NETWORK WORKING GROUP B. Tung Internet-Draft USC Information Sciences Institute Expires: March 16, 2006 L. Zhu Microsoft Corporation September 12, 2005 Public Key Cryptography for Initial Authentication in Kerberos draft-ietf-cat-kerberos-pk-init-28 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on March 16, 2006. Copyright Notice Copyright (C) The Internet Society (2005). Abstract This document describes protocol extensions (hereafter called PKINIT) to the Kerberos protocol specification. These extensions provide a method for integrating public key cryptography into the initial authentication exchange, by using asymmetric-key signature and/or encryption algorithms in pre-authentication data fields. Tung & Zhu Expires March 16, 2006 [Page 1] Internet-Draft PKINIT September 2005 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions Used in This Document . . . . . . . . . . . . . . 3 3. Extensions . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.1. Definitions, Requirements, and Constants . . . . . . . . . 4 3.1.1. Required Algorithms . . . . . . . . . . . . . . . . . 4 3.1.2. Defined Message and Encryption Types . . . . . . . . . 5 3.1.3. Algorithm Identifiers . . . . . . . . . . . . . . . . 6 3.2. PKINIT Pre-authentication Syntax and Use . . . . . . . . . 7 3.2.1. Generation of Client Request . . . . . . . . . . . . . 7 3.2.2. Receipt of Client Request . . . . . . . . . . . . . . 10 3.2.3. Generation of KDC Reply . . . . . . . . . . . . . . . 14 3.2.4. Receipt of KDC Reply . . . . . . . . . . . . . . . . . 19 3.3. Interoperability Requirements . . . . . . . . . . . . . . 20 3.4. KDC Indication of PKINIT Support . . . . . . . . . . . . . 21 4. Security Considerations . . . . . . . . . . . . . . . . . . . 21 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24 7.1. Normative References . . . . . . . . . . . . . . . . . . . 24 7.2. Informative References . . . . . . . . . . . . . . . . . . 25 Appendix A. PKINIT ASN.1 Module . . . . . . . . . . . . . . . . . 25 Appendix B. Test Vectors . . . . . . . . . . . . . . . . . . . . 31 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 33 Intellectual Property and Copyright Statements . . . . . . . . . . 34 Tung & Zhu Expires March 16, 2006 [Page 2] Internet-Draft PKINIT September 2005 1. Introduction A client typically authenticates itself to a service in Kerberos using three distinct though related exchanges. First, the client requests a ticket-granting ticket (TGT) from the Kerberos authentication server (AS). Then, it uses the TGT to request a service ticket from the Kerberos ticket-granting server (TGS). Usually, the AS and TGS are integrated in a single device known as a Kerberos Key Distribution Center, or KDC. Finally, the client uses the service ticket to authenticate itself to the service. The advantage afforded by the TGT is that the client exposes his long-term secrets only once. The TGT and its associated session key can then be used for any subsequent service ticket requests. One result of this is that all further authentication is independent of the method by which the initial authentication was performed. Consequently, initial authentication provides a convenient place to integrate public-key cryptography into Kerberos authentication. As defined in [RFC4120], Kerberos authentication exchanges use symmetric-key cryptography, in part for performance. One disadvantage of using symmetric-key cryptography is that the keys must be shared, so that before a client can authenticate itself, he must already be registered with the KDC. Conversely, public-key cryptography (in conjunction with an established Public Key Infrastructure) permits authentication without prior registration with a KDC. Adding it to Kerberos allows the widespread use of Kerberized applications by clients without requiring them to register first with a KDC--a requirement that has no inherent security benefit. As noted above, a convenient and efficient place to introduce public- key cryptography into Kerberos is in the initial authentication exchange. This document describes the methods and data formats for integrating public-key cryptography into Kerberos initial authentication. 2. Conventions Used in This Document 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]. Both the AS and the TGS are referred to as the KDC. In this document, the encryption key used to encrypt the enc-part Tung & Zhu Expires March 16, 2006 [Page 3] Internet-Draft PKINIT September 2005 field of the KDC-REP in the AS-REP [RFC4120] is referred to as the AS reply key. 3. Extensions This section describes extensions to [RFC4120] for supporting the use of public-key cryptography in the initial request for a ticket. Briefly, this document defines the following extensions to [RFC4120]: 1. The client indicates the use of public-key authentication by including a special preauthenticator in the initial request. This preauthenticator contains the client's public-key data and a signature. 2. The KDC tests the client's request against its authentication policy and trusted Certification Authorities (CAs). 3. If the request passes the verification tests, the KDC replies as usual, but the reply is encrypted using either: a. a key generated through a Diffie-Hellman (DH) key exchange [RFC2631] [IEEE1363] with the client, signed using the KDC's signature key; or b. a symmetric encryption key, signed using the KDC's signature key and encrypted using the client's public key. Any keying material required by the client to obtain the encryption key for decrypting the KDC reply is returned in a pre- authentication field accompanying the usual reply. 4. The client validates the KDC's signature, obtains the encryption key, decrypts the reply, and then proceeds as usual. Section 3.1 of this document enumerates the required algorithms and necessary extension message types. Section 3.2 describes the extension messages in greater detail. 3.1. Definitions, Requirements, and Constants 3.1.1. Required Algorithms All PKINIT implementations MUST support the following algorithms: Tung & Zhu Expires March 16, 2006 [Page 4] Internet-Draft PKINIT September 2005 o AS reply key enctype: aes128-cts-hmac-sha1-96 and aes256-cts-hmac- sha1-96 [RFC3962]. o Signature algorithm: sha-1WithRSAEncryption [RFC3279]. o AS reply key delivery method: Diffie-Hellman key exchange [RFC2631]. In addition, implementations of this specification MUST be capable of processing the Extended Key Usage (EKU) extension and the id-pksan (as defined in Section 3.2.2) otherName of the Subject Alternative Name (SAN) extension in X.509 certificates [RFC3280], if present. 