances of the signerInfo MUST NOT include multiple instances of the
SMIMEEncryptionKeyPreference attribute. CMS defines the ASN.1 syntax SMIMEEncryptionKeyPreference attribute. CMS defines the ASN.1 syntax
for Attribute to include attrValues SET OF AttributeValue. A for Attribute to include attrValues SET OF AttributeValue. A
SMIMEEncryptionKeyPreference attribute MUST only include a single SMIMEEncryptionKeyPreference attribute MUST only include a single
instance of AttributeValue. There MUST NOT be zero or multiple instance of AttributeValue. There MUST NOT be zero or multiple
instances of AttributeValue present in the attrValues SET OF instances of AttributeValue present in the attrValues SET OF
AttributeValue. AttributeValue.
The sending agent SHOULD include the referenced certificate in the set The sending agent SHOULD include the referenced certificate in the
of certificates included in the signed message if this attribute is set of certificates included in the signed message if this attribute
used. The certificate may be omitted if it has been previously made is used. The certificate may be omitted if it has been previously
available to the receiving agent. Sending agents SHOULD use this made available to the receiving agent. Sending agents SHOULD use
attribute if the commonly used or preferred encryption certificate is this attribute if the commonly used or preferred encryption
not the same as the certificate used to sign the message. certificate is not the same as the certificate used to sign the
message.
Receiving agents SHOULD store the preference data if the signature on Receiving agents SHOULD store the preference data if the signature on
the message is valid and the signing time is greater than the the message is valid and the signing time is greater than the
currently stored value. (As with the SMIMECapabilities, the clock currently stored value. (As with the SMIMECapabilities, the clock
skew should be checked and the data not used if the skew is too skew should be checked and the data not used if the skew is too
great.) Receiving agents SHOULD respect the sender's encryption key great.) Receiving agents SHOULD respect the sender's encryption key
preference attribute if possible. This however represents only a preference attribute if possible. This however represents only a
preference and the receiving agent may use any certificate in replying preference and the receiving agent may use any certificate in
to the sender that is valid. replying to the sender that is valid.
2.5.3.1 Selection of Recipient Key Management Certificate 2.5.3.1 Selection of Recipient Key Management Certificate
In order to determine the key management certificate to be used when In order to determine the key management certificate to be used when
sending a future CMS envelopedData message for a particular recipient, sending a future CMS envelopedData message for a particular
the following steps SHOULD be followed: recipient, the following steps SHOULD be followed:
- If an SMIMEEncryptionKeyPreference attribute is found in a - If an SMIMEEncryptionKeyPreference attribute is found in a
signedData object received from the desired recipient, this identifies signedData object received from the desired recipient, this
the X.509 certificate that should be used as the X.509 key management identifies the X.509 certificate that should be used as the X.509
certificate for the recipient. key management certificate for the recipient.
- If an SMIMEEncryptionKeyPreference attribute is not found in a - If an SMIMEEncryptionKeyPreference attribute is not found in a
signedData object received from the desired recipient, the set of signedData object received from the desired recipient, the set of
X.509 certificates should be searched for a X.509 certificate with the X.509 certificates should be searched for a X.509 certificate with
same subject name as the signing X.509 certificate which can be used the same subject name as the signing X.509 certificate which can
for key management. be used for key management.
- Or use some other method of determining the user's key management - Or use some other method of determining the user's key management
key. If a X.509 key management certificate is not found, then key. If a X.509 key management certificate is not found, then
encryption cannot be done with the signer of the message. If multiple encryption cannot be done with the signer of the message. If multiple
X.509 key management certificates are found, the S/MIME agent can make X.509 key management certificates are found, the S/MIME agent can
an arbitrary choice between them. make an arbitrary choice between them.
2.6 SignerIdentifier SignerInfo Type 2.6 SignerIdentifier SignerInfo Type
S/MIME v3 requires the use of SignerInfo version 1, that is the S/MIME v3 requires the use of SignerInfo version 1, that is the
issuerAndSerialNumber CHOICE MUST be used for SignerIdentifier. issuerAndSerialNumber CHOICE MUST be used for SignerIdentifier.
2.7 ContentEncryptionAlgorithmIdentifier 2.7 ContentEncryptionAlgorithmIdentifier
Sending and receiving agents MUST support encryption and decryption Sending and receiving agents MUST support encryption and decryption
with DES EDE3 CBC, hereinafter called "tripleDES" [3DES] [DES]. with DES EDE3 CBC, hereinafter called "tripleDES" [3DES] [DES].
Receiving agents SHOULD support encryption and decryption using the Receiving agents SHOULD support encryption and decryption using the
RC2 [RC2] or a compatible algorithm at a key size of 40 bits, RC2 [RC2] or a compatible algorithm at a key size of 40 bits,
hereinafter called "RC2/40". hereinafter called "RC2/40".
2.7.1 Deciding Which Encryption Method To Use 2.7.1 Deciding Which Encryption Method To Use
When a sending agent creates an encrypted message, it has to decide When a sending agent creates an encrypted message, it has to decide
which type of encryption to use. The decision process involves using which type of encryption to use. The decision process involves using
information garnered from the capabilities lists included in messages information garnered from the capabilities lists included in messages
received from the recipient, as well as out-of-band information such received from the recipient, as well as out-of-band information such
as private agreements, user preferences, legal restrictions, and so as private agreements, user preferences, legal restrictions, and so
on. on.
Section 2.5 defines a method by which a sending agent can optionally Section 2.5 defines a method by which a sending agent can optionally
announce, among other things, its decrypting capabilities in its order announce, among other things, its decrypting capabilities in its
of preference. The following method for processing and remembering the order of preference. The following method for processing and
encryption capabilities attribute in incoming signed messages SHOULD remembering the encryption capabilities attribute in incoming signed
be used. messages SHOULD be used.
- If the receiving agent has not yet created a list of capabilities - If the receiving agent has not yet created a list of capabilities
for the sender's public key, then, after verifying the signature for the sender's public key, then, after verifying the signature
on the incoming message and checking the timestamp, the receiving on the incoming message and checking the timestamp, the receiving
agent SHOULD create a new list containing at least the signing agent SHOULD create a new list containing at least the signing
time and the symmetric capabilities. time and the symmetric capabilities.
- If such a list already exists, the receiving agent SHOULD verify
that the signing time in the incoming message is greater than
the signing time stored in the list and that the signature is
valid. If so, the receiving agent SHOULD update both the signing
time and capabilities in the list. Values of the signing time that
lie far in the future (that is, a greater discrepancy than any
reasonable clock skew), or a capabilities list in messages whose
signature could not be verified, MUST NOT be accepted.
The list of capabilities SHOULD be stored for future use in creating - If such a list already exists, the receiving agent SHOULD verify
messages. that the signing time in the incoming message is greater than
the signing time stored in the list and that the signature is
valid. If so, the receiving agent SHOULD update both the signing
time and capabilities in the list. Values of the signing time that
lie far in the future (that is, a greater discrepancy than any
reasonable clock skew), or a capabilities list in messages whose
signature could not be verified, MUST NOT be accepted.
Before sending a message, the sending agent MUST decide whether it is The list of capabilities SHOULD be stored for future use in creating
willing to use weak encryption for the particular data in the message. messages.
If the sending agent decides that weak encryption is unacceptable for
this data, then the sending agent MUST NOT use a weak algorithm such
as RC2/40. The decision to use or not use weak encryption overrides
any other decision in this section about which encryption algorithm to
use.
Sections 2.7.2.1 through 2.7.2.4 describe the decisions a sending Before sending a message, the sending agent MUST decide whether it is
agent SHOULD use in deciding which type of encryption should be willing to use weak encryption for the particular data in the
applied to a message. These rules are ordered, so the sending agent message. If the sending agent decides that weak encryption is
SHOULD make its decision in the order given. unacceptable for this data, then the sending agent MUST NOT use a
weak algorithm such as RC2/40. The decision to use or not use weak
encryption overrides any other decision in this section about which
encryption algorithm to use.
Sections 2.7.2.1 through 2.7.2.4 describe the decisions a sending
agent SHOULD use in deciding which type of encryption should be
applied to a message. These rules are ordered, so the sending agent
SHOULD make its decision in the order given.
2.7.1.1 Rule 1: Known Capabilities 2.7.1.1 Rule 1: Known Capabilities
If the sending agent has received a set of capabilities from the If the sending agent has received a set of capabilities from the
recipient for the message the agent is about to encrypt, then the recipient for the message the agent is about to encrypt, then the
sending agent SHOULD use that information by selecting the first sending agent SHOULD use that information by selecting the first
capability in the list (that is, the capability most preferred by the capability in the list (that is, the capability most preferred by the
intended recipient) for which the sending agent knows how to encrypt. intended recipient) for which the sending agent knows how to encrypt.
The sending agent SHOULD use one of the capabilities in the list if The sending agent SHOULD use one of the capabilities in the list if
the agent reasonably expects the recipient to be able to decrypt the the agent reasonably expects the recipient to be able to decrypt the
message. message.
