Network Working Group J. Schaad Internet-Draft Soaring Hawk Consulting Intended status: Informational November 19, 2010 Expires: May 23, 2011 S/MIME Capabilities for Public Key Definitions draft-ietf-pkix-pubkey-caps-00 Abstract This document defines a set of S/MIME Capability types for ASN.1 encoding for the current set of public keys define in the PKIX working group. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on May 23, 2011. Copyright Notice Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. 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. Schaad Expires May 23, 2011 [Page 1] Internet-Draft PubKey Caps November 2010 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Notation . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. RSA Public Keys . . . . . . . . . . . . . . . . . . . . . . . 5 2.1. Generic RSA Public Keys . . . . . . . . . . . . . . . . . 5 2.2. RSASSA-PSS Signature Public Keys . . . . . . . . . . . . . 6 3. Diffie-Hellman Keys . . . . . . . . . . . . . . . . . . . . . 7 3.1. Diffie-Hellman Signature Public Key . . . . . . . . . . . 7 4. Elliptical Curve Keys . . . . . . . . . . . . . . . . . . . . 9 4.1. Generic Elliptical Curve Keys . . . . . . . . . . . . . . 9 5. RSASSA-PSS Signature Algorithm Capability . . . . . . . . . . 10 6. Security Considerations . . . . . . . . . . . . . . . . . . . 12 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14 8.1. Normative References . . . . . . . . . . . . . . . . . . . 14 8.2. Informative References . . . . . . . . . . . . . . . . . . 14 Appendix A. 2008 ASN.1 Module . . . . . . . . . . . . . . . . . . 15 Appendix B. Future Work . . . . . . . . . . . . . . . . . . . . . 18 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 19 Schaad Expires May 23, 2011 [Page 2] Internet-Draft PubKey Caps November 2010 1. Introduction In the process of dealing with the OCSP agility issues in [I-D.ietf-pkix-ocspagility] it was noted that we really wanted to describe some information to be used in selecting a public key, but we did not currently have any way of doing so at the current time. This document fills that hole by defining a set of S/MIME Capability types for a small set of public key representations. 1.1. Notation The main body of the text is written using snippets of ASN.1 that are extracted from the ASN.1 2008 module in Appendix A. This is because I am a strong advocate of moving the current versions of ASN.1 as they can contain meta-data which is not representable in the 1988 version of ASN.1. In keeping with the current policy of the PKIX working group, the 1988 module is still to be considered the normative module in the event of a conflict between the contents of the two modules. When reading this document, it is assumed that you will have a degree of familiarity with tthe basic object module that is presented in section 3 of RFC 5912 ([RFC5912]). We use the SMIME-CAPS object in this document, it assoicates two fields together in a single object. SMIME-CAPS ::= CLASS { &id OBJECT IDENTIFIER UNIQUE, &Type OPTIONAL } WITH SYNTAX { [TYPE &Type] IDENTIFIED BY &id } These fields are: &id contains an object identifier. When placed in an objet set, this element is tagged so that no two elements can be placed in the set that have the same value in the &id field. &Type optionally contains an ASN.1 type identifier. If the field &Type is not defined then the optional parameters field of the AlgorithmIdentifier type would be omitted. The class also has a specialized syntax for how to define an object in this class. The all upper case fields TYPE IDENTIFIER and BY are syntaxtic sugar to make it easier to read and identify what pieces are manditory and which are optional. One of the things that can be done is to reference the fields of an object while defining other objects. This means that if an object Schaad Expires May 23, 2011 [Page 3] Internet-Draft PubKey Caps November 2010 call foo has a field name &value, the same value can be directly referenced as foo.