draft-ietf-smime-aes-alg-04.txt   draft-ietf-smime-aes-alg-05.txt 
S/MIME Working Group J. Schaad S/MIME Working Group J. Schaad
Internet Draft Soaring Hawk Consulting Internet Draft Soaring Hawk Consulting
Document: draft-ietf-smime-aes-alg-04.txt R. Housley Document: draft-ietf-smime-aes-alg-05.txt
Expires: July 2002 RSA Laboratories Expires: May 2003 November 2002
January 2002
Use of the AES Encryption Algorithm and RSA-OAEP Key Transport in CMS Use of the AES Encryption Algorithm in CMS
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026. all provisions of Section 10 of RFC 2026.
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Abstract Abstract
This document specifies the conventions for using the Advanced This document specifies the conventions for using the Advanced
Encryption Standard (AES) algorithm [AES] for encryption and the Encryption Standard (AES) algorithm [AES] for encryption with the
RSAES-OAEP key transport algorithm [PKCS#1v2.0] for key management Cryptographic Message Syntax (CMS) [CMS].
with the Cryptographic Message Syntax (CMS) [CMS].
Conventions used in this document Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC 2119 this document are to be interpreted as described in RFC 2119
[MUSTSHOULD]. [MUSTSHOULD].
1 Overview 1 Overview
This document specifies the conventions for using the RSAES-OAEP key This document specifies the conventions for using Advanced Encryption
transport algorithm and Advanced Encryption Standard (AES) content
encryption algorithm with the Cryptographic Message Syntax [CMS]
enveloped-data and encrypted-data content types.
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This document presents the use of the two algorithms together, since
we anticipate that they will be used together. However, the two
algorithms can be used independently. For example, RSA-OAEP could be
used to transport Triple-DES keys, and AES keys could be distributed Standard (AES) content encryption algorithm with the Cryptographic
out-of-band for use with mail lists. Message Syntax [CMS] enveloped-data and encrypted-data content types.
CMS values are generated using ASN.1 [X.208-88], using the Basic CMS values are generated using ASN.1 [X.208-88], using the Basic
Encoding Rules (BER) [X.209-88] and the Distinguished Encoding Rules Encoding Rules (BER) [X.209-88] and the Distinguished Encoding Rules
(DER) [X.509-88]. (DER) [X.509-88].
1.1 AES 1.1 AES
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The Advanced Encryption Standard (AES) [AES] was developed to replace The Advanced Encryption Standard (AES) [AES] was developed to replace
DES [DES]. The AES Federal Information Processing Standard (FIPS) DES [DES]. The AES Federal Information Processing Standard (FIPS)
Publication specifies a cryptographic algorithm for use by U.S. Publication specifies a cryptographic algorithm for use by U.S.
Government organizations. However, the AES will also be widely used Government organizations. However, the AES will also be widely used
by organizations, institutions, and individuals outside of the U.S. by organizations, institutions, and individuals outside of the U.S.
Government. Government.
Two researchers who developed and submitted the Rijndael algorithm Two researchers who developed and submitted the Rijndael algorithm
skipping to change at line 105 skipping to change at line 94
it also demonstrates excellent performance. The Rijndael algorithm it also demonstrates excellent performance. The Rijndael algorithm
operations are among the easiest to defend against power and timing operations are among the easiest to defend against power and timing
attacks. Additionally, it appears that some defense can be provided attacks. Additionally, it appears that some defense can be provided
against such attacks without significantly impacting the algorithm's against such attacks without significantly impacting the algorithm's
performance. Finally, the algorithm's internal round structure performance. Finally, the algorithm's internal round structure
appears to have good potential to benefit from instruction-level appears to have good potential to benefit from instruction-level
parallelism. parallelism.
The AES specifies three key sizes: 128, 192 and 256 bits. The AES specifies three key sizes: 128, 192 and 256 bits.
1.2 RSA-OAEP
When the variant of the RSA key transport algorithm specified in PKCS
#1 Version 1.5 [PKCS#1v1.5] is used for key management, it is
vulnerable to adaptive chosen ciphertext attacks. This attack is
described in [RSALAB] and [CRYPTO98]. The use of PKCS #1 Version 1.5
key transport in interactive applications is especially vulnerable,
but countermeasures are described in [MMA]. Exploitation of this
identified vulnerability, revealing the result of a particular RSA
decryption, requires access to an oracle which will respond to
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hundreds of thousands of ciphertexts, which are constructed
adaptively in response to previously-received replies providing
information on the successes or failures of attempted decryption
operations.
The attack appears significantly less feasible in store-and-forward
environments, such as S/MIME. When PKCS #1 Version 1.5 key transport
is applied as an intermediate encryption layer within an interactive
request-response communications environment, exploitation could be
more feasible. However, Secure Sockets Layer (SSL) [SSL] and
Transport Layer Security (TLS) [TLS] protocol implementations could
include countermeasures that detect and prevent Bleichenbacher's and
other chosen-ciphertext attacks, without changing the way the RSA key
transport algorithm is used. These countermeasures are performed
within the protocol level. In the interest of long-term security
assurance, it is prudent to adopt an improved cryptographic technique
rather than embedding countermeasures in protocols.
An updated version of PKCS #1 has been published: PKCS #1 Version 2.0
[PKCS#1v2.0]. This new document supersedes RFC 2313 [PKCS#1v1.5].
PKCS #1 Version 2.0 preserves support for the encryption padding
format defined in PKCS #1 Version 1.5 [PKCS#1v1.5], and it also
defines a new alternative. To resolve the adaptive chosen ciphertext
vulnerability, the PKCS #1 Version 2.0 specifies and recommends use
of Optimal Asymmetric Encryption Padding (OAEP) when RSA encryption
is used to provide confidentiality, such as key transport.
This document specifies the use of RSAES-OAEP key transport algorithm
in the Cryptographic Message Syntax (CMS) [CMS]. CMS can be used in
either a store-and-forward or an interactive request-response
environment.