2.7.1.2 Rule 2: Unknown Capabilities, Known Use of Encryption 2.7.1.2 Rule 2: Unknown Capabilities, Known Use of Encryption
If: If:
- the sending agent has no knowledge of the encryption capabilities - the sending agent has no knowledge of the encryption capabilities
of the recipient, of the recipient,
- and the sending agent has received at least one message from the - and the sending agent has received at least one message from the
recipient, recipient,
- and the last encrypted message received from the recipient had a - and the last encrypted message received from the recipient had a
trusted signature on it, trusted signature on it,
then the outgoing message SHOULD use the same encryption algorithm as
was used on the last signed and encrypted message received from the then the outgoing message SHOULD use the same encryption algorithm as
recipient. was used on the last signed and encrypted message received from the
recipient.
2.7.1.3 Rule 3: Unknown Capabilities, Unknown Version of S/MIME 2.7.1.3 Rule 3: Unknown Capabilities, Unknown Version of S/MIME
If: If:
- the sending agent has no knowledge of the encryption capabilities
of the recipient, - the sending agent has no knowledge of the encryption capabilities
- and the sending agent has no knowledge of the version of S/MIME of the recipient,
of the recipient, - and the sending agent has no knowledge of the version of S/MIME
then the sending agent SHOULD use tripleDES because it is a stronger of the recipient,
algorithm and is required by S/MIME v3. If the sending agent chooses
not to use tripleDES in this step, it SHOULD use RC2/40. then the sending agent SHOULD use tripleDES because it is a stronger
algorithm and is required by S/MIME v3. If the sending agent chooses
not to use tripleDES in this step, it SHOULD use RC2/40.
2.7.2 Choosing Weak Encryption 2.7.2 Choosing Weak Encryption
Like all algorithms that use 40 bit keys, RC2/40 is considered by many Like all algorithms that use 40 bit keys, RC2/40 is considered by
to be weak encryption. A sending agent that is controlled by a human many to be weak encryption. A sending agent that is controlled by a
SHOULD allow a human sender to determine the risks of sending data human SHOULD allow a human sender to determine the risks of sending
using RC2/40 or a similarly weak encryption algorithm before sending data using RC2/40 or a similarly weak encryption algorithm before
the data, and possibly allow the human to use a stronger encryption sending the data, and possibly allow the human to use a stronger
method such as tripleDES. encryption method such as tripleDES.
2.7.3 Multiple Recipients 2.7.3 Multiple Recipients
If a sending agent is composing an encrypted message to a group of If a sending agent is composing an encrypted message to a group of
recipients where the encryption capabilities of some of the recipients recipients where the encryption capabilities of some of the
do not overlap, the sending agent is forced to send more than one recipients do not overlap, the sending agent is forced to send more
message. It should be noted that if the sending agent chooses to send than one message. It should be noted that if the sending agent
a message encrypted with a strong algorithm, and then send the same chooses to send a message encrypted with a strong algorithm, and then
message encrypted with a weak algorithm, someone watching the send the same message encrypted with a weak algorithm, someone
communications channel may be able to learn the contents of the watching the communications channel may be able to learn the contents
strongly-encrypted message simply by decrypting the weakly-encrypted of the strongly-encrypted message simply by decrypting the weakly-
message. encrypted message.
3. Creating S/MIME Messages 3. Creating S/MIME Messages
This section describes the S/MIME message formats and how they are This section describes the S/MIME message formats and how they are
created. S/MIME messages are a combination of MIME bodies and CMS created. S/MIME messages are a combination of MIME bodies and CMS
objects. Several MIME types as well as several CMS objects are used. objects. Several MIME types as well as several CMS objects are used.
The data to be secured is always a canonical MIME entity. The MIME The data to be secured is always a canonical MIME entity. The MIME
entity and other data, such as certificates and algorithm identifiers, entity and other data, such as certificates and algorithm
are given to CMS processing facilities which produces a CMS object. identifiers, are given to CMS processing facilities which produces a
The CMS object is then finally wrapped in MIME. The Enhanced Security CMS object. The CMS object is then finally wrapped in MIME. The
Services for S/MIME [ESS] document provides examples of how nested, Enhanced Security Services for S/MIME [ESS] document provides
secured S/MIME messages are formatted. ESS provides an example of how examples of how nested, secured S/MIME messages are formatted. ESS
a triple-wrapped S/MIME message is formatted using multipart/signed provides an example of how a triple-wrapped S/MIME message is
and application/pkcs7-mime for the signatures. formatted using multipart/signed and application/pkcs7-mime for the
signatures.
S/MIME provides one format for enveloped-only data, several formats S/MIME provides one format for enveloped-only data, several formats
for signed-only data, and several formats for signed and enveloped for signed-only data, and several formats for signed and enveloped
data. Several formats are required to accommodate several data. Several formats are required to accommodate several
environments, in particular for signed messages. The criteria for environments, in particular for signed messages. The criteria for
choosing among these formats are also described. choosing among these formats are also described.
The reader of this section is expected to understand MIME as described The reader of this section is expected to understand MIME as
in [MIME-SPEC] and [MIME-SECURE]. described in [MIME-SPEC] and [MIME-SECURE].
3.1 Preparing the MIME Entity for Signing or Enveloping 3.1 Preparing the MIME Entity for Signing or Enveloping
S/MIME is used to secure MIME entities. A MIME entity may be a sub- S/MIME is used to secure MIME entities. A MIME entity may be a sub-
part, sub-parts of a message, or the whole message with all its sub- part, sub-parts of a message, or the whole message with all its sub-
parts. A MIME entity that is the whole message includes only the MIME parts. A MIME entity that is the whole message includes only the MIME
headers and MIME body, and does not include the RFC-822 headers. Note headers and MIME body, and does not include the RFC-822 headers.
that S/MIME can also be used to secure MIME entities used in Note that S/MIME can also be used to secure MIME entities used in
applications other than Internet mail. applications other than Internet mail.
The MIME entity that is secured and described in this section can be
thought of as the "inside" MIME entity. That is, it is the "innermost"
object in what is possibly a larger MIME message. Processing "outside"
MIME entities into CMS objects is described in Section 3.2, 3.4 and
elsewhere.
The procedure for preparing a MIME entity is given in [MIME-SPEC]. The The MIME entity that is secured and described in this section can be
same procedure is used here with some additional restrictions when thought of as the "inside" MIME entity. That is, it is the
signing. Description of the procedures from [MIME-SPEC] are repeated "innermost" object in what is possibly a larger MIME message.
here, but the reader should refer to that document for the exact Processing "outside" MIME entities into CMS objects is described in
procedure. This section also describes additional requirements. Section 3.2, 3.4 and elsewhere.
A single procedure is used for creating MIME entities that are to be The procedure for preparing a MIME entity is given in [MIME-SPEC].
signed, enveloped, or both signed and enveloped. Some additional steps The same procedure is used here with some additional restrictions
are recommended to defend against known corruptions that can occur when signing. Description of the procedures from [MIME-SPEC] are
during mail transport that are of particular importance for clear- repeated here, but the reader should refer to that document for the
signing using the multipart/signed format. It is recommended that exact procedure. This section also describes additional requirements.
these additional steps be performed on enveloped messages, or signed
and enveloped messages in order that the message can be forwarded to
any environment without modification.
These steps are descriptive rather than prescriptive. The implementor A single procedure is used for creating MIME entities that are to be
is free to use any procedure as long as the result is the same. signed, enveloped, or both signed and enveloped. Some additional
steps are recommended to defend against known corruptions that can
occur during mail transport that are of particular importance for
clear- signing using the multipart/signed format. It is recommended
that these additional steps be performed on enveloped messages, or
signed and enveloped messages in order that the message can be
forwarded to any environment without modification.
Step 1. The MIME entity is prepared according to the local conventions These steps are descriptive rather than prescriptive. The implementor
is free to use any procedure as long as the result is the same.
Step 2. The leaf parts of the MIME entity are converted to canonical Step 1. The MIME entity is prepared according to the local
form conventions
Step 3. Appropriate transfer encoding is applied to the leaves of the Step 2. The leaf parts of the MIME entity are converted to canonical
MIME entity form
Step 3. Appropriate transfer encoding is applied to the leaves of the
MIME entity
When an S/MIME message is received, the security services on the When an S/MIME message is received, the security services on the
message are processed, and the result is the MIME entity. That MIME message are processed, and the result is the MIME entity. That MIME
entity is typically passed to a MIME-capable user agent where, it is entity is typically passed to a MIME-capable user agent where, it is
further decoded and presented to the user or receiving application. further decoded and presented to the user or receiving application.
3.1.1 Canonicalization 3.1.1 Canonicalization
Each MIME entity MUST be converted to a canonical form that is Each MIME entity MUST be converted to a canonical form that is
uniquely and unambiguously representable in the environment where the uniquely and unambiguously representable in the environment where the
signature is created and the environment where the signature will be signature is created and the environment where the signature will be
verified. MIME entities MUST be canonicalized for enveloping as well verified. MIME entities MUST be canonicalized for enveloping as well
as signing. as signing.
The exact details of canonicalization depend on the actual MIME type The exact details of canonicalization depend on the actual MIME type
and subtype of an entity, and are not described here. Instead, the and subtype of an entity, and are not described here. Instead, the
standard for the particular MIME type should be consulted. For standard for the particular MIME type should be consulted. For
example, canonicalization of type text/plain is different from example, canonicalization of type text/plain is different from
canonicalization of audio/basic. Other than text types, most types canonicalization of audio/basic. Other than text types, most types
have only one representation regardless of computing platform or have only one representation regardless of computing platform or
environment which can be considered their canonical representation. In environment which can be considered their canonical representation.
general, canonicalization will be performed by the non-security part In general, canonicalization will be performed by the non-security
of the sending agent rather than the S/MIME implementation. part of the sending agent rather than the S/MIME implementation.
