INTERNET-DRAFT Brian Tung draft-ietf-cat-kerberos-pk-init-19.txt Clifford Neuman Updates: RFC 1510bis USC/ISI expires September 30, 2004 Matthew Hur Ari Medvinsky Microsoft Corporation Sasha Medvinsky Motorola, Inc. John Wray Iris Associates, Inc. Jonathan Trostle Public Key Cryptography for Initial Authentication in Kerberos 0. Status Of This Memo This document is an Internet-Draft and is in full conformance with all provision of Section 10 of RFC 2026. 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 The distribution of this memo is unlimited. It is filed as draft-ietf-cat-kerberos-pk-init-19.txt and expires September 30, 2004. Please send comments to the authors. 1. Abstract This document describes protocol extensions (hereafter called PKINIT) to the Kerberos protocol specification (RFC 1510bis [1]). These extensions provide a method for integrating public key cryptography into the initial authentication exchange, by passing digital certificates and associated authenticators in preauthentication data fields. 2. 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. (In this document, we will refer to both the AS and the TGS as the KDC.) Finally, the client uses the service ticket to authenticate itself to the service. The advantage afforded by the TGT is that the client need explicitly request a ticket and expose his credentials only once. The TGT and its associated session key can then be used for any subsequent 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, Kerberos authentication exchanges use symmetric-key cryptography, in part for performance. One cost 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. 3. Extensions This section describes extensions to RFC 1510bis for supporting the use of public-key cryptography in the initial request for a ticket. Briefly, this document defines the following extensions to RFC 1510bis: 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. 2. The KDC tests the client's request against its 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 symmetric encryption key, signed using the KDC?s signature key and encrypted using the client?s encryption key; or b. a key generated through a Diffie-Hellman exchange with the client, signed using the KDC's signature key. Any keying material required by the client to obtain the Encryption key is returned in a preauthentication field in the usual reply. 4. The client obtains the encryption key, decrypts the reply, and then proceeds as usual. Section 3.1 of this document defines the necessary message formats. Section 3.2 describes their syntax and use in greater detail. 3.1. Definitions 3.1.1. Required Algorithms All PKINIT implementations MUST support the following algorithms: - Reply key (or DH-derived key): AES256-CTS-HMAC-SHA1-96 etype; - Signature algorithm: SHA-1 digest and RSA; - Reply key delivery method: ephemeral-ephemeral Diffie-Hellman with a non-zero nonce; - Unkeyed checksum type for the paChecksum member of PKAuthenticator: SHA1 (unkeyed). 3.1.2. Defined Message and Encryption Types PKINIT makes use of the following new preauthentication types: PA-PK-AS-REQ TBD PA-PK-AS-REP TBD PA-PK-OCSP-REQ TBD PA-PK-OCSP-REP TBD PKINIT also makes use of the following new authorization data type: AD-INITIAL-VERIFIED-CAS TBD PKINIT introduces the following new error codes: KDC_ERR_CLIENT_NOT_TRUSTED 62 KDC_ERR_KDC_NOT_TRUSTED 63 KDC_ERR_INVALID_SIG 64 KDC_ERR_KEY_SIZE 65 KDC_ERR_CERTIFICATE_MISMATCH 66 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 PKINIT uses the following typed data types for errors: TD-DH-PARAMETERS TBD TD-TRUSTED-CERTIFIERS 104 TD-CERTIFICATE-INDEX 105 PKINIT defines the following encryption types, for use in the AS-REQ message (to indicate acceptance of the corresponding encryption OIDs in PKINIT): 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 above encryption types are used by the client only within the KDC-REQ-BODY to indicate which CMS [2] algorithms it supports. Their use within Kerberos EncryptedData structures is not specified by this document. 