CMS supports variety of architectures for certificate-based key
management, particularly the one defined by the PKIX working group
[PROFILE]. PKCS #1 Version 1.5 and PKCS #1 Version 2.0 require the
same RSA public key information. Thus, a certified RSA public key
may be used with either RSA key transport technique.
2 Enveloped-data Conventions 2 Enveloped-data Conventions
The CMS enveloped-data content type consists of encrypted content and The CMS enveloped-data content type consists of encrypted content and
wrapped content-encryption keys for one or more recipients. The wrapped content-encryption keys for one or more recipients. The AES
RSAES-OAEP key transport algorithm is used to wrap the content- algorithm is used to encrypt the content.
encryption key for one recipient. The AES algorithm is used to
encrypt the content.
Compliant software MUST meet the requirements for constructing an Compliant software MUST meet the requirements for constructing an
enveloped-data content type stated in [CMS] Section 6, "Enveloped- enveloped-data content type stated in [CMS] Section 6, "Enveloped-
data Content Type". data Content Type".
An AES content-encryption key MUST be randomly generated for each The AES content-encryption key MUST be randomly generated for each
instance of an enveloped-data content type. The content-encryption instance of an enveloped-data content type. The content-encryption
key (CEK) is used to encrypt the content. key (CEK) is used to encrypt the content.
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AES can be used with the enveloped-data content type using any of the AES can be used with the enveloped-data content type using any of the
following key management techniques defined in [CMS] Section 6. following key management techniques defined in [CMS] Section 6.
1) Key Transport: The AES CEK is uniquely wrapped for each recipient 1) Key Transport: The AES CEK is uniquely wrapped for each recipient
using the recipient's public RSA key and other values. Section 2.2 using the recipient's public RSA key and other values. Section 2.2
provides additional details. provides additional details.
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2) Key Agreement: The AES CEK is uniquely wrapped for each recipient 2) Key Agreement: The AES CEK is uniquely wrapped for each recipient
using a pairwise symmetric key-encryption key (KEK) generated using using a pairwise symmetric key-encryption key (KEK) generated using
DH-ES [DH] using the originator's randomly generated private key, the DH-ES [DH] using the originator's randomly generated private key, the
recipient's public DH key, and other values. Section 2.3 provides recipient's public DH key, and other values. Section 2.3 provides
additional details. additional details.
3) Previously Distributed Symmetric KEK: The AES CEK is wrapped 3) Previously Distributed Symmetric KEK: The AES CEK is wrapped
using a previously distributed symmetric KEK (such as a Mail List using a previously distributed symmetric KEK (such as a Mail List
Key). The methods by which the symmetric KEK is generated and Key). The methods by which the symmetric KEK is generated and
distributed are beyond the scope of this document. Section 2.4 distributed are beyond the scope of this document. Section 2.4
provides additional details. provides additional details.
4) Password Encryption: The AES CEK is wrapped using a KEK derived 4) Password Encryption: The AES CEK is wrapped using a KEK derived
from a password or other shared secret. Section 2.5 provides from a password or other shared secret. Section 2.5 provides
additional details. additional details.
Documents defining the use of the Other Recipient Info structure will
need to define the proper use for the AES algorithm if desired.
2.1 EnvelopedData Fields 2.1 EnvelopedData Fields
The enveloped-data content type is ASN.1 encoded using the The enveloped-data content type is ASN.1 encoded using the
EnvelopedData syntax. The fields of the EnvelopedData syntax MUST be EnvelopedData syntax. The fields of the EnvelopedData syntax MUST be
populated as follows: populated as follows:
The EnvelopedData version is determined based on a number of factors. The EnvelopedData version is determined based on a number of factors.
See [CMS] section 6.1 for the algorithm to determine this value. See [CMS] section 6.1 for the algorithm to determine this value.
The EnvelopedData originatorInfo field is not used for the RSAES-OAEP
key transport algorithm. However, this field MAY be present to
support recipients using other key management algorithms.
The EnvelopedData recipientInfos CHOICE is dependent on the key The EnvelopedData recipientInfos CHOICE is dependent on the key
management technique used. Section 2.2, 2.3, 2.4 and 2.5 provide management technique used. Section 2.2, 2.3, 2.4 and 2.5 provide
additional information. additional information.
The EnvelopedData encryptedContentInfo contentEncryptionAlgorithm The EnvelopedData encryptedContentInfo contentEncryptionAlgorithm
field MUST specify a symmetric encryption algorithm. Implementations field MUST specify a symmetric encryption algorithm. Implementations
MUST support content encryption with AES, but implementations MAY MUST support content encryption with AES, but implementations MAY
support other algorithms as well. support other algorithms as well.
The EnvelopedData unprotectedAttrs MAY be present. The EnvelopedData unprotectedAttrs MAY be present.
2.2 KeyTransRecipientInfo Fields 2.2 KeyTransRecipientInfo Fields
The enveloped-data content type is ASN.1 encoded using the The enveloped-data content type is ASN.1 encoded using the
EnvelopedData syntax. The fields of the EnvelopedData syntax MUST be EnvelopedData syntax. The fields of the EnvelopedData syntax MUST be
populated as follows: populated as follows:
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The KeyTransRecipientInfo version MUST be either 0 or 2. If the The KeyTransRecipientInfo version MUST be either 0 or 2. If the
RecipientIdentifier is the CHOICE issuerAndSerialNumber, then the RecipientIdentifier is the CHOICE issuerAndSerialNumber, then the
version MUST be 0. If the RecipientIdentifier is version MUST be 0. If the RecipientIdentifier is
subjectKeyIdentifier, then the version MUST be 2. subjectKeyIdentifier, then the version MUST be 2.
