Network Working Group Sean Turner, IECA
Internet Draft November 9, 2009
Intended Status: Standard Track
Expires: March 9, 2010
Algorithms for Asymmetric Key Package Content Type
draft-turner-asymmetrickeyformat-algs-00.txt
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Abstract
This document describes the conventions for using several
cryptographic algorithms with the EncryptedPrivateKeyInfo structure,
as defined in RFC 5208. It also includes conventions necessary to
protect the AsymmetricKeyPackage content type with SignedData,
EnvelopedData, EncryptedData, AuthenticatedData, and
AuthEnvelopedData.
1. Introduction
This document describes the conventions for using several
cryptographic algorithms with the EncryptedPrivateKeyInfo structure
[RFC5208]. The EncryptedPrivateKeyInfo is used by [P12] to encrypt
PrivateKeyInfo [RFCTBD1]. It is similar to EncryptedData [RFC3852] in
that it has no recipients, no originators, and no content encryption
keys and requires keys be managed by other means.
This document also includes conventions necessary to protect the
AsymmetricKeyPackage content type [RFCTBD1] with Cryptographic
Message Syntax (CMS) protecting content types: SignedData [RFC3852],
EnvelopedData [RFC3852], EncryptedData [RFC3852], AuthenticatedData
[RFC3852], 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.
1.1. Terminology
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].
2. EncryptedPrivateKeyInfo
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
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PKCS#5 [RFC2898] and PKCS#12 [P12]. The major difference between PKCS
#5 and PKCS #12 being the supported encoding for the password: ASCII
for PKCS #5 and Unicode for PKCS #12. [RFC2898] specifies two
mechanisms PBE Schemes (PBES) 1 and 2, the defacto is PBES 1. The
notation for the PBES 1 is: PBEWith<digest>And<encryption>. The
following schemes are defined in PKCS #5: PBEWithMD2AndDES-CBC,
PBEWithMD2AndRC2, PBEWithMD5AndDES-CBC, PBEWithMD5AndRC2,
PBEWithSHA1AndDES-CBC, PBEWithSHA1AndRC2. The following schemes are
defined in PKCS #12: PBEWithSHAAnd3-KeyTripleDES-CBC, PBEWithSHAAnd2-
KeyTripleDES-CBC, PBEWithSHAAnd128BitRC2-CBC, PBEWithSHAAnd40BitRC2-
CBC, PBEWithSHAAnd128BitRC4, and PBEWithSHAAnd40BitRC4.
Implementation defaults vary.
The PBES 1 algorithms require salt and iteration count values. The
salt length in PKCS #5 is 8-octets while there is no restriction on
the length of the salt in PKCS #12, but PKCS #12 recommends the salt
be as long as the digest algorithms output (e.g., 20-octets for SHA-
1). The iteration count in PKCS #5 is recommended to be at least
1000 and PKCS #12 recommends at least 1024.
It is RECOMMENDED that implementations support AES-128 Key Wrap with
Padding [RFC5649] or AES-256 Key Wrap with Padding [RFC5649].
3. AsymmetricKeyPackage
As noted in Asymmetric Key Packages [RFCTBD1], CMS can be used to
protect the AsymmetricKeyPackage. The following provides guidance
for SignedData [RFC3852], EnvelopedData [RFC3852], EncryptedData
[RFC3852], AuthenticatedData [RFC3852], and AuthEnvelopedData
[RFC5083].
3.1. SignedData
If an implementation supports SignedData, then it MUST support RSA
[RFC3370], SHOULD support RSASSA-PSS [RFC4056], and SHOULD support
DSA [RFC3370]. Additionally, implementations MUST support SHA-256
[RFCTBD3] and SHOULD support SHA-1 [RFC3370].
3.2. EnvelopedData
If an implementation supports EnvelopedData, then it MUST implement
the key transport and it MAY implement the key agreement mechanism.
When key transport is used, RSA encryption [RFC3370] MUST be
supported and RSAES-OAEP [RFC3560] SHOULD be supported.
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When key agreement is used, Diffie-Hellman ephemeral-static [RFC3370]
SHOULD be supported.
Regardless of the key management technique choice, implementations
MUST support AES-128 Key Wrap with Padding [RFC5649].
Implementations SHOULD support AES-256 Key Wrap with Padding
[RFC5649].
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 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].
3.3. EncryptedData
If an implementation supports EncryptedData, then it MUST implement
AES-128 Key Wrap with Padding [RFC5649] and MAY implement AES-256 Key
Wrap with Padding [RFC5649].
NOTE: EncryptedData requires that keys be managed by means other than
EncryptedData; therefore, the only algorithm specified is the content
encryption algorithm.
3.4. AuthenticatedData
If an implementation supports AuthenticatedData, then it MUST
implement SHA-256 [RFCTBD3] 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.
3.5. AuthEnvelopedData
If an implementation supports AuthenticatedData, then it MUST
implement the EnvelopedData recommendations except for the content
encryption algorithm, which in this case is MUST be either 128-bit
AES-CCM or AES-GCM [RFC5084] or SHOULD BE 256-bit AES-CCM or AES-GCM
[RFC5084].
4. Public Key Sizes
The easiest way to implement the key transport requirement for
EnvelopedData and AuthenticatedData is with public key certificates
[RFC5280]. If an implementation support RSA, RSAES-OAEP, or DH, then
it MUST support key lengths from 1024-bit to 2048-bit, inclusive.
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5. SMIMECapabilities Attribute
[RFCTBD4] 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], [RFC3394], [RFC3560],
[RFC3852], [RFC4056], [RFC4231], [RFC5083], [RFC5084], [RFC5649],
[RFCTBD1], and [RFCTBD3] apply.
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 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 better.
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-CCM or AES-
GCM mode that provides the same properties. If an AES-CCM and AES-
GCM 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.
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7. IANA Considerations
None. Please remove this section prior to publication as an RFC.
8. References
8.1. 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.
[RFC3394] Housley, R., and J. Schaad, "Advanced Encryption Standard
(AES) Key Wrap Algorithm", RFC 3394, September 2002.
[RFC3560] Housley, R., "Use of the RSAES-OAEP Key Transport
Algorithm in the Cryptographic Message Syntax (CMS)", RFC
3560, July 2003.
[RFC3852] Housley, R., "Cryptographic Message Syntax (CMS)", RFC
3852, July 2004.
[RFC4056] Schaad, J., "Use of 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,
November 2007.
[RFC5084] Housley, R., "Using AES-CCM and AES-GCM Authenticated
Encryption in the Cryptographic Message Syntax (CMS)",
RFC 5084, November 2007.
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[RFC5208] Kaliski, B., "Public-Key Cryptography Standards (PKCS)
#8: Private-Key Information Syntax Specification Version
1.2", RFC 5208, May 2008.
[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, August 2009.
[RFCTBD1] Turners, S., "Asymmetric Key Packages", draft-turner-
asymmetrickeyformat-02.txt, work-in-progress.
[RFCTBD3] Turner, S., "Using SHA2 Algorithms with Cryptographic
Message Syntax", draft-ietf-smime-sha2-11.txt, work-in-
progress.
[RFCTBD4] Turner, S., and B. Ramsdell, "Secure/Multipurpose
Internet Mail Extensions (S/MIME) Version 3.2 Message
Specification", draft-ietf-smime-3851bis-11.txt, work-in-
progress.
8.2. 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.
Authors' Addresses
Sean Turner
IECA, Inc.
3057 Nutley Street, Suite 106
Fairfax, VA 22031
USA
EMail: turners@ieca.com
Turner Expires March 9, 2010 [Page 7]