3.1.3. Algorithm Identifiers PKINIT does not define, but does make use of, the following algorithm identifiers. PKINIT uses the following algorithm identifier for Diffie-Hellman key agreement [9]: dhpublicnumber PKINIT uses the following signature algorithm identifiers [8, 12]: sha-1WithRSAEncryption (RSA with SHA1) md5WithRSAEncryption (RSA with MD5) id-dsa-with-sha1 (DSA with SHA1) PKINIT uses the following encryption algorithm identifiers [5] 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 [2] for encrypting the reply key with the temporary key: des-ede3-cbc (three-key 3DES, CBC mode) rc2-cbc (RC2, CBC mode) Kerberos data structures require the use of integer etypes, while CMS objects use OIDs. Therefore, each cryptographic algorithm supported by PKINIT is identified both by a CMS OID and by an equivalent Kerberos etype (defined in section 3.1.2). 3.2. PKINIT Preauthentication Syntax and Use This section defines the syntax and use of the various preauthentication fields employed by PKINIT. 3.2.1. Client Request The initial authentication request (AS-REQ) is sent as per RFC 1510bis; the preauthentication field contains data signed by the client's private signature key as follows: PA-PK-AS-REQ ::= SEQUENCE { signedAuthPack [0] ContentInfo, -- Defined in CMS [2]. -- Type is SignedData. -- Content is AuthPack -- (defined below). trustedCertifiers [1] SEQUENCE OF TrustedCA OPTIONAL, -- A list of CAs, trusted by -- the client, used to certify -- KDCs. kdcCert [2] IssuerAndSerialNumber OPTIONAL, -- Defined in CMS [2]. -- Identifies a particular KDC -- certificate, if the client -- already has it. encryptionCert [3] IssuerAndSerialNumber OPTIONAL, -- May identify the client's -- Diffie-Hellman certificate, -- or an RSA encryption key -- certificate. ... } TrustedCA ::= CHOICE { caName [0] Name, -- Fully qualified X.500 name -- as defined in RFC 3280 [4]. issuerAndSerial [1] IssuerAndSerialNumber, -- Identifies a specific CA -- certificate. ... } AuthPack ::= SEQUENCE { pkAuthenticator [0] PKAuthenticator, clientPublicValue [1] SubjectPublicKeyInfo OPTIONAL, -- Defined in RFC 3280 [4]. -- Present only if the client -- is using ephemeral-ephemeral -- Diffie-Hellman. ... } PKAuthenticator ::= SEQUENCE { cusec [0] INTEGER, ctime [1] KerberosTime, -- cusec and ctime are used as -- in RFC 1510bis, for replay -- prevention. nonce [2] INTEGER, -- Binds reply to request, -- MUST be zero when client -- will accept cached -- Diffie-Hellman parameters -- from KDC. MUST NOT be -- zero otherwise. -- MUST be 0 <= nonce < 2^32. paChecksum [3] Checksum, -- Defined in RFC 1510bis [1]. -- Performed over KDC-REQ-BODY, -- MUST be unkeyed. ... } IMPORTS -- from RFC 3280 [4] SubjectPublicKeyInfo, AlgorithmIdentifier, Name FROM PKIX1Explicit88 { iso (1) identified-organization (3) dod (6) internet (1) security (5) mechanisms (5) pkix (7) id-mod (0) id-pkix1-explicit (18) } IMPORTS -- from RFC 2630 [2] ContentInfo, IssuerAndSerialNumber FROM CryptographicMessageSyntax { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) modules(0) cms(1) } IMPORTS -- from RFC 1510bis [1] KerberosTime, Checksum FROM KerberosV5Spec2 { iso(1) identified-organization(3) dod(6) internet(1) security(5) kerberosV5(2) modules(4) krb5spec2(2) } The ContentInfo in the signedAuthPack is filled out as follows: 1. The eContent field contains data of type AuthPack. It MUST contain the pkAuthenticator, and MAY also contain the client's Diffie-Hellman public value (clientPublicValue). 2. The eContentType field MUST contain the OID value for pkauthdata: { iso (1) org (3) dod (6) internet (1) security (5) kerberosv5 (2) pkinit (3) pkauthdata (1)} 3. The signerInfos field MUST contain the signature over the AuthPack. 4. The certificates field MUST contain at least a signature verification certificate chain that the KDC can use to verify the signature over the AuthPack. Additionally, the client MAY insert an encryption certificate chain, if (for example) the client is not using ephemeral-ephemeral Diffie-Hellman. 5. If a Diffie-Hellman key is being used, the parameters SHOULD be chosen from the First or Second defined Oakley Groups. (See RFC 2409 [10].) 6. The KDC may wish to use cached Diffie-Hellman parameters. To indicate acceptance of caching, the client sends zero in the nonce field of the pkAuthenticator. Zero is not a valid value for this field under any other circumstances. Since zero is used to indicate acceptance of cached parameters, message binding in this case is performed using only the nonce in the main request. 