The KeyTransRecipientInfo RecipientIdentifier provides two The KeyTransRecipientInfo RecipientIdentifier provides two
alternatives for specifying the recipient's certificate, and thereby alternatives for specifying the recipient's certificate, and thereby
the recipient's public key. The recipient's certificate MUST contain the recipient's public key. The recipient's certificate MUST contain
a RSA public key. The CEK is encrypted with the recipient's RSA a RSA public key. The CEK is encrypted with the recipient's RSA
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public key. The issuerAndSerialNumber alternative identifies the public key. The issuerAndSerialNumber alternative identifies the
recipient's certificate by the issuer's distinguished name and the recipient's certificate by the issuer's distinguished name and the
certificate serial number; the subjectKeyIdentifier identifies the certificate serial number; the subjectKeyIdentifier identifies the
recipient's certificate by the X.509 subjectKeyIdentifier extension recipient's certificate by the X.509 subjectKeyIdentifier extension
value. value.
The KeyTransRecipientInfo keyEncryptionAlgorithm field specifies the The KeyTransRecipientInfo keyEncryptionAlgorithm field specifies the
RSAES-OAEP algorithm, and the associated parameters used to encrypt key transport algorithm (i.e. RSAES-OAEP [RSA-OAEP]), and the
the CEK for the recipient. The key encryption process is described associated parameters used to encrypt the CEK for the recipient.
in [PKCS#1v2.0]. See section 4.2 of this document for the algorithm
identifier and the parameter syntax.
The KeyTransRecipientInfo encryptedKey is the result of encrypting The KeyTransRecipientInfo encryptedKey is the result of encrypting
the CEK with the recipient's RSA public key using the RSAES-OAEP the CEK with the recipient's RSA public key.
algorithm.
Note: When using a Triple-DES CEK, implementations MUST adjust the
parity bits for each DES key comprising the Triple-DES key prior to
RSAES-OAEP encryption.
Note: The same key wrap algorithm is used for both Two-key Triple-DES
and Three-key Triple-DES CEK keys. When a Two-key Triple-DES key is
to be wrapped, a third DES key with the same value as the first DES
key is created. Thus, all wrapped Triple-DES keys include three DES
keys.
2.3 KeyAgreeRecipientInfo Fields 2.3 KeyAgreeRecipientInfo Fields
This section describes the conventions for using ES-DH and AES with This section describes the conventions for using ES-DH and AES with
the CMS enveloped-data content type to support key agreement. When the CMS enveloped-data content type to support key agreement. When
key agreement is used, then the RecipientInfo keyAgreeRecipientInfo key agreement is used, then the RecipientInfo keyAgreeRecipientInfo
CHOICE MUST be used. CHOICE MUST be used.
The KeyAgreeRecipient version MUST be 3. The KeyAgreeRecipient version MUST be 3.
The EnvelopedData originatorInfo field MUST be the originatorKey The EnvelopedData originatorInfo field MUST be the originatorKey
alternative. The originatoryKey algorithm fields MUST contain the alternative. The originatorKey algorithm fields MUST contain the dh-
dh-public-number object identifier with absent parameters. The public-number object identifier with absent parameters. The
originatorKey publicKey MUST contain the originator's ephemeral originatorKey publicKey MUST contain the originator's ephemeral
public key. public key.
The EnvelopedData ukm MAY be present. The EnvelopedData ukm MAY be present.
The EnvelopedData keyEncrytionAlgorithm MUST be the id-alg-ESDH The EnvelopedData keyEncrytionAlgorithm MUST be the id-alg-ESDH
algorithm identifier [CMSALG]. algorithm identifier [CMSALG].
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2.3.1 ES-DH/AES Key Derivation 2.3.1 ES-DH/AES Key Derivation
Generation of the AES KEK to be used with the AES -key wrap algorithm Generation of the AES KEK to be used with the AES -key wrap algorithm
is done using the method described in [DH]. is done using the method described in [DH].
2.3.1.1 Example 1 2.3.1.1 Example 1
ZZ is the 20 bytes 00 01 02 03 04 05 06 07 08 09 ZZ is the 20 bytes 00 01 02 03 04 05 06 07 08 09
0a 0b 0c 0d 0e 0f 10 11 12 13 0a 0b 0c 0d 0e 0f 10 11 12 13
The key wrap algorithm is AES-128 wrap, so we need 128 bits (16 The key wrap algorithm is AES-128 wrap, so we need 128 bits (16
bytes) of keying material. bytes) of keying material.
skipping to change at line 328 skipping to change at line 238
bytes) of keying material. bytes) of keying material.
No partyAInfo is used. No partyAInfo is used.