The most common and important canonicalization is for text, which is The most common and important canonicalization is for text, which is
often represented differently in different environments. MIME entities often represented differently in different environments. MIME
of major type "text" must have both their line endings and character entities of major type "text" must have both their line endings and
set canonicalized. The line ending must be the pair of characters character set canonicalized. The line ending must be the pair of
<CR><LF>, and the charset should be a registered charset [CHARSETS]. characters <CR><LF>, and the charset should be a registered charset
The details of the canonicalization are specified in [MIME-SPEC]. The [CHARSETS]. The details of the canonicalization are specified in
chosen charset SHOULD be named in the charset parameter so that [MIME-SPEC]. The chosen charset SHOULD be named in the charset
the receiving agent can unambiguously determine the charset used. parameter so that the receiving agent can unambiguously determine the
charset used.
Note that some charsets such as ISO-2022 have multiple representations Note that some charsets such as ISO-2022 have multiple
for the same characters. When preparing such text for signing, the representations for the same characters. When preparing such text for
canonical representation specified for the charset MUST be used. signing, the canonical representation specified for the charset MUST
be used.
3.1.2 Transfer Encoding 3.1.2 Transfer Encoding
When generating any of the secured MIME entities below, except the When generating any of the secured MIME entities below, except the
signing using the multipart/signed format, no transfer encoding at all signing using the multipart/signed format, no transfer encoding at
is required. S/MIME implementations MUST be able to deal with binary all is required. S/MIME implementations MUST be able to deal with
MIME objects. If no Content-Transfer-Encoding header is present, the binary MIME objects. If no Content-Transfer-Encoding header is
transfer encoding should be considered 7BIT. present, the transfer encoding should be considered 7BIT.
S/MIME implementations SHOULD however use transfer encoding described S/MIME implementations SHOULD however use transfer encoding described
in section 3.1.3 for all MIME entities they secure. The reason for in section 3.1.3 for all MIME entities they secure. The reason for
securing only 7-bit MIME entities, even for enveloped data that are securing only 7-bit MIME entities, even for enveloped data that are
not exposed to the transport, is that it allows the MIME entity to be not exposed to the transport, is that it allows the MIME entity to be
handled in any environment without changing it. For example, a trusted handled in any environment without changing it. For example, a
gateway might remove the envelope, but not the signature, of a trusted gateway might remove the envelope, but not the signature, of
message, and then forward the signed message on to the end recipient a message, and then forward the signed message on to the end
so that they can verify the signatures directly. If the transport recipient so that they can verify the signatures directly. If the
internal to the site is not 8-bit clean, such as on a wide-area transport internal to the site is not 8-bit clean, such as on a
network with a single mail gateway, verifying the signature will not wide-area network with a single mail gateway, verifying the signature
be possible unless the original MIME entity was only 7-bit data. will not be possible unless the original MIME entity was only 7-bit
data.
3.1.3 Transfer Encoding for Signing Using multipart/signed 3.1.3 Transfer Encoding for Signing Using multipart/signed
If a multipart/signed entity is EVER to be transmitted over the If a multipart/signed entity is EVER to be transmitted over the
standard Internet SMTP infrastructure or other transport that is standard Internet SMTP infrastructure or other transport that is
constrained to 7-bit text, it MUST have transfer encoding applied so constrained to 7-bit text, it MUST have transfer encoding applied so
that it is represented as 7-bit text. MIME entities that are 7-bit that it is represented as 7-bit text. MIME entities that are 7-bit
data already need no transfer encoding. Entities such as 8-bit text data already need no transfer encoding. Entities such as 8-bit text
and binary data can be encoded with quoted-printable or base-64 and binary data can be encoded with quoted-printable or base-64
transfer encoding. transfer encoding.
The primary reason for the 7-bit requirement is that the Internet mail The primary reason for the 7-bit requirement is that the Internet
transport infrastructure cannot guarantee transport of 8-bit or binary mail transport infrastructure cannot guarantee transport of 8-bit or
data. Even though many segments of the transport infrastructure now binary data. Even though many segments of the transport
handle 8-bit and even binary data, it is sometimes not possible to infrastructure now handle 8-bit and even binary data, it is sometimes
know whether the transport path is 8-bit clear. If a mail message with not possible to know whether the transport path is 8-bit clear. If a
8-bit data were to encounter a message transfer agent that can not mail message with 8-bit data were to encounter a message transfer
transmit 8-bit or binary data, the agent has three options, none of agent that can not transmit 8-bit or binary data, the agent has three
which are acceptable for a clear-signed message: options, none of which are acceptable for a clear-signed message:
- The agent could change the transfer encoding; this would invalidate - The agent could change the transfer encoding; this would invalidate
the signature. the signature.
- The agent could transmit the data anyway, which would most likely - The agent could transmit the data anyway, which would most likely
result in the 8th bit being corrupted; this too would invalidate the result in the 8th bit being corrupted; this too would invalidate the
signature. signature.
- The agent could return the message to the sender. - The agent could return the message to the sender.
[MIME-SECURE] prohibits an agent from changing the transfer encoding [MIME-SECURE] prohibits an agent from changing the transfer encoding
of the first part of a multipart/signed message. If a compliant agent of the first part of a multipart/signed message. If a compliant agent
that can not transmit 8-bit or binary data encounters a that can not transmit 8-bit or binary data encounters a
multipart/signed message with 8-bit or binary data in the first part, multipart/signed message with 8-bit or binary data in the first part,
it would have to return the message to the sender as undeliverable. it would have to return the message to the sender as undeliverable.
3.1.4 Sample Canonical MIME Entity 3.1.4 Sample Canonical MIME Entity
This example shows a multipart/mixed message with full transfer This example shows a multipart/mixed message with full transfer
encoding. This message contains a text part and an attachment. The encoding. This message contains a text part and an attachment. The
sample message text includes characters that are not US-ASCII and thus sample message text includes characters that are not US-ASCII and
must be transfer encoded. Though not shown here, the end of each line thus must be transfer encoded. Though not shown here, the end of each
is <CR><LF>. The line ending of the MIME headers, the text, and line is <CR><LF>. The line ending of the MIME headers, the text, and
transfer encoded parts, all must be <CR><LF>. transfer encoded parts, all must be <CR><LF>.
Note that this example is not of an S/MIME message. Note that this example is not of an S/MIME message.
Content-Type: multipart/mixed; boundary=bar Content-Type: multipart/mixed; boundary=bar
--bar --bar
Content-Type: text/plain; charset=iso-8859-1 Content-Type: text/plain; charset=iso-8859-1
Content-Transfer-Encoding: quoted-printable Content-Transfer-Encoding: quoted-printable
=A1Hola Michael! =A1Hola Michael!
How do you like the new S/MIME specification? How do you like the new S/MIME specification?
I agree. It's generally a good idea to encode lines that begin I agree. It's generally a good idea to encode lines that begin with
with From=20 because some mail transport agents will insert a
From=20because some mail transport agents will insert a greater- greater-than (>) sign, thus invalidating the signature.
than (>) sign, thus invalidating the signature.
Also, in some cases it might be desirable to encode any =20 Also, in some cases it might be desirable to encode any =20
trailing whitespace that occurs on lines in order to ensure =20 trailing whitespace that occurs on lines in order to ensure =20
that the message signature is not invalidated when passing =20 that the message signature is not invalidated when passing =20
a gateway that modifies such whitespace (like BITNET). =20 a gateway that modifies such whitespace (like BITNET). =20
--bar --bar
Content-Type: image/jpeg Content-Type: image/jpeg
Content-Transfer-Encoding: base64 Content-Transfer-Encoding: base64
iQCVAwUBMJrRF2N9oWBghPDJAQE9UQQAtl7LuRVndBjrk4EqYBIb3h5QXIX/LC// iQCVAwUBMJrRF2N9oWBghPDJAQE9UQQAtl7LuRVndBjrk4EqYBIb3h5QXIX/LC//
jJV5bNvkZIGPIcEmI5iFd9boEgvpirHtIREEqLQRkYNoBActFBZmh9GC3C041WGq jJV5bNvkZIGPIcEmI5iFd9boEgvpirHtIREEqLQRkYNoBActFBZmh9GC3C041WGq
uMbrbxc+nIs1TIKlA08rVi9ig/2Yh7LFrK5Ein57U/W72vgSxLhe/zhdfolT9Brn uMbrbxc+nIs1TIKlA08rVi9ig/2Yh7LFrK5Ein57U/W72vgSxLhe/zhdfolT9Brn
HOxEa44b+EI= HOxEa44b+EI=
--bar-- --bar--
3.2 The application/pkcs7-mime Type 3.2 The application/pkcs7-mime Type
The application/pkcs7-mime type is used to carry CMS objects of The application/pkcs7-mime type is used to carry CMS objects of
several types including envelopedData and signedData. The details of several types including envelopedData and signedData. The details of
constructing these entities is described in subsequent sections. This constructing these entities is described in subsequent sections. This
section describes the general characteristics of the application/pkcs7- section describes the general characteristics of the
mime type. application/pkcs7-mime type.