3.1.2. Defined Message and Encryption Types PKINIT makes use of the following new pre-authentication types: PA_PK_AS_REQ 16 PA_PK_AS_REP 17 PKINIT also makes use of the following new authorization data type: AD_INITIAL_VERIFIED_CAS 9 PKINIT introduces the following new error codes: KDC_ERR_CLIENT_NOT_TRUSTED 62 KDC_ERR_INVALID_SIG 64 KDC_ERR_DH_KEY_PARAMETERS_NOT_ACCEPTED 65 KDC_ERR_CANT_VERIFY_CERTIFICATE 70 KDC_ERR_INVALID_CERTIFICATE 71 KDC_ERR_REVOKED_CERTIFICATE 72 KDC_ERR_REVOCATION_STATUS_UNKNOWN 73 KDC_ERR_CLIENT_NAME_MISMATCH 75 KDC_ERR_INCONSISTENT_KEY_PURPOSE 76 PKINIT uses the following typed data types for errors: TD_TRUSTED_CERTIFIERS 104 TD_INVALID_CERTIFICATES 105 TD_DH_PARAMETERS 109 PKINIT defines the following encryption types, for use in the AS-REQ message to indicate acceptance of the corresponding algorithms that can used by Cryptographic Message Syntax (CMS) [RFC3852] messages in the reply: Tung & Zhu Expires March 16, 2006 [Page 5] Internet-Draft PKINIT September 2005 dsaWithSHA1-CmsOID 9 md5WithRSAEncryption-CmsOID 10 sha1WithRSAEncryption-CmsOID 11 rc2CBC-EnvOID 12 rsaEncryption-EnvOID (PKCS1 v1.5) 13 rsaES-OAEP-EnvOID (PKCS1 v2.0) 14 des-ede3-cbc-EnvOID 15 The ASN.1 module for all structures defined in this document (plus IMPORT statements for all imported structures) is given in Appendix A. All structures defined in or imported into this document MUST be encoded using Distinguished Encoding Rules (DER) [X690] (unless otherwise noted). All data structures carried in OCTET STRINGs must be encoded according to the rules specified in corresponding specifications. Interoperability note: Some implementations may not be able to decode wrapped CMS objects encoded with BER but not DER; specifically, they may not be able to decode infinite length encodings. To maximize interoperability, implementers SHOULD encode CMS objects used in PKINIT with DER. 3.1.3. Algorithm Identifiers PKINIT does not define, but does make use of, the following algorithm identifiers. PKINIT uses the following algorithm identifier(s) for Diffie-Hellman key agreement [RFC3279]: dhpublicnumber (Modular Exponential Diffie-Hellman [RFC2631]) PKINIT uses the following signature algorithm identifiers [RFC3279]: sha-1WithRSAEncryption (RSA with SHA1) md5WithRSAEncryption (RSA with MD5) id-dsa-with-sha1 (DSA with SHA1) PKINIT uses the following encryption algorithm identifiers [RFC3447] for encrypting the temporary key with a public key: rsaEncryption (PKCS1 v1.5) id-RSAES-OAEP (PKCS1 v2.0) PKINIT uses the following algorithm identifiers [RFC3370] [RFC3565] for encrypting the AS reply key with the temporary key: Tung & Zhu Expires March 16, 2006 [Page 6] Internet-Draft PKINIT September 2005 des-ede3-cbc (three-key 3DES, CBC mode) rc2-cbc (RC2, CBC mode) id-aes256-CBC (AES-256, CBC mode) 3.2. PKINIT Pre-authentication Syntax and Use This section defines the syntax and use of the various pre- authentication fields employed by PKINIT. 3.2.1. Generation of Client Request The initial authentication request (AS-REQ) is sent as per [RFC4120]; in addition, a pre-authentication data element, whose padata-type is PA_PK_AS_REQ and whose padata-value contains the DER encoding of the type PA-PK-AS-REQ, is included. PA-PK-AS-REQ ::= SEQUENCE { signedAuthPack [0] IMPLICIT OCTET STRING, -- Contains a CMS type ContentInfo encoded -- according to [RFC3852]. -- The contentType field of the type ContentInfo -- is id-signedData (1.2.840.113549.1.7.2), -- and the content field is a SignedData. -- The eContentType field for the type SignedData is -- id-pkauthdata (1.3.6.1.5.2.3.1), and the -- eContent field contains the DER encoding of the -- type AuthPack. -- AuthPack is defined below. trustedCertifiers [1] SEQUENCE OF ExternalPrincipalIdentifier OPTIONAL, -- A list of CAs, trusted by the client, that can -- be used to certify the KDC. -- Each ExternalPrincipalIdentifier identifies a CA -- or a CA certificate (thereby its public key). -- The information contained in the -- trustedCertifiers SHOULD be used by the KDC as -- hints to guide its selection of an appropriate -- certificate chain to return to the client. kdcPkId [2] IMPLICIT OCTET STRING OPTIONAL, -- Contains a CMS type SignerIdentifier encoded -- according to [RFC3852]. -- Identifies, if present, a particular KDC -- public key that the client already has. ... } DHNonce ::= OCTET STRING Tung & Zhu Expires March 16, 2006 [Page 7] Internet-Draft PKINIT September 2005 ExternalPrincipalIdentifier ::= SEQUENCE { subjectName [0] IMPLICIT OCTET STRING OPTIONAL, -- Contains a PKIX type Name encoded according to -- [RFC3280]. -- Identifies the certificate subject by the -- distinguished subject name. -- REQUIRED when there is a distinguished subject -- name present in the certificate. issuerAndSerialNumber [1] IMPLICIT OCTET STRING OPTIONAL, -- Contains a CMS type IssuerAndSerialNumber encoded -- according to [RFC3852]. -- Identifies a certificate of the subject. -- REQUIRED for TD-INVALID-CERTIFICATES and -- TD-TRUSTED-CERTIFIERS. subjectKeyIdentifier [2] IMPLICIT OCTET STRING OPTIONAL, -- Identifies the subject's public key by a key -- identifier. When an X.509 certificate is -- referenced, this key identifier matches the X.509 -- subjectKeyIdentifier extension value. When other -- certificate formats are referenced, the documents -- that specify the certificate format and their use -- with the CMS must include details on matching the -- key identifier to the appropriate certificate -- field. -- RECOMMENDED for TD-TRUSTED-CERTIFIERS. ... } AuthPack ::= SEQUENCE { pkAuthenticator [0] PKAuthenticator, clientPublicValue [1] SubjectPublicKeyInfo OPTIONAL, -- Type SubjectPublicKeyInfo is defined in -- [RFC3280]. -- Specifies Diffie-Hellman domain parameters -- and the client's public key value [IEEE1363]. -- The DH public key value is encoded as a BIT -- STRING according to [RFC3279]. -- This field is present only if the client wishes -- to use the Diffie-Hellman key agreement method. supportedCMSTypes [2] SEQUENCE OF AlgorithmIdentifier OPTIONAL, -- Type AlgorithmIdentifier is defined in -- [RFC3280]. -- List of CMS encryption types supported by the -- client in order of (decreasing) preference. clientDHNonce [3] DHNonce OPTIONAL, -- Present only if the client indicates that it -- wishes to reuse DH keys or to allow the KDC to Tung & Zhu Expires March 16, 2006 [Page 8] Internet-Draft PKINIT September 2005 -- do so (see Section 3.2.3.1). ... } PKAuthenticator ::= SEQUENCE { cusec [0] INTEGER (0..999999), ctime [1] KerberosTime, -- cusec and ctime are used as in [RFC4120], for -- replay prevention. nonce [2] INTEGER (0..4294967295), -- Chosen randomly; This nonce does not need to -- match with the nonce in the KDC-REQ-BODY. paChecksum [3] OCTET STRING, -- Contains the SHA1 checksum, performed over -- KDC-REQ-BODY. ... } The ContentInfo [RFC3852] structure for the signedAuthPack field is filled out as follows: 1. The contentType field of the type ContentInfo is id-signedData (as defined in [RFC3852]), and the content field is a SignedData (as defined in [RFC3852]). 2. The eContentType field for the type SignedData is id-pkauthdata: { iso(1) org(3) dod(6) internet(1) security(5) kerberosv5(2) pkinit(3) pkauthdata(1) }. 3. The eContent field for the type SignedData contains the DER encoding of the type AuthPack. 4. The signerInfos field of the type SignedData contains a single signerInfo, which contains the signature over the type AuthPack. 5. The certificates field of the type SignedData contains certificates intended to facilitate certification path construction, so that the KDC can verify the signature over the type AuthPack. For path validation, these certificates SHOULD be sufficient to construct at least one certification path from the client certificate to one trust anchor acceptable by the KDC [CAPATH]. The client MUST be capable of including such a set of certificates if configured to do so. The certificates field MUST NOT contain "root" CA certificates. 