Consequently, the input to SHA-1 is: Consequently, the input to SHA-1 is:
00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 ; ZZ 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 ; ZZ
30 1b 30 1b
30 11 30 11
06 09 60 86 48 01 65 03 04 01 05 ; AES-128 wrap OID 06 09 60 86 48 01 65 03 04 01 05 ; AES-128 wrap OID
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04 04 04 04
00 00 00 01 ; Counter 00 00 00 01 ; Counter
a2 06 a2 06
04 04 04 04
00 00 00 80 ; key length 00 00 00 80 ; key length
And the output is the 32 bytes: And the output is the 32 bytes:
d6 d6 b0 94 c1 02 7a 7d e6 e3 11 72 94 a3 53 64 49 08 50 f9 d6 d6 b0 94 c1 02 7a 7d e6 e3 11 72 94 a3 53 64 49 08 50 f9
skipping to change at line 360 skipping to change at line 273
The partyAInfo used is the 64 bytes The partyAInfo used is the 64 bytes
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
Consequently, the input to first invocation of SHA-1 is: Consequently, the input to first invocation of SHA-1 is:
00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 ; ZZ 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 ; ZZ
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30 5f 30 5f
30 11 30 11
06 09 60 86 48 01 65 03 04 01 2c ; AES-256 wrap OID 06 09 60 86 48 01 65 03 04 01 2c ; AES-256 wrap OID
04 04 04 04
00 00 00 01 ; Counter 00 00 00 01 ; Counter
a0 42 a0 42
04 40 04 40
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 ; partyAInfo 01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 ; partyAInfo
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
skipping to change at line 386 skipping to change at line 296
04 04 04 04
00 00 01 00 ; key length 00 00 01 00 ; key length
And the output is the 20 bytes: And the output is the 20 bytes:
6f da b9 fa 67 09 30 3e 7e 2f 68 50 29 6f 28 fb 1b a6 4e 2a 6f da b9 fa 67 09 30 3e 7e 2f 68 50 29 6f 28 fb 1b a6 4e 2a
The input to second invocation of SHA-1 is: The input to second invocation of SHA-1 is:
00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 ; ZZ 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 ; ZZ
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30 5f 30 5f
30 11 30 11
06 09 60 86 48 01 65 03 04 01 2c ; AES-256 wrap OID 06 09 60 86 48 01 65 03 04 01 2c ; AES-256 wrap OID
04 04 04 04
00 00 00 02 ; Counter 00 00 00 02 ; Counter
a0 42 a0 42
04 40 04 40
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 ; partyAInfo 01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 ; partyAInfo
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
skipping to change at line 417 skipping to change at line 330
K = 6f da b9 fa 67 09 30 3e 7e 2f 68 50 29 6f 28 fb 1b a6 4e 2a K = 6f da b9 fa 67 09 30 3e 7e 2f 68 50 29 6f 28 fb 1b a6 4e 2a
73 36 a5 ae 90 33 31 39 cb 3f 0e 90 73 36 a5 ae 90 33 31 39 cb 3f 0e 90
2.3.2 AES CEK Wrap Process 2.3.2 AES CEK Wrap Process
The AES key wrap algorithm encrypts one AES key in another AES key. The AES key wrap algorithm encrypts one AES key in another AES key.
The algorithm produces an output 64-bits longer than the input AES The algorithm produces an output 64-bits longer than the input AES
CEK, the additional bits are a checksum. The algorithm uses 6*n AES CEK, the additional bits are a checksum. The algorithm uses 6*n AES
encryption/decryption operations where n is number of 64-bit blocks encryption/decryption operations where n is number of 64-bit blocks
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in the AES CEK. Full details of the AES key wrap algorithm are in the AES CEK. Full details of the AES key wrap algorithm are
available at [AES-WRAP]. available at [AES-WRAP].
NIST has assigned the following OIDs to define the AES key wrap NIST has assigned the following OIDs to define the AES key wrap
algorithm. algorithm.
id-aes128-wrap OBJECT IDENTIFIER ::= { aes 5 } id-aes128-wrap OBJECT IDENTIFIER ::= { aes 5 }
id-aes192-wrap OBJECT IDENTIFIER ::= { aes 25 } id-aes192-wrap OBJECT IDENTIFIER ::= { aes 25 }
id-aes256-wrap OBJECT IDENTIFIER ::= { aes 45 } id-aes256-wrap OBJECT IDENTIFIER ::= { aes 45 }
skipping to change at line 445 skipping to change at line 354
length. If different lengths are supported, the KEK MUST be of equal length. If different lengths are supported, the KEK MUST be of equal
or greater length than the CEK. or greater length than the CEK.
2.4 KEKRecipientInfo Fields 2.4 KEKRecipientInfo Fields
This section describes the conventions for using AES with the CMS This section describes the conventions for using AES with the CMS
enveloped-data content type to support previously distributed enveloped-data content type to support previously distributed
symmetric KEKs. When a previously distributed symmetric KEK is used symmetric KEKs. When a previously distributed symmetric KEK is used
to wrap the AES CEK, then the RecipientInfo KEKRecipientInfo CHOICE to wrap the AES CEK, then the RecipientInfo KEKRecipientInfo CHOICE
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MUST be used. The methods used to generate and distribute the MUST be used. The methods used to generate and distribute the
symmetric KEK are beyond the scope of this document. One possible symmetric KEK are beyond the scope of this document. One possible
method of distributing keys is documented in [SYMKEYDIST]. method of distributing keys is documented in [SYMKEYDIST].
The KEKRecipientInfo fields MUST be populated as specified in [CMS] The KEKRecipientInfo fields MUST be populated as specified in [CMS]
Section 6.2.3, KEKRecipientInfo Type. Section 6.2.3, KEKRecipientInfo Type.
The KEKRecipientInfo keyEncryptionAlgorithm algorithm field MUST be The KEKRecipientInfo keyEncryptionAlgorithm algorithm field MUST be
one of the OIDs defined in section 2.3.2 indicating that the AES wrap one of the OIDs defined in section 2.3.2 indicating that the AES wrap
skipping to change at line 478 skipping to change at line 390
RecipientInfo PasswordRecipientInfo CHOICE MUST be used. RecipientInfo PasswordRecipientInfo CHOICE MUST be used.
The keyEncryptionAlgorithm algorithm field MUST be one of the OIDs The keyEncryptionAlgorithm algorithm field MUST be one of the OIDs
defined in section 2.3.2 indicating the AES wrap function is used to defined in section 2.3.2 indicating the AES wrap function is used to
wrap the AES CEK. The keyEncryptionAlgorithm parameters field MUST wrap the AES CEK. The keyEncryptionAlgorithm parameters field MUST
be absent. be absent.
The encryptedKey field MUST be the result of the AES key wrap The encryptedKey field MUST be the result of the AES key wrap
algorithm applied to the AES CEK value. algorithm applied to the AES CEK value.
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3 Encrypted-data Conventions 3 Encrypted-data Conventions
The CMS encrypted-data content type consists of encrypted content
with implicit key management. The AES algorithm is used to encrypt
the content.
Compliant software MUST meet the requirements for constructing an
enveloped-data content type stated in [CMS] Section 8, "Encrypted-
data Content Type".