The carried CMS object always contains a MIME entity that is prepared The carried CMS object always contains a MIME entity that is prepared
as described in section 3.1 if the eContentType is id-data. Other as described in section 3.1 if the eContentType is id-data. Other
contents may be carried when the eContentType contains different contents may be carried when the eContentType contains different
values. See [ESS] for an example of this with signed receipts. values. See [ESS] for an example of this with signed receipts.
Since CMS objects are binary data, in most cases base-64 transfer Since CMS objects are binary data, in most cases base-64 transfer
encoding is appropriate, in particular when used with SMTP transport. encoding is appropriate, in particular when used with SMTP transport.
The transfer encoding used depends on the transport through which the The transfer encoding used depends on the transport through which the
object is to be sent, and is not a characteristic of the MIME type. object is to be sent, and is not a characteristic of the MIME type.
Note that this discussion refers to the transfer encoding of the CMS Note that this discussion refers to the transfer encoding of the CMS
object or "outside" MIME entity. It is completely distinct from, and object or "outside" MIME entity. It is completely distinct from, and
unrelated to, the transfer encoding of the MIME entity secured by the unrelated to, the transfer encoding of the MIME entity secured by the
CMS object, the "inside" object, which is described in section 3.1. CMS object, the "inside" object, which is described in section 3.1.
Because there are several types of application/pkcs7-mime objects, a Because there are several types of application/pkcs7-mime objects, a
sending agent SHOULD do as much as possible to help a receiving agent sending agent SHOULD do as much as possible to help a receiving agent
know about the contents of the object without forcing the receiving know about the contents of the object without forcing the receiving
agent to decode the ASN.1 for the object. The MIME headers of all agent to decode the ASN.1 for the object. The MIME headers of all
application/pkcs7-mime objects SHOULD include the optional "smime- application/pkcs7-mime objects SHOULD include the optional "smime-
type" parameter, as described in the following sections. type" parameter, as described in the following sections.
3.2.1 The name and filename Parameters 3.2.1 The name and filename Parameters
For the application/pkcs7-mime, sending agents SHOULD emit the For the application/pkcs7-mime, sending agents SHOULD emit the
optional "name" parameter to the Content-Type field for compatibility optional "name" parameter to the Content-Type field for compatibility
with older systems. Sending agents SHOULD also emit the optional with older systems. Sending agents SHOULD also emit the optional
Content-Disposition field [CONTDISP] with the "filename" parameter. If Content-Disposition field [CONTDISP] with the "filename" parameter.
a sending agent emits the above parameters, the value of the If a sending agent emits the above parameters, the value of the
parameters SHOULD be a file name with the appropriate extension: parameters SHOULD be a file name with the appropriate extension:
MIME Type File Extension MIME Type File Extension
Application/pkcs7-mime (signedData, .p7m Application/pkcs7-mime (signedData, .p7m
envelopedData) envelopedData)
Application/pkcs7-mime (degenerate .p7c Application/pkcs7-mime (degenerate .p7c
signedData "certs-only" message) signedData "certs-only" message)
Application/pkcs7-signature .p7s Application/pkcs7-signature .p7s
In addition, the file name SHOULD be limited to eight characters In addition, the file name SHOULD be limited to eight characters
followed by a three letter extension. The eight character filename followed by a three letter extension. The eight character filename
base can be any distinct name; the use of the filename base "smime" base can be any distinct name; the use of the filename base "smime"
SHOULD be used to indicate that the MIME entity is associated with SHOULD be used to indicate that the MIME entity is associated with
S/MIME. S/MIME.
Including a file name serves two purposes. It facilitates easier use Including a file name serves two purposes. It facilitates easier use
of S/MIME objects as files on disk. It also can convey type of S/MIME objects as files on disk. It also can convey type
information across gateways. When a MIME entity of type information across gateways. When a MIME entity of type
application/pkcs7-mime (for example) arrives at a gateway that has no application/pkcs7-mime (for example) arrives at a gateway that has no
special knowledge of S/MIME, it will default the entity's MIME type to special knowledge of S/MIME, it will default the entity's MIME type
application/octet-stream and treat it as a generic attachment, thus to application/octet-stream and treat it as a generic attachment,
losing the type information. However, the suggested filename for an thus losing the type information. However, the suggested filename for
attachment is often carried across a gateway. This often allows the an attachment is often carried across a gateway. This often allows
receiving systems to determine the appropriate application to hand the the receiving systems to determine the appropriate application to
attachment off to, in this case a stand-alone S/MIME processing hand the attachment off to, in this case a stand-alone S/MIME
application. Note that this mechanism is provided as a convenience for processing application. Note that this mechanism is provided as a
implementations in certain environments. A proper S/MIME convenience for implementations in certain environments. A proper
implementation MUST use the MIME types and MUST NOT rely on the file S/MIME implementation MUST use the MIME types and MUST NOT rely on
extensions. the file extensions.
3.2.2 The smime-type parameter 3.2.2 The smime-type parameter
The application/pkcs7-mime content type defines the optional "smime- The application/pkcs7-mime content type defines the optional "smime-
type" parameter. The intent of this parameter is to convey details type" parameter. The intent of this parameter is to convey details
about the security applied (signed or enveloped) along with infomation about the security applied (signed or enveloped) along with
about the contained content. This draft defines the following smime- infomation about the contained content. This memo defines the
types. following smime-types.
Name Security Inner Content Name Security Inner Content
enveloped-data EnvelopedData id-data enveloped-data EnvelopedData id-data
signed-data SignedData id-data signed-data SignedData id-data
certs-only SignedData none certs-only SignedData none
In order that consistency can be obtained with future, the following In order that consistency can be obtained with future, the following
guidelines should be followed when assigning a new smime-type guidelines should be followed when assigning a new smime-type
parameter. parameter.
1. If both signing and encryption can be applied to the content, then 1. If both signing and encryption can be applied to the content, then
two values for smime-type SHOULD be assigned "signed-*" and "encrypted- two values for smime-type SHOULD be assigned "signed-*" and
*". If one operation can be assigned then this may be omitted. Thus "encrypted-*". If one operation can be assigned then this may be
since "certs-only" can only be signed, "signed-" is omitted. omitted. Thus since "certs-only" can only be signed, "signed-" is
omitted.
2. A common string for a content oid should be assigned. We use "data" 2. A common string for a content oid should be assigned. We use
for the id-data content OID when MIME is the inner content. "data" for the id-data content OID when MIME is the inner content.
3. If no common string is assigned. Then the common string of 3. If no common string is assigned. Then the common string of
"OID.<oid>" is recommended (for example, "OID.1.3.6.1.5.5.7.6.1" would "OID.<oid>" is recommended (for example, "OID.1.3.6.1.5.5.7.6.1"
be DES40). would be DES40).
3.3 Creating an Enveloped-only Message 3.3 Creating an Enveloped-only Message
This section describes the format for enveloping a MIME entity without This section describes the format for enveloping a MIME entity
signing it. It is important to note that sending enveloped but not without signing it. It is important to note that sending enveloped
signed messages does not provide for data integrity. It is possible to but not signed messages does not provide for data integrity. It is
replace ciphertext in such a way that the processed message will still possible to replace ciphertext in such a way that the processed
be valid, but the meaning may be altered. message will still be valid, but the meaning may be altered.
Step 1. The MIME entity to be enveloped is prepared according to Step 1. The MIME entity to be enveloped is prepared according to
section 3.1. section 3.1.
Step 2. The MIME entity and other required data is processed into a Step 2. The MIME entity and other required data is processed into a
CMS object of type envelopedData. In addition to encrypting a copy of CMS object of type envelopedData. In addition to encrypting a copy of
the content-encryption key for each recipient, a copy of the content the content-encryption key for each recipient, a copy of the content
encryption key SHOULD be encrypted for the originator and included in encryption key SHOULD be encrypted for the originator and included in
the envelopedData (see CMS Section 6). the envelopedData (see CMS Section 6).
Step 3. The CMS object is inserted into an application/pkcs7-mime MIME Step 3. The CMS object is inserted into an application/pkcs7-mime
entity. MIME entity.
The smime-type parameter for enveloped-only messages is "enveloped- The smime-type parameter for enveloped-only messages is "enveloped-
data". The file extension for this type of message is ".p7m". data". The file extension for this type of message is ".p7m".
A sample message would be: A sample message would be:
Content-Type: application/pkcs7-mime; smime-type=enveloped-data; Content-Type: application/pkcs7-mime; smime-type=enveloped-data;
name=smime.p7m name=smime.p7m
Content-Transfer-Encoding: base64 Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=smime.p7m Content-Disposition: attachment; filename=smime.p7m
rfvbnj756tbBghyHhHUujhJhjH77n8HHGT9HG4VQpfyF467GhIGfHfYT6 rfvbnj756tbBghyHhHUujhJhjH77n8HHGT9HG4VQpfyF467GhIGfHfYT6
7n8HHGghyHhHUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H 7n8HHGghyHhHUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H
f8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4 f8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
0GhIGfHfQbnj756YT64V 0GhIGfHfQbnj756YT64V
3.4 Creating a Signed-only Message 3.4 Creating a Signed-only Message
There are two formats for signed messages defined for S/MIME: There are two formats for signed messages defined for S/MIME:
application/pkcs7-mime with SignedData, and multipart/signed. In application/pkcs7-mime with SignedData, and multipart/signed. In
general, the multipart/signed form is preferred for sending, and general, the multipart/signed form is preferred for sending, and
receiving agents SHOULD be able to handle both. receiving agents SHOULD be able to handle both.
