S/MIME Working Group B. Kaliski
Internet Draft RSA Laboratories
Document: draft-ietf-smime-cms-rsa-kem-01.txt October 2003
Category: Standards
Use of the RSA-KEM Key Transport Algorithm in CMS
Status of this Memo
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Abstract
The RSA-KEM Key Transport Algorithm is a one-pass (store-and-forward)
mechanism for transporting keying data to a recipient using the
recipient's RSA public key. This document specifies the conventions
for using the RSA-KEM Key Transport Algorithm with the Cryptographic
Message Syntax (CMS). This version (-01) updates the ASN.1 syntax to
align with the latest drafts of ANS X9.44 and ISO/IEC 18033-2, and
adds material on certificate conventions and S/MIME capabilities.
Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC 2119
[STDWORDS].
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1. Introduction
The RSA-KEM Key Transport Algorithm is a one-pass (store-and-forward)
mechanism for transporting keying data to a recipient using the
recipient's RSA public key.
Most previous key transport algorithms based on the RSA public-key
cryptosystem (e.g., the popular PKCS #1 v1.5 algorithm [PKCS1]) have
the following general form:
1. Format or "pad" the keying data to obtain an integer m.
2. Encrypt the integer m with the recipient's RSA public key:
c = m^e mod n
3. Output c as the encrypted keying data.
The RSA-KEM Key Transport Algorithm takes a different approach that
provides higher security assurance, by encrypting a _random_ integer
with the recipient's public key, and using a symmetric key-wrapping
scheme to encrypt the keying data. It has the following form:
1. Generate a random integer z between 0 and n-1.
2. Encrypt the integer z with the recipient's RSA public key:
c = z^e mod n.
3. Derive a key-encrypting key KEK from the integer z.
4. Wrap the keying data using KEK to obtain wrapped keying data
WK.
5. Output c and WK as the encrypted keying data.
This different approach provides higher security assurance because
the input to the underlying RSA operation is random and independent
of the message, and the key-encrypting key KEK is derived from it in
a strong way. As a result, the algorithm enjoys a "tight" security
proof in the random oracle model. It is also architecturally
convenient because the public-key operations are separate from the
symmetric operations on the keying data. One benefit is that the
length of the keying data is bounded only by the symmetric key-
wrapping scheme, not the size of the RSA modulus.
The RSA-KEM Key Transport Algorithm in various forms is being adopted
in several draft standards including the draft ANS X9.44 [ANS-X9.44]
and the draft ISO/IEC 18033-2 [ISO-IEC-18033-2]. It has also been
recommended by the NESSIE project [NESSIE]. Although the other
standards are still in development, the algorithm is stable across
the drafts. For completeness, a specification of the algorithm is
given in Appendix A of this document; ASN.1 syntax is given in
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Appendix B.
NOTE: The term KEM stands for "key encapsulation mechanism" and
refers to the first three steps of the process above. The
formalization of key transport algorithms (or more generally,
asymmetric encryption schemes) in terms of key encapsulation
mechanisms is described further in research by Victor Shoup leading
to the development of the ISO/IEC 18033-2 standard [SHOUP].
2. Use in CMS
The RSA-KEM Key Transport Algorithm MAY be employed for one or more
recipients in the CMS enveloped-data content type (Section 6 of
[CMS]), where the keying data processed by the algorithm is the CMS
content-encryption key.
The RSA-KEM Key Transport Algorithm SHOULD be considered for new
CMS-based applications as a replacement for the widely implemented
RSA encryption algorithm specified originally in PKCS #1 v1.5 (see
[PKCS1] and Section 4.2.1 of [CMSALGS]), which is vulnerable to
chosen-ciphertext attacks. The RSAES-OAEP Key Transport Algorithm
has also been proposed as a replacement (see [PKCS1] and [CMS-
OAEP]). RSA-KEM has the advantage over RSAES-OAEP of a tighter
security proof, but the disadvantage of slightly longer encrypted
keying data.
2.1 Underlying Components
A CMS implementation that supports the RSA-KEM Key Transport
Algorithm MUST support at least the following underlying components:
* For the key derivation function, KDF2 (see [ANS-X9.44][IEEE-
P1363a]) based on SHA-1 (see [FIPS-180-2]) (this function is
also specified as the key derivation function in [ANS-X9.63])
* For the key-wrapping scheme, AES-Wrap-128, i.e., the AES Key
Wrap with a 128-bit key encrypting key (see [AES-WRAP])
An implementation SHOULD also support KDF2 based on SHA-256 (see
[FIPS-180-2]), and the Triple-DES Key Wrap (see [3DES-WRAP]). It MAY
support other underlying components.