The encrypted-data content type is ASN.1 encoded using the The encrypted-data content type is ASN.1 encoded using the
EncryptededData syntax. The fields of the EncryptedData syntax MUST EncryptededData syntax. The fields of the EncryptedData syntax MUST
be populated as follows: be populated as follows:
The EncryptedData version is determined based on a number of factors. The EncryptedData version is determined based on a number of factors.
See [CMS] section 9.1 for the algorithm to determine this value. See [CMS] section 9.1 for the algorithm to determine this value.
The EncryptedData encryptedContentInfo contentEncryptionAlgorithm The EncryptedData encryptedContentInfo contentEncryptionAlgorithm
field MUST specify a symmetric encryption algorithm. Implementations field MUST specify a symmetric encryption algorithm. Implementations
MUST support encryption using AES, but implementations MAY support MUST support encryption using AES, but implementations MAY support
other algorithms as well. other algorithms as well.
The EncryptedData unprotectedAttrs MAY be present. The EncryptedData unprotectedAttrs MAY be present.
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4 Algorithm Identifiers and Parameters 4 Algorithm Identifiers and Parameters
This section specified algorithm identifiers for the AES encryption This section specified algorithm identifiers for the AES encryption
algorithm and the RSAES-OAEP key transport algorithm. algorithm.
4.1 AES Algorithm Identifiers and Parameters 4.1 AES Algorithm Identifiers and Parameters
The AES algorithm is defined in [AES]. RSA #1 v1.5 [PKCS#1v1.5] The AES algorithm is defined in [AES]. RSAES-OAEP [RSA-OAEP] MAY be
MUST NOT be used to transport AES keys. RSAES-OAEP [PKCS#1v2.0] MAY used to transport AES keys.
be used to transport AES keys.
AES is added to the set of symmetric content encryption algorithms AES is added to the set of symmetric content encryption algorithms
defined in [CMSALG]. The AES content-encryption algorithm, in Cipher defined in [CMSALG]. The AES content-encryption algorithm, in Cipher
Block Chaining (CBC) mode, for the three different key sizes are Block Chaining (CBC) mode, for the three different key sizes are
identified by the following object identifiers: identified by the following object identifiers:
id-aes128-CBC OBJECT IDENTIFIER ::= { aes 2 } id-aes128-CBC OBJECT IDENTIFIER ::= { aes 2 }
id-aes192-CBC OBJECT IDENTIFIER ::= { aes 22 } id-aes192-CBC OBJECT IDENTIFIER ::= { aes 22 }
id-aes256-CBC OBJECT IDENTIFIER ::= { aes 42 } id-aes256-CBC OBJECT IDENTIFIER ::= { aes 42 }
skipping to change at line 535 skipping to change at line 453
Content encryption algorithm identifiers are located in the Content encryption algorithm identifiers are located in the
EnvelopedData EncryptedContentInfo contentEncryptionAlgorithm and the EnvelopedData EncryptedContentInfo contentEncryptionAlgorithm and the
EncryptedData EncryptedContentInfo contentEncryptionAlgorithm fields. EncryptedData EncryptedContentInfo contentEncryptionAlgorithm fields.
Content encryption algorithms are used to encrypt the content located Content encryption algorithms are used to encrypt the content located
in the EnvelopedData EncryptedContentInfo encryptedContent and the in the EnvelopedData EncryptedContentInfo encryptedContent and the
EncryptedData EncryptedContentInfo encryptedContent fields. EncryptedData EncryptedContentInfo encryptedContent fields.
4.2 RSAES-OAEP Algorithm Identifiers and Parameters
The RSAES-OAEP key transport algorithm is the RSA encryption scheme
defined in RFC 2437 [PKCS#1v2.0], where the message to be encrypted
is the content-encryption key.
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Use of the AES Algorithm in CMS February 2002
The RSA key is identified in a certificate using the rsaEncryption
object identifier:
pkcs-1 OBJECT IDENTIFIER ::= {
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) }
rsaEncryption OBJECT IDENTIFIER ::= { pkcs-1 1 }
Note: This is the same algorithm identifier used by RSAES-PKCS1-v1_5.
This means that the existence of an RSA key in a certificate cannot
be used to infer that a recipient can decrypt an RSAES-OAEP encrypted
content-encryption key.
The object identifier for RSAES-OAEP is:
id-RSAES-OAEP OBJECT IDENTIFIER ::= { pkcs-1 7 }
The AlgorithmIdentifier parameters field MUST be present, and the
parameters field MUST contain RSAES-OAEP-params. RSAES-OAEP-params
have the following syntax:
RSAES-OAEP-params ::= SEQUENCE {
hashFunc [0] AlgorithmIdentifier DEFAULT sha1Identifier,
maskGenFunc [1] AlgorithmIdentifier DEFAULT mgf1SHA1Identifier,
pSourceFunc [2] AlgorithmIdentifier
DEFAULT pSpecifiedEmptyIdentifier }
sha1Identifier ::= AlgorithmIdentifier {
id-sha1, NULL }
mgf1SHA1Identifier ::= AlgorithmIdentifier {
id-mgf1, sha1Identifier }
pSpecifiedEmptyIdentifier ::= AlgorithmIdentifier {
id-pSpecified, nullOctetString }
id-sha1 OBJECT IDENTIFIER ::= {
iso(1) identified-organization(3) oiw(14) secsig(3)
algorithms(2) 26 }
id-mgf1 OBJECT IDENTIFIER ::= { pkcs-1 8 }
id-pSpecified OBJECT IDENTIFIER ::= { pkcs-1 9 }
nullOctetString OCTET STRING (SIZE (0)) ::= { ''H }
The fields of type RSAES-OAEP-params have the following meanings:
hashFunc identifies the one-way hash function. Implementations MUST
support SHA-1 [SHA1]. The SHA-1 algorithm identifier is comprised of
the id-sha1 object identifier and a parameter of NULL.