3.2.2. Validation 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 must look for a client certificate in the signedAuthPack. If it cannot find one signed by a CA it trusts, it sends back an error of type KDC_ERR_CANT_VERIFY_CERTIFICATE. The accompanying e-data for this error is a SEQUENCE OF TYPED-DATA: TYPED-DATA ::= SEQUENCE { -- As defined in RFC 1510bis. data-type [0] INTEGER, data-value [1] OCTET STRING } IMPORTS -- from RFC 1510bis [1] TYPED-DATA, Checksum FROM KerberosV5Spec2 { iso(1) identified-organization(3) dod(6) internet(1) security(5) kerberosV5(2) modules(4) krb5spec2(2) } For this error, the data-type is TD-TRUSTED-CERTIFIERS, and the data-value is an OCTET STRING containing the DER encoding of TrustedCertifiers ::= SEQUENCE OF Name If, while verifying the certificate chain, the KDC determines that the signature on one of the certificates in the signedAuthPack is invalid, it returns an error of type KDC_ERR_INVALID_CERTIFICATE. The accompanying e-data for this error is a SEQUENCE OF TYPED-DATA, whose data-type is TD-CERTIFICATE-INDEX, and whose data-value is an OCTET STRING containing the DER encoding of the index into the CertificateSet field, ordered as sent by the client: CertificateIndex ::= IssuerAndSerialNumber -- IssuerAndSerialNumber of -- certificate with invalid signature If more than one certificate signature is invalid, the KDC MAY send one TYPED-DATA per invalid signature. The KDC MAY also check whether any of the certificates in the client's chain have been revoked. If any of them have been revoked, the KDC MUST return an error of type KDC_ERR_REVOKED_CERTIFICATE; if the KDC attempts to determine the revocation status but is unable to do so, it SHOULD return an error of type KDC_ERR_REVOCATION_STATUS_UNKNOWN. The certificate or certificates affected are identified exactly as for an error of type KDC_ERR_INVALID_CERTIFICATE (see above). In addition to validating the certificate chain, the KDC MUST also check that the certificate properly maps to the client's principal name as specified in the AS-REQ as follows: 1. If the KDC has its own mapping from the name in the certificate to a Kerberos name, it uses that Kerberos name. 2. Otherwise, if the certificate contains a SubjectAltName extension with a Kerberos name in the otherName field, it uses that name. The otherName field (of type AnotherName) in the SubjectAltName extension MUST contain the following: The type-id is: krb5PrincipalName OBJECT IDENTIFIER ::= { iso (1) org (3) dod (6) internet (1) security (5) kerberosv5 (2) 2 } The value is: KRB5PrincipalName ::= SEQUENCE { realm [0] Realm, principalName [1] PrincipalName } IMPORTS -- from RFC 3280 [4] GeneralName FROM PKIX1Explicit88 { iso (1) identified-organization (3) dod (6) internet (1) security (5) mechanisms (5) pkix (7) id-mod (0) id-pkix1-explicit (18) } IMPORTS -- from RFC 1510bis [1] PrincipalName, Realm FROM KerberosV5Spec2 { iso(1) identified-organization(3) dod(6) internet(1) security(5) kerberosV5(2) modules(4) krb5spec2(2) } If the KDC does not have its own mapping and there is no Kerberos name present in the certificate, or if the name in the request does not match the name in the certificate (including the realm name), or if there is no name in the request, the KDC MUST return error code KDC_ERR_CLIENT_NAME_MISMATCH. There is no accompanying e-data for this error. If the name in the request is [special "blank" name], the KDC MAY insert a different name in the reply. Even if the chain is validated, and the names in the certificate and the request match, the KDC may decide not to trust the client. For example, the certificate may include an Enxtended Key Usage (EKU) OID in the extensions field. As a matter of local policy, the KDC may decide to reject requests on the basis of the absence or presence of specific EKU OIDs. In this case, the KDC MUST return error code KDC_ERR_CLIENT_NOT_TRUSTED. The PKINIT EKU OID is defined as: { iso (1) org (3) dod (6) internet (1) security (5) kerberosv5 (2) pkinit (3) pkekuoid (4) } If the client's signature on the signedAuthPack fails to verify, the KDC MUST return error KDC_ERR_INVALID_SIG. There is no accompanying e-data for this error. 