2.2 RecipientInfo Conventions
When the RSA-KEM Key Transport Algorithm is employed for a recipient,
recipient, the RecipientInfo alternative for that recipient MUST be
KeyTransRecipientInfo. The algorithm-specific fields of the
KeyTransRecipientInfo value MUST have the following values:
* keyEncryptionAlgorithm.algorithm MUST be id-ac-generic-hybrid
(see Appendix B)
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* keyEncryptionAlgorithm.parameters MUST be a value of type
GenericHybridParameters, identifying the RSA-KEM key
encapsulation mechanism (see Appendix B)
* encryptedKey MUST be the encrypted keying data output by the
algorithm (see Appendix A)
2.3 Certificate Conventions
The conventions specified in this section augment RFC 3280 [PROFILE].
A recipient who employs the RSA-KEM Key Transport Algorithm MAY
identify the public key in a certificate by the same
AlgorithmIdentifier as for the PKCS #1 v1.5 algorithm, i.e., using
the rsaEncryption object identifier [PKCS1].
If the recipient wishes only to employ the RSA-KEM Key Transport
Algorithm with a given public key, the recipient MUST identify the
public key in the certificate using the id-ac-generic-hybrid object
identifier (see Appendix B) where the associated
GenericHybridParameters value indicates the underlying components
with which the algorithm is to be employed. The certificate user MUST
perform the RSA-KEM Key Transport algorithm using only those
components.
Regardless of the AlgorithmIdentifier used, the RSA public key is
encoded in the same manner in the subject public key information.
The RSA public key MUST be encoded using the type RSAPublicKey type:
RSAPublicKey ::= SEQUENCE {
modulus INTEGER, -- n
publicExponent INTEGER -- e
}
Here, the modulus is the modulus n, and publicExponent is the public
exponent e. The DER encoded RSAPublicKey is carried in the
subjectPublicKey BIT STRING within the subject public key
information.
The intended application for the key MAY be indicated in the key
usage certificate extension (see [PROFILE], Section 4.2.1.3). If the
keyUsage extension is present in a certificate that conveys an RSA
public key with the id-ac-generic-hybrid object identifier as
discussed above, then the key usage extension MUST contain the
following value:
keyEncipherment.
dataEncipherment SHOULD NOT be present. That is, a key intended to be
employed only with the RSA-KEM Key Transport Algorithm SHOULD NOT
also be employed for data encryption.
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2.4 SMIMECapabilities Attribute Conventions
RFC 2633 [MSG], Section 2.5.2 defines the SMIMECapabilities signed
attribute (defined as a SEQUENCE of SMIMECapability SEQUENCEs) to be
used to specify a partial list of algorithms that the software
announcing the SMIMECapabilities can support. When constructing a
signedData object, compliant software MAY include the
SMIMECapabilities signed attribute announcing that it supports the
RSA-KEM Key Transport algorithm.
The SMIMECapability SEQUENCE representing the RSA-KEM Key Transport
Algorithm MUST include the id-ac-generic-hybrid object identifier
(see Appendix B) in the capabilityID field and MUST include a
GenericHybridParameters value in the parameters field identifying the
components with which the algorithm is to be employed.
The DER encoding of a SMIMECapability SEQUENCE is the same as the DER
encoding of an AlgorithmIdentifier. Example DER encodings for typical
sets of components are given in Appendix B.4.
3. Security Considerations
The security of the RSA-KEM Key Transport Algorithm described in
this document can be shown to be tightly related to the difficulty
of either solving the RSA problem or breaking the underlying
symmetric key-wrapping scheme, if the underlying key derivation
function is modeled as a random oracle, and assuming that the
symmetric key-wrapping scheme satisfies the properties of a data
encapsulation mechanism [SHOUP]. While in practice a random-oracle
result does not provide an actual security proof for any particular
key derivation function, the result does provide assurance that the
general construction is reasonable; a key derivation function would
need to be particularly weak to lead to an attack that is not
possible in the random oracle model.
The RSA key size and the underlying components should be selected
consistent with the desired symmetric security level for an
application. Several security levels have been identified in [NIST-
GUIDELINE]. For brevity, the first three levels are mentioned here:
* 80-bit security. The RSA key size SHOULD be at least 1024 bits,
the hash function underlying KDF2 SHOULD be SHA-1 or above, and
the symmetric key-wrapping scheme SHOULD be AES Key Wrap or
Triple-DES Key Wrap.