Implementations that perform key encryption MUST omit the hashFunc
field when SHA-1 is used, indicating that the default algorithm was
Schaad, Housley 10
Use of the AES Algorithm in CMS February 2002
used. Implementations that perform key decryption MUST recognize
both the id-sha1 object identifier and an absent hashFunc field as an
indication that SHA-1 was used.
maskGenFunc identifies the mask generation function. Implementations
MUST support MFG1 [PKCS#1v2.0]. MFG1 requires a one-way hash
function, and it is identified in the parameter field of the MFG1
algorithm identifier. Implementations MUST support SHA-1 [SHA1].
The MFG1 algorithm identifier is comprised of the id-mgf1 object
identifier and a parameter that contains the algorithm identifier of
the one-way hash function employed with MFG1. The SHA-1 algorithm
identifier is comprised of the id-sha1 object identifier and a
parameter of NULL. Implementations that perform key encryption MUST
omit the maskGenFunc field when MFG1 with SHA-1 is used, indicating
that the default algorithm was used. Implementations that perform
key decryption MUST recognize both the id-mgf1 and id-sha1 object
identifiers as well as an absent maskGenFunc field as an indication
that MFG1 with SHA-1 was used.
pSourceFunc identifies the source (and possibly the value) of the
encoding parameters, commonly called P. Implementations MUST
represent P by an algorithm identifier, id-pSpecified, indicating
that P is explicitly provided as an OCTET STRING in the parameters.
The default value for P is an empty string. In this case, pHash in
EME-OAEP contains the hash of a zero length string. Implementations
MUST support a zero length P value. Implementations that perform key
encryption MUST omit the pSourceFunc field when a zero length P value
is used, indicating that the default value was used. Implementations
that perform key decryption MUST recognize both the id-pSpecified
object identifier and an absent pSourceFunc field as an indication
that a zero length P value was used.
5 SMIMECapabilities Attribute Conventions 5 SMIMECapabilities Attribute Conventions
An S/MIME client SHOULD announce the set of cryptographic functions An S/MIME client SHOULD announce the set of cryptographic functions
it supports by using the S/MIME capabilities attribute. This it supports by using the S/MIME capabilities attribute. This
attribute provides a partial list of object identifiers of attribute provides a partial list of object identifiers of
cryptographic functions and MUST be signed by the client. The cryptographic functions and MUST be signed by the client. The
algorithm OIDs SHOULD be logically separated in functional categories algorithm OIDs SHOULD be logically separated in functional categories
and MUST be ordered with respect to their preference. and MUST be ordered with respect to their preference.
RFC 2633 [MSG], Section 2.5.2 defines the SMIMECapabilities signed RFC 2633 [MSG], Section 2.5.2 defines the SMIMECapabilities signed
attribute (defined as a SEQUENCE of SMIMECapability SEQUENCEs) to be attribute (defined as a SEQUENCE of SMIMECapability SEQUENCEs) to be
used to specify a partial list of algorithms that the software used to specify a partial list of algorithms that the software
announcing the SMIMECapabilities can support. announcing the SMIMECapabilities can support.
5.1 RSAES-OEAP SMIMECapability Attribute 5.1 AES S/MIME Capability Attributes
When constructing a signedData object, compliant software MAY include
the SMIMECapabilities signed attribute announcing that it supports
the RSAES-OAEP algorithm.
The SMIMECapability SEQUENCE representing RSAES-OAEP MUST include the
id-RSAES-OAEP object identifier in the capabilityID field and MUST
Schaad, Housley 11
Use of the AES Algorithm in CMS February 2002
include the RSAES-OAEP-Default-Identifier SEQUENCE in the parameters
field.
RSAES-OAEP-Default-Identifier ::= AlgorithmIdentifier {
id-RSAES-OAEP, { sha1Identifier, mgf1SHA1Identifier,
pSpecifiedEmptyIdentifier } }
When all of the default settings are selected, the SMIMECapability
SEQUENCE representing RSAES-OAEP MUST be DER-encoded as:
30 0D 06 09 2A 86 48 86 F7 0D 01 01 07 30 00
5.2 AES S/MIME Capability Attributes
If an S/MIME client is required to support symmetric encryption with If an S/MIME client is required to support symmetric encryption with
AES, the capabilities attribute MUST contain the AES object AES, the capabilities attribute MUST contain the AES object
identifier specified above in the category of symmetric algorithms. identifier specified above in the category of symmetric algorithms.
The parameter associated with this object identifier MUST is The parameter associated with this object identifier MUST is
AESSMimeCapability. AESSMimeCapability.
Schaad 8
Use of the AES Algorithm in CMS July 2002
AESSMimeCapabilty ::= NULL AESSMimeCapabilty ::= NULL
The encodings for the mandatory key sizes are: The encodings for the mandatory key sizes are:
Key Size Capability Key Size Capability
128 30 0D 06 09 60 86 48 01 65 03 04 01 02 30 00 128 30 0D 06 09 60 86 48 01 65 03 04 01 02 30 00
196 30 0D 06 09 60 86 48 01 65 03 04 01 16 30 00 196 30 0D 06 09 60 86 48 01 65 03 04 01 16 30 00
256 30 0D 06 09 60 86 48 01 65 03 04 01 2A 30 00 256 30 0D 06 09 60 86 48 01 65 03 04 01 2A 30 00
skipping to change at line 708 skipping to change at line 499
process involves information obtained from the capabilities lists process involves information obtained from the capabilities lists
included in messages received from the recipient, as well as other included in messages received from the recipient, as well as other
information such as private agreements, user preferences, legal information such as private agreements, user preferences, legal
restrictions, and so on. If users require AES for symmetric restrictions, and so on. If users require AES for symmetric
encryption, the S/MIME clients on both the sending and receiving side encryption, the S/MIME clients on both the sending and receiving side
MUST support it, and it MUST be set in the user preferences. MUST support it, and it MUST be set in the user preferences.