6. The client's Diffie-Hellman public value (clientPublicValue) is included if and only if the client wishes to use the Diffie- Hellman key agreement method. The Diffie-Hellman domain Tung & Zhu Expires March 16, 2006 [Page 9] Internet-Draft PKINIT September 2005 parameters [IEEE1363] for the client's public key are specified in the algorithm field of the type SubjectPublicKeyInfo [RFC3279] and the client's Diffie-Hellman public key value is mapped to a subjectPublicKey (a BIT STRING) according to [RFC3279]. When using the Diffie-Hellman key agreement method, implementations MUST support Oakley 1024-bit Modular Exponential (MODP) well- known group 2 [RFC2412] and Oakley 2048-bit MODP well-known group 14 [RFC3526], and SHOULD support Oakley 4096-bit MODP well-known group 16 [RFC3526]. The Diffie-Hellman field size should be chosen so as to provide sufficient cryptographic security [RFC3766]. When MODP Diffie-Hellman is used, the exponents should have at least twice as many bits as the symmetric keys that will be derived from them [ODL99]. 7. The client may wish to reuse DH keys or to allow the KDC to do so (see Section 3.2.3.1). If so, then the client includes the clientDHNonce field. This nonce string needs to be as long as the longest key length of the symmetric key types that the client supports. This nonce MUST be chosen randomly. 3.2.2. Receipt of Client Request Upon receiving the client's request, the KDC validates it. This section describes the steps that the KDC MUST (unless otherwise noted) take in validating the request. The KDC verifies the client's signature in the signedAuthPack field according to [RFC3852]. If, while validating the client's X.509 certificate [RFC3280], the KDC cannot build a certification path to validate the client's certificate, it sends back a KRB-ERROR [RFC4120] message with the code KDC_ERR_CANT_VERIFY_CERTIFICATE. The accompanying e-data for this error message is a TYPED-DATA (as defined in [RFC4120]) that contains an element whose data-type is TD_TRUSTED_CERTIFIERS, and whose data-value contains the DER encoding of the type TD-TRUSTED- CERTIFIERS: TD-TRUSTED-CERTIFIERS ::= SEQUENCE OF ExternalPrincipalIdentifier -- Identifies a list of CAs trusted by the KDC. -- Each ExternalPrincipalIdentifier identifies a CA -- or a CA certificate (thereby its public key). Tung & Zhu Expires March 16, 2006 [Page 10] Internet-Draft PKINIT September 2005 Upon receiving this error message, the client SHOULD retry only if it has a different set of certificates (from those of the previous requests) that form a certification path (or a partial path) from one of the trust anchors acceptable by the KDC to its own certificate. If, while processing the certification path, the KDC determines that the signature on one of the certificates in the signedAuthPack field is invalid, it returns a KRB-ERROR [RFC4120] message with the code KDC_ERR_INVALID_CERTIFICATE. The accompanying e-data for this error message is a TYPED-DATA that contains an element whose data-type is TD_INVALID_CERTIFICATES, and whose data-value contains the DER encoding of the type TD-INVALID-CERTIFICATES: TD-INVALID-CERTIFICATES ::= SEQUENCE OF ExternalPrincipalIdentifier -- Each ExternalPrincipalIdentifier identifies a -- certificate (sent by the client) with an invalid -- signature. If more than one X.509 certificate signature is invalid, the KDC MAY include one IssuerAndSerialNumber per invalid signature within the TD-INVALID-CERTIFICATES. The client's X.509 certificate is validated according to [RFC3280]. Based on local policy, the KDC may also check whether any X.509 certificates in the certification path validating the client's certificate have been revoked. If any of them have been revoked, the KDC MUST return an error message with the code KDC_ERR_REVOKED_CERTIFICATE; if the KDC attempts to determine the revocation status but is unable to do so, it SHOULD return an error message with the code KDC_ERR_REVOCATION_STATUS_UNKNOWN. The certificate or certificates affected are identified exactly as for the error code KDC_ERR_INVALID_CERTIFICATE (see above). Note that the TD_INVALID_CERTIFICATES error data is only used to identify invalid certificates sent by the client in the request. The client's public key is then used to verify the signature. If the signature fails to verify, the KDC MUST return an error message with the code KDC_ERR_INVALID_SIG. There is no accompanying e-data for this error message. In addition to validating the client's signature, the KDC MUST also check that the client's public key used to verify the client's signature is bound to the client's principal name as specified in the AS-REQ as follows: Tung & Zhu Expires March 16, 2006 [Page 11] Internet-Draft PKINIT September 2005 1. If the KDC has its own binding between either the client's signature-verification public key or the client's certificate and the client's Kerberos principal name, it uses that binding. 2. Otherwise, if the client's X.509 certificate contains a Subject Alternative Name (SAN) extension carrying a KRB5PrincipalName (defined below) in the otherName field of the type GeneralName [RFC3280], it binds the client's X.509 certificate to that name. The type of the otherName field is AnotherName. The type-id field of the type AnotherName is id-pksan: id-pksan OBJECT IDENTIFIER ::= { iso(1) org(3) dod(6) internet(1) security(5) kerberosv5(2) x509-sanan (2) } And the value field of the type AnotherName is a KRB5PrincipalName. KRB5PrincipalName ::= SEQUENCE { realm [0] Realm, principalName [1] PrincipalName } If the KDC does not have its own binding and there is no KRB5PrincipalName name present in the client's X.509 certificate, or if the Kerberos name in the request does not match the KRB5PrincipalName in the client's X.509 certificate (including the realm name), the KDC MUST return an error message with the code KDC_ERR_CLIENT_NAME_MISMATCH. There is no accompanying e-data for this error message. Even if the certification path is validated and the certificate is mapped to the client's principal name, the KDC may decide not to accept the client's certificate, depending on local policy. The KDC MAY require the presence of an Extended Key Usage (EKU) KeyPurposeId [RFC3280] id-pkekuoid in the extensions field of the client's X.509 certificate: id-pkekuoid OBJECT IDENTIFIER ::= { iso(1) org(3) dod(6) internet(1) security(5) kerberosv5(2) pkinit(3) pkekuoid(4) } -- PKINIT client authentication. -- Key usage bits that MUST be consistent: -- digitalSignature. If this EKU KeyPurposeId is required but it is not present or if the Tung & Zhu Expires March 16, 2006 [Page 12] Internet-Draft PKINIT September 2005 client certificate is restricted not to be used for PKINIT client authentication per Section 4.2.1.13 of [RFC3280], the KDC MUST return an error message of the code KDC_ERR_INCONSISTENT_KEY_PURPOSE. There is no accompanying e-data for this error message. KDCs implementing this requirement SHOULD also accept the EKU KeyPurposeId id-ms-sc- logon (1.3.6.1.4.1.311.20.2.2) as meeting the requirement, as there are a large number of X.509 client