3.4.1 Choosing a Format for Signed-only Messages 3.4.1 Choosing a Format for Signed-only Messages
There are no hard-and-fast rules when a particular signed-only format There are no hard-and-fast rules when a particular signed-only format
should be chosen because it depends on the capabilities of all the should be chosen because it depends on the capabilities of all the
receivers and the relative importance of receivers with S/MIME receivers and the relative importance of receivers with S/MIME
facilities being able to verify the signature versus the importance of facilities being able to verify the signature versus the importance
receivers without S/MIME software being able to view the message. of receivers without S/MIME software being able to view the message.
Messages signed using the multipart/signed format can always be viewed Messages signed using the multipart/signed format can always be
by the receiver whether they have S/MIME software or not. They can viewed by the receiver whether they have S/MIME software or not. They
also be viewed whether they are using a MIME-native user agent or they can also be viewed whether they are using a MIME-native user agent or
have messages translated by a gateway. In this context, "be viewed" they have messages translated by a gateway. In this context, "be
means the ability to process the message essentially as if it were not viewed" means the ability to process the message essentially as if it
a signed message, including any other MIME structure the message might were not a signed message, including any other MIME structure the
have. message might have.
Messages signed using the signedData format cannot be viewed by a Messages signed using the signedData format cannot be viewed by a
recipient unless they have S/MIME facilities. However, if they have recipient unless they have S/MIME facilities. However, if they have
S/MIME facilities, these messages can always be verified if they were S/MIME facilities, these messages can always be verified if they were
not changed in transit. not changed in transit.
3.4.2 Signing Using application/pkcs7-mime with SignedData 3.4.2 Signing Using application/pkcs7-mime with SignedData
This signing format uses the application/pkcs7-mime MIME type. The This signing format uses the application/pkcs7-mime MIME type. The
steps to create this format are: steps to create this format are:
Step 1. The MIME entity is prepared according to section 3.1
Step 2. The MIME entity and other required data is processed into a Step 1. The MIME entity is prepared according to section 3.1
CMS object of type signedData
Step 3. The CMS object is inserted into an application/pkcs7-mime MIME Step 2. The MIME entity and other required data is processed into a
entity CMS object of type signedData
The smime-type parameter for messages using application/pkcs7-mime Step 3. The CMS object is inserted into an application/pkcs7-mime
with SignedData is "signed-data". The file extension for this type of MIME entity
message is ".p7m".
A sample message would be: The smime-type parameter for messages using application/pkcs7-mime
with SignedData is "signed-data". The file extension for this type of
message is ".p7m".
Content-Type: application/pkcs7-mime; smime-type=signed-data; A sample message would be:
name=smime.p7m
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=smime.p7m
567GhIGfHfYT6ghyHhHUujpfyF4f8HHGTrfvhJhjH776tbB9HG4VQbnj7 Content-Type: application/pkcs7-mime; smime-type=signed-data;
77n8HHGT9HG4VQpfyF467GhIGfHfYT6rfvbnj756tbBghyHhHUujhJhjH name=smime.p7m
HUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H7n8HHGghyHh Content-Transfer-Encoding: base64
6YT64V0GhIGfHfQbnj75 Content-Disposition: attachment; filename=smime.p7m
567GhIGfHfYT6ghyHhHUujpfyF4f8HHGTrfvhJhjH776tbB9HG4VQbnj7
77n8HHGT9HG4VQpfyF467GhIGfHfYT6rfvbnj756tbBghyHhHUujhJhjH
HUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H7n8HHGghyHh
6YT64V0GhIGfHfQbnj75
3.4.3 Signing Using the multipart/signed Format 3.4.3 Signing Using the multipart/signed Format
This format is a clear-signing format. Recipients without any S/MIME This format is a clear-signing format. Recipients without any S/MIME
or CMS processing facilities are able to view the message. It makes or CMS processing facilities are able to view the message. It makes
use of the multipart/signed MIME type described in [MIME-SECURE]. The use of the multipart/signed MIME type described in [MIME-SECURE]. The
multipart/signed MIME type has two parts. The first part contains the multipart/signed MIME type has two parts. The first part contains the
MIME entity that is signed; the second part contains the "detached MIME entity that is signed; the second part contains the "detached
signature" CMS SignedData object in which the encapContentInfo signature" CMS SignedData object in which the encapContentInfo
eContent field is absent. eContent field is absent.
3.4.3.1 The application/pkcs7-signature MIME Type 3.4.3.1 The application/pkcs7-signature MIME Type
This MIME type always contains a single CMS object of type signedData. This MIME type always contains a single CMS object of type
The signedData encapContentInfo eContent field MUST be absent. The signedData. The signedData encapContentInfo eContent field MUST be
signerInfos field contains the signatures for the MIME entity. absent. The signerInfos field contains the signatures for the MIME
entity.
The file extension for signed-only messages using application/pkcs7- The file extension for signed-only messages using application/pkcs7-
signature is ".p7s". signature is ".p7s".
3.4.3.2 Creating a multipart/signed Message 3.4.3.2 Creating a multipart/signed Message
Step 1. The MIME entity to be signed is prepared according to section Step 1. The MIME entity to be signed is prepared according to section
3.1, taking special care for clear-signing. 3.1, taking special care for clear-signing.
Step 2. The MIME entity is presented to CMS processing in order to Step 2. The MIME entity is presented to CMS processing in order to
obtain an object of type signedData in which the encapContentInfo obtain an object of type signedData in which the encapContentInfo
eContent field is absent. eContent field is absent.
Step 3. The MIME entity is inserted into the first part of a Step 3. The MIME entity is inserted into the first part of a
multipart/signed message with no processing other than that described multipart/signed message with no processing other than that described
in section 3.1. in section 3.1.
Step 4. Transfer encoding is applied to the "detached signature" CMS Step 4. Transfer encoding is applied to the "detached signature" CMS
SignedData object and it is inserted into a MIME entity of type SignedData object and it is inserted into a MIME entity of type
application/pkcs7-signature. application/pkcs7-signature.
Step 5. The MIME entity of the application/pkcs7-signature is inserted Step 5. The MIME entity of the application/pkcs7-signature is
into the second part of the multipart/signed entity. inserted into the second part of the multipart/signed entity.
The multipart/signed Content type has two required parameters: the The multipart/signed Content type has two required parameters: the
protocol parameter and the micalg parameter. protocol parameter and the micalg parameter.
The protocol parameter MUST be "application/pkcs7-signature". Note The protocol parameter MUST be "application/pkcs7-signature". Note
that quotation marks are required around the protocol parameter that quotation marks are required around the protocol parameter
because MIME requires that the "/" character in the parameter value because MIME requires that the "/" character in the parameter value
MUST be quoted. MUST be quoted.
The micalg parameter allows for one-pass processing when the signature The micalg parameter allows for one-pass processing when the
is being verified. The value of the micalg parameter is dependent on signature is being verified. The value of the micalg parameter is
the message digest algorithm(s) used in the calculation of the Message dependent on the message digest algorithm(s) used in the calculation
Integrity Check. If multiple message digest algorithms are used they of the Message Integrity Check. If multiple message digest algorithms
MUST be separated by commas per [MIME-SECURE]. The values to be placed are used they MUST be separated by commas per [MIME-SECURE]. The
in the micalg parameter SHOULD be from the following: values to be placed in the micalg parameter SHOULD be from the
following:
Algorithm Value Algorithm Value
used used
MD5 md5 MD5 md5
SHA-1 sha1 SHA-1 sha1
Any other unknown Any other unknown
(Historical note: some early implementations of S/MIME emitted and (Historical note: some early implementations of S/MIME emitted and
expected "rsa-md5" and "rsa-sha1" for the micalg parameter.) Receiving expected "rsa-md5" and "rsa-sha1" for the micalg parameter.)
agents SHOULD be able to recover gracefully from a micalg parameter Receiving agents SHOULD be able to recover gracefully from a micalg
value that they do not recognize. parameter value that they do not recognize.
3.4.3.3 Sample multipart/signed Message 3.4.3.3 Sample multipart/signed Message
Content-Type: multipart/signed; Content-Type: multipart/signed;
protocol="application/pkcs7-signature"; protocol="application/pkcs7-signature";
micalg=sha1; boundary=boundary42 micalg=sha1; boundary=boundary42
--boundary42 --boundary42
Content-Type: text/plain Content-Type: text/plain
This is a clear-signed message. This is a clear-signed message.