6 Security Considerations 6 Security Considerations
If RSA-OAEP and RSA #1 v1.5 are both used to transport the same CEK, If RSA-OAEP [PKCS#1v2.0] and RSA #1 v1.5 [RSA#1v1.5] are both used to
then an attacker can still use the Bleichenbacher attack against the
RSA #1 v1.5 encrypted key. It is generally unadvisable to mix both transport the same CEK, then an attacker can still use the
RSA-OAEP and RSA #1 v1.5 in the same set of recipients. Bleichenbacher attack against the RSA #1 v1.5 encrypted key. It is
generally unadvisable to mix both RSA-OAEP and RSA #1 v1.5 in the
same set of recipients.
Implementations must protect the RSA private key and the CEK. Implementations must protect the RSA private key and the CEK.
Compromise of the RSA private key may result in the disclosure of all Compromise of the RSA private key may result in the disclosure of all
messages protected with that key. Compromise of the CEK may result messages protected with that key. Compromise of the CEK may result
in disclosure of the associated encrypted content. in disclosure of the associated encrypted content.
The generation of AES CEKs, RSA public/private key pairs, and MGF The generation of AES CEKs relies on random numbers. The use of
seeds rely on random numbers. The use of inadequate pseudo-random inadequate pseudo-random number generators (PRNGs) to generate these
number generators (PRNGs) to generate these values can result in values can result in little or no security. An attacker may find it
little or no security. An attacker may find it much easier to much easier to reproduce the PRNG environment that produced the keys,
Schaad, Housley 12
Use of the AES Algorithm in CMS February 2002
reproduce the PRNG environment that produced the keys, searching the searching the resulting small set of possibilities, rather than brute
resulting small set of possibilities, rather than brute force
searching the whole key space. The generation of quality random
numbers is difficult. RFC 1750 [RANDOM] offers important guidance in
this area. force searching the whole key space. The generation of quality
random numbers is difficult. RFC 1750 [RANDOM] offers important
guidance in this area.
When wrapping a CEK with a KEK, the KEK MUST always be at least the When wrapping a CEK with a KEK, the KEK MUST always be at least the
same length as the CEK. An attacker will generally work at the same length as the CEK. An attacker will generally work at the
weakest point in an encryption system. This would be the smaller of weakest point in an encryption system. This would be the smaller of
the two key sizes for a brute force attack. the two key sizes for a brute force attack.
References References
AES National Institute of Standards. AES National Institute of Standards.
FIPS Pub 197: Advanced Encryption Standard (AES). FIPS Pub 197: Advanced Encryption Standard (AES).
26 November 2001. 26 November 2001.
AES-WRAP Schaad, J., R. Housley, "AES Key Wrap Algorithm", AES-WRAP Schaad, J., R. Housley, "Advanced Encryption Standard (AES)
Draft-ietf-smime-aes-key-wrap-00.txt Schaad 9
Use of the AES Algorithm in CMS July 2002
CMS Housley, R., Cryptographic Message Syntax. Key Wrap Algorithm", RFC 3394, September 2002
draft-ietf-smime-rfc2630bis-06.txt.
CMSALG Housley, R., Cryptographic Message Syntax (CMS) Algorithms, CMS Housley, R., "Cryptographic Message Syntax (CMS)", RFC
draft-ietf-smime-cmsalg-07.txt. 3369, August 2002.
CRYPTO98 Bleichenbacher, D., "Chosen Ciphertext Attacks Against CMSALG Housley, R., "Cryptographic Message Syntax (CMS)
Protocols Based on the RSA Encryption Standard PKCS #1," Algorithms, RFC 3370, August 2002.
in H. Krawczyk (editor), Advances in Cryptology - CRYPTO'98
Proceedings, Lecture Notes in Computer Science 1462 (1998),
Springer-Verlag, pp. 1-12.
DES National Institute of Standards and Technology. DES National Institute of Standards and Technology.
FIPS Pub 46: Data Encryption Standard. 15 January 1977. FIPS Pub 46: Data Encryption Standard. 15 January 1977.
DH Rescorla, E., Diffie-Hellman Key Agreement Method, RFC DH Rescorla, E., Diffie-Hellman Key Agreement Method, RFC
2631, June 1999. 2631, June 1999.
MUSTSHOULD Bradner, S., Key Words for Use in RFCs to Indicate MUSTSHOULD Bradner, S., Key Words for Use in RFCs to Indicate
Requirement Levels. BCP 14, RFC 2119. March 1997. Requirement Levels. BCP 14, RFC 2119. March 1997.
MMA Rescorla, E., Preventing the Million Message Attack
on CMS, RFC 3218, January 2002.
MSG Ramsdell, B., Editor. S/MIME Version 3 Message MSG Ramsdell, B., Editor. S/MIME Version 3 Message
Specification. RFC 2633. June 1999. Specification. RFC 2633. June 1999.
PKCS#1v1.5 Kaliski, B. PKCS #1: RSA Encryption, Version 1.5. PKCS#1v1.5 Kaliski, B. PKCS #1: RSA Encryption, Version 1.5.
RFC 2313. March 1998. RFC 2313. March 1998.
PKCS#1v2.0 Kaliski, B. PKCS #1: RSA Encryption, Version 2.0. PKCS#1v2.0 Kaliski, B. PKCS #1: RSA Encryption, Version 2.0.
RFC 2437. October 1998. RFC 2437. October 1998.
Schaad, Housley 13
Use of the AES Algorithm in CMS February 2002
PROFILE Housley, R., W. Ford, W. Polk, and D. Solo. Internet
X.509 Public Key Infrastructure: Certificate and CRL
Profile. <draft-ietf-smime-new-part1.txt>.
RANDOM Eastlake, D., S. Crocker, and J. Schiller. Randomness RANDOM Eastlake, D., S. Crocker, and J. Schiller. Randomness
Recommendations for Security. RFC 1750. December 1994. Recommendations for Security. RFC 1750. December 1994.
