PKIX Working Group R. Housley (SPYRUS) Internet Draft W. Polk (NIST) expires in six months August 5, 1998 Internet X.509 Public Key Infrastructure Representation of Key Exchange Algorithm (KEA) Keys in Internet X.509 Public Key Infrastructure Certificates Status of this Memo This document is an Internet-Draft. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. 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." To view the entire list of current Internet-Drafts, please check the "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast). Copyright (C) The Internet Society (1998). All Rights Reserved. Housley & Polk [Page 1] INTERNET DRAFT August 5 1998 Representation of Key Exchange Algorithm (KEA) Keys in Internet X.509 Public Key Infrastructure Certificates Table of Contents Abstract .......................................................... 3 1. Executive Summary ............................................. 3 2. Requirements and Assumptions .................................. 3 2.1. Communication and Topology .................................. 3 2.2. Acceptability Criteria ...................................... 4 2.3. User Expectations ........................................... 4 2.4. Administrator Expectations .................................. 4 3. KEA Algorithm Support ......................................... 5 3.1. Subject Public Key Info ..................................... 5 3.1.1. Algorithm Identifier and Parameters ....................... 5 3.1.2. Encoding of KEA Public Keys ............................... 6 3.2. Key Usage Extension in KEA certificates ..................... 6 4. ASN.1 Modules .................................................. 7 4.1 1988 Syntax ................................................... 7 4.2 1993 Syntax ................................................... 7 5. References ..................................................... 8 6. Patent Statements .............................................. 8 7. Security Considerations ........................................ 8 8. Author Addresses ............................................... 9 Housley & Polk [Page 2] INTERNET DRAFT August 5 1998 Abstract This is the third draft of a profile for specification of Key Exchange Algorithm (KEA) keys in Internet Public Key Infrastructure X.509 certificates. There are only minor changes in content from the second draft. Several modifications are required now that KEA has been declassified. A patent statement is required, and a published reference for the KEA algorithm is required. After these modifica- tions, the document will be complete. Please send comments on this document to the ietf-pkix@imc.org mail list. 1. Executive Summary This specification contains guidance on the use of the Internet Pub- lic Key Infrastructure certificates to convey Key Exchange Algorithm (KEA) keys. This specification is an addendum to RFC xxxx, "Internet X.509 Public Key Infrastructure: Certificate and CRL Profile". Implementations of this specification must also conform to RFC xxxx. Implementations of this specification are not required to conform to other parts from that series. The Key Exchange Algorithm (KEA) is a classified algorithm for exchanging keys. This specification profiles the format and seman- tics of fields in X.509 V3 certificates containing KEA keys. The specification addresses the subjectPublicKeyInfo field and the keyUsage extension. 2. Requirements and Assumptions The goal is to augment the X.509 certificate profile presented in Part 1 to facilitate the management of KEA keys for those communities which use this algorithm. 2.1. Communication and Topology This profile, as presented in Part 1 and augmented by this specifica- tion, supports users without high bandwidth, real-time IP connec- tivity, or high connection availablity. In addition, the profile allows for the presence of firewall or other filtered communication. This profile does not assume the deployment of an X.500 Directory system. The profile does not prohibit the use of an X.500 Directory, Housley & Polk [Page 3] INTERNET DRAFT August 5 1998 but other means of distributing certificates and certificate revoca- tion lists (CRLs) are supported. 2.2. Acceptability Criteria The goal of the Internet Public Key Infrastructure (PKI) is to meet the needs of deterministic, automated identification, authentication, access control, and authorization functions. Support for these ser- vices determines the attributes contained in the certificate as well as the ancillary control information in the certificate such as pol- icy data and certification path constraints. The goal of this document is to profile KEA certificates, specifying the contants and semantics of attributes which were not fully speci- fied by Part 1. If not specifically addressed by this document, the contents and semantics of the fields and extensions must be as described in Part 1. 