--boundary42 --boundary42
Content-Type: application/pkcs7-signature; name=smime.p7s Content-Type: application/pkcs7-signature; name=smime.p7s
Content-Transfer-Encoding: base64 Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=smime.p7s Content-Disposition: attachment; filename=smime.p7s
ghyHhHUujhJhjH77n8HHGTrfvbnj756tbB9HG4VQpfyF467GhIGfHfYT6 ghyHhHUujhJhjH77n8HHGTrfvbnj756tbB9HG4VQpfyF467GhIGfHfYT6
4VQpfyF467GhIGfHfYT6jH77n8HHGghyHhHUujhJh756tbB9HGTrfvbnj 4VQpfyF467GhIGfHfYT6jH77n8HHGghyHhHUujhJh756tbB9HGTrfvbnj
n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4 n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
7GhIGfHfYT64VQbnj756 7GhIGfHfYT64VQbnj756
--boundary42-- --boundary42--
3.5 Signing and Encrypting 3.5 Signing and Encrypting
To achieve signing and enveloping, any of the signed-only and To achieve signing and enveloping, any of the signed-only and
encrypted-only formats may be nested. This is allowed because the encrypted-only formats may be nested. This is allowed because the
above formats are all MIME entities, and because they all secure MIME above formats are all MIME entities, and because they all secure MIME
entities. entities.
An S/MIME implementation MUST be able to receive and process An S/MIME implementation MUST be able to receive and process
arbitrarily nested S/MIME within reasonable resource limits of the arbitrarily nested S/MIME within reasonable resource limits of the
recipient computer. recipient computer.
It is possible to either sign a message first, or to envelope the It is possible to either sign a message first, or to envelope the
message first. It is up to the implementor and the user to choose. message first. It is up to the implementor and the user to choose.
When signing first, the signatories are then securely obscured by the When signing first, the signatories are then securely obscured by the
enveloping. When enveloping first the signatories are exposed, but it enveloping. When enveloping first the signatories are exposed, but it
is possible to verify signatures without removing the enveloping. This is possible to verify signatures without removing the enveloping.
may be useful in an environment were automatic signature verification This may be useful in an environment were automatic signature
is desired, as no private key material is required to verify a verification is desired, as no private key material is required to
signature. verify a signature.
There are security ramifications to choosing whether to sign first or There are security ramifications to choosing whether to sign first or
encrypt first. A recipient of a message that is encrypted and then encrypt first. A recipient of a message that is encrypted and then
signed can validate that the encrypted block was unaltered, but cannot signed can validate that the encrypted block was unaltered, but
determine any relationship between the signer and the unencrypted cannot determine any relationship between the signer and the
contents of the message. A recipient of a message that is signed-then- unencrypted contents of the message. A recipient of a message that is
encrypted can assume that the signed message itself has not been signed-then-encrypted can assume that the signed message itself has
altered, but that a careful attacker may have changed the not been altered, but that a careful attacker may have changed the
unauthenticated portions of the encrypted message. unauthenticated portions of the encrypted message.
3.6 Creating a Certificates-only Message 3.6 Creating a Certificates-only Message
The certificates only message or MIME entity is used to transport The certificates only message or MIME entity is used to transport
certificates, such as in response to a registration request. This certificates, such as in response to a registration request. This
format can also be used to convey CRLs. format can also be used to convey CRLs.
Step 1. The certificates are made available to the CMS generating Step 1. The certificates are made available to the CMS generating
process which creates a CMS object of type signedData. The signedData process which creates a CMS object of type signedData. The signedData
encapContentInfo eContent field MUST be absent and signerInfos field encapContentInfo eContent field MUST be absent and signerInfos field
MUST be empty. MUST be empty.
Step 2. The CMS signedData object is enclosed in an application/pkcs7- Step 2. The CMS signedData object is enclosed in an
mime MIME entity application/pkcs7-mime MIME entity
The smime-type parameter for a certs-only message is "certs-only". The smime-type parameter for a certs-only message is "certs-only".
The file extension for this type of message is ".p7c". The file extension for this type of message is ".p7c".
3.7 Registration Requests 3.7 Registration Requests
A sending agent that signs messages MUST have a certificate for the A sending agent that signs messages MUST have a certificate for the
signature so that a receiving agent can verify the signature. There signature so that a receiving agent can verify the signature. There
are many ways of getting certificates, such as through an exchange are many ways of getting certificates, such as through an exchange
with a certificate authority, through a hardware token or diskette, with a certificate authority, through a hardware token or diskette,
and so on. and so on.
S/MIME v2 [SMIMEV2] specified a method for "registering" public keys S/MIME v2 [SMIMEV2] specified a method for "registering" public keys
with certificate authorities using an application/pkcs10 body part. with certificate authorities using an application/pkcs10 body part.
The IETF's PKIX Working Group is preparing another method for The IETF's PKIX Working Group is preparing another method for
requesting certificates; however, that work was not finished at the requesting certificates; however, that work was not finished at the
time of this draft. S/MIME v3 does not specify how to request a time of this memo. S/MIME v3 does not specify how to request a
certificate, but instead mandates that every sending agent already has
a certificate. Standardization of certificate management is being certificate, but instead mandates that every sending agent already
pursued separately in the IETF. has a certificate. Standardization of certificate management is being
pursued separately in the IETF.
3.8 Identifying an S/MIME Message 3.8 Identifying an S/MIME Message
Because S/MIME takes into account interoperation in non-MIME Because S/MIME takes into account interoperation in non-MIME
environments, several different mechanisms are employed to carry the environments, several different mechanisms are employed to carry the
type information, and it becomes a bit difficult to identify S/MIME type information, and it becomes a bit difficult to identify S/MIME
messages. The following table lists criteria for determining whether messages. The following table lists criteria for determining whether
or not a message is an S/MIME message. A message is considered an or not a message is an S/MIME message. A message is considered an
S/MIME message if it matches any below. S/MIME message if it matches any below.
The file suffix in the table below comes from the "name" parameter in The file suffix in the table below comes from the "name" parameter in
the content-type header, or the "filename" parameter on the content- the content-type header, or the "filename" parameter on the content-
disposition header. These parameters that give the file suffix are not disposition header. These parameters that give the file suffix are
listed below as part of the parameter section. not listed below as part of the parameter section.
MIME type: application/pkcs7-mime MIME type: application/pkcs7-mime
parameters: any parameters: any
file suffix: any file suffix: any
MIME type: multipart/signed MIME type: multipart/signed
parameters: protocol="application/pkcs7-signature" parameters: protocol="application/pkcs7-signature"
file suffix: any file suffix: any
MIME type: application/octet-stream MIME type: application/octet-stream
parameters: any parameters: any
file suffix: p7m, p7s, p7c file suffix: p7m, p7s, p7c
4. Certificate Processing 4. Certificate Processing
A receiving agent MUST provide some certificate retrieval mechanism in A receiving agent MUST provide some certificate retrieval mechanism
order to gain access to certificates for recipients of digital in order to gain access to certificates for recipients of digital
envelopes. This draft does not cover how S/MIME agents handle envelopes. This memo does not cover how S/MIME agents handle
certificates, only what they do after a certificate has been validated certificates, only what they do after a certificate has been
or rejected. S/MIME certification issues are covered in [CERT3]. validated or rejected. S/MIME certification issues are covered in
[CERT3].
At a minimum, for initial S/MIME deployment, a user agent could At a minimum, for initial S/MIME deployment, a user agent could
automatically generate a message to an intended recipient requesting automatically generate a message to an intended recipient requesting
that recipient's certificate in a signed return message. Receiving and that recipient's certificate in a signed return message. Receiving
sending agents SHOULD also provide a mechanism to allow a user to and sending agents SHOULD also provide a mechanism to allow a user to
"store and protect" certificates for correspondents in such a way so "store and protect" certificates for correspondents in such a way so
as to guarantee their later retrieval. as to guarantee their later retrieval.
4.1 Key Pair Generation 4.1 Key Pair Generation
If an S/MIME agent needs to generate a key pair, then the S/MIME agent If an S/MIME agent needs to generate a key pair, then the S/MIME
or some related administrative utility or function MUST be capable of agent or some related administrative utility or function MUST be
generating separate DH and DSS public/private key pairs on behalf of capable of generating separate DH and DSS public/private key pairs on
the user. Each key pair MUST be generated from a good source of non- behalf of the user. Each key pair MUST be generated from a good
deterministic random input [RANDOM] and the private key MUST be source of non-deterministic random input [RANDOM] and the private key
protected in a secure fashion. MUST be protected in a secure fashion.
If an S/MIME agent needs to generate a key pair, then the S/MIME agent If an S/MIME agent needs to generate a key pair, then the S/MIME
or some related administrative utility or function SHOULD generate RSA agent or some related administrative utility or function SHOULD
key pairs. generate RSA key pairs.
A user agent SHOULD generate RSA key pairs at a minimum key size of A user agent SHOULD generate RSA key pairs at a minimum key size of
768 bits. A user agent MUST NOT generate RSA key pairs less than 512 768 bits. A user agent MUST NOT generate RSA key pairs less than 512
bits long. Creating keys longer than 1024 bits may cause some older bits long. Creating keys longer than 1024 bits may cause some older
S/MIME receiving agents to not be able to verify signatures, but gives S/MIME receiving agents to not be able to verify signatures, but
better security and is therefore valuable. A receiving agent SHOULD be gives better security and is therefore valuable. A receiving agent
able to verify signatures with keys of any size over 512 bits. Some SHOULD be able to verify signatures with keys of any size over 512
agents created in the United States have chosen to create 512 bit keys bits. Some agents created in the United States have chosen to create
in order to get more advantageous export licenses. However, 512 bit 512 bit keys in order to get more advantageous export licenses.
keys are considered by many to be cryptographically insecure. However, 512 bit keys are considered by many to be cryptographically
Implementors should be aware that multiple (active) key pairs may be insecure. Implementors should be aware that multiple (active) key
associated with a single individual. For example, one key pair may be pairs may be associated with a single individual. For example, one
used to support confidentiality, while a different key pair may be key pair may be used to support confidentiality, while a different
used for authentication. key pair may be used for authentication.