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 clock skew times in RFC 1510bis [1] apply here. If the check fails, the KDC MUSTreturn 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 ephemeral-ephemeral Diffie-Hellman, the KDC checks to see if the parameters satisfy its policy. If they do not, it MUST return error code KDC_ERR_KEY_SIZE. The accompanying e-data is a SEQUENCE OF TYPED-DATA, whose data-type is TD-DH-PARAMETERS, and whose data-value is an OCTET STRING containing the DER encoding of a DomainParameters (see [3]), including appropriate Diffie-Hellman parameters with which to retry the request. The KDC MUST return error code KDC_ERR_CERTIFICATE_MISMATCH if the client included a kdcCert field in the PA-PK-AS-REQ and the KDC does not have the corresponding certificate. The KDC MUST return error code KDC_ERR_KDC_NOT_TRUSTED if the client did not include a kdcCert field, but did include a trustedCertifiers field, and the KDC does not possesses a certificate issued by one of the listed certifiers. 3.2.3. KDC Reply Assuming that the client's request has been properly validated, the KDC proceeds as per RFC 1510bis, except as follows. The KDC MUST set the initial flag and include an authorization data of type AD-INITIAL-VERIFIED-CAS in the issued ticket. The value is an OCTET STRING containing the DER encoding of InitialVerifiedCAs: InitialVerifiedCAs ::= SEQUENCE OF SEQUENCE { ca [0] Name, Validated [1] BOOLEAN, ... } The KDC MAY wrap any AD-INITIAL-VERIFIED-CAS data in AD-IF-RELEVANT containers if the list of CAs satisfies the KDC's realm's policy. (This corresponds to the TRANSITED-POLICY-CHECKED ticket flag.) Furthermore, any TGS must copy such authorization data from tickets used in a PA-TGS-REQ of the TGS-REQ to the resulting ticket, including the AD-IF-RELEVANT container, if present. AP 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.) 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 AS-REP is otherwise unchanged from RFC 1510bis. The KDC encrypts the reply as usual, but not with the client's long-term key. Instead, it encrypts it with either a generated encryption key, or a key derived from a Diffie-Hellman exchange. The contents of the PA-PK-AS-REP indicate the type of encryption key that was used: PA-PK-AS-REP ::= CHOICE { dhSignedData [0] ContentInfo, -- Type is SignedData. -- Content is KDCDHKeyInfo -- (defined below). encKeyPack [1] ContentInfo, -- Type is SignedData. -- Content is ReplyKeyPack -- (defined below). ... } KDCDHKeyInfo ::= SEQUENCE { subjectPublicKey [0] BIT STRING, -- Equals public exponent -- (g^a mod p). -- INTEGER encoded as payload -- of BIT STRING. nonce [1] INTEGER, -- Binds reply to request. -- Exception: A value of zero -- indicates that the KDC is -- using cached values. dhKeyExpiration [2] KerberosTime OPTIONAL, -- Expiration time for KDC's -- cached values. ... } The fields of the ContentInfo for dhSignedData are to be filled in as follows: 1. The eContent field contains data of type KDCDHKeyInfo. 2. The eContentType field contains the OID value for pkdhkeydata: { iso (1) org (3) dod (6) internet (1) security (5) kerberosv5 (2) pkinit (3) pkdhkeydata (2) } 3. The signerInfos field contains a single signerInfo, which is the signature of the KDCDHKeyInfo. 4. The certificates field contains a signature verification certificate chain that the client will use to verify the KDC's signature over the KDCDHKeyInfo. This field may only be left empty if the client did include a kdcCert field in the PA-PK-AS-REQ, indicating that it has the KDC's certificate. 5. If the client and KDC agree to use cached parameters, the KDC MUST return a zero in the nonce field and include the expiration time of the cached values in the dhKeyExpiration field. If this time is exceeded, the client MUST NOT use the reply. If the time is absent, the client MUST NOT use the reply and MAY resubmit a request with a non-zero nonce, thus indicating non-acceptance of the cached parameters. The key is derived as follows: Both the KDC and the client calculate the value g^(ab) mod p, where a and b are the client's and KDC's private exponents, respectively. They both take the first k bits of this secret value as a key generation seed, where the parameter k (the size of the seed) is dependent on the selected key type, as specified in [6]. The seed is then converted into a protocol key by applying to it a random-to-key function, which is also dependent on key type. 1. For example, if the encryption type is DES with MD4, k = 64 bits and the random-to-key function consists of replacing some of the bits with parity bits, according to FIPS PUB 74 [9]. 