2.3. User Expectations Users of the Internet PKI are people and processes who use client software and are the subjects named in certificates. These uses include readers and writers of electronic mail, the clients for WWW browsers, WWW servers, and the key manager for IPSEC within a router. This profile recognizes the limitations of the platforms these users employ and the sophistication/attentiveness of the users themselves. This manifests itself in minimal user configuration responsibility (e.g., root keys, rules), explicit platform usage constraints within the certificate, certification path constraints which shield the user from many malicious actions, and applications which sensibly automate validation functions. 2.4. Administrator Expectations As with users, the Internet PKI profile is structured to support the individuals who generally operate Certification Authorities (CAs). Providing administrators with unbounded choices increases the chances that a subtle CA administrator mistake will result in broad comprom- ise or unnecessarily limit interoperability. This profile defines the object identifiers and data formats that must be supported to intepret KEA public keys. Housley & Polk [Page 4] INTERNET DRAFT August 5 1998 3. KEA Algorithm Support This section describes object identifiers and data formats which may be used with PKIX certicate profile to describe X.509 certificates containing a KEA public key. Conforming CAs are required to use the object identifiers and data formats when issuing KEA certificates. Conforming applications shall recognize the object identifiers and process the data formats when processing such certificates. 3.1. Subject Public Key Info The certificate identifies the KEA algorithm, conveys optional param- eters, and specifies the KEA public key in the subjectPublicKeyInfo field. The subjectPublicKeyInfo field is a SEQUENCE of an algorithm identifier and the subjectPublicKey field. The certificate indicates the algorithm through an algorithm identif- ier. This algorithm identifier consists of an object identifier (OID) and optional associated parameters. Section 3.1.1 identifies the preferred OID and parameters for the KEA algorithm. Conforming CAs shall use the identified OID when issuing certificates containing public keys for the KEA algorithm. Conforming applications supporting the KEA algorithm shall, at a minimum, recognize the OID identified in section 3.1.1. The certificate conveys the KEA public key through the subjectPub- licKey field. This subjectPublicKey field is a BIT STRING. Section 3.1.2 specifies the method for encoding a KEA public key as a BIT STRING. Conforming CAs shall encode the KEA public key as described in Section 3.1.2 when issuing certificates containing public keys for the KEA algorithm. Conforming applications supporting the KEA algo- rithm shall decode the subjectPublicKey as described in section 3.1.2 when the algorithm identifier is the one presented in 3.1.1. 3.1.1. Algorithm Identifier and Parameters The Key Exchange Algorithm (KEA) is a classified algorithm for exchanging keys. A KEA "pairwise key" may be generated between two users if their KEA public keys were generated with the same KEA parameters. The KEA parameters are not included in a certificate; instead a "domain identifier" is supplied in the parameters field. When the subjectPublicKeyInfo field contains a KEA key, the algorithm Housley & Polk [Page 5] INTERNET DRAFT August 5 1998 identifier and parameters shall be as defined in [sdn.701r]: id-keyExchangeAlgorithm OBJECT IDENTIFIER ::= { 2 16 840 1 101 2 1 1 22 } KEA-Parms-Id ::= OCTET STRING CAs shall populate the parameters field of the AlgorithmIdentifier within the subjectPublicKeyInfo field of each certificate containing a KEA public key with an 80-bit parameter identifier (OCTET STRING), also known as the domain identifier. The domain identifier will be computed in three steps: (1) the KEA parameters are DER encoded using the Dss-Parms structure; (2) a 160-bit SHA-1 hash is generated from the parameters; and (3) the 160-bit hash is reduced to 80-bits by performing an "exclusive or" of the 80 high order bits with the 80 low order bits. The resulting value is encoded such that the most significant byte of the 80-bit value is the first octet in the octet string. The Dss-Parms is provided in [RFC xxx] and reproduced below for com- pleteness. Dss-Parms ::= SEQUENCE { p INTEGER, q INTEGER, g INTEGER } 3.1.2. Encoding of KEA Public Keys A KEA public key, y, is conveyed in the subjectPublicKey BIT STRING such that the most significant bit (MSB) of y becomes the MSB of the BIT STRING value field and the least significant bit (LSB) of y becomes the LSB of the BIT STRING value field. This results in the following encoding: BIT STRING tag, BIT STRING length, 0 (indicating that there are zero unused bits in the final octet of y), BIT STRING value field including y. 3.2. Key Usage Extension in KEA certificates The key usage extension may optionally appear in a KEA certificate. If a KEA certificate includes the keyUsage extension, only the following values may be asserted: Housley & Polk [Page 6] INTERNET DRAFT August 5 1998 keyAgreement; encipherOnly; and decipherOnly. The encipherOnly and decipherOnly values may only be asserted if the keyAgreement value is also asserted. At most one of encipherOnly and decipherOnly shall be asserted in keyUsage extension. 