&value. This means that we would automatically get any updates to values or types and we do not need to do any replication of the data. Schaad Expires May 23, 2011 [Page 4] Internet-Draft PubKey Caps November 2010 2. RSA Public Keys There are currently three different public key object identifiers for RSA public keys. These are RSA, RSA-OCSP and RSA-PSS. 2.1. Generic RSA Public Keys Almost all RSA keys that are contained in certificates today use the generic RSA public key format and identifier. This allows for the public key to be used both for key transport and for signature validation (assuming it is compatible with the bits in the key usage extension). The only reason for using one of more specific public key identifiers is if the user wants to restrict the usage of the RSA public key with a specific algorithm. For the generic RSA public key, the S/MIME capibility that is advertised is a request for a specific key size to be used. This would normally be used for dealing with a request on the key to be used for a signature that the client would then verify. In general the user would provide a specific key when a key transport algorithm is being considered. The ASN.1 that is used for the generic RSA public key is defined as below: scap-pk-rsa SMIME-CAPS ::= { TYPE RSAKeyCapabilities IDENTIFIED BY pk-rsa.&id } RSAKeyCapabilities ::= SEQUENCE { minKeySize RSAKeySize, maxKeySize RSAKeySize OPTIONAL } RSAKeySize ::= INTEGER (1024 | 2048 | 3072 | 7680 | 15360) From the above we can see that: o We use the same object identifer as the public key to identify the S/MIME capability field. o We define a new type RSAKeyCapabilities that is used as the type field for the S/MIME capability. For the structure RSAKeyCapabilities, the fields are used as follows: Schaad Expires May 23, 2011 [Page 5] Internet-Draft PubKey Caps November 2010 minKeySize contains the minimum length of the RSA modulus to be used. maxKeySize contains the maximum length of the RSA modules that should be used. If this field is absent then no maximum length is requested/expected. 2.2. RSASSA-PSS Signature Public Keys While most of the time one will use the generic RSA public key identifier in a certificate, the RSA SSA-PSS identifier can be used if the owner of the key desires to restrict the usage of the key to just this algorithm. The ASN.1 that is used for the RSA SSA-PSS public key is defined below: scap-pk-rsaSSA-PSS SMIME-CAPS ::= { TYPE RSAKeyCapabilities IDENTIFIED BY pk-rsaSSA-PSS.&id } From the above we can see that: o We use the same object identifier as the public key to identify the S/MIME capability field. o We use the same type for the S/MIME capability as is used above for the generic RSA public key. Schaad Expires May 23, 2011 [Page 6] Internet-Draft PubKey Caps November 2010 3. Diffie-Hellman Keys There is current two Diffie-Hellman public key object identifiers. These are DH and DSA. 3.1. Diffie-Hellman Signature Public Key This public key type is used for the validation of DSA signatures. The ASN.1 that is used for DSA keys is defined below: scap-pk-dsa ::= { TYPE DSAKeyCapabilities IDENTIFIED BY pk-dsa.&id } DSAKeyCapabilites ::= CHOICE { keySizes [0] SEQUENCE { minKeySize DSAKeySize, maxKeySize DSAKeySize OPTIONAL }, keyParams [1] pk-dsa.&Type } DSAKeySize ::= INTEGER (1024 | 2048 | 3072 | 7680 | 15360 ) From the above we can see that: o We use the same object identifer as the public key to identify the S/MIME capability field. o We define a new type DSAKeyCapabilities that is used as the type field for the S/MIME capability. For the structure DSAKeyCapabilities, the fields are used as follows: keySizes is used when only a key size is needed to be specified and not a specific group. It is expected that ths would be the most commonly used of the two options. In key sizes the fields are used as follows: minKeySize contains the minimum length of the DSA modulus to be used. maxKeySize contains the maximum length of the DSA modules that should be used. If this field is absent then no maximum length is requested/expected. Schaad Expires May 23, 2011 [Page 7] Internet-Draft PubKey Caps November 2010 keyParams contains the exact set of DSA for the key used to sign the message. NOTE: In the original discussions the option keyParams would not have existed in this structure, and they may not exist in a future version of the structure. The issue is that we really only need to have the key size fields, but there seems to be a mis-match between this structure and that used for ECC where we don't specify anything about key sizes, but do specify the exact group to be used. We should probably have a discussion about rationalizing these together. Schaad Expires May 23, 2011 [Page 8] Internet-Draft PubKey Caps November 2010 4. Elliptical Curve Keys There are currently three Eliptical Curve public key object identifiers. These are EC, EC-DH and EC-MQV 4.1. Generic Elliptical Curve Keys All most all ECC keys that are contained in certificates today use the generic ECC public key format and identifier. This allows for the public key to be used both for key agreement and for signature validation (assuming the appropriate bits are in the certificate). The only reason for using one of the more specific public key identifier is if the user wants to restrict the usage of the ECC public key with a specific algorithm. For the generic ECC public key, the S/MIME capability that is advertized is a request for a specific group to be used. The ASN.1 that is used for the generic ECC public key is defined as below: scap-pk-ec SMIME-CAPS ::= { TYPE pk-ec.&Type IDENTIFIED BY pk-ec.&id } From the above we can see that: o We use the same object identifier as the public key to identify the S/MIME capability field. o We use the same data type for the S/MIME capability as is used for the public key. Schaad Expires May 23, 2011 [Page 9] Internet-Draft PubKey Caps November 2010 5. RSASSA-PSS Signature Algorithm Capability This document defines a new S/MIME Capability for the RSA-SSA-PSS signature algorithm. THere already exists one in [RFC4055] where the parameters field is not used. When the S/MIME group defined a S/MIME Capability for the RSA-SSA-PSS signature algorithm, it was done so in the context of how S/MIME defines and uses S/MIME Capabilities. When placed in an S/MIME message [RFC3851] or in a certificate [RFC4262] it is always placed in a sequence of capabilities. This meant that one can place the identifier for RSA-SSA-PSS in the sequence along with the identifier for MD5, SHA-1 and SHA-256. The assumption was then made that one could compute the matrix of all answers and the publisher would support all elements in the matrix. This has the possiblity that the publisher could accendently publish a point in the matrix that is not actually supported. In this situation, there is only a single item that is published. This means that we need to publish all of the assoicated information along with the identifier for the signature algorrithm in a single entity. For this reason we now define a new parameter type to be used as the S/MIME capaiblity type which contains a hash identifier and a mask identifier. The actual ASN.1 used for this is as follows: scap-sa-rsaSSA-PSS SMIME-CAPS ::= { TYPE RsaSsa-Pss-sig-caps IDENTIFIED BY sa-rsaSSA-PSS.&id } RsaSsa-Pss-sig-caps ::= SEQUENCE { hashAlg SMimeCapability{ HashAlgorithmSet }, maskAlg SMimeCapability{ MaskAlgorithmSet }, trailerField INTEGER DEFAULT 1 } From the above we can see that: o We use the same object identifier as the public key to identify the S/MIME capability field. o We define a new type RsaSsa-Pss-sig-caps which contains the information that needs to defined to describe a specific instance of the signature algorithm. For the type RsaSsa-Pss-sig-caps, the fields are used as follows: Schaad Expires May 23, 2011 [Page 10] Internet-Draft PubKey Caps November 2010 hashAlg contains the S/MIME capability for the hash algorithm we are declaring we support with the RSA-SSA-PSS signature algorithm. maskAlg contains the S/MIME capability for the mask algorithm we are declaring we support with the RSA-SSA-PSS signature algorithm. trailerField specifies which trailer field algorithm is being supported. This MUST be the value 1. NOTE: In at least one iteration of the design we used a sequence of hash identifiers and a sequence of masking functions and again made the assumption that entire matrix would be supported. This has been removed at this point since the original intent of S/MIME capabilities is that one should be able to do a binary comparison of the DER encoding of the field and determine a specific capability was published. We could return back to using the sequence if we wanted to lose the ability to do a binary compare but needed to shorten the encodings. This does not currently appear to be an issue at this point. Schaad Expires May 23, 2011 [Page 11] Internet-Draft PubKey Caps November 2010 6. Security Considerations To Be Supplied. Schaad Expires May 23, 2011 [Page 12] Internet-Draft PubKey Caps November 2010 7. IANA