* 112-bit security. The RSA key size SHOULD be at least 2048
bits, the hash function underlying KDF2 SHOULD be SHA-224 or
above, and the symmetric key-wrapping scheme SHOULD be AES Key
Wrap or Triple-DES Key Wrap.
* 128-bit security. The RSA key size SHOULD be at least 3072
bits, the hash function underlying KDF2 SHOULD be SHA-256 or
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above, and the symmetric key-wrapping scheme SHOULD be AES Key
Wrap.
Note that the AES Key Wrap MAY be used at all three of these levels;
the use of AES does not require a 128-bit security level for other
components.
Implementations MUST protect the RSA private key and the content-
encryption key. Compromise of the RSA private key may result in the
disclosure of all messages protected with that key. Compromise of the
content-encryption key may result in disclosure of the associated
encrypted content.
Additional considerations related to key management may be found in
[NIST-GUIDELINE].
The security of the algorithm also depends on the strength of the
random number generator, which SHOULD have a comparable security
level. For further discussion on random number generation, please
see [RANDOM].
Implementations SHOULD NOT reveal information about intermediate
values or calculations, whether by timing or other "side channels",
or otherwise an opponent may be able to determine information about
the keying data and/or the recipient's private key. Although not all
intermediate information may be useful to an opponent, it is
preferable to conceal as much information as is practical, unless
analysis specifically indicates that the information would not be
useful.
Generally, good cryptographic practice employs a given RSA key pair
in only one scheme. This practice avoids the risk that vulnerability
in one scheme may compromise the security of the other, and may be
essential to maintain provable security. While RSA public keys have
often been employed for multiple purposes such as key transport and
digital signature without any known bad interactions, for increased
security assurance, such combined use of an RSA key pair is NOT
RECOMMENDED in the future (unless the different schemes are
specifically designed to be used together).
Accordingly, an RSA key pair used for the RSA-KEM Key Transport
Algorithm SHOULD NOT also be used for digital signatures. (Indeed,
ASC X9 requires such a separation between key establishment key pairs
and digital signature key pairs.) Continuing this principle of key
separation, a key pair used for the RSA-KEM Key Transport Algorithm
SHOULD NOT be used with other key establishment schemes, or for data
encryption, or with more than one set of underlying algorithm
components.
Parties MAY wish to formalize the assurance that one another's
implementations are correct through implementation validation, e.g.
NIST's Cryptographic Module Validation Program (CMVP).
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4. References
4.1 Normative References
3DES-WRAP Housley, R. Triple-DES and RC2 Key Wrapping. RFC
3217. December 2001.
AES-WRAP Schaad, J. and R. Housley. Advanced Encryption
Standard (AES) Key Wrap Algorithm. RFC 3394.
September 2002.
ANS-X9.63 American National Standard X9.63-2002: Public Key
Cryptography for the Financial Services Industry:
Key Agreement and Key Transport Using Elliptic
Curve Cryptography.
CMS Housley, R. Cryptographic Message Syntax. RFC
3369. August 2002.
CMSALGS Housley, R. Cryptographic Message Syntax (CMS)
Algorithms. RFC 3370. August 2002.
FIPS-180-2 National Institute of Standards and Technology
(NIST). FIPS 180-2: Secure Hash Standard. August
2002.
MSG Ramsdell, B. S/MIME Version 3 Message
Specification. RFC 2633. June 1999.
PROFILE Housley, R., Polk, W., Ford, W. and D. Solo.
Internet X.509 Public Key Infrastructure:
Certificate and Certificate Revocation List (CRL)
Profile. RFC 3280. April 2002.
STDWORDS Bradner, S. Key Words for Use in RFCs to Indicate
Requirement Levels. RFC 2119. March 1997.
4.2 Informative References
ANS-X9.44 ASC X9F1 Working Group. Draft American National
Standard X9.44: Public Key Cryptography for the
Financial Services Industry -- Key Establishment
Using Integer Factorization Cryptography. Draft D6,
October 15, 2003.
CMS-OAEP Housley, R. Use of the RSAES-OAEP Key Transport
Algorithm in the Cryptographic Message Syntax
(CMS). RFC 3560. July 2003.
IEEE-P1363a IEEE P1363 Working Group. IEEE P1363a: Standard
Specifications for Public Key Cryptography:
Additional Techniques. Draft D12, May 12, 2003.
Available via http://grouper.ieee.org/groups/1363.
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ISO-IEC-18033-2 ISO/IEC 18033-2: Information technology -- Security
techniques -- Encryption algorithms – Part 2:
Asymmetric Ciphers. 2nd Committee Draft, July 10,
2003.
