Network Working Group Sean Turner, IECA Internet Draft February 1, 2010 Intended Status: Standard Track Expires: August 1, 2010 Algorithms for Asymmetric Key Package Content Type draft-turner-asymmetrickeyformat-algs-01.txt Abstract This document describes the conventions for using several cryptographic algorithms with the EncryptedPrivateKeyInfo structure, as defined in RFC TBD1. It also includes conventions necessary to protect the AsymmetricKeyPackage content type with SignedData, EnvelopedData, EncryptedData, AuthenticatedData, and AuthEnvelopedData. Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. 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Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. 1. Introduction This document describes the conventions for using several cryptographic algorithms with the EncryptedPrivateKeyInfo structure [RFCTBD1]. The EncryptedPrivateKeyInfo is used by [P12] to encrypt PrivateKeyInfo [RFCTBD1]. It is similar to EncryptedData [RFC5652] 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 [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. 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]. Turner Expires August 1, 2010 [Page 2] Internet-Draft Algorithms for Asymmetric Key Packages February 2010 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 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. [RFC2898] specifies two PBE Schemes (PBES) 1 and 2, the defacto is PBES 1. The notation for the PBES 1 is: PBEWithAnd. 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 [RFC5652], EnvelopedData [RFC5652], EncryptedData [RFC5652], AuthenticatedData [RFC5652], and AuthEnvelopedData [RFC5083]. 3.1. SignedData If an implementation supports SignedData, then it MUST support the signature scheme RSA [RFC3370] and SHOULD support the signature schemes RSASSA-PSS [RFC4056] and DSA [RFC3370]. Additionally, implementations MUST support in concert with these signature schemes the hash function SHA-256 [RFC5754] and it SHOULD support the hash function SHA-1 [RFC3370]. Turner Expires August 1, 2010 [Page 3] Internet-Draft Algorithms for Asymmetric Key Packages February 2010 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. 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 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]. 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 other means; therefore, the only algorithm specified is the content encryption algorithm. 3.4. AuthenticatedData 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. 3.5. AuthEnvelopedData 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 Turner Expires August 1, 2010 [Page 4] Internet-Draft Algorithms for Asymmetric Key Packages February 2010 SHOULD be implemented. Implementations MAY also support for AES-CCM [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. 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], [RFC3394], [RFC3560], [RFC5652], [RFC4056], [RFC4231], [RFC5083], [RFC5084], [RFC5649], [RFC5754], and [RFCTBD1] 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 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 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- Turner Expires August 1, 2010 [Page 5] Internet-Draft Algorithms for Asymmetric Key Packages February 2010 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. 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. [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. Turner Expires August 1, 2010 [Page 6] Internet-Draft Algorithms for Asymmetric Key Packages February 2010 [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, August 2009. [RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", RFC 5652, September 2009. [RFC5751] Turner, S., and B. Ramsdell, "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. [RFCTBD1] Turners, S., "Asymmetric Key Packages", draft-turner- asymmetrickeyformat-03.txt, work-in-progress. /** RFC Editor: Please replace "RFCTBD1" with "RFC####" where #### is the number of the published RFC. Please do this in both the references and the text. **/ 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. Turner Expires August 1, 2010 [Page 7] Internet-Draft Algorithms for Asymmetric Key Packages February 2010 Authors' Addresses Sean Turner IECA, Inc. 3057 Nutley Street, Suite 106 Fairfax, VA 22031 USA EMail: turners@ieca.com Turner Expires August 1, 2010 [Page 8]