Internet Engineering Task Force (IETF) S. Turner
Request for Comments: 5959 IECA
Category: Standards Track August 2010
Algorithms for Asymmetric Key Package Content Type
This document describes the conventions for using several
cryptographic algorithms with the EncryptedPrivateKeyInfo structure,
as defined in RFC 5958. It also includes conventions necessary to
protect the AsymmetricKeyPackage content type with SignedData,
EnvelopedData, EncryptedData, AuthenticatedData, and
Status of This Memo
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This document describes the conventions for using several
cryptographic algorithms with the EncryptedPrivateKeyInfo structure
[RFC5958]. The EncryptedPrivateKeyInfo is used by [P12] to encrypt
PrivateKeyInfo [RFC5958]. It is similar to EncryptedData [RFC5652]
in that it has no recipients, no originators, and no content
encryption keys and requires keys to be managed by other means.
This document also includes conventions necessary to protect the
AsymmetricKeyPackage content type [RFC5958] with Cryptographic
Message Syntax (CMS) protecting content types: SignedData [RFC5652],
EnvelopedData [RFC5652], EncryptedData [RFC5652], AuthenticatedData
[RFC5652], and AuthEnvelopedData [RFC5083]. Implementations of
AsymmetricKeyPackage do not require support for any CMS protecting
content type; however, if the AsymmetricKeyPackage is CMS protected
it is RECOMMENDED that conventions defined herein be followed.
This document does not define any new algorithms instead it refers to
previously defined algorithms.
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 [RFC2119].
The de facto standard used to encrypt the PrivateKeyInfo structure,
which is subsequently placed in the EncryptedPrivateKeyInfo
encryptedData field, is Password Based Encryption (PBE) based on PKCS
#5 [RFC2898] and PKCS #12 [P12]. The major difference between PKCS
#5 and PKCS #12 is the supported encoding for the password: ASCII for
PKCS #5 and Unicode for PKCS #12, encoded as specified in Section B.1
of [P12]. [RFC2898] specifies two PBE Schemes (PBES) 1 and 2;
[RFC2898] recommends PBES2 for new specification. PBES2 with a key
derivation algorithm of PBKDF2 using HMAC with SHA-256 [RFC5754] and
an encryption algorithm of AES Key Wrap with Padding as defined in
[RFC5649] MUST be supported. AES-256 Key Wrap with Padding [RFC5649]
MAY also be supported as an encryption algorithm.
As noted in Asymmetric Key Packages [RFC5958], CMS can be used to
protect the AsymmetricKeyPackage. The following provides guidance
for SignedData [RFC5652], EnvelopedData [RFC5652], EncryptedData
[RFC5652], AuthenticatedData [RFC5652], and AuthEnvelopedData
If an implementation supports SignedData, then it MUST support the
signature scheme RSA [RFC3370] [RFC5754] and SHOULD support the
signature schemes RSASSA-PSS [RFC4056] and DSA [RFC3370] [RFC5754].
Additionally, implementations MUST support in concert with these
signature schemes the hash function SHA-256 [RFC5754] and SHOULD
support the hash function SHA-1 [RFC3370].
If an implementation supports EnvelopedData, then it MUST implement
key transport and it MAY implement key agreement.
When key transport is used, RSA encryption [RFC3370] MUST be
supported and RSAES-OAEP (RSA Encryption Scheme - Optimal Asymmetric
Encryption Padding) [RFC3560] SHOULD be supported.
When key agreement is used, Diffie-Hellman (DH) ephemeral-static
[RFC3370] MUST be supported.
Since the content type is used to carry a cryptographic key and its
attributes, an algorithm that is traditionally used to encrypt one
key with another is employed. Regardless of the key management
technique choice, implementations MUST support AES-128 Key Wrap with
Padding [RFC5649] as the content encryption algorithm.
Implementations SHOULD support AES-256 Key Wrap with Padding
[RFC5649] as the content encryption algorithm.
When key agreement is used, a key wrap algorithm is also specified to
wrap the content encryption key. If the content encryption algorithm
is AES-128 Key Wrap with Padding, then the key wrap algorithm MUST be
AES-128 Key Wrap with Padding [RFC5649]. If the content encryption
algorithm is AES-256 Key Wrap with Padding, then the key wrap
algorithm MUST be AES-256 Key Wrap with Padding [RFC5649].
If an implementation supports EncryptedData, then it MUST implement
AES-128 Key Wrap with Padding [RFC5649] and SHOULD implement AES-256
Key Wrap with Padding [RFC5649].
NOTE: EncryptedData requires that keys be managed by other means;
therefore, the only algorithm specified is the content encryption
algorithm. Since the content type is used to carry a cryptographic
key and its attributes, an algorithm that is traditionally used to
encrypt one key with another is employed.
If an implementation supports AuthenticatedData, then it MUST
implement SHA-256 [RFC5754] and SHOULD support SHA-1 [RFC3370] as the
message digest algorithm. Additionally, HMAC with SHA-256 [RFC4231]
MUST be supported and HMAC with SHA-1 [RFC3370] SHOULD be supported.
