PKINIT Algorithm Agility

The Public Key Cryptography for Initial Authentication in Kerberos (PKINIT) standard defines several protocol structures that are either tied to SHA-1 , or do not support negotiation or discovery, but are instead based on local policy: The checksum algorithm in the authentication request is hardwired to use SHA-1. The acceptable digest algorithms for signing the authentication data are not discoverable. The key derivation function in Section 3.2.3.1 of is hardwired to use SHA-1. The acceptable digest algorithms for signing the client X.509 certificates are not discoverable. In August 2004, Xiaoyun Wang's research group reported MD4 collisions generated using hand calculation , alongside attacks on later hash function designs in the MD4, MD5 and SHA family. These attacks and their consequences are discussed in . These discoveries challenged the security of protocols relying on the collision resistance properties of these hashes. The Internet Engineering Task Force (IETF) called for actions to update existing protocols to provide crypto algorithm agility so that protocols support multiple cryptographic algorithms (including hash functions) and provide clean, tested transition strategies between algorithms, as recommended by BCP 201 . To address these concerns, new key derivation functions (KDFs), identified by object identifiers, are defined. The PKINIT client provides a list of KDFs in the request and the Key Distribution Center (KDC) picks one in the response, thus a mutually-supported KDF is negotiated. Furthermore, structures are defined to allow the client to discover the Cryptographic Message Syntax (CMS) digest algorithms supported by the KDC for signing the pre-authentication data and signing the client X.509 certificate.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

The paChecksum defined in Section 3.2.1 of provides a cryptographic binding between the client's pre-authentication data and the corresponding Kerberos request body. This also prevents the KDC-REQ body from being tampered with. SHA-1 is the only allowed checksum algorithm defined in . This facility relies on the collision resistance properties of the SHA-1 checksum . When the reply key delivery mechanism is based on public key encryption as described in Section 3.2.3.2 of , the asChecksum in the KDC reply provides the binding between the pre-authentication and the ticket request and response messages, and integrity protection for the unauthenticated clear text in these messages. However, if the reply key delivery mechanism is based on the Diffie-Hellman key agreement as described in Section 3.2.3.1 of , the security provided by using SHA-1 in the paChecksum is weak, and nothing else cryptographically binds the AS request to the ticket response. In this case, the new KDF selected by the KDC as described in provides the cryptographic binding and integrity protection.

Section 3.2.2 of is updated to add optional typed data to the KDC_ERR_DIGEST_IN_SIGNED_DATA_NOT_ACCEPTED error. When a KDC implementation conforming to this specification returns this error code, it MAY include in a list of supported CMS types signifying the digest algorithms supported by the KDC, in the decreasing preference order. This is accomplished by including a TD_CMS_DATA_DIGEST_ALGORITHMS typed data element in the error data.

The corresponding data for the TD_CMS_DATA_DIGEST_ALGORITHMS contains the ASN.1 Distinguished Encoding Rules (DER) encoded TD-CMS-DIGEST-ALGORITHMS-DATA structure defined as follows:

The algorithm identifiers in the TD-CMS-DIGEST-ALGORITHMS identifiy digest algorithms supported by the KDC. This information sent by the KDC via TD_CMS_DATA_DIGEST_ALGORITHMS can facilitate trouble-shooting when none of the digest algorithms supported by the client is supported by the KDC.

Section 3.2.2 of is updated to add optional typed data to the KDC_ERR_DIGEST_IN_CERT_NOT_ACCEPTED error. When a KDC conforming to this specification returns this error, it MAY send a list of digest algorithms acceptable to the KDC for use by the Certificate Authority (CA) in signing the client's X.509 certificate, in the decreasing preference order. This is accomplished by including a TD_CERT_DIGEST_ALGORITHMS typed data element in the error data. The corresponding data contains the ASN.1 DER encoding of the structure TD-CERT-DIGEST-ALGORITHMS-DATA defined as follows:

The KDC fills in the allowedAlgorithm field with the list of algorithm identifiers indicating digest algorithms that are used by the CA to sign the client's X.509 certificate and are acceptable to the KDC in the process of validating the client's X.509 certificate, in the order of decreasing preference. The rejectedAlgorithm field identifies the signing algorithm for use in signing the client's X.509 certificate that has been rejected by the KDC in the process of validating the client's certificate .