2. If the encryption type is three-key 3DES with HMAC-SHA1, k = 168 bits and the random-to-key function is DES3random-to-key as defined in [6]. This function inserts parity bits to create a 192-bit 3DES protocol key that is compliant with FIPS PUB 74 [9]. This key is used to generate additional keys Ke and Ki, for encryption and integrity protection, respectively, using the key usage value of 3, as per [6] for the handling of the encrypted part of the AS-REP. If the KDC and client are not using Diffie-Hellman, the KDC encrypts the reply with an encryption key, packed in the encKeyPack, which contains data of type ReplyKeyPack: ReplyKeyPack ::= SEQUENCE { replyKey [0] EncryptionKey, -- Defined in RFC 1510bis. -- Used to encrypt main reply. -- MUST be at least as strong -- as session key. (Using the -- same enctype and a strong -- prng should suffice, if no -- stronger encryption system -- is available.) nonce [1] INTEGER, -- Binds reply to request. -- MUST be 0 < nonce < 2^32. ... } IMPORTS -- from RFC 1510bis [1] EncryptionKey FROM KerberosV5Spec2 { iso(1) identified-organization(3) dod(6) internet(1) security(5) kerberosV5(2) modules(4) krb5spec2(2) } The fields of the ContentInfo for encKeyPack MUST be filled in as follows: 1. The content is of type SignedData. The eContent for the SignedData is of type ReplyKeyPack. 2. The eContentType for the SignedData contains the OID value for pkrkeydata: { iso (1) org (3) dod (6) internet (1) security (5) kerberosv5 (2) pkinit (3) pkrkeydata (3) } 3. The signerInfos field contains a single signerInfo, which is the signature of the ReplyKeyPack. 4. The certificates field contains a signature verification certificate chain that the client will use to verify the KDC's signature over the ReplyKeyPack. This field may only be left empty if the client did include a kdcCert field in the PA-PK-AS-REQ, indicating that it has the KDC's certificate. 5. The encryptedContentType for the EnvelopedData contains the OID value for id-signedData: { iso (1) member-body (2) us (840) rsadsi (113549) pkcs (1) pkcs7 (7) signedData (2) } 6. The recipientInfos field is a SET which MUST contain exactly one member of type KeyTransRecipientInfo. The encryptedKey for this member contains the temporary key which is encrypted using the client's public key. 7. The unprotectedAttrs or originatorInfo fields MAY be present. 3.2.4. Validation of KDC Reply Upon receipt of the KDC's reply, the client proceeds as follows. If the PA-PK-AS-REP contains a dhSignedData, the client obtains and verifies the Diffie-Hellman parameters, and obtains the shared key as described above. Otherwise, the message contains an encKeyPack, and the client decrypts and verifies the temporary encryption key. In either case, the client then decrypts the main reply with the resulting key, and then proceeds as described in RFC 1510bis. 3.2.5. Support for OCSP OCSP (Online Certificate Status Protocol) [8] allows the use of on-line requests for a client or server to determine the validity of each other's certificates. It is particularly useful for clients authenticating each other across a constrained network. These clients will not have to download the entire CRL to check for the validity of the KDC's certificate. In these cases, the KDC generally has better connectivity to the OCSP server, and it therefore processes the OCSP request and response and sends the results to the client. The mechanism defined in this section allow a client to request an OCSP response from the KDC when using PKINIT. This is similar to the way that OCSP is handled in [7]. OCSP support is provided in PKINIT through the use of additional preauthentication data. The following new preauthentication types are defined: PA-PK-OCSP-REQ ::= SEQUENCE { -- PAType TBD responderIDList [0] SEQUENCE of ResponderID OPTIONAL, -- ResponderID is a DER-encoded -- ASN.1 type defined in [8] requestExtensions [1] Extensions OPTIONAL -- Extensions is a DER-encoded -- ASN.1 type defined in [8] } PA-PK-OCSP-REP ::= SEQUENCE of OCSPResponse -- OCSPResponse is a DER-encoded -- ASN.1 type defined in [8] A KDC that receives a PA-PK-OCSP-REQ MAY send a PA-PK-OCSP-REP. KDCs MUST NOT send a PA-PK-OCSP-REP if they do not first receive a PA-PK-OCSP-REQ from the client. The KDC MAY either send a cached OCSP response or send an on-line request to the OCSP server. In the case that a responderIDList is not sent or is empty, the OCSP response must be signed by the authority that issued the certificate, unless specified otherwise by a mutually agreed policy between the client and the KDC. When using OCSP, the response is signed by the OCSP server, which is trusted by the client. Depending on local policy, further verification of the validity of the OCSP server may need to be done. 4. Security Considerations 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. 