Considerations This document has no IANA considerations. Schaad Expires May 23, 2011 [Page 13] Internet-Draft PubKey Caps November 2010 8. References 8.1. Normative References [RFC3279] Bassham, L., Polk, W., and R. Housley, "Algorithms and Identifiers for the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 3279, April 2002. [RFC3370] Housley, R., "Cryptographic Message Syntax (CMS) Algorithms", RFC 3370, August 2002. [RFC4055] Schaad, J., Kaliski, B., and R. Housley, "Additional Algorithms and Identifiers for RSA Cryptography for use in the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 4055, June 2005. [RFC5912] Hoffman, P. and J. Schaad, "New ASN.1 Modules for the Public Key Infrastructure Using X.509 (PKIX)", RFC 5912, June 2010. 8.2. Informative References [I-D.ietf-pkix-ocspagility] Hallam-Baker, P. and S. Santesson, "OCSP Algorithm Agility", draft-ietf-pkix-ocspagility-08 (work in progress), March 2010. [RFC3851] Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.1 Message Specification", RFC 3851, July 2004. [RFC4262] Santesson, S., "X.509 Certificate Extension for Secure/ Multipurpose Internet Mail Extensions (S/MIME) Capabilities", RFC 4262, December 2005. Schaad Expires May 23, 2011 [Page 14] Internet-Draft PubKey Caps November 2010 Appendix A. 2008 ASN.1 Module PUBLIC-KEY-SMIME-CAPIBLITIES --TBD Add module number-- DEFINITIONS ::= BEGIN SMIME-CAPS, PUBLIC-KEY FROM AlgoritrithmInformation-2009 {iso(1) identified-organization(3) dod(6) internet(1) security(5) mechanisms(5) pkix(7) id-mod(0) id-mod-algorithmInformation-02(58)} pk-rsa, pk-dsa, pk-dh, pk-ec, pk-edDH, pk-ecMQV FROM PKIXAlgs-2009 { iso(1) identified-organization(3) dod(6) internet(1) security(5) mechanisms(5) pkix(7) id-mod(0) id-mod-pkix1-algorithms2008-02(56) } pk-rsaSSA-PSS, pk-rsaES-OAEP, sa-rsaSSA-PSS FROM PKIX1-PSS-OAEP-Algorithms-2009 {iso(1) identified-organization(3) dod(6) internet(1) security(5) mechanisms(5) pkix(7) id-mod(0) id-mod-pkix1-rsa-pkalgs-02(54)} ; -- -- Define a set containing all of the S/MIME capabilties defined -- by this document -- SMimeCaps SMIME-CAPS ::= { scap-pk-rsa | scap-pk-rsaSSA-PSS | scap-pk-dsa | scap-pk-ec | scap-pk-ecDH | scap-sa-rsaSSA-PSS } -- -- We defined RSA keys from the modules RFC3279 and -- scap-pk-rsa SMIME-CAPS ::= { TYPE RSAKeyCapabilities IDENTIFIED BY pk-rsa.&id } RSAKeyCapabilities ::= SEQUENCE { minKeySize RSAKeySize, maxKeySize RSAKeySize OPTIONAL } Schaad Expires May 23, 2011 [Page 15] Internet-Draft PubKey Caps November 2010 RSAKeySize ::= INTEGER (1024 | 2048 | 3072 | 7680 | 15360) scap-pk-rsaES-OAEP SMIME-CAPS ::= { TYPE NULL IDENTIFIED BY pk-rsaES-OAEP.&id } scap-pk-rsaSSA-PSS SMIME-CAPS ::= { TYPE RSAKeyCapabilities IDENTIFIED BY pk-rsaSSA-PSS.&id } scap-sa-rsaSSA-PSS SMIME-CAPS ::= { TYPE RsaSsa-Pss-sig-caps IDENTIFIED BY sa-rsaSSA-PSS.&id } RsaSsa-Pss-sig-caps ::= SEQUENCE { hashAlg SMimeCapability{ HashAlgorithmSet }, maskAlg SMimeCapability{ MaskAlgorithmSet }, trailerField INTEGER DEFAULT 1 } -- -- we define DH/DSA keys from the module RFC3279 -- scap-pk-dsa ::= { TYPE DSAKeyCapabilities IDENTIFIED BY pk-dsa.&id } DSAKeyCapabilites ::= CHOICE { keySizes [0] SEQUENCE { minKeySize DSAKeySize, maxKeySize DSAKeySize OPTIONAL }, keyParams [1] pk-dsa.&Type } DSAKeySize ::= INTEGER (1024 | 2048 | 3072 | 7680 | 15360 ) scap-pk-dh ::= { TYPE INTEGER IDENTIFIED BY pk-dh.&id } Schaad Expires May 23, 2011 [Page 16] Internet-Draft PubKey Caps November 2010 -- -- we define Eliptical Curve keys from the module RFC3279 -- scap-pk-ec SMIME-CAPS ::= { TYPE pk-ec.&Type IDENTIFIED BY pk-ec.&id } scap-pk-ecDH SMIME-CAPS ::= { TYPE pk-ecDH.&Type IDENTIFIED BY pk-ecDH.&id } scap-pk-ecMQV SMIME-CAPS ::= { TYPE pk-ecMQV.&Type IDENTIFIED BY pk-ecMQV.&id } END Schaad Expires May 23, 2011 [Page 17] Internet-Draft PubKey Caps November 2010 Appendix B. Future Work A future revision of [RFC5912] should be done at some point which expands the definition of the PUBLIC-KEY class and allows for an S/MIME Capability to be included in the class defintion. This would encourage people to think about this as an issue when defining new public key structures in the future. Schaad Expires May 23, 2011 [Page 18] Internet-Draft PubKey Caps November 2010 Author's Address Jim Schaad Soaring Hawk Consulting Email: jimsch@augustcellars.com Schaad Expires May 23, 2011 [Page 19]