Section 3.2.3.1 of is updated to define additional Key Derivation Functions (KDFs) to derive a Kerberos protocol key based on the secret value generated by the Diffie-Hellman key exchange. Section 3.2.1 of is updated to add a new field to the AuthPack structure to indicate which new KDFs are supported by the client. Section 3.2.3 of is updated to add a new field to the DHRepInfo structure to indicate which KDF is selected by the KDC. The KDF algorithm described in this document (based on ) can be implemented using any cryptographic hash function. A new KDF for PKINIT usage is identified by an object identifier. The following KDF object identifiers are defined:

Where id-pkinit is defined in . All key derivation functions specified above use the one-step key derivation method described in Section 5.8.2.1 of , using the ASN.1 format for FixedInfo, and Section 4.1 of , using option 1 for the auxiliary function H. id-pkinit-kdf-ah-sha1 uses SHA-1 as the hash function. id-pkinit-kdf-ah-sha256, id-pkinit-kdf-ah-sha356, and id-pkinit-kdf-ah-sha512 use SHA-256 , SHA-384 ( and SHA-512 respectively. To name the input parameters, an abbreviated version of the key derivation method is described below. reps = ceiling(L/H_outputBits) Initialize a 32-bit, big-endian bit string counter as 1. For i = 1 to reps by 1, do the following: Compute Hashi = H(counter || Z || OtherInfo). Increment counter (not to exceed 2^32-1) Set key_material = Hash1 || Hash2 || ... so that the length of key_material is L bits, truncating the last block as necessary. The above KDF produces a bit string of length L in bits as the keying material. The AS reply key is the output of random-to-key() using that keying material as the input. The input parameters for these KDFs are provided as follows: H_outputBits is 160 bits for id-pkinit-kdf-ah-sha1, 256 bits for id-pkinit-kdf-ah-sha256, 384 bits for id-pkinit-kdf-ah-sha384, and 512 bits for id-pkinit-kdf-ah-sha512. max_H_inputBits is 2^64. The secret value (Z) is the shared secret value generated by the Diffie-Hellman exchange. The Diffie-Hellman shared value is first padded with leading zeros such that the size of the secret value in octets is the same as that of the modulus, then represented as a string of octets in big-endian order. The key data length (L) is the key-generation seed length in bits for the Authentication Service (AS) reply key. The enctype of the AS reply key is selected according to . The algorithm identifier (algorithmID) input parameter is the identifier of the respective KDF. For example, this is id-pkinit-kdf-ah-sha1 if the KDF uses SHA-1 as the hash. The initiator identifier (partyUInfo) contains the ASN.1 DER encoding of the KRB5PrincipalName that identifies the client as specified in the AS-REQ in the request. The recipient identifier (partyVInfo) contains the ASN.1 DER encoding of the KRB5PrincipalName that identifies the TGS as specified in the AS-REQ in the request. The supplemental public information (suppPubInfo) is the ASN.1 DER encoding of the structure PkinitSuppPubInfo as defined later in this section. The supplemental private information (suppPrivInfo) is absent. OtherInfo is the ASN.1 DER encoding of the following sequence:

The structure PkinitSuppPubInfo is defined as follows:

The PkinitSuppPubInfo structure contains mutually-known public information specific to the authentication exchange. The enctype field is the enctype of the AS reply key as selected according to . The as-REQ field contains the DER encoding of the type AS-REQ in the request sent from the client to the KDC. Note that the as-REQ field does not include the wrapping 4 octet length field when TCP is used. The pk-as-rep field contains the DER encoding of the type PA-PK-AS-REP in the KDC reply. The PkinitSuppPubInfo provides a cryptographic bindings between the pre-authentication data and the corresponding ticket request and response, thus addressing the concerns described in . The KDF is negotiated between the client and the KDC. The client sends an unordered set of supported KDFs in the request, and the KDC picks one from the set in the reply. To accomplish this, the AuthPack structure in is extended as follows:

The new field supportedKDFs contains an unordered set of KDFs supported by the client. The KDFAlgorithmId structure contains an object identifier that identifies a KDF. The algorithm of the KDF and its parameters are defined by the corresponding specification of that KDF. The DHRepInfo structure in is extended as follows:

The new field kdf in the extended DHRepInfo structure identifies the KDF picked by the KDC. If the supportedKDFs field is present in the request, a KDC conforming to this specification MUST choose one of the KDFs supported by the client and indicate its selection in the kdf field in the reply. If the supportedKDFs field is absent in the request, the KDC MUST omit the kdf field in the reply and use the key derivation function from Section 3.2.3.1 of . If none of the KDFs supported by the client is acceptable to the KDC, the KDC MUST reply with the new error code KDC_ERR_NO_ACCEPTABLE_KDF: KDC_ERR_NO_ACCEPTABLE_KDF 100 If the client fills the supportedKDFs field in the request, but the kdf field in the reply is not present, the client can deduce that the KDC is not updated to conform with this specification, or that the exchange was subjected to a downgrade attack. It is a matter of local policy on the client whether to reject the reply when the kdf field is absent in the reply; if compatibility with non-updated KDCs is not a concern, the reply should be rejected. Implementations conforming to this specification MUST support id-pkinit-kdf-ah-sha256.

