Anonymity Support for Kerberos

In certain situations, the Kerberos client may wish to authenticate a server and/or protect communications without revealing the client's own identity. For example, consider an application that provides read access to a research database and that permits queries by arbitrary requesters. A client of such a service might wish to authenticate the service, to establish trust in the information received from it, but might not wish to disclose the client's identity to the service for privacy reasons. Extensions to Kerberos are specified in this document by which a client can authenticate the Key Distribution Center (KDC) and request an anonymous ticket. The client can use the anonymous ticket to authenticate the server and protect subsequent client-server communications. By using the extensions defined in this specification, the client can request an anonymous ticket where the client may reveal the client's identity to the client's own KDC, or the client can hide the client's identity completely by using anonymous Public Key Cryptography for Initial Authentication in Kerberos (PKINIT) as defined in . Using the returned anonymous ticket, the client remains anonymous in subsequent Kerberos exchanges thereafter to KDCs on the cross-realm authentication path and to the server with which it communicates. In this specification, the client realm in the anonymous ticket is the anonymous realm name when anonymous PKINIT is used to obtain the ticket. The client realm is the client's real realm name if the client is authenticated using the client's long-term keys. Note that a membership in a realm can imply a member of the community represented by the realm. The interaction with Generic Security Service Application Program Interface (GSS-API) is described after the protocol description. This specification replaces RFC 6112 to correct technical errors in that specification. RFC 6112 is classified is historic; implementation of RFC 6112 is NOT RECOMMENDED: existing implementations comply with this specification and not RFC 6112.

In , the pepper2 string is corrected to comply with the string actually used by implementations. The requirement for the anonymous option to be used when an anonymous ticket is used in a TGS request is reduced from a MUST to a SHOULD.

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 .

The anonymous Kerberos realm name is defined as a well-known realm name based on , and the value of this well-known realm name is the literal "WELLKNOWN:ANONYMOUS". The anonymous Kerberos principal name is defined as a well-known Kerberos principal name based on . The value of the name-type field is KRB_NT_WELLKNOWN , and the value of the name-string field is a sequence of two KerberosString components: "WELLKNOWN", "ANONYMOUS". The anonymous ticket flag is defined as bit 16 (with the first bit being bit 0) in the TicketFlags:

TicketFlags ::= KerberosFlags -- anonymous(16) -- TicketFlags and KerberosFlags are defined in [RFC4120] This is a new ticket flag that is used to indicate that a ticket is an anonymous one. An anonymous ticket is a ticket that has all of the following properties: The cname field contains the anonymous Kerberos principal name. The crealm field contains the client's realm name or the anonymous realm name. The anonymous ticket contains no information that can reveal the client's identity. However, the ticket may contain the client realm, intermediate realms on the client's authentication path, and authorization data that may provide information related to the client's identity. For example, an anonymous principal that is identifiable only as being in a particular group of users can be implemented using authorization data. Such authorization data, if included in the anonymous ticket, would disclose the that the client is a member of the group observed. The anonymous ticket flag is set. The anonymous KDC option is defined as bit 16 (with the first bit being bit 0) in the KDCOptions:

KDCOptions ::= KerberosFlags -- anonymous(16) -- KDCOptions and KerberosFlags are defined in [RFC4120] As described in , the anonymous KDC option is set to request an anonymous ticket in an Authentication Service (AS) request or a Ticket Granting Service (TGS) request.

In order to request an anonymous ticket, the client sets the anonymous KDC option in an AS request or a TGS request. The rest of this section is organized as follows: it first describes protocol actions specific to AS exchanges, then it describes those of TGS exchanges. These are then followed by the description of protocol actions common to both AS and TGS and those in subsequent exchanges.