certificates deployed for use with PKINIT which have this EKU. As a matter of local policy, the KDC MAY decide to reject requests on the basis of the absence or presence of other specific EKU OID's. If the client's public key is not accepted, the KDC returns an error message with the code KDC_ERR_CLIENT_NOT_TRUSTED. The KDC MUST check the timestamp to ensure that the request is not a replay, and that the time skew falls within acceptable limits. The recommendations for clock skew times in [RFC4120] apply here. If the check fails, the KDC MUST return error code KRB_AP_ERR_REPEAT or KRB_AP_ERR_SKEW, respectively. If the clientPublicValue is filled in, indicating that the client wishes to use the Diffie-Hellman key agreement method, the KDC SHOULD check to see if the key parameters satisfy its policy. If they do not, it MUST return an error message with the code KDC_ERR_DH_KEY_PARAMETERS_NOT_ACCEPTED. The accompanying e-data is a TYPED-DATA that contains an element whose data-type is TD_DH_PARAMETERS, and whose data-value contains the DER encoding of the type TD-DH-PARAMETERS: TD-DH-PARAMETERS ::= SEQUENCE OF AlgorithmIdentifier -- Each AlgorithmIdentifier specifies a set of -- Diffie-Hellman domain parameters [IEEE1363]. -- This list is in decreasing preference order. TD-DH-PARAMETERS contains a list of Diffie-Hellman domain parameters that the KDC supports in decreasing preference order, from which the client SHOULD pick one to retry the request. If the client included a kdcPkId field in the PA-PK-AS-REQ and the KDC does not possess the corresponding key, the KDC MUST ignore the kdcPkId field as if the client did not include one. If there is a supportedCMSTypes field in the AuthPack, the KDC must check to see if it supports any of the listed types. If it supports more than one of the types, the KDC SHOULD use the one listed first. If it does not support any of them, it MUST return an error message with the code KDC_ERR_ETYPE_NOSUPP [RFC4120]. Tung & Zhu Expires March 16, 2006 [Page 13] Internet-Draft PKINIT September 2005 3.2.3. Generation of KDC Reply Assuming that the client's request has been properly validated, the KDC proceeds as per [RFC4120], except as follows. The KDC MUST set the initial flag and include an authorization data element of ad-type [RFC4120] AD_INITIAL_VERIFIED_CAS in the issued ticket. The ad-data [RFC4120] field contains the DER encoding of the type AD-INITIAL-VERIFIED-CAS: AD-INITIAL-VERIFIED-CAS ::= SEQUENCE OF ExternalPrincipalIdentifier -- Identifies the certification path based on which -- the client certificate was validated. -- Each ExternalPrincipalIdentifier identifies a CA -- or a CA certificate (thereby its public key). The AS wraps any AD-INITIAL-VERIFIED-CAS data in AD-IF-RELEVANT containers if the list of CAs satisfies the AS' realm's local policy (this corresponds to the TRANSITED-POLICY-CHECKED ticket flag [RFC4120]). Furthermore, any TGS MUST copy such authorization data from tickets used within a PA-TGS-REQ of the TGS-REQ into the resulting ticket. If the list of CAs satisfies the local KDC's realm's policy, the TGS MAY wrap the data into the AD-IF-RELEVANT container, otherwise it MAY unwrap the authorization data out of the AD-IF-RELEVANT container. Application servers that understand this authorization data type SHOULD apply local policy to determine whether a given ticket bearing such a type *not* contained within an AD-IF-RELEVANT container is acceptable. (This corresponds to the AP server checking the transited field when the TRANSITED-POLICY-CHECKED flag has not been set [RFC4120].) If such a data type is contained within an AD-IF- RELEVANT container, AP servers MAY apply local policy to determine whether the authorization data is acceptable. The content of the AS-REP is otherwise unchanged from [RFC4120]. The KDC encrypts the reply as usual, but not with the client's long-term key. Instead, it encrypts it with either a shared key derived from a Diffie-Hellman exchange, or a generated encryption key. The contents of the PA-PK-AS-REP indicate which key delivery method is used: PA-PK-AS-REP ::= CHOICE { dhInfo [0] DHRepInfo, -- Selected when Diffie-Hellman key exchange is -- used. encKeyPack [1] IMPLICIT OCTET STRING, -- Selected when public key encryption is used. Tung & Zhu Expires March 16, 2006 [Page 14] Internet-Draft PKINIT September 2005 -- Contains a CMS type ContentInfo encoded -- according to [RFC3852]. -- The contentType field of the type ContentInfo is -- id-envelopedData (1.2.840.113549.1.7.3). -- The content field is an EnvelopedData. -- The contentType field for the type EnvelopedData -- is id-signedData (1.2.840.113549.1.7.2). -- The eContentType field for the inner type -- SignedData (when unencrypted) is id-pkrkeydata -- (1.2.840.113549.1.7.3) and the eContent field -- contains the DER encoding of the type -- ReplyKeyPack. -- ReplyKeyPack is defined in Section 3.2.3.2. ... } DHRepInfo ::= SEQUENCE { dhSignedData [0] IMPLICIT OCTET STRING, -- Contains a CMS type ContentInfo encoded according -- to [RFC3852]. -- The contentType field of the type ContentInfo is -- id-signedData (1.2.840.113549.1.7.2), and the -- content field is a SignedData. -- The eContentType field for the type SignedData is -- id-pkdhkeydata (1.3.6.1.5.2.3.2), and the -- eContent field contains the DER encoding of the -- type KDCDHKeyInfo. -- KDCDHKeyInfo is defined below. serverDHNonce [1] DHNonce OPTIONAL -- Present if and only if dhKeyExpiration is -- present in the KDCDHKeyInfo. } KDCDHKeyInfo ::= SEQUENCE { subjectPublicKey [0] BIT STRING, -- KDC's DH public key. -- The DH public key value is encoded as a BIT -- STRING according to [RFC3279]. nonce [1] INTEGER (0..4294967295), -- Contains the nonce in the PKAuthenticator of the -- request if DH keys are NOT reused, -- 0 otherwise. dhKeyExpiration [2] KerberosTime OPTIONAL, -- Expiration time for KDC's key pair, -- present if and only if DH keys are reused. If -- this field is omitted then the serverDHNonce -- field MUST also be omitted. See Section 3.2.3.1. ... Tung & Zhu Expires March 16, 2006 [Page 15] Internet-Draft PKINIT September 2005 } 3.2.3.1. Using Diffie-Hellman Key Exchange In this case, the PA-PK-AS-REP contains a DHRepInfo structure. The ContentInfo [RFC3852] structure for the dhSignedData field is filled in as follows: 1. The contentType field of the type ContentInfo is id-signedData (as defined in [RFC3852]), and the content field is a SignedData (as defined in [RFC3852]). 2. The eContentType field for the type SignedData is the OID value for id-pkdhkeydata: { iso(1) org(3) dod(6) internet(1) security(5) kerberosv5(2) pkinit(3) pkdhkeydata(2) }. 3. The eContent field for the type SignedData contains the DER encoding of the type KDCDHKeyInfo. 4. The signerInfos field of the type SignedData contains a single signerInfo, which contains the signature over the type KDCDHKeyInfo. 