5. Security 5. Security
This entire draft discusses security. Security issues not covered in This entire memo discusses security. Security issues not covered in
other parts of the draft include: other parts of the memo include:
40-bit encryption is considered weak by most cryptographers. Using 40-bit encryption is considered weak by most cryptographers. Using
weak cryptography in S/MIME offers little actual security over sending weak cryptography in S/MIME offers little actual security over
plaintext. However, other features of S/MIME, such as the sending plaintext. However, other features of S/MIME, such as the
specification of tripleDES and the ability to announce stronger specification of tripleDES and the ability to announce stronger
cryptographic capabilities to parties with whom you communicate, allow cryptographic capabilities to parties with whom you communicate,
senders to create messages that use strong encryption. Using weak allow senders to create messages that use strong encryption. Using
cryptography is never recommended unless the only alternative is no weak cryptography is never recommended unless the only alternative is
cryptography. When feasible, sending and receiving agents should no cryptography. When feasible, sending and receiving agents should
inform senders and recipients the relative cryptographic strength of inform senders and recipients the relative cryptographic strength of
messages. messages.
It is impossible for most software or people to estimate the value of It is impossible for most software or people to estimate the value of
a message. Further, it is impossible for most software or people to a message. Further, it is impossible for most software or people to
estimate the actual cost of decrypting a message that is encrypted estimate the actual cost of decrypting a message that is encrypted
with a key of a particular size. Further, it is quite difficult to with a key of a particular size. Further, it is quite difficult to
determine the cost of a failed decryption if a recipient cannot decode determine the cost of a failed decryption if a recipient cannot
a message. Thus, choosing between different key sizes (or choosing decode a message. Thus, choosing between different key sizes (or
whether to just use plaintext) is also impossible. However, decisions choosing whether to just use plaintext) is also impossible. However,
based on these criteria are made all the time, and therefore this decisions based on these criteria are made all the time, and
draft gives a framework for using those estimates in choosing therefore this memo gives a framework for using those estimates in
algorithms. choosing algorithms.
If a sending agent is sending the same message using different If a sending agent is sending the same message using different
strengths of cryptography, an attacker watching the communications strengths of cryptography, an attacker watching the communications
channel may be able to determine the contents of the strongly- channel may be able to determine the contents of the strongly-
encrypted message by decrypting the weakly-encrypted version. In other encrypted message by decrypting the weakly-encrypted version. In
words, a sender should not send a copy of a message using weaker other words, a sender should not send a copy of a message using
cryptography than they would use for the original of the message. weaker cryptography than they would use for the original of the
message.
Modification of the ciphertext can go undetected if authentication is Modification of the ciphertext can go undetected if authentication is
not also used, which is the case when sending EnvelopedData without not also used, which is the case when sending EnvelopedData without
wrapping it in SignedData or enclosing SignedData within it. wrapping it in SignedData or enclosing SignedData within it.
A. ASN.1 Module A. ASN.1 Module
SecureMimeMessageV3 SecureMimeMessageV3
{ iso(1) member-body(2) us(840) rsadsi(113549) { iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) smime(4) } pkcs(1) pkcs-9(9) smime(16) modules(0) smime(4) }
DEFINITIONS IMPLICIT TAGS ::= DEFINITIONS IMPLICIT TAGS ::=
BEGIN BEGIN
IMPORTS IMPORTS
-- Cryptographic Message Syntax -- Cryptographic Message Syntax
SubjectKeyIdentifier, IssuerAndSerialNumber, SubjectKeyIdentifier, IssuerAndSerialNumber,
RecipientKeyIdentifier RecipientKeyIdentifier
FROM CryptographicMessageSyntax FROM CryptographicMessageSyntax
{ iso(1) member-body(2) us(840) rsadsi(113549) { iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) cms(1) }; pkcs(1) pkcs-9(9) smime(16) modules(0) cms(1) };
-- id-aa is the arc with all new authenticated and unauthenticated -- id-aa is the arc with all new authenticated and unauthenticated
attributes -- attributes produced the by S/MIME Working Group
id-aa OBJECT IDENTIFIER ::= {iso(1) member-body(2) usa(840) id-aa OBJECT IDENTIFIER ::= {iso(1) member-body(2) usa(840)
rsadsi(113549) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) attributes(2)} pkcs(1) pkcs-9(9) smime(16) attributes(2)}
-- S/MIME Capabilities provides a method of broadcasting the symetric -- S/MIME Capabilities provides a method of broadcasting the symetric
capabilities -- capabilities understood. Algorithms should be ordered by preference
grouped -- and grouped by type
by type
smimeCapabilities OBJECT IDENTIFIER ::= smimeCapabilities OBJECT IDENTIFIER ::=
{iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 15} {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 15}
SMIMECapability ::= SEQUENCE { SMIMECapability ::= SEQUENCE {
capabilityID OBJECT IDENTIFIER, capabilityID OBJECT IDENTIFIER,
parameters ANY DEFINED BY capabilityID OPTIONAL } parameters ANY DEFINED BY capabilityID OPTIONAL }
SMIMECapabilities ::= SEQUENCE OF SMIMECapability SMIMECapabilities ::= SEQUENCE OF SMIMECapability
-- Encryption Key Preference provides a method of broadcasting the -- Encryption Key Preference provides a method of broadcasting the
prefered -- preferred encryption certificate.
id-aa-encrypKeyPref OBJECT IDENTIFIER ::= {id-aa 11} id-aa-encrypKeyPref OBJECT IDENTIFIER ::= {id-aa 11}
SMIMEEncryptionKeyPreference ::= CHOICE { SMIMEEncryptionKeyPreference ::= CHOICE {
issuerAndSerialNumber [0] IssuerAndSerialNumber, issuerAndSerialNumber [0] IssuerAndSerialNumber,
receipentKeyId [1] RecipientKeyIdentifier, receipentKeyId [1] RecipientKeyIdentifier,
subjectAltKeyIdentifier [2] SubjectKeyIdentifier subjectAltKeyIdentifier [2] SubjectKeyIdentifier
} }
-- The Content Encryption Algorithms defined for SMIME are: -- The Content Encryption Algorithms defined for SMIME are:
-- Triple-DES is the manditory algorithm with CBCParameter being the -- Triple-DES is the manditory algorithm with CBCParameter being the
parameters -- parameters
dES-EDE3-CBC OBJECT IDENTIFIER ::= dES-EDE3-CBC OBJECT IDENTIFIER ::=
{iso(1) member-body(2) us(840) rsadsi(113549) {iso(1) member-body(2) us(840) rsadsi(113549)
encryptionAlgorithm(3) 7} encryptionAlgorithm(3) 7}
CBCParameter ::= IV CBCParameter ::= IV
IV ::= OCTET STRING (SIZE (8..8)) IV ::= OCTET STRING (SIZE (8..8))
-- RC2 (or compatable) is an optional algorithm w/ RC2-CBC-paramter -- RC2 (or compatable) is an optional algorithm w/ RC2-CBC-paramter
as the -- as the parameter
parameter
rC2-CBC OBJECT IDENTIFIER ::= rC2-CBC OBJECT IDENTIFIER ::=
{iso(1) member-body(2) us(840) rsadsi(113549) {iso(1) member-body(2) us(840) rsadsi(113549)
encryptionAlgorithm(3) 2} encryptionAlgorithm(3) 2}
-- For the effective-key-bits (key size) greater than 32 and less than -- For the effective-key-bits (key size) greater than 32 and less than
-- 256, the RC2-CBC algorithm parameters are encoded as: -- 256, the RC2-CBC algorithm parameters are encoded as:
RC2-CBC-parameter ::= SEQUENCE { RC2-CBC-parameter ::= SEQUENCE {
rc2ParameterVersion INTEGER, rc2ParameterVersion INTEGER,
iv IV} iv IV}
-- For the effective-key-bits of 40, 64, and 128, the -- For the effective-key-bits of 40, 64, and 128, the
rc2ParameterVersion -- rc2ParameterVersion values are 160, 120, 58 respectively.