RSALABS Bleichenbacher, D., B. Kaliski, and J. Staddon. RSA-OAEP Housley, R. "Use of the RSAES-OAEP Key Transport Algorithm
Recent Results on PKCS #1: RSA Encryption Standard. in CMS", draft-ietf-smime-cms-rsaes-oaep-03.txt, June 2002.
RSA Laboratories' Bulletin No. 7, June 26, 1998.
[At http://www.rsasecurity.com/rsalabs/bulletins]
SHA1 National Institute of Standards and Technology.
FIPS Pub 180-1: Secure Hash Standard. 17 April 1995.
SSL Freier, A., P. Karlton, and P. Kocher. The SSL Protocol,
Version 3.0. Netscape Communications. November 1996.
[At http://www.netscape.com/eng/ssl3/draft302.txt]
SYMKEYDIST Turner, S. CMS Symmetric Key Management and Distribution. SYMKEYDIST Turner, S. CMS Symmetric Key Management and Distribution.
RFC TDB. Date TBD. RFC TDB. Date TBD.
<draft-ietf-smime-symkeydist-06.txt> <draft-ietf-smime-symkeydist-06.txt>
TLS Dierks, T. and C. Allen. The TLS Protocol Version 1.0.
RFC 2246. January 1999.
X.208-88 CCITT. Recommendation X.208: Specification of Abstract X.208-88 CCITT. Recommendation X.208: Specification of Abstract
Syntax Notation One (ASN.1). 1988. Syntax Notation One (ASN.1). 1988.
X.209-88 CCITT. Recommendation X.209: Specification of Basic X.209-88 CCITT. Recommendation X.209: Specification of Basic
Encoding Rules for Abstract Syntax Notation One (ASN.1). Encoding Rules for Abstract Syntax Notation One (ASN.1).
1988. 1988.
X.509-88 CCITT. Recommendation X.509: The Directory - X.509-88 CCITT. Recommendation X.509: The Directory -
Authentication Framework. 1988. Authentication Framework. 1988.
Acknowledgements Acknowledgements
This document is the result of contributions from many This document is the result of contributions from many
professionals. We appreciate the hard work of all members of the professionals. We appreciate the hard work of all members of the
IETF S/MIME Working Group. We wish to extend a special thanks to IETF S/MIME Working Group.
Burt Kaliski.
Author's Addresses Author's Addresses
Schaad 10
Use of the AES Algorithm in CMS July 2002
Jim Schaad Jim Schaad
Soaring Hawk Consulting Soaring Hawk Consulting
Email: jimsch@exmsft.com Email: jimsch@exmsft.com
Russell Housley
RSA Laboratories
918 Spring Knoll Drive
Schaad, Housley 14
Use of the AES Algorithm in CMS February 2002
Herndon, VA 20170
USA
Email: rhousley@rsasecurity.com
Appendix A ASN.1 Module Appendix A ASN.1 Module
CMSAesRsaesOaep {iso(1) member-body(2) us(840) rsadsi(113549) CMSAesRsaesOaep {iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) aes-rsaes-oaep(19) } pkcs(1) pkcs-9(9) smime(16) modules(0) id-mod-cms-aes(19) }
DEFINITIONS IMPLICIT TAGS ::= DEFINITIONS IMPLICIT TAGS ::=
BEGIN BEGIN
-- EXPORTS ALL -- -- EXPORTS ALL --
IMPORTS IMPORTS
-- PKIX -- PKIX
AlgorithmIdentifier AlgorithmIdentifier
FROM PKIXExplicit88 {iso(1) identified-organization(3) dod(6) FROM PKIXExplicit88 {iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7) id-mod(0) internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
skipping to change at line 879 skipping to change at line 634
id-aes256-CBC OBJECT IDENTIFIER ::= { aes 42 } id-aes256-CBC OBJECT IDENTIFIER ::= { aes 42 }
-- AES-IV is a the parameter for all the above object identifiers. -- AES-IV is a the parameter for all the above object identifiers.
AES-IV ::= OCTET STRING (SIZE(16)) AES-IV ::= OCTET STRING (SIZE(16))
-- AES S/MIME Capabilty parameter for all the above object identifiers -- AES S/MIME Capabilty parameter for all the above object identifiers
AESSMimeCapability ::= NULL AESSMimeCapability ::= NULL
pkcs-1 OBJECT IDENTIFIER ::= {iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs-1(1) }
id-RSAES-OAEP OBJECT IDENTIFIER ::= { pkcs-1 7 }
RSAES-OAEP-params ::= SEQUENCE {
hashFunc [0] AlgorithmIdentifier DEFAULT sha1Identifier,
maskGenFunc [1] AlgorithmIdentifier DEFAULT mgf1SHA1Identifier,
pSourceFunc [2] AlgorithmIdentifier
DEFAULT pSpecifiedEmptyIdentifier }
sha1Identifier AlgorithmIdentifier ::= { id-sha1, NULL }
Schaad, Housley 15
Use of the AES Algorithm in CMS February 2002
mgf1SHA1Identifier AlgorithmIdentifier ::= {id-mgf1, sha1Identifier }
nullOctetString OCTET STRING (SIZE (0)) ::= { ''H }
pSpecifiedEmptyIdentifier AlgorithmIdentifier ::= { id-pSpecified,
nullOctetString }
id-sha1 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
oiw(14) secsig(3) algorithms(2) 26 }
id-mgf1 OBJECT IDENTIFIER ::= { pkcs-1 8 }
id-pSpecified OBJECT IDENTIFIER ::= { pkcs-1 9 }
rSAES-OAEP-Default-Identifier AlgorithmIdentifier ::= {
id-RSAES-OAEP, { sha1Identifier, mgf1SHA1Identifier,
pSpecifiedEmptyIdentifier } }
END END
Schaad, Housley 16 Schaad 11
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