5. The certificates field of the type SignedData contains certificates intended to facilitate certification path construction, so that the client can verify the KDC's signature over the type KDCDHKeyInfo. The information contained in the trustedCertifiers in the request SHOULD be used by the KDC as hints to guide its selection of an appropriate certificate chain to return to the client. This field may only. be left empty if the KDC public key specified by the kdcPkId field in the PA-PK- AS-REQ was used for signing. Otherwise, for path validation, these certificates SHOULD be sufficient to construct at least one certification path from the KDC certificate to one trust anchor acceptable by the client [CAPATH]. The KDC MUST be capable of including such a set of certificates if configured to do so. The certificates field MUST NOT contain "root" CA certificates. 6. If the client included the clientDHNonce field, then the KDC may choose to reuse its DH keys (see Section 3.2.3.1). If the server reuses DH keys then it MUST include an expiration time in the dhKeyExpiration field. Past the point of the expiration time, the signature over the type DHRepInfo is considered expired/ invalid. When the server reuses DH keys then it MUST include a serverDHNonce at least as long as the length of keys for the symmetric encryption system used to encrypt the AS reply. Note that including the serverDHNonce changes how the client and Tung & Zhu Expires March 16, 2006 [Page 16] Internet-Draft PKINIT September 2005 server calculate the key to use to encrypt the reply; see below for details. The KDC SHOULD NOT reuse DH keys unless the clientDHNonce field is present in the request. The AS reply key is derived as follows: 1. Both the KDC and the client calculate the shared secret value as follows: a) When MODP Diffie-Hellman is used, let DHSharedSecret be the shared secret value. DHSharedSecret is the value ZZ as described in Section 2.1.1 of [RFC2631]. DHSharedSecret is first padded with leading zeros such that the size of DHSharedSecret in octets is the same as that of the modulus, then represented as a string of octets in big-endian order. Implementation note: Both the client and the KDC can cache the triple (ya, yb, DHSharedSecret), where ya is the client's public key and yb is the KDC's public key. If both ya and yb are the same in a later exchange, the cached DHSharedSecret can be used. 2. Let K be the key-generation seed length [RFC3961] of the AS reply key whose enctype is selected according to [RFC4120]. 3. Define the function octetstring2key() as follows: octetstring2key(x) == random-to-key(K-truncate( SHA1(0x00 | x) | SHA1(0x01 | x) | SHA1(0x02 | x) | ... )) where x is an octet string; | is the concatenation operator; 0x00, 0x01, 0x02, etc., are each represented as a single octet; random- to-key() is an operation that generates a protocol key from a bitstring of length K; and K-truncate truncates its input to the first K bits. Both K and random-to-key() are as defined in the kcrypto profile [RFC3961] for the enctype of the AS reply key. 4. When DH keys are reused, let n_c be the clientDHNonce, and n_k be the serverDHNonce; otherwise, let both n_c and n_k be empty octet strings. Tung & Zhu Expires March 16, 2006 [Page 17] Internet-Draft PKINIT September 2005 5. The AS reply key k is: k = octetstring2key(DHSharedSecret | n_c | n_k) 3.2.3.2. Using Public Key Encryption In this case, the PA-PK-AS-REP contains a ContentInfo structure wrapped in an OCTET STRING. The AS reply key is encrypted in the encKeyPack field, which contains data of type ReplyKeyPack: ReplyKeyPack ::= SEQUENCE { replyKey [0] EncryptionKey, -- Contains the session key used to encrypt the -- enc-part field in the AS-REP. asChecksum [1] Checksum, -- Contains the checksum of the AS-REQ -- corresponding to the containing AS-REP. -- The checksum is performed over the type AS-REQ. -- The protocol key [RFC3961] of the checksum is the -- replyKey and the key usage number is 6. -- If the replyKey's enctype is "newer" [RFC4120] -- [RFC4121], the checksum is the required -- checksum operation [RFC3961] for that enctype. -- The client MUST verify this checksum upon receipt -- of the AS-REP. ... } The ContentInfo [RFC3852] structure for the encKeyPack field is filled in as follows: 1. The contentType field of the type ContentInfo is id-envelopedData (as defined in [RFC3852]), and the content field is an EnvelopedData (as defined in [RFC3852]). 2. The contentType field for the type EnvelopedData is id- signedData: { iso (1) member-body (2) us (840) rsadsi (113549) pkcs (1) pkcs7 (7) signedData (2) }. 3. The eContentType field for the inner type SignedData (when decrypted from the encryptedContent field for the type EnvelopedData) is id-pkrkeydata: { iso(1) org(3) dod(6) internet(1) security(5) kerberosv5(2) pkinit(3) pkrkeydata(3) }. 4. The eContent field for the inner type SignedData contains the DER encoding of the type ReplyKeyPack. Tung & Zhu Expires March 16, 2006 [Page 18] Internet-Draft PKINIT September 2005 5. The signerInfos field of the inner type SignedData contains a single signerInfo, which contains the signature over the type ReplyKeyPack. 6. The certificates field of the inner type SignedData contains certificates intended to facilitate certification path construction, so that the client can verify the KDC's signature over the type ReplyKeyPack. The information contained in the trustedCertifiers in the request SHOULD be used by the KDC as hints to guide its selection of an appropriate certificate chain to return to the client. This field may only be left empty if the KDC public key specified by the kdcPkId field in the PA-PK- AS-REQ was used for signing. Otherwise, for path validation, these certificates SHOULD be sufficient to construct at least one certification path from the KDC certificate to one trust anchor acceptable by the client [CAPATH]. The KDC MUST be capable of including such a set of certificates if configured to do so. The certificates field MUST NOT contain "root" CA certificates. 7. The recipientInfos field of the type EnvelopedData is a SET which MUST contain exactly one member of type KeyTransRecipientInfo. The encryptedKey of this member contains the temporary key which is encrypted using the client's public key. 8. The unprotectedAttrs or originatorInfo fields of the type EnvelopedData MAY be present. Implementations of this RSA encryption key delivery method are RECOMMENDED to support for RSA keys at least 2048 bits in size. 3.2.4. Receipt of KDC Reply Upon receipt of the KDC's reply, the client proceeds as follows. If the PA-PK-AS-REP contains the dhSignedData field, the client derives the AS reply key using the same procedure used by the KDC as defined in Section 3.2.3.1. Otherwise, the message contains the encKeyPack field, and the client decrypts and extracts the temporary key in the encryptedKey field of the member KeyTransRecipientInfo, and then uses that as the AS reply key. If the public key encrytion method is used, the client MUST verify the asChecksum contained in the ReplyKeyPack. In either case, the client MUST verify the signature in the SignedData according to [RFC3852]. The KDC's X.509 certificate MUST be validated according to [RFC3280]. In addition, unless the client can otherwise verify that the public key used to verify the KDC's signature is bound to the KDC of the target realm, the KDC's X.509 Tung & Zhu Expires March 16, 2006 [Page 19] Internet-Draft PKINIT September 2005 certificate MUST contain a Subject Alternative Name extension [RFC3280] carrying an AnotherName whose type-id is id-pksan (as defined in Section 3.2.2) and whose value is a KRB5PrincipalName that matches the name of the TGS of the target realm (as defined in Section 7.3 of [RFC4120]). Unless the client knows by some other means that the KDC certificate is intended for a Kerberos KDC, the client MUST require that the KDC certificate contains the EKU KeyPurposeId [RFC3280] id-pkkdcekuoid: id-pkkdcekuoid OBJECT IDENTIFIER ::= { iso(1) org(3) dod(6) internet(1) security(5) kerberosv5(2) pkinit(3) pkkdcekuoid(5) } -- Signing KDC responses. -- Key usage bits that MUST be consistent: -- digitalSignature. If the KDC certificate contains the Kerberos TGS name encoded as an id-pksan SAN, this certificate is certified by the issuing CA as a KDC certificate, therefore the id-pkkdcekuoid EKU is not required. If all applicable checks are satisfied, the client then decrypts the enc-part field of the KDC-REP in the AS-REP using the AS reply key, and then proceeds as described in [RFC4120]. Implementation note: CAs issuing KDC certificates SHOULD place all "short" and "fully-qualified" Kerberos realm names of the KDC (one per GeneralName [RFC3280]) into the KDC certificate to allow maximum flexibility. 3.3. Interoperability Requirements The client MUST be capable of sending a set of certificates sufficient to allow the KDC to construct a certification path for the client's certificate, if the correct set of certificates is provided through configuration or policy. If the client sends all the X.509 certificates on a certification path to a trust anchor acceptable by the KDC, and the KDC can not verify the client's public key otherwise, the KDC MUST be able to process path validation for the client's certificate based on the certificates in the request. The KDC MUST be capable of sending a set of certificates sufficient to allow the client to construct a certification path for the KDC's certificate, if the correct set of certificates is provided through configuration or policy. Tung & Zhu Expires March 16, 2006 [Page 20] Internet-Draft PKINIT September 2005 If the KDC sends all the X.509 certificates on a certification path to a trust anchor acceptable by the client, and the client can not verify the KDC's public key otherwise, the client MUST be able to process path validation for the KDC's certificate based on the certificates in the reply. 3.4. KDC Indication of PKINIT Support If pre-authentication is required, but was not present in the request, per [RFC4120] an error message with the code KDC_ERR_PREAUTH_FAILED is returned and a METHOD-DATA object will be stored in the e-data field of the KRB-ERROR message to specify which pre-authentication mechanisms are acceptable. The KDC can then indicate the support of PKINIT by including an empty element whose padata-type is PA_PK_AS_REQ in that METHOD-DATA object. Otherwise if it is required by the KDC's local policy that the client must be pre-authenticated using the pre-authentication mechanism specified in this document, but no PKINIT pre-authentication was present in the request, an error message with the code KDC_ERR_PREAUTH_FAILED SHOULD be returned. KDCs MUST leave the padata-value field of the PA_PK_AS_REQ element in the KRB-ERROR's METHOD-DATA empty (i.e., send a zero-length OCTET STRING), and clients MUST ignore this and any other value. Future extensions to this protocol may specify other data to send instead of an empty OCTET STRING. 4. Security Considerations The symmetric reply key size and Diffie-Hellman field size or RSA modulus size should be chosen so as to provide sufficient cryptographic security [RFC3766]. When MODP Diffie-Hellman is used, the exponents should have at least twice as many bits as the symmetric keys that will be derived from them [ODL99]. PKINIT raises certain security considerations beyond those that can be regulated strictly in protocol definitions. We will address them in this section. PKINIT extends the cross-realm model to the public-key infrastructure. Users of PKINIT must understand security policies and procedures appropriate to the use of Public Key Infrastructures [RFC3280]. Tung & Zhu Expires March 16, 2006 [Page 21] Internet-Draft PKINIT September 2005 In order to trust a KDC certificate that is certified by a CA as a KDC certificate for a target realm (for example, by asserting the TGS name of that Kerberos realm as an id-pksan SAN and/or restricting the certificate usage by using the id-pkkdcekuoid EKU, as described in Section 3.2.4), the client MUST verify that the KDC certificate's issuing CA is authorized to issue KDC certificates for that target realm. Otherwise, the binding between the KDC certificate and the KDC of the target realm is not established. How to validate this authorization is a matter of local policy. A way to achieve this is the configuration of specific sets of intermediary CAs and trust anchors, one of which must be on the KDC certificate's certification path [RFC3280]; and for each CA or trust anchor the realms for which it is allowed to issue certificates. In addition, if any CA is trusted to issue KDC certificates can also issue other kinds of certificates, then local policy must be able to distinguish between them: for example, it could require that KDC certificates contain the id-pkkdcekuoid EKU or that the realm be specified with the id-pksan SAN. It is the responsibility of the PKI administrators for an organization to ensure that KDC certificates are only issued to KDCs, and that clients can ascertain this using their local policy. Standard Kerberos allows the possibility of interactions between cryptosystems of varying strengths; this document adds interactions with public-key cryptosystems to Kerberos. Some administrative policies may allow the use of relatively weak public keys. Using such keys to wrap data encrypted under stronger conventional cryptosystems may be inappropriate. PKINIT requires keys for symmetric cryptosystems to be generated. Some such systems contain "weak" keys. For recommendations regarding these weak keys, see [RFC4120]. PKINIT allows the use of the same RSA key pair for encryption and signing when doing RSA encryption based key delivery. This is not recommended usage of RSA keys [RFC3447], by using DH based key delivery this is avoided. Care should be taken in how certificates are chosen for the purposes of authentication using PKINIT. Some local policies may require that key escrow be used for certain certificate types. Deployers of PKINIT should be aware of the implications of using certificates that have escrowed keys for the purposes of authentication. Because signing only certificates are normally not escrowed, by using DH based key delivery this is avoided. Tung & Zhu Expires March 16, 2006 [Page 22] Internet-Draft PKINIT September 2005 PKINIT does not provide for a "return routability" test to prevent attackers from mounting a denial-of-service attack on the KDC by causing it to perform unnecessary and expensive public-key operations. Strictly speaking, this is also true of standard Kerberos, although the potential cost is not as great, because standard Kerberos does not make use of public-key cryptography. By using DH based key delivery and reusing DH keys, the necessary crypto processing cost per request can be minimized. The syntax for the AD-INITIAL-VERIFIED-CAS authorization data does permit empty SEQUENCEs to be encoded. Such empty sequences may only be used if the KDC itself vouches for the user's certificate. 