NESSIE NESSIE Consortium. Portfolio of Recommended
Cryptographic Primitives. February 27, 2003.
Available via http://www.cryptonessie.org/.
NIST-GUIDELINE National Institute of Standards and Technology.
Special Publication 800-57: Recommendation for Key
Management. Part 1: General Guideline. Draft,
January 2003. Available via
http://csrc.nist.gov/CryptoToolkit/tkkeymgmt.html.
PKCS1 Jonsson, J. and B. Kaliski. PKCS #1: RSA
Cryptography Specifications Version 2.1. RFC 3447.
February 2003.
RANDOM Eastlake, D., S. Crocker, and J. Schiller.
Randomness Recommendations for Security. RFC 1750.
December 1994.
SHOUP Shoup, V. A Proposal for an ISO Standard for
Public Key Encryption. Version 2.1, December 20,
2001. Available via http://www.shoup.net/papers/.
5. IANA Considerations
Within the CMS, algorithms are identified by object identifiers
(OIDs). With one exception, all of the OIDs used in this document
were assigned in other IETF documents, in ISO/IEC standards
documents, by the National Institute of Standards and Technology
(NIST), and in Public-Key Cryptography Standards (PKCS) documents.
The one exception is that the ASN.1 module's identifier (see Appendix
B.3) is assigned in this document. No further action by the IANA is
necessary for this document or any anticipated updates.
6. Acknowledgments
This document is one part of a strategy to align algorithm standards
produced by ASC X9, ISO/IEC JTC1 SC27, NIST, and the IETF. I would
like to thank the members of the ASC X9F1 working group for their
contributions to drafts of ANS X9.44 which led to this specification.
My thanks as well to Russ Housley as well for his guidance and
encouragement. I also appreciate the helpful direction I've received
from Blake Ramsdell and Jim Schaad in bringing this document to
fruition.
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7. Author's Address
Burt Kaliski
RSA Laboratories
174 Middlesex Turnpike
Bedford, MA 01730
USA
bkaliski@rsasecurity.com
Appendix A. RSA-KEM Key Transport Algorithm
The RSA-KEM Key Transport Algorithm is a one-pass (store-and-forward)
mechanism for transporting keying data to a recipient using the
recipient's RSA public key.
With this type of algorithm, a sender encrypts the keying data using
the recipient's public key to obtain encrypted keying data. The
recipient decrypts the encrypted keying data using the recipient's
private key to recover the keying data.
A.1 Underlying Components
The algorithm has the following underlying components:
* KDF, a key derivation function, which derives keying data of a
specified length from a shared secret value
* Wrap, a symmetric key-wrapping scheme, which encrypts keying
data using a key-encrypting key
In the following, kekLen denotes the length in bytes of the key-
encrypting key for the underlying symmetric key-wrapping scheme.
In this scheme, the length of the keying data to be transported MUST
be among the lengths supported by the underlying symmetric key-
wrapping scheme. (The AES Key Wrap, for instance, requires the length
of the keying data to be a multiple of 8 bytes, and at least 16
bytes.) Usage and formatting of the keying data (e.g., parity
adjustment for Triple-DES keys) is outside the scope of this
algorithm.
With some key derivation functions, it is possible to include other
information besides the shared secret value in the input to the
function. Also, with some symmetric key-wrapping schemes, it is
possible to associate a label with the keying data. Such uses are
outside the scope of this document, as they are not directly
supported by CMS.
A.2 Sender's Operations
Let (n,e) be the recipient's RSA public key (see [PKCS1] for details)
and let K be the keying data to be transported.
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Let nLen denote the length in bytes of the modulus n, i.e., the least
integer such that 2^{8*nLen} > n.
The sender performs the following operations:
1. Generate a random integer z between 0 and n-1 (see Note), and
convert z to a byte string Z of length nLen, most significant
byte first:
z = RandomInteger (0, n-1)
Z = IntegerToString (z, nLen)
2. Encrypt the random integer z using the recipient's public key
(n,e) and convert the resulting integer c to a ciphertext C, a
byte string of length nLen:
c = z^e mod n
C = IntegerToString (c, nLen)
3. Derive a key-encrypting key KEK of length kekLen bytes from the
byte string Z using the underlying key derivation function:
KEK = KDF (Z, kekLen)
4. Wrap the keying data K with the key-encrypting key KEK using
the underlying key-wrapping scheme to obtain wrapped keying
data WK:
WK = Wrap (KEK, K)
5. Concatenate the ciphertext C and the wrapped keying data WK to
obtain the encrypted keying data EK:
EK = C || WK
6. Output the encrypted keying data EK.
NOTE: The random integer z MUST be generated independently at random
for different encryption operations, whether for the same or
different recipients.