4. ASN.1 Modules 4.1 1988 Syntax PKIXkea88 {iso(1) identified-organization(3) dod(6) inter- net(1) security(5) mechanisms(5) pkix(7) id-mod(0) to be assigned(?) } BEGIN ::= -- EXPORTS ALL -- -- IMPORTS NONE -- id-keyExchangeAlgorithm OBJECT IDENTIFIER ::= { 2 16 840 1 101 2 1 1 22 } KEA-Parms-Id ::= OCTET STRING END 4.2 1993 Syntax PKIXkea93 {iso(1) identified-organization(3) dod(6) inter- net(1) security(5) mechanisms(5) pkix(7) id-mod(0) to be assigned(?) } BEGIN ::= -- EXPORTS ALL -- IMPORTS ALGORITHM-ID FROM PKIX1Explicit93 {iso(1) identified-organization(3) dod(6) internet(1) security(5) mechanisms(5) pkix(7) id-mod(0) id-pkix1-explicit-93(3) } KeaPublicKey ALGORTHM-ID ::= { OID id-keyExchangeAlgorithm PARMS KEA-Parms-Id } id-keyExchangeAlgorithm OBJECT IDENTIFIER ::= { 2 16 840 1 101 2 1 1 22 } Housley & Polk [Page 7] INTERNET DRAFT August 5 1998 KEA-Parms-Id ::= OCTET STRING END 5. References [KEA] "Skipjack and KEA Algorithm Specification", Version 2.0, 29 May 1998. available from http://csrc.nist.gov/encryption/skipjack-kea.htm [SDN.701R] SDN.701, "Message Security Protocol", Revision 4.0 1996-06-07 with "Corrections to Message Security Protocol, SDN.701, Rev 4.0, 96-06-07." August 30, 1996. [RFC xxxx] R. Housley, W. Ford, W. Polk and D. Solo "Internet X.509 Public Key Infrastructure: X.509 Certificate and CRL Profile", July 28, 1998. 6. Patent Statements To be added. 7. Security Considerations This specification is devoted to the format and encoding of KEA keys in X.509 certificates. Since certificates are digitally signed, no additional integrity service is necessary. Certificates need not be kept secret, and unrestricted and anonymous access to certificates and CRLs has no security implications. However, security factors outside the scope of this specification will affect the assurance provided to certificate users. This sec- tion highlights critical issues that should be considered by imple- mentors, administrators, and users. The procedures performed by CAs and RAs to validate the binding of the subject's identity of their public key greatly affect the assurance that should be placed in the certificate. Relying parties may wish to review the CA's certificate practice statement. The protection afforded private keys is a critical factor in main- taining security. Failure of users to protect their KEA private keys will permit an attacker to masquerade as them, or decrypt their per- sonal information. The availability and freshness of revocation information will affect the degree of assurance that should be placed in a certificate. Housley & Polk [Page 8] INTERNET DRAFT August 5 1998 While certificates expire naturally, events may occur during its natural lifetime which negate the binding between the subject and public key. If revocation information is untimely or unavailable, the assurance associated with the binding is clearly reduced. Simi- larly, implementations of the Path Validation mechanism described in section 6 that omit revocation checking provide less assurance than those that support it. The path validation algorithm specified in [RFC xxxx] depends on the certain knowledge of the public keys (and other information) about one or more trusted CAs. The decision to trust a CA is an important decision as it ultimately determines the trust afforded a certifi- cate. The authenticated distribution of trusted CA public keys (usu- ally in the form of a "self-signed" certificate) is a security criti- cal out of band process that is beyond the scope of this specifica- tion. In addition, where a key compromise or CA failure occurs for a trusted CA, the user will need to modify the information provided to the path validation routine. Selection of too many trusted CAs will make the trusted CA information difficult to maintain. On the other hand, selection of only one trusted CA may limit users to a closed community of users until a global PKI emerges. The quality of implementations that process certificates may also affect the degree of assurance provided. The path validation algo- rithm described in section 6 relies upon the integrity of the trusted CA information, and especially the integrity of the public keys asso- ciated with the trusted CAs. By substituting public keys for which an attacker has the private key, an attacker could trick the user into accepting false certificates. The binding between a key and certificate subject cannot be stronger than the cryptographic module implementation and algorithms used to generate the signature. 8. Author Addresses: Russell Housley SPYRUS PO Box 1198 Herndon, VA 20172 USA housley@spyrus.com Tim Polk NIST Building 820, Room 426 Housley & Polk [Page 9] INTERNET DRAFT August 5 1998 Gaithersburg, MD 20899 USA wpolk@nist.gov Housley & Polk [Page 10]