5. Acknowledgements The following people have made significant contributions to this draft: Paul Leach, Kristin Lauter, Sam Hartman, Love Hornquist Astrand, Ken Raeburn, Nicolas Williams, John Wray, Jonathan Trostle, Tom Yu, Jeffrey Hutzelman, David Cross, Dan Simon, Karthik Jaganathan, Chaskiel M Grundman, Stefan Santesson, Andre Scedrov and Aaron D. Jaggard. Special thanks to Clifford Neuman, Matthew Hur, Sasha Medvinsky and Jonathan Trostle who wrote earlier versions of this document. The authors are indebted to the Kerberos working group chair Jeffrey Hutzelman who kept track of various issues and was enormously helpful during the creation of this document. Some of the ideas on which this document is based arose during discussions over several years between members of the SAAG, the IETF CAT working group, and the PSRG, regarding integration of Kerberos and SPX. Some ideas have also been drawn from the DASS system. These changes are by no means endorsed by these groups. This is an attempt to revive some of the goals of those groups, and this document approaches those goals primarily from the Kerberos perspective. Lastly, comments from groups working on similar ideas in DCE have been invaluable. 6. IANA Considerations This document has no actions for IANA. Tung & Zhu Expires March 16, 2006 [Page 23] Internet-Draft PKINIT September 2005 7. References 7.1. Normative References [IEEE1363] IEEE, "Standard Specifications for Public Key Cryptography", IEEE 1363, 2000. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2412] Orman, H., "The OAKLEY Key Determination Protocol", RFC 2412, November 1998. [RFC2631] Rescorla, E., "Diffie-Hellman Key Agreement Method", RFC 2631, June 1999. [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. [RFC3280] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 3280, April 2002. [RFC3370] Housley, R., "Cryptographic Message Syntax (CMS) Algorithms", RFC 3370, August 2002. [RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1", RFC 3447, February 2003. [RFC3526] Kivinen, T. and M. Kojo, "More Modular Exponential (MODP) Diffie-Hellman groups for Internet Key Exchange (IKE)", RFC 3526, May 2003. [RFC3565] Schaad, J., "Use of the Advanced Encryption Standard (AES) Encryption Algorithm in Cryptographic Message Syntax (CMS)", RFC 3565, July 2003. [RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For Public Keys Used For Exchanging Symmetric Keys", BCP 86, RFC 3766, April 2004. [RFC3852] Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3852, July 2004. Tung & Zhu Expires March 16, 2006 [Page 24] Internet-Draft PKINIT September 2005 [RFC3961] Raeburn, K., "Encryption and Checksum Specifications for Kerberos 5", RFC 3961, February 2005. [RFC3962] Raeburn, K., "Advanced Encryption Standard (AES) Encryption for Kerberos 5", RFC 3962, February 2005. [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The Kerberos Network Authentication Service (V5)", RFC 4120, July 2005. [RFC4121] Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos Version 5 Generic Security Service Application Program Interface (GSS-API) Mechanism: Version 2", RFC 4121, July 2005. [X.509-97] ITU-T. Recommendation X.509: The Directory - Authentication Framework. 1997. [X690] ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER), ITU-T Recommendation X.690 (1997) | ISO/IEC International Standard 8825-1:1998. 7.2. Informative References [CAPATH] RFC-Editor: To be replaced by RFC number for draft-ietf- pkix-certpathbuild. Work in Progress. [LENSTRA] Lenstra, A. and E. Verheul, "Selecting Cryptographic Key Sizes", Journal of Cryptology 14 (2001) 255-293. [ODL99] Odlyzko, A., "Discrete logarithms: The past and the future, Designs, Codes, and Cryptography (1999)". Appendix A. PKINIT ASN.1 Module KerberosV5-PK-INIT-SPEC { iso(1) identified-organization(3) dod(6) internet(1) security(5) kerberosV5(2) modules(4) pkinit(5) } DEFINITIONS EXPLICIT TAGS ::= BEGIN IMPORTS Tung & Zhu Expires March 16, 2006 [Page 25] Internet-Draft PKINIT September 2005 SubjectPublicKeyInfo, AlgorithmIdentifier FROM PKIX1Explicit88 { iso (1) identified-organization (3) dod (6) internet (1) security (5) mechanisms (5) pkix (7) id-mod (0) id-pkix1-explicit (18) } -- As defined in RFC 3280. KerberosTime, PrincipalName, Realm, EncryptionKey FROM KerberosV5Spec2 { iso(1) identified-organization(3) dod(6) internet(1) security(5) kerberosV5(2) modules(4) krb5spec2(2) } ; id-pkinit OBJECT IDENTIFIER ::= { iso (1) org (3) dod (6) internet (1) security (5) kerberosv5 (2) pkinit (3) } id-pkauthdata OBJECT IDENTIFIER ::= { id-pkinit 1 } id-pkdhkeydata OBJECT IDENTIFIER ::= { id-pkinit 2 } id-pkrkeydata OBJECT IDENTIFIER ::= { id-pkinit 3 } id-pkekuoid OBJECT IDENTIFIER ::= { id-pkinit 4 } id-pkkdcekuoid OBJECT IDENTIFIER ::= { id-pkinit 5 } id-pksan OBJECT IDENTIFIER ::= { iso(1) org(3) dod(6) internet(1) security(5) kerberosv5(2) x509-sanan (2) } pa-pk-as-req INTEGER ::= 16 pa-pk-as-rep INTEGER ::= 17 ad-initial-verified-cas INTEGER ::= 9 td-trusted-certifiers INTEGER ::= 104 td-invalid-certificates INTEGER ::= 105 td-dh-parameters INTEGER ::= 109 PA-PK-AS-REQ ::= SEQUENCE { signedAuthPack [0] IMPLICIT OCTET STRING, -- Contains a CMS type ContentInfo encoded -- according to [RFC3852]. -- The contentType field of the type ContentInfo -- is id-signedData (1.2.840.113549.1.7.2), -- and the content field is a SignedData. -- The eContentType field for the type SignedData is -- id-pkauthdata (1.3.6.1.5.2.3.1), and the -- eContent field contains the DER encoding of the -- type AuthPack. -- AuthPack is defined below. trustedCertifiers [1] SEQUENCE OF Tung & Zhu Expires March 16, 2006 [Page 26] Internet-Draft PKINIT September 2005 ExternalPrincipalIdentifier OPTIONAL, -- A list of CAs, trusted by the client, that can -- be used to certify the KDC. -- Each ExternalPrincipalIdentifier identifies a CA -- or a CA certificate (thereby its public key). -- The information contained in the -- trustedCertifiers SHOULD be used by the KDC as -- hints to guide its selection of an appropriate -- certificate chain to return to the client. kdcPkId [2] IMPLICIT OCTET STRING OPTIONAL, -- Contains a CMS type SignerIdentifier encoded -- according to [RFC3852]. -- Identifies, if present, a particular KDC -- public key that the client already has. ... } DHNonce ::= OCTET STRING ExternalPrincipalIdentifier ::= SEQUENCE { subjectName [0] IMPLICIT OCTET STRING OPTIONAL, -- Contains a PKIX type Name encoded according to -- [RFC3280]. -- Identifies the certificate subject by the -- distinguished subject name. -- REQUIRED when there is a distinguished subject -- name present in the certificate. issuerAndSerialNumber [1] IMPLICIT OCTET STRING OPTIONAL, -- Contains a CMS type IssuerAndSerialNumber encoded -- according to [RFC3852]. -- Identifies a certificate of the subject. -- REQUIRED for TD-INVALID-CERTIFICATES and -- TD-TRUSTED-CERTIFIERS. subjectKeyIdentifier [2] IMPLICIT OCTET STRING OPTIONAL, -- Identifies the subject's public key by a key -- identifier. When an X.509 certificate