A.3 Recipient's Operations
Let (n,d) be the recipient's RSA private key (see [PKCS1]; other
private key formats are allowed) and let EK be the encrypted keying
data.
Let nLen denote the length in bytes of the modulus n.
The recipient performs the following operations:
1. Separate the encrypted keying data EK into a ciphertext C of
length nLen bytes and wrapped keying data WK:
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C || WK = EK
If the length of the encrypted keying data is less than nLen
bytes, output "decryption error" and stop.
2. Convert the ciphertext C to an integer c, most significant
byte first. Decrypt the integer c using the recipient's
private key (n,d) to recover an integer z (see Note):
c = StringToInteger (C)
z = c^d mod n
If the integer c is not between 0 and n-1, output "decryption
error" and stop.
3. Convert the integer z to a byte string Z of length nLen, most
significant byte first (see Note):
Z = IntegerToString (z, nLen)
4. Derive a key-encrypting key KEK of length kekLen bytes from
the byte string Z using the underlying key derivation function
(see Note):
KEK = KDF (Z, kekLen)
5. Unwrap the wrapped keying data WK with the key-encrypting key
KEK using the underlying key-wrapping scheme to recover the
keying data K:
K = Unwrap (KEK, WK)
If the unwrapping operation outputs an error, output
"decryption error" and stop.
6. Output the keying data K.
NOTE: Implementations SHOULD NOT reveal information about the integer
z and the string Z, nor about the calculation of the exponentiation
in Step 2, the conversion in Step 3, or the key derivation in Step 4,
whether by timing or other "side channels". The observable behavior
of the implementation SHOULD be the same at these steps for all
ciphertexts C that are in range. (For example, IntegerToString
conversion should take the same amount of time regardless of the
actual value of the integer z.) The integer z, the string Z and other
intermediate results MUST be securely deleted when they are no longer
needed.
Appendix B. ASN.1 Syntax
The ASN.1 syntax for identifying the RSA-KEM Key Transport Algorithm
is an extension of the syntax for the "generic hybrid cipher" in the
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draft ISO/IEC 18033-2 [ISO-IEC-18033-2], and is the same as employed
in the draft ANS X9.44 [ANS-X9.44]. The syntax for the scheme is
given in Section B.1. The syntax for selected underlying components
including those mentioned above is given in B.2.
The following object identifier prefixes are used in the definitions
below:
is18033-2 OID ::= { iso(1) standard(0) is18033(18033) part2(2) }
nistAlgorithm OID ::= {
joint-iso-itu-t(2) country(16) us(840) organization(1)
gov(101) csor(3) nistAlgorithm(4)
}
pkcs-1 OID ::= {
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1)
}
NullParms is a more descriptive synonym for NULL when an algorithm
identifier has null parameters:
NullParms ::= NULL
The material in this Appendix is based on a draft standard and is
SUBJECT TO CHANGE as that standard is developed.
B.1 RSA-KEM Key Transport Algorithm
The object identifier for the RSA-KEM Key Transport Algorithm is the
same as for the "generic hybrid cipher" in the draft ANS ISO/IEC
18033-2, id-ac-generic-hybrid, which is defined in the draft as
id-ac-generic-hybrid OID ::= {
is18033-2 asymmetric-cipher(1) generic-hybrid(2)
}
The associated parameters for id-ac-generic-hybrid have type
GenericHybridParameters:
GenericHybridParameters ::= {
kem KeyEncapsulationMechanism,
dem DataEncapsulationMechanism
}
The fields of type GenericHybridParameters have the following
meanings:
* kem identifies the underlying key encapsulation mechanism. For
the RSA-KEM Key Transport Algorithm, the scheme is RSA-KEM from
the draft ISO/IEC 18033-2.
The object identifier for RSA-KEM (as a key encapsulation
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mechanism) is id-kem-rsa, which is defined in the draft ISO/IEC
18033-2 as
id-kem-rsa OID ::= {
is18033-2 key-encapsulation-mechanism(2) rsa(4)
}
The associated parameters for id-kem-rsa have type
RsaKemParameters:
RsaKemParameters ::= {
keyDerivationFunction KeyDerivationFunction,
keyLength KeyLength
}
The fields of type RsaKemParameters have the following
meanings:
* keyDerivationFunction identifies the underlying key
derivation function. For alignment with the draft ANS
X9.44, it MUST be KDF2. However, other key derivation
functions MAY be used with CMS. Please see B.2.1 for the
syntax for KDF2.
KeyDerivationFunction ::=
AlgorithmIdentifier {{KDFAlgorithms}}
KDFAlgorithms ALGORITHMS ::= {
kdf2,
... -- implementations may define other methods
}
* keyLength is the length in bytes of the key-encrypting
key, which depends on the underlying symmetric key-
wrapping scheme.
KeyLength ::= INTEGER (1..MAX)
* dem identifies the underlying data encapsulation mechanism.
For alignment with the draft ANS X9.44, it MUST be an X9-
approved symmetric key-wrapping scheme. (See Note.) However,
other symmetric key-wrapping schemes MAY be used with CMS.
Please see B.2.2 for the syntax for the AES and Triple-DES Key
Wraps.
DataEncapsulationMechanism ::=
AlgorithmIdentifier {{DEMAlgorithms}}
DEMAlgorithms ALGORITHM ::= {
X9-SymmetricKeyWrappingSchemes,
... -- implementations may define other methods
}
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X9-SymmetricKeyWrappingSchemes ALGORITHM ::= {
aes128-Wrap | aes192-Wrap | aes256-Wrap | tdes-Wrap,
... -- allows for future expansion
}
NOTE: The generic hybrid cipher in the draft ISO/IEC 18033-2 can
encrypt arbitrary data, hence the term "data encapsulation
mechanism". The symmetric key-wrapping schemes take the role of data
encapsulation mechanisms in the RSA-KEM Key Transport Algorithm. The
draft ISO/IEC 18033-2 currently allows only three particular data
encapsulation mechanisms, not including any of these symmetric key-
wrapping schemes. However, the ASN.1 syntax in that document expects
that additional algorithms will be allowed.
B.2 Selected Underlying Components
B.2.1 Key Derivation Functions
The object identifier for KDF2 (see [ISO-IEC-18033-2]) is
id-kdf-kdf2 OID ::= {
is18033-2 key-derivation-functions(5) kdf2(2)
}
The associated parameters identify the underlying hash function. For
alignment with the draft ANS X9.44, the hash function MUST be an ASC
X9-approved hash function. (See Note.) However, other hash functions
MAY be used with CMS.
kdf2 ALGORITHM ::= {{ OID id-kdf-kdf2 PARMS KDF2-HashFunction }}
KDF2-HashFunction ::= AlgorithmIdentifier {{KDF2-HashFunctions}}
KDF2-HashFunctions ALGORITHM ::= {
X9-HashFunctions,
... -- implementations may define other methods
}
X9-HashFunctions ALGORITHM ::= {
sha1 | sha224 | sha256 | sha384 | sha512,
... -- allows for future expansion
}
The object identifier for SHA-1 is
id-sha1 OID ::= {
iso(1) identified-organization(3) oiw(14) secsig(3)
algorithms(2) sha1(26)
}
The object identifiers for SHA-256, SHA-384 and SHA-512 are
id-sha256 OID ::= { nistAlgorithm hashAlgs(2) sha256(1) }
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id-sha384 OID ::= { nistAlgorithm hashAlgs(2) sha384(2) }
id-sha512 OID ::= { nistAlgorithm hashAlgs(2) sha512(3) }
There has been some confusion over whether the various SHA object
identifiers have a NULL parameter, or no associated parameters. As
also discussed in [PKCS1], implementations SHOULD generate algorithm
identifiers without parameters, and MUST accept algorithm identifiers
either without parameters, or with NULL parameters.
sha1 ALGORITHM ::= {{ OID id-sha1 }} -- NULLParms MUST be
sha224 ALGORITHM ::= {{ OID id-sha224 }} -- accepted for these
sha256 ALGORITHM ::= {{ OID id-sha256 }} -- OIDs
sha384 ALGORITHM ::= {{ OID id-sha384 }} –- ""
sha512 ALGORITHM ::= {{ OID id-sha512 }} –- ""
NOTE: As of this writing, only SHA-1 is an ASC X9-approved hash
function; SHA-224 and above are in the process of being approved. The
object identifier for SHA-224 has not yet been assigned.