-- The following list the OIDs to be used with S/MIME V3
-- Digest Algorithms: -- Digest Algorithms:
-- md5 OBJECT IDENTIFIER ::= -- md5 OBJECT IDENTIFIER ::=
digestAlgorithm(2) 5} -- {iso(1) member-body(2) us(840) rsadsi(113549)
-- digestAlgorithm(2) 5}
-- sha-1 OBJECT IDENTIFIER ::= -- sha-1 OBJECT IDENTIFIER ::=
algorithm(2) -- {iso(1) identified-organization(3) oiw(14) secsig(3)
-- algorithm(2) 26}
-- Asymmetric Encryption Algorithms -- Asymmetric Encryption Algorithms
-- --
-- rsaEncryption OBJECT IDENTIFIER ::= -- rsaEncryption OBJECT IDENTIFIER ::=
1} -- {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1)
-- 1}
-- --
-- rsa OBJECT IDENTIFIER ::= -- rsa OBJECT IDENTIFIER ::=
-- {joint-iso-ccitt(2) ds(5) algorithm(8) encryptionAlgorithm(1) 1}
-- --
-- id-dsa OBJECT IDENTIFIER ::= -- id-dsa OBJECT IDENTIFIER ::=
-- {iso(1) member-body(2) us(840) x9-57(10040) x9cm(4) 1 } -- {iso(1) member-body(2) us(840) x9-57(10040) x9cm(4) 1 }
-- Signature Algorithms -- Signature Algorithms
-- --
-- md2WithRSAEncryption OBJECT IDENTIFIER ::= -- md2WithRSAEncryption OBJECT IDENTIFIER ::=
-- {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) -- {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1)
2} -- 2}
-- --
-- md5WithRSAEncryption OBJECT IDENTIFIER ::= -- md5WithRSAEncryption OBJECT IDENTIFIER ::=
-- {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) -- {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1)
4} -- 4}
-- --
-- sha-1WithRSAEncryption OBJECT IDENTIFIER ::= -- sha-1WithRSAEncryption OBJECT IDENTIFIER ::=
-- {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) -- {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1)
5} -- 5}
-- --
-- id-dsa-with-sha1 OBJECT IDENTIFIER ::= -- id-dsa-with-sha1 OBJECT IDENTIFIER ::=
-- {iso(1) member-body(2) us(840) x9-57(10040) x9cm(4) 3} -- {iso(1) member-body(2) us(840) x9-57(10040) x9cm(4) 3}
-- Other Signed Attributes -- Other Signed Attributes
-- --
-- signingTime OBJECT IDENTIFIER ::= -- signingTime OBJECT IDENTIFIER ::=
-- {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) -- {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
5} -- 5}
-- See [CMS] for a description of how to encode the attribute -- See [CMS] for a description of how to encode the attribute
value. -- value.
END END
B. References B. References
[3DES] ANSI X9.52-1998, "Triple Data Encryption Algorithm Modes of [3DES] ANSI X9.52-1998, "Triple Data Encryption Algorithm
Operation", American National Standards Institute, 1998. Modes of Operation", American National Standards
Institute, 1998.
[CERT3] "S/MIME Version 3 Certificate Handling", Internet Draft draft- [CERT3] Ramsdell, B., Editor, "S/MIME Version 3 Certificate
ietf-smime-cert-*.txt. Handling", RFC 2632, June 1999.
[CHARSETS] Character sets assigned by IANA. See <ftp://ftp.isi.edu/in- [CHARSETS] Character sets assigned by IANA. See
notes/iana/assignments/character-sets>. <ftp://ftp.isi.edu/in-
notes/iana/assignments/character-sets>.
[CMS] "Cryptographic Message Syntax", Internet Draft draft-ietf-smime- [CMS] Housley, R., "Cryptographic Message Syntax", RFC 2630,
cms-*.txt. June 1999.
[CONTDISP] "Communicating Presentation Information in Internet [CONTDISP] Troost, R., Dorner, S. and K. Moore, "Communicating
Messages: The Content-Disposition Header Field", RFC 2183 Presentation Information in Internet Messages: The
Content-Disposition Header Field", RFC 2183, August
1997.
[DES] ANSI X3.106, "American National Standard for Information Systems- [DES] ANSI X3.106, "American National Standard for
Data Link Encryption," American National Standards Institute, 1983. Information Systems- Data Link Encryption," American
National Standards Institute, 1983.
[DH] "Diffie-Hellman Key Agreement Method", Internet Draft draft-ietf- [DH] Rescorla, E., "Diffie-Hellman Key Agreement Method",
smime-x942-*.txt RFC 2631, June 1999.
[DSS] NIST FIPS PUB 186, "Digital Signature Standard", 18 May 1994. [DSS] NIST FIPS PUB 186, "Digital Signature Standard", 18
May 1994.
[ESS] "Enhanced Security Services for S/MIME", Internet draft, draft- [ESS] Hoffman, P., Editor "Enhanced Security Services for
ietf-smime-ess-*.txt. S/MIME", RFC 2634, June 1999.
[MD5] "The MD5 Message Digest Algorithm", RFC 1321 [MD5] Rivest, R., "The MD5 Message Digest Algorithm", RFC
1321, April 1992.
[MIME-SPEC] The primary definition of MIME. "MIME Part 1: Format of [MIME-SPEC] The primary definition of MIME. "MIME Part 1: Format
Internet Message Bodies", RFC 2045; "MIME Part 2: Media Types", RFC of Internet Message Bodies", RFC 2045; "MIME Part 2:
2046; "MIME Part 3: Message Header Extensions for Non-ASCII Text", RFC Media Types", RFC 2046; "MIME Part 3: Message Header
2047; "MIME Part 4: Registration Procedures", RFC 2048; "MIME Part 5: Extensions for Non-ASCII Text", RFC 2047; "MIME Part
Conformance Criteria and Examples", RFC 2049 4: Registration Procedures", RFC 2048; "MIME Part 5:
Conformance Criteria and Examples", RFC 2049,
September 1993.
[MIME-SECURE] "Security Multiparts for MIME: Multipart/Signed and [MIME-SECURE] Galvin, J., Murphy, S., Crocker, S. and N. Freed,
Multipart/Encrypted", RFC 1847 "Security Multiparts for MIME: Multipart/Signed and
Multipart/Encrypted", RFC 1847, October 1995.
[MUSTSHOULD] "Key words for use in RFCs to Indicate Requirement [MUSTSHOULD] Bradner, S., "Key words for use in RFCs to Indicate
Levels", RFC 2119 Requirement Levels", BCP14, RFC 2119, March 1997.
[PKCS-1] "PKCS #1: RSA Encryption Version 1.5", RFC 2313 [PKCS-1] Kaliski, B., "PKCS #1: RSA Encryption Version 2.0",
RFC 2437, October 1998.
[PKCS-7] "PKCS #7: Cryptographic Message Syntax Version 1.5", RFC 2315 [PKCS-7] Kaliski, B., "PKCS #7: Cryptographic Message Syntax
Version 1.5", RFC 2315, March 1998.
[RANDOM] "Randomness Recommendations for Security", RFC 1750 [RANDOM] Eastlake, 3rd, D., Crocker, S. and J. Schiller,
"Randomness Recommendations for Security", RFC 1750,
December 1994.
[RC2] "A Description of the RC2 (r) Encryption Algorithm", RFC 2268 [RC2] Rivest, R., "A Description of the RC2 (r) Encryption
Algorithm", RFC 2268, January 1998.
[SHA1] NIST FIPS PUB 180-1, "Secure Hash Standard," National Institute [SHA1] NIST FIPS PUB 180-1, "Secure Hash Standard," National
of Standards and Technology, U.S. Department of Commerce, DRAFT, 31May Institute of Standards and Technology, U.S. Department
1994. of Commerce, DRAFT, 31May 1994.
[SMIMEV2] "S/MIME Version 2 Message Specification", RFC 2311 [SMIMEV2] Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L.
and L. Repka, "S/MIME Version 2 Message
Specification", RFC 2311, March 1998.
C. Acknowledgements C. Acknowledgements
<TBD> Many thanks go out to the other authors of the S/MIME Version 2
Message Specification RFC: Steve Dusse, Paul Hoffman, Laurence
Lundblade and Lisa Repka. Without v2, there wouldn't be a v3.
D. Changes from last draft A number of the members of the S/MIME Working Group have also worked
very hard and contributed to this document. Any list of people is
doomed to omission, and for that I apologize. In alphabetical order,
the following people stand out in my mind due to the fact that they
made direct contributions to this document.
Clarified section 2.4.1 in the case of multipart/signed (eContent is Dave Crocker
absent in that case) (Jim Schaad) Bill Flanigan
Removed receipt request attribute from section 2.5 (Jim Schaad) Paul Hoffman
Capitalized MUST for use of the issuerAndSerialNumber CHOICE in Russ Housley
section 2.6 (Jim Schaad) John Pawling
Capitalized NOT in section 3.2.1 regarding reliance on file extensions Jim Schaad
(Jim Schaad)
Changed [DH] reference to refer to draft-ietf-smime-x942-*.txt (Jim
Schaad)
Replaced section A with ASN.1 module (Jim Schaad)
Rewording of 2.7.3 to explain that the content of the strongly-
encrypted message can be learned by decrypting the weaker message
(Russ Housley)
Provided example OID. string for new smime-type values in section
3.2.2 (Russ Housley)
Rewording of section 5 regarding sending two messages with different
levels of encryption (Russ Housley)
Added [RANDOM] reference in section 4.1 and to section B (Russ
Housley)
Explained in section 2.5.2 that section A contains all of the MUST and
SHOULD OIDs (Russ Housley)
Added language to 2.2 and 2.3 about S/MIME v2 clients only have
rsaEncryption (Paul Hoffman)
F. Edito's address Editor's Address
Blake Ramsdell Blake Ramsdell
Worldtalk Worldtalk
13122 NE 20th St., Suite C 17720 NE 65th St Ste 201
Bellevue, WA 98005 Redmond, WA 98052
(425) 882-8861
blaker@deming.com Phone: +1 425 376 0225
EMail: blaker@deming.com
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