is -- referenced, this key identifier matches the X.509 -- subjectKeyIdentifier extension value. When other -- certificate formats are referenced, the documents -- that specify the certificate format and their use -- with the CMS must include details on matching the -- key identifier to the appropriate certificate -- field. -- RECOMMENDED for TD-TRUSTED-CERTIFIERS. ... } Tung & Zhu Expires March 16, 2006 [Page 27] Internet-Draft PKINIT September 2005 AuthPack ::= SEQUENCE { pkAuthenticator [0] PKAuthenticator, clientPublicValue [1] SubjectPublicKeyInfo OPTIONAL, -- Type SubjectPublicKeyInfo is defined in -- [RFC3280]. -- Specifies Diffie-Hellman domain parameters -- and the client's public key value [IEEE1363]. -- The DH public key value is encoded as a BIT -- STRING according to [RFC3279]. -- This field is present only if the client wishes -- to use the Diffie-Hellman key agreement method. supportedCMSTypes [2] SEQUENCE OF AlgorithmIdentifier OPTIONAL, -- Type AlgorithmIdentifier is defined in -- [RFC3280]. -- List of CMS encryption types supported by the -- client in order of (decreasing) preference. clientDHNonce [3] DHNonce OPTIONAL, -- Present only if the client indicates that it -- wishes to reuse DH keys or to allow the KDC to -- do so. ... } PKAuthenticator ::= SEQUENCE { cusec [0] INTEGER (0..999999), ctime [1] KerberosTime, -- cusec and ctime are used as in [RFC4120], for -- replay prevention. nonce [2] INTEGER (0..4294967295), -- Chosen randomly; This nonce does not need to -- match with the nonce in the KDC-REQ-BODY. paChecksum [3] OCTET STRING, -- Contains the SHA1 checksum, performed over -- KDC-REQ-BODY. ... } TD-TRUSTED-CERTIFIERS ::= SEQUENCE OF ExternalPrincipalIdentifier -- Identifies a list of CAs trusted by the KDC. -- Each ExternalPrincipalIdentifier identifies a CA -- or a CA certificate (thereby its public key). TD-INVALID-CERTIFICATES ::= SEQUENCE OF ExternalPrincipalIdentifier -- Each ExternalPrincipalIdentifier identifies a -- certificate (sent by the client) with an invalid Tung & Zhu Expires March 16, 2006 [Page 28] Internet-Draft PKINIT September 2005 -- signature. KRB5PrincipalName ::= SEQUENCE { realm [0] Realm, principalName [1] PrincipalName } AD-INITIAL-VERIFIED-CAS ::= SEQUENCE OF ExternalPrincipalIdentifier -- Identifies the certification path based on which -- the client certificate was validated. -- Each ExternalPrincipalIdentifier identifies a CA -- or a CA certificate (thereby its public key). PA-PK-AS-REP ::= CHOICE { dhInfo [0] DHRepInfo, -- Selected when Diffie-Hellman key exchange is -- used. encKeyPack [1] IMPLICIT OCTET STRING, -- Selected when public key encryption is used. -- Contains a CMS type ContentInfo encoded -- according to [RFC3852]. -- The contentType field of the type ContentInfo is -- id-envelopedData (1.2.840.113549.1.7.3). -- The content field is an EnvelopedData. -- The contentType field for the type EnvelopedData -- is id-signedData (1.2.840.113549.1.7.2). -- The eContentType field for the inner type -- SignedData (when unencrypted) is id-pkrkeydata -- (1.2.840.113549.1.7.3) and the eContent field -- contains the DER encoding of the type -- ReplyKeyPack. -- ReplyKeyPack is defined below. ... } DHRepInfo ::= SEQUENCE { dhSignedData [0] IMPLICIT OCTET STRING, -- Contains a CMS type ContentInfo encoded according -- to [RFC3852]. -- The contentType field of the type ContentInfo is -- id-signedData (1.2.840.113549.1.7.2), and the -- content field is a SignedData. -- The eContentType field for the type SignedData is -- id-pkdhkeydata (1.3.6.1.5.2.3.2), and the -- eContent field contains the DER encoding of the -- type KDCDHKeyInfo. -- KDCDHKeyInfo is defined below. Tung & Zhu Expires March 16, 2006 [Page 29] Internet-Draft PKINIT September 2005 serverDHNonce [1] DHNonce OPTIONAL -- Present if and only if dhKeyExpiration is -- present. } KDCDHKeyInfo ::= SEQUENCE { subjectPublicKey [0] BIT STRING, -- KDC's DH public key. -- The DH public key value is encoded as a BIT -- STRING according to [RFC3279]. nonce [1] INTEGER (0..4294967295), -- Contains the nonce in the PKAuthenticator of the -- request if DH keys are NOT reused, -- 0 otherwise. dhKeyExpiration [2] KerberosTime OPTIONAL, -- Expiration time for KDC's key pair, -- present if and only if DH keys are reused. If -- this field is omitted then the serverDHNonce -- field MUST also be omitted. ... } ReplyKeyPack ::= SEQUENCE { replyKey [0] EncryptionKey, -- Contains the session key used to encrypt the -- enc-part field in the AS-REP. asChecksum [1] Checksum, -- Contains the checksum of the AS-REQ -- corresponding to the containing AS-REP. -- The checksum is performed over the type AS-REQ. -- The protocol key [RFC3961] of the checksum is the -- replyKey and the key usage number is 6. -- If the replyKey's enctype is "newer" [RFC4120] -- [RFC4121], the checksum is the required -- checksum operation [RFC3961] for that enctype. -- The client MUST verify this checksum upon receipt -- of the AS-REP. ... } TD-DH-PARAMETERS ::= SEQUENCE OF AlgorithmIdentifier -- Each AlgorithmIdentifier specifies a set of -- Diffie-Hellman domain parameters [IEEE1363]. -- This list is in decreasing preference order. END Tung & Zhu Expires March 16, 2006 [Page 30] Internet-Draft PKINIT September 2005 Appendix B. Test Vectors Function octetstring2key() is defined in Section 3.2.3.1. This section describes a few sets of test vectors that would be useful for implementers of octetstring2key(). Set 1 ===== Input octet string x is: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 Output of K-truncate() when the key size is 32 octets: 5e e5 0d 67 5c 80 9f e5 9e 4a 77 62 c5 4b 65 83 75 47 ea fb 15 9b d8 cd c7 5f fc a5 91 1e 4c 41 Set 2: ===== Input octet string x is: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 Output of K-truncate() when the key size is 32 octets: Tung & Zhu Expires March 16, 2006 [Page 31] Internet-Draft PKINIT September 2005 ac f7 70 7c 08 97 3d df db 27 cd 36 14 42 cc fb a3 55 c8 88 4c b4 72 f3 7d a6 36 d0 7d 56 78 7e Set 3: ====== Input octet string x is: 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 00 01 02 03 04 05 06 07 08 Output of K-truncate() when the key size is 32 octets: c4 42 da 58 5f cb 80 e4 3b 47 94 6f 25 40 93 e3 73 29 d9 90 01 38 0d b7 83 71 db 3a cf 5c 79 7e Set 4: ===== Input octet string x is: 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 00 01 02 03 04 05 06 07 08 Output of K-truncate() when the key size is 32 octets: 00 53 95 3b 84 c8 96 f4 eb 38 5c 3f 2e 75 1c 4a 59 0e d6 ff ad ca 6f f6 4f 47 eb eb 8d 78 0f fc Tung & Zhu Expires March 16, 2006 [Page 32] Internet-Draft PKINIT September 2005 Authors' Addresses Brian Tung USC Information Sciences Institute 4676 Admiralty Way Suite 1001, Marina del Rey CA Marina del Rey, CA 90292 US Email: brian@isi.edu Larry Zhu Microsoft Corporation One Microsoft Way Redmond, WA 98052 US Email: lzhu@microsoft.com Tung & Zhu Expires March 16, 2006 [Page 33] Internet-Draft PKINIT September 2005 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. 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Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2005). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Tung & Zhu Expires March 16, 2006 [Page 34]