An old client interoperating with a new KDC will not recognize a TD-CMS-DIGEST-ALGORITHMS-DATA element in a KDC_ERR_DIGEST_IN_SIGNED_DATA_NOT_ACCEPTED error, or a TD-CERT-DIGEST-ALGORITHMS-DATA element in a KDC_ERR_DIGEST_IN_CERT_NOT_ACCEPTED error. Because the error data is encoded as typed data, the client will ignore the unrecognized elements. An old KDC interoperating with a new client will not include a TD-CMS-DIGEST-ALGORITHMS-DATA element in a KDC_ERR_DIGEST_IN_SIGNED_DATA_NOT_ACCEPTED error, or a TD-CERT-DIGEST-ALGORITHMS-DATA element in a KDC_ERR_DIGEST_IN_CERT_NOT_ACCEPTED error. To the client this appears just as if a new KDC elected not to include a list of digest algorithms. An old client interoperating with a new KDC will not include the supportedKDFs field in the request. The KDC MUST omit the kdf field in the reply and use the KDF as expected by the client, or reject the request if local policy forbids use of the old KDF. A new client interoperating with an old KDC will include the supportedKDFs field in the request; this field will be ignored as an unknown extension by the KDC. The KDC will omit the kdf field in the reply and will use the KDF. The client can deduce from the omitted kdf field that the KDC is not updated to conform to this specification, or that the exchange was subjected to a downgrade attack. The client MUST use the KDF, or reject the reply if local policy forbids the use of the old KDF.

This section contains test vectors for the KDF defined above.

This document describes negotiation of checksum types, key derivation functions and other cryptographic functions. If a given negotiation is unauthenticated, care must be taken to accept only secure values; to do otherwise allows an active attacker to perform a downgrade attack. The discovery method described in uses a Kerberos error message, which is unauthenticated in a typical exchange. An attacker may attempt to downgrade a client to a weaker CMS type by forging a KDC_ERR_DIGEST_IN_SIGNED_DATA_NOT_ACCEPTED error. It is a matter of local policy whether a client accepts a downgrade to a weaker CMS type, and whether the KDC accepts the weaker CMS type. A client may reasonably assume that the real KDC implements all hash functions used in the client's X.509 certificate, and refuse attempts to downgrade to weaker hash functions. The discovery method described in also uses a Kerberos error message. An attacker may attempt to downgrade a client to a certificate using a weaker signing algorithm by forging a KDC_ERR_DIGEST_IN_CERT_NOT_ACCEPTED error. It is a matter of local policy whether a client accepts a downgrade to a weaker certificate, and whether the KDC accepts the weaker certificate. This attack is only possible if the client device possesses multiple client certificates of varying strength. In the KDF negotiation method described in , the client supportedKDFs value is protected by the signature on the signedAuthPack field in the request. If this signature algorithm is weak to collision attacks, an attacker may attempt to downgrade the negotiation by substituting an AuthPack with a different or absent supportedKDFs value, using a PKINIT freshness token to partially control the legitimate AuthPack value. A client performing anonymous PKINIT does not sign the AuthPack, so an attacker can easily remove the supportedKDFs value in this case. Finally, the kdf field in the DHRepInfo of the KDC response is unauthenticated, so could be altered or removed by an attacker, although this alteration will likely result in a decryption failure by the client rather than a successful downgrade. It is a matter of local policy whether a client accepts a downgrade to the old KDF, and whether the KDC allows the use of the old KDF. The paChecksum field, which binds the client pre-authentication data to the Kerberos request body, remains fixed at SHA-1. If an attacker substitutes a different request body using an attack against SHA-1 (a second preimage attack is likely required as the attacker does not control any part of the legitimate request body), the KDC will not detect the substitution. Instead, if a new KDF is negotiated, the client will detect the substitution by failing to decrypt the reply. An attacker may attempt to impersonate the KDC to the client via an attack on the hash function used in the dhSignedData signature, substituting the attacker's subjectPublicKey for the legitimate one without changing the hash value. It is a matter of local policy which hash function the KDC uses in its signature and which hash functions the client will accept in the KDC signature. A KDC may reasonably assume that the client implements all hash functions used in the KDF algorithms listed the supportedKDFs field of the request.

Jeffery Hutzelman, Shawn Emery, Tim Polk, Kelley Burgin, Ben Kaduk, Scott Bradner, and Eric Rescorla reviewed the document and provided suggestions for improvements.

IANA is requested to update the following registrations in the Kerberos Pre-authentication and Typed Data Registry created by section 7.1 of RFC 6113 to refer to this specification. These values were reserved for this specification in the initial registrations.