B.2.2 Symmetric Key-Wrapping Schemes
The object identifiers for the AES Key Wrap depends on the size of
the key encrypting key. There are three object identifiers (see
[AES-WRAP]):
id-aes128-Wrap OID ::= { nistAlgorithm aes(1) aes128-Wrap(5) }
id-aes192-Wrap OID ::= { nistAlgorithm aes(1) aes192-Wrap(25) }
id-aes256-Wrap OID ::= { nistAlgorithm aes(1) aes256-Wrap(45) }
These object identifiers have no associated parameters.
aes128-Wrap ALGORITHM ::= {{ OID id-aes128-wrap }}
aes192-Wrap ALGORITHM ::= {{ OID id-aes192-wrap }}
aes256-Wrap ALGORITHM ::= {{ OID id-aes256-wrap }}
The object identifier for the Triple-DES Key Wrap (see [3DES-WRAP])
is
id-alg-CMS3DESwrap OBJECT IDENTIFIER ::= {
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
smime(16) alg(3) 6
}
This object identifier has a NULL parameter.
tdes-Wrap ALGORITHM ::=
{{ OID id-alg-CMS3DESwrap PARMS NullParms }}
NOTE: As of this writing, the AES Key Wrap and the Triple-DES Key
Wrap are in the process of being approved by ASC X9.
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B.3 ASN.1 module
CMS-RSA-KEM
{ iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
pkcs-9(9) smime(16) modules(0) cms-rsa-kem(21) } [[check]]
BEGIN
-- EXPORTS ALL
-- IMPORTS None
-- Useful types and definitions
OID ::= OBJECT IDENTIFIER -- alias
-- Unless otherwise stated, if an object identifier has associated
-- parameters (i.e., the PARMS element is specified), the parameters
-- field shall be included in algorithm identifier values. The
-- parameters field shall be omitted if and only if the object
-- identifier does not have associated parameters (i.e., the PARMS
-- element is omitted), unless otherwise stated.
ALGORITHM ::= CLASS {
&id OBJECT IDENTIFIER UNIQUE,
&Type OPTIONAL
}
WITH SYNTAX { OID &id [PARMS &Type] }
AlgorithmIdentifier { ALGORITHM:IOSet } ::= SEQUENCE {
algorithm ALGORITHM.&id( {IOSet} ),
parameters ALGORITHM.&Type( {IOSet}{@algorithm} ) OPTIONAL
}
NullParms ::= NULL
-- ISO/IEC 18033-2 arc
is18033-2 OID ::= { iso(1) standard(0) is18033(18033) part2(2) }
-- NIST algorithm arc
nistAlgorithm OID ::= {
joint-iso-itu-t(2) country(16) us(840) organization(1)
gov(101) csor(3) nistAlgorithm(4)
}
-- PKCS #1 arc
pkcs-1 OID ::= {
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1)
}
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-- RSA-KEM Key Transport Algorithm, based on Generic Hybrid Cipher
id-ac-generic-hybrid OID ::= {
is18033-2 asymmetric-cipher(1) generic-hybrid(2)
}
GenericHybridParameters ::= {
kem KeyEncapsulationMechanism,
dem DataEncapsulationMechanism
}
id-kem-rsa OID ::= {
is18033-2 key-encapsulation-mechanism(2) rsa(4)
}
RsaKemParameters ::= {
keyDerivationFunction KeyDerivationFunction,
keyLength KeyLength
}
KeyDerivationFunction ::= AlgorithmIdentifier {{KDFAlgorithms}}
KDFAlgorithms ALGORITHMS ::= {
kdf2,
... -- implementations may define other methods
}
KeyLength ::= INTEGER (1..MAX)
DataEncapsulationMechanism ::= AlgorithmIdentifier {{DEMAlgorithms}}
DEMAlgorithms ALGORITHM ::= {
X9-SymmetricKeyWrappingSchemes,
... -- implementations may define other methods
}
X9-SymmetricKeyWrappingSchemes ALGORITHM ::= {
aes128-Wrap | aes192-Wrap | aes256-Wrap | tdes-Wrap,
... -- allows for future expansion
}
-- Key Derivation Functions
id-kdf-kdf2 OID ::= { is18033-2 key-derivation-functions(5) kdf2(2) }
kdf2 ALGORITHM ::= {{ OID id-kdf-kdf2 PARMS KDF2-HashFunction }}
KDF2-HashFunction ::= AlgorithmIdentifier {{KDF2-HashFunctions}}
KDF2-HashFunctions ALGORITHM ::= {
X9-HashFunctions,
... -- implementations may define other methods
}