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 RFC 1510bis. PKINIT allows the use of a zero nonce in the PKAuthenticator when cached Diffie-Hellman keys are used. In this case, message binding is performed using the nonce in the main request in the same way as it is done for ordinary AS-REQs (without the PKINIT pre-authenticator). The nonce field in the KDC request body is signed through the checksum in the PKAuthenticator, which cryptographically binds the PKINIT pre-authenticator to the main body of the AS Request and also provides message integrity for the full AS Request. However, when a PKINIT pre-authenticator in the AS-REP has a zero-nonce, and an attacker has somehow recorded this pre-authenticator and discovered the corresponding Diffie-Hellman private key (e.g., with a brute-force attack), the attacker will be able to fabricate his own AS-REP messages that impersonate the KDC with this same pre-authenticator. This compromised pre-authenticator will remain valid as long as its expiration time has not been reached and it is therefore important for clients to check this expiration time and for the expiration time to be reasonably short, which depends on the size of the Diffie-Hellman group. If a client also caches its Diffie-Hellman keys, then the session key could remain the same during multiple AS-REQ/AS-REP exchanges and an attacker which compromised the session key could fabricate his own AS-REP messages with a pre-recorded pre-authenticator until the client starts using a new Diffie-Hellman key pair and while the KDC pre-authenticator has not yet expired. It is therefore not recommended for KDC clients to also cache their Diffie-Hellman keys. 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. 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. 5. Acknowledgements 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. Expiration Date This draft expires September 30, 2004. 7. Bibliography [1] RFC-Editor: To be replaced by RFC number for draft-ietf-krb-wg-kerberos-clarifications. [2] R. Housley. Cryptographic Message Syntax., April 1999. Request For Comments 2630. [3] W. Polk, R. Housley, and L. Bassham. Algorithms and Identifiers for the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile, April 2002. Request For Comments 3279. [4] R. Housley, W. Polk, W. Ford, D. Solo. Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile, April 2002. Request for Comments 3280. [5] B. Kaliski, J. Staddon. PKCS #1: RSA Cryptography Specifications, October 1998. Request for Comments 2437. [6] RFC-Editor: To be replaced by RFC number for draft-ietf-krb-wg-crypto. [7] S. Blake-Wilson, M. Nystrom, D. Hopwood, J. Mikkelsen, and T. Wright. Transport Layer Security (TLS) Extensions, June 2003. Request for Comments 3546. [8] M. Myers, R. Ankney, A. Malpani, S. Galperin, and C. Adams. Internet X.509 Public Key Infrastructure: Online Certificate Status Protocol - OCSP, June 1999. Request for Comments 2560. [9] NIST, Guidelines for Implementing and Using the NBS Encryption Standard, April 1981. FIPS PUB 74. [10] D. Harkins and D. Carrel. The Internet Key Exchange (IKE), November 1998. Request for Comments 2409. 8. Authors Brian Tung Clifford Neuman USC Information Sciences Institute 4676 Admiralty Way Suite 1001 Marina del Rey CA 90292-6695 Phone: +1 310 822 1511 E-mail: {brian,bcn}@isi.edu Matthew Hur Ari Medvinsky Microsoft Corporation One Microsoft Way Redmond WA 98052 Phone: +1 425 707 3336 E-mail: matthur@microsoft.com, arimed@windows.microsoft.com Sasha Medvinsky Motorola, Inc. 6450 Sequence Drive San Diego, CA 92121 +1 858 404 2367 E-mail: smedvinsky@motorola.com John Wray Iris Associates, Inc. 5 Technology Park Dr. Westford, MA 01886 E-mail: John_Wray@iris.com Jonathan Trostle E-mail: jtrostle@world.std.com