If an implementation supports AuthEnvelopedData, then it MUST
implement the EnvelopedData recommendations except for the content
encryption algorithm, which in this case MUST be AES-GCM [RFC5084];
the 128-bit version MUST be implemented and the 256-bit version
SHOULD be implemented. Implementations MAY also support for AES-CCM
4. Public Key Sizes
The easiest way to implement the SignedData, EnvelopedData, and
AuthEnvelopedData is with public key certificates [RFC5280]. If an
implementation support RSA, RSASSA-PSS, DSS, RSAES-OAEP, or DH, then
it MUST support key lengths from 1024-bit to 2048-bit, inclusive.
5. SMIMECapabilities Attribute
[RFC5751] defines the SMIMECapabilities attribute as a mechanism for
recipients to indicate their supported capabilities including the
algorithms they support. The following are values for the
SMIMECapabilities attribute for AES Key Wrap with Padding [RFC5649]
when used as a content encryption algorithm:
AES-128 KW with Padding: 30 0d 06 09 60 86 48 01 65 03 04 01 08
AES-192 KW with Padding: 30 0d 06 09 60 86 48 01 65 03 04 01 1C
AES-256 KW with Padding: 30 0d 06 09 60 86 48 01 65 03 04 01 30
6. Security Considerations
The security considerations from [RFC3370], [RFC3560], [RFC4056],
[RFC4231], [RFC5083], [RFC5084], [RFC5649], [RFC5652], [RFC5754], and
The strength of any encryption scheme is only as good as its weakest
link, which in the case of a PBES is the password. Passwords need to
provide sufficient entropy to ensure they cannot be easily guessed.
The U.S. National Institute of Standards and Technology (NIST)
Electronic Authentication Guidance [SP800-63] provides some
information on password entropy. [SP800-63] indicates that a user-
chosen 20-character password from a 94-character keyboard with no
checks provides 36 bits of entropy. If the 20-character password is
randomly chosen, then the amount of entropy is increased to roughly
131 bits of entropy. The amount of entropy in the password does not
correlate directly to bits of security but in general the more than
The choice of content encryption algorithms for this document was
based on [RFC5649]: "In the design of some high assurance
cryptographic modules, it is desirable to segregate cryptographic
keying material from other data. The use of a specific cryptographic
mechanism solely for the protection of cryptographic keying material
can assist in this goal". Unfortunately, there is no AES-GCM or AES-
CCM mode that provides the same properties. If an AES-GCM and AES-
CCM mode that provides the same properties is defined, then this
document will be updated to adopt that algorithm.
[SP800-57] provides comparable bits of security for some algorithms
and key sizes. [SP800-57] also provides time frames during which
certain numbers of bits of security are appropriate and some
environments may find these time frames useful.
7.1. Normative References
[P12] RSA Laboratories, "PKCS #12 v1.0: Personal Information
Exchange Syntax", June 1999.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2898] Kaliski, B., "PKCS #5: Password-Based Cryptography
Specification Version 2.0", RFC 2898, September 2000.
[RFC3370] Housley, R., "Cryptographic Message Syntax (CMS)
Algorithms", RFC 3370, August 2002.
[RFC3560] Housley, R., "Use of the RSAES-OAEP Key Transport
Algorithm in Cryptographic Message Syntax (CMS)", RFC
3560, July 2003.
[RFC4056] Schaad, J., "Use of the RSASSA-PSS Signature Algorithm in
Cryptographic Message Syntax (CMS)", RFC 4056, June 2005.
[RFC4231] Nystrom, M., "Identifiers and Test Vectors for HMAC-
SHA-224, HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512",
RFC 4231, December 2005.
[RFC5083] Housley, R., "Cryptographic Message Syntax (CMS)
Authenticated-Enveloped-Data Content Type", RFC 5083,
[RFC5084] Housley, R., "Using AES-CCM and AES-GCM Authenticated
Encryption in the Cryptographic Message Syntax (CMS)",
RFC 5084, November 2007.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation
List (CRL) Profile", RFC 5280, May 2008.
[RFC5649] Housley, R. and M. Dworkin, "Advanced Encryption Standard
(AES) Key Wrap with Padding Algorithm", RFC 5649,
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD
70, RFC 5652, September 2009.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, January 2010.
[RFC5754] Turner, S., "Using SHA2 Algorithms with Cryptographic
Message Syntax", RFC 5754, January 2010.
[RFC5958] Turner, S., "Asymmetric Key Packages", RFC 5958, August
7.2. Informative References
[SP800-57] National Institute of Standards and Technology (NIST),
Special Publication 800-57: Recommendation for Key
Management - Part 1 (Revised), March 2007.
[SP800-63] National Institute of Standards and Technology (NIST),
Special Publication 800-63: Electronic Authentication
Guidance, April 2006.
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