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-- Hash Functions
X9-HashFunctions ALGORITHM ::= {
sha1 | sha224 | sha256 | sha384 | sha512,
... -- allows for future expansion
}
id-sha1 OID ::= {
iso(1) identified-organization(3) oiw(14) secsig(3)
algorithms(2) sha1(26)
}
id-sha256 OID ::= { nistAlgorithm hashAlgs(2) sha256(1) }
id-sha384 OID ::= { nistAlgorithm hashAlgs(2) sha384(2) }
id-sha512 OID ::= { nistAlgorithm hashAlgs(2) sha512(3) }
sha1 ALGORITHM ::= {{ OID id-sha1 }} -- NullParms MUST be
sha224 ALGORITHM ::= {{ OID id-sha224 }} -- accepted for these
sha256 ALGORITHM ::= {{ OID id-sha256 }} -- OIDs
sha384 ALGORITHM ::= {{ OID id-sha384 }} –- ""
sha512 ALGORITHM ::= {{ OID id-sha512 }} –- ""
-- Symmetric Key-Wrapping Schemes
id-aes128-Wrap OID ::= { nistAlgorithm aes(1) aes128-Wrap(5) }
id-aes192-Wrap OID ::= { nistAlgorithm aes(1) aes192-Wrap(25) }
id-aes256-Wrap OID ::= { nistAlgorithm aes(1) aes256-Wrap(45) }
aes128-Wrap ALGORITHM ::= {{ OID id-aes128-wrap }}
aes192-Wrap ALGORITHM ::= {{ OID id-aes192-wrap }}
aes256-Wrap ALGORITHM ::= {{ OID id-aes256-wrap }}
id-alg-CMS3DESwrap OBJECT IDENTIFIER ::= {
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
smime(16) alg(3) 6
}
tdes-Wrap ALGORITHM ::= {{ OID id-alg-CMS3DESwrap PARMS NullParms }}
B.4 Examples
As an example, if the key derivation function is KDF2 based on
SHA-256 and the symmetric key-wrapping scheme is the AES Key Wrap
with a 128-bit KEK, the AlgorithmIdentifier for the RSA-KEM Key
Transport Algorithm will have the following value:
SEQUENCE {
id-ac-generic-hybrid, -- generic cipher
SEQUENCE { -- GenericHybridParameters
SEQUENCE { -- key encapsulation mechanism
id-kem-rsa, -- RSA-KEM
SEQUENCE { -- RsaKemParameters
SEQUENCE { -- key derivation function
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id-kdf-kdf2, -- KDF2
SEQUENCE { -- KDF2-HashFunction
id-sha256 -- SHA-256; no parameters (preferred)
},
16 -- KEK length in bytes
},
SEQUENCE { -- data encapsulation mechanism
id-aes128-Wrap -- AES-128 Wrap; no parameters
}
}
}
This AlgorithmIdentifier value has the following DER encoding:
30 4f
06 07 28 81 8c 71 02 01 02 -- id-ac-generic-hybrid
30 44
30 25
06 07 28 81 8c 71 02 02 04 -- id-kem-rsa
30 1a
30 16
06 07 28 81 8c 71 02 05 02 -- id-kdf-kdf2
30 0b
06 09 60 86 48 01 65 03 04 02 01 -- id-sha256
02 10 -- 16 bytes
30 0b
06 09 60 86 48 01 65 03 04 01 05 -- id-aes128-Wrap
The DER encodings for other typical sets of underlying components are
as follows:
* KDF2 based on SHA-384, AES Key Wrap with a 192-bit KEK
30 4f 06 07 28 81 8c 71 02 01 02 30 44 30 25 06
07 28 81 8c 71 02 02 04 30 1a 30 16 06 07 28 81
8c 71 02 05 02 30 0b 06 09 60 86 48 01 65 03 04
02 02 02 18 30 0b 06 09 60 86 48 01 65 03 04 01
19
* KDF2 based on SHA-512, AES Key Wrap with a 256-bit KEK
30 4f 06 07 28 81 8c 71 02 01 02 30 44 30 25 06
07 28 81 8c 71 02 02 04 30 1a 30 16 06 07 28 81
8c 71 02 05 02 30 0b 06 09 60 86 48 01 65 03 04
02 03 02 20 30 0b 06 09 60 86 48 01 65 03 04 01
2d
* KDF2 based on SHA-1, Triple-DES Key Wrap with a 128-bit KEK
(two-key triple-DES)
30 4f 06 07 28 81 8c 71 02 01 02 30 44 30 21 06
07 28 81 8c 71 02 02 04 30 16 30 12 06 07 28 81
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8c 71 02 05 02 30 07 06 05 2b 0e 03 02 1a 02 10
30 0f 06 0b 2a 86 48 86 f7 0d 01 09 10 03 06 05
00
* KDF2 based on SHA-224, Triple-DES Key Wrap with a 192-bit
KEK (three-key triple-DES)
[[to be defined, awaiting OID for SHA-224]]
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The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
Kaliski Standards - Exp: April 2004 [Page 20]