The client requests an anonymous ticket by setting the anonymous KDC option in an AS exchange. The Kerberos client can use the client's long-term keys, the client's X.509 certificates , or any other pre-authentication data, to authenticate to the KDC and request an anonymous ticket in an AS exchange where the client's identity is known to the KDC. If the client in the AS request is anonymous, the anonymous KDC option MUST be set in the request. Otherwise, the KDC MUST return a KRB-ERROR message with the code KDC_ERR_BADOPTION. If the client is anonymous and the KDC does not have a key to encrypt the reply (this can happen when, for example, the KDC does not support PKINIT ), the KDC MUST return an error message with the code KDC_ERR_NULL_KEY . When policy allows, the KDC issues an anonymous ticket. If the client name in the request is the anonymous principal, the client realm (crealm) in the reply is the anonymous realm, otherwise, the client realm is the realm of the AS. As specified by , the client name and the client realm in the EncTicketPart of the reply MUST match with the corresponding client name and the client realm of the KDC reply; the client MUST use the client name and the client realm returned in the KDC-REP in subsequent message exchanges when using the obtained anonymous ticket. Care MUST be taken by the KDC not to reveal the client's identity in the authorization data of the returned ticket when populating the authorization data in a returned anonymous ticket. The AD_INITIAL_VERIFIED_CAS authorization data, as defined in , contains the issuer name of the client certificate. This authorization is not applicable and MUST NOT be present in the returned anonymous ticket when anonymous PKINIT is used. When the client is authenticated (i.e., anonymous PKINIT is not used), if it is undesirable to disclose such information about the client's identity, the AD_INITIAL_VERIFIED_CAS authorization data SHOULD be removed from the returned anonymous ticket. The client can use the client's key to mutually authenticate with the KDC and request an anonymous Ticket Granting Ticket (TGT) in the AS request. In that case, the reply key is selected as normal, according to Section 3.1.3 of .

This sub-section defines anonymous PKINIT. As described earlier in this section, the client can request an anonymous ticket by authenticating to the KDC using the client's identity; alternatively, without revealing the client's identity to the KDC, the Kerberos client can request an anonymous ticket as follows: the client sets the client name as the anonymous principal in the AS exchange and provides PA_PK_AS_REQ pre-authentication data where the signerInfos field of the SignedData of the PA_PK_AS_REQ is empty, and the certificates field is absent. Because the anonymous client does not have an associated asymmetric key pair, the client MUST choose the Diffie-Hellman key agreement method by filling in the Diffie-Hellman domain parameters in the clientPublicValue . This use of the anonymous client name in conjunction with PKINIT is referred to as anonymous PKINIT. If anonymous PKINIT is used, the realm name in the returned anonymous ticket MUST be the anonymous realm. Upon receiving the anonymous PKINIT request from the client, the KDC processes the request, according to Section 3.1.2 of . The KDC skips the checks for the client's signature and the client's public key (such as the verification of the binding between the client's public key and the client name), but performs otherwise applicable checks, and proceeds as normal, according to . For example, the AS MUST check if the client's Diffie-Hellman domain parameters are acceptable. The Diffie-Hellman key agreement method MUST be used and the reply key is derived according to Section 3.2.3.1 of . If the clientPublicValue is not present in the request, the KDC MUST return a KRB-ERROR with the code KDC_ERR_PUBLIC_KEY_ENCRYPTION_NOT_SUPPORTED . If all goes well, an anonymous ticket is generated, according to Section 3.1.3 of , and PA_PK_AS_REP pre-authentication data is included in the KDC reply, according to . If the KDC does not have an asymmetric key pair, it MAY reply anonymously or reject the authentication attempt. If the KDC replies anonymously, the signerInfos field of the SignedData of PA_PK_AS_REP in the reply is empty, and the certificates field is absent. The server name in the anonymous KDC reply contains the name of the TGS. Upon receipt of the KDC reply that contains an anonymous ticket and PA_PK_AS_REP pre-authentication data, the client can then authenticate the KDC based on the KDC's signature in the PA_PK_AS_REP. If the KDC's signature is missing in the KDC reply (the reply is anonymous), the client MUST reject the returned ticket if it cannot authenticate the KDC otherwise. A KDC that supports anonymous PKINIT MUST indicate the support of PKINIT, according to Section 3.4 of . In addition, such a KDC MUST indicate support for anonymous PKINIT by including a padata element of padata-type PA_PKINIT_KX and empty padata-value when including PA-PK-AS-REQ in an error reply. When included in a KDC error, PA_PKINIT_KX indicates support for anonymous PKINIT. As discussed in Section 7, when included in an AS-REP, PA_PKINIT_KX proves that the KDC and client both contributed to the session key for any use of Diffie-Hellman key agreement with PKINIT. Note that in order to obtain an anonymous ticket with the anonymous realm name, the client MUST set the client name as the anonymous principal in the request when requesting an anonymous ticket in an AS exchange. Anonymous PKINIT is the only way via which an anonymous ticket with the anonymous realm as the client realm can be generated in this specification.

The client requests an anonymous ticket by setting the anonymous KDC option in a TGS exchange, and in that request the client can use a normal Ticket Granting Ticket (TGT) with the client's identity, or an anonymous TGT, or an anonymous cross-realm TGT. If the client uses a normal TGT, the client's identity is known to the TGS. Note that the client can completely hide the client's identity in an AS exchange using anonymous PKINIT, as described in the previous section. If the ticket in the PA-TGS-REQ of the TGS request is an anonymous one, the anonymous KDC option SHOULD be set in the request. When policy allows, the KDC issues an anonymous ticket. If the ticket in the TGS request is an anonymous one, the client name and the client realm are copied from that ticket; otherwise, the ticket in the TGS request is a normal ticket, the returned anonymous ticket contains the client name as the anonymous principal and the client realm as the true realm of the client. In all cases, according to the client name and the client realm in the EncTicketPart of the reply MUST match with the corresponding client name and the client realm of the anonymous ticket in the reply; the client MUST use the client name and the client realm returned in the KDC-REP in subsequent message exchanges when using the obtained anonymous ticket. Care MUST be taken by the TGS not to reveal the client's identity in the authorization data of the returned ticket. When propagating authorization data in the ticket or in the enc-authorization-data field of the request, the TGS MUST ensure that the client confidentiality is not violated in the returned anonymous ticket. The TGS MUST process the authorization data recursively, according to Section 5.2.6 of , beyond the container levels such that all embedded authorization elements are interpreted. The TGS SHOULD NOT populate identity-based authorization data into an anonymous ticket in that such authorization data typically reveals the client's identity. The specification of a new authorization data type MUST specify the processing rules of the authorization data when an anonymous ticket is returned. If there is no processing rule defined for an authorization data element or the authorization data element is unknown, the TGS MUST process it when an anonymous ticket is returned as follows: If the authorization data element may reveal the client's identity, it MUST be removed unless otherwise specified. If the authorization data element, that could reveal the client's identity, is intended to restrict the use of the ticket or limit the rights otherwise conveyed in the ticket, it cannot be removed in order to hide the client's identity. In this case, the authentication attempt MUST be rejected, and the TGS MUST return an error message with the code KDC_ERR_POLICY. Note this is applicable to both critical and optional authorization data. If the authorization data element is unknown, the TGS MAY remove it, or transfer it into the returned anonymous ticket, or reject the authentication attempt, based on local policy for that authorization data type unless otherwise specified. If there is no policy defined for a given unknown authorization data type, the authentication MUST be rejected. The error code is KDC_ERR_POLICY when the authentication is rejected. The AD_INITIAL_VERIFIED_CAS authorization data, as defined in , contains the issuer name of the client certificate. If it is undesirable to disclose such information about the client's identity, the AD_INITIAL_VERIFIED_CAS authorization data SHOULD be removed from an anonymous ticket. The TGS encodes the name of the previous realm into the transited field, according to Section 3.3.3.2 of . Based on local policy, the TGS MAY omit the previous realm, if the cross realm TGT is an anonymous one, in order to hide the authentication path of the client. The unordered set of realms in the transited field, if present, can reveal which realm may potentially be the realm of the client or the realm that issued the anonymous TGT. The anonymous Kerberos realm name MUST NOT be present in the transited field of a ticket. The true name of the realm that issued the anonymous ticket MAY be present in the transited field of a ticket.

In both AS and TGS exchanges, the realm field in the KDC request is always the realm of the target KDC, not the anonymous realm when the client requests an anonymous ticket. Absent other information, the KDC MUST NOT include any identifier in the returned anonymous ticket that could reveal the client's identity to the server. Unless anonymous PKINIT is used, if a client requires anonymous communication, then the client MUST check to make sure that the ticket in the reply is actually anonymous by checking the presence of the anonymous ticket flag in the flags field of the EncKDCRepPart. This is because KDCs ignore unknown KDC options. A KDC that does not understand the anonymous KDC option will not return an error, but will instead return a normal ticket. The subsequent client and server communications then proceed as described in . Note that the anonymous principal name and realm are only applicable to the client in Kerberos messages, the server cannot be anonymous in any Kerberos message per this specification. A server accepting an anonymous service ticket may assume that subsequent requests using the same ticket originate from the same client. Requests with different tickets are likely to originate from different clients. Upon receipt of an anonymous ticket, the transited policy check is performed in the same way as that of a normal ticket if the client's realm is not the anonymous realm; if the client realm is the anonymous realm, absent other information any realm in the authentication path is allowed by the cross-realm policy check.

Conforming implementations MUST support the anonymous principal with a non-anonymous realm, and they MAY support the anonymous principal with the anonymous realm using anonymous PKINIT.

GSS-API defines the name_type GSS_C_NT_ANONYMOUS to represent the anonymous identity. In addition, Section 2.1.1 of defines the single string representation of a Kerberos principal name with the name_type GSS_KRB5_NT_PRINCIPAL_NAME. The anonymous principal with the anonymous realm corresponds to the GSS- API anonymous principal. A principal with the anonymous principal name and a non-anonymous realm is an authenticated principal; hence, such a principal does not correspond to the anonymous principal in GSS-API with the GSS_C_NT_ANONYMOUS name type. The name syntax for GSS_KRB5_NT_PRINCIPAL_NAME MUST be used for importing the anonymous principal name with a non-anonymous realm name and for displaying and exporting these names. In addition, this syntax must be used along with the name type GSS_C_NT_ANONYMOUS for displaying and exporting the anonymous principal with the anonymous realm. At the GSS-API level, an initiator/client requests the use of an anonymous principal with the anonymous realm by asserting the "anonymous" flag when calling GSS_Init_Sec_Context(). The GSS-API implementation MAY provide implementation-specific means for requesting the use of an anonymous principal with a non-anonymous realm. GSS-API does not know or define "anonymous credentials", so the (printable) name of the anonymous principal will rarely be used by or relevant for the initiator/client. The printable name is relevant for the acceptor/server when performing an authorization decision based on the initiator name that is returned from the acceptor side upon the successful security context establishment. A GSS-API initiator MUST carefully check the resulting context attributes from the initial call to GSS_Init_Sec_Context() when requesting anonymity, because (as in the GSS-API tradition and for backwards compatibility) anonymity is just another optional context attribute. It could be that the mechanism doesn't recognize the attribute at all or that anonymity is not available for some other reasons -- and in that case the initiator MUST NOT send the initial security context token to the acceptor, because it will likely reveal the initiators identity to the acceptor, something that can rarely be "un-done". Portable initiators are RECOMMENDED to use default credentials whenever possible, and request anonymity only through the input anon_req_flag to GSS_Init_Sec_Context().

The definition in this section was motivated by protocol analysis of anonymous PKINIT (defined in this document) in building secure channels and subsequent channel bindings . In order to enable applications of anonymous PKINIT to form secure channels, all implementations of anonymous PKINIT need to meet the requirements of this section. There is otherwise no connection to the rest of this document. PKINIT is useful for constructing secure channels. To ensure that an attacker cannot create a channel by observing exchanges, it is desirable that neither the KDC nor the client unilaterally determine the ticket session key. The specific reason why the ticket session key is derived jointly is discussed at the end of this section. To achieve that end, a KDC conforming to this definition MUST encrypt a randomly generated key, called the KDC contribution key, in the PA_PKINIT_KX padata (defined next in this section). The KDC contribution key is then combined with the reply key to form the ticket session key of the returned ticket. These two keys are then combined using the KRB-FX-CF2 operation defined in , where K1 is the KDC contribution key, K2 is the reply key, the input pepper1 is American Standard Code for Information Interchange (ASCII) string "PKINIT", and the input pepper2 is ASCII string "KEYEXCHANGE".

PA_PKINIT_KX 147 -- padata for PKINIT that contains an encrypted -- KDC contribution key. PA-PKINIT-KX ::= EncryptedData -- EncryptionKey -- Contains an encrypted key randomly -- generated by the KDC (known as the KDC contribution key). -- Both EncryptedData and EncryptionKey are defined in [RFC4120] The PA_PKINIT_KX padata MUST be included in the KDC reply when anonymous PKINIT is used; it SHOULD be included if PKINIT is used with the Diffie-Hellman key exchange but the client is not anonymous; it MUST NOT be included otherwise (e.g., when PKINIT is used with the public key encryption as the key exchange). The padata-value field of the PA-PKINIT-KX type padata contains the DER encoding of the Abstract Syntax Notation One (ASN.1) type PA-PKINIT-KX. The PA-PKINIT-KX structure is an EncryptedData. The cleartext data being encrypted is the DER-encoded KDC contribution key randomly generated by the KDC. The encryption key is the reply key and the key usage number is KEY_USAGE_PA_PKINIT_KX (44). The client then decrypts the KDC contribution key and verifies the ticket session key in the returned ticket is the combined key of the KDC contribution key and the reply key as described above. A conforming client MUST reject anonymous PKINIT authentication if the PA_PKINIT_KX padata is not present in the KDC reply or if the ticket session key of the returned ticket is not the combined key of the KDC contribution key and the reply key when PA-PKINIT-KX is present in the KDC reply. This protocol provides a binding between the party which generated the session key and the DH exchange used to generate they reply key. Hypothetically, if the KDC did not use PA-PKINIT-KX, the client and KDC would perfrom a DH key exchange to determine a shared key, and that key would be used as a reply key. The KDC would then generate a ticket with a session key encrypting the reply with the DH agreement. A MITM attacker would just decrypt the session key + ticket using the DH key from the attacker and KDC DH exchange, and re-encrypt it using the key from the attacker and client DH exchange, while keeping a copy of the session key and ticket. By requiring the session key in a way that can be verified by the client, this protocol binds the ticket to the DH exchange and prevents the MITM attack.

KRB-FX-CF2() combines two protocol keys based on the pseudo-random() function defined in . Given two input keys, K1 and K2, where K1 and K2 can be of two different enctypes, the output key of KRB-FX-CF2(), K3, is derived as follows:

KRB-FX-CF2(protocol key, protocol key, octet string, octet string) -> (protocol key) PRF+(K1, pepper1) -> octet-string-1 PRF+(K2, pepper2) -> octet-string-2 KRB-FX-CF2(K1, K2, pepper1, pepper2) -> random-to-key(octet-string-1 ^ octet-string-2) Where ^ denotes the exclusive-OR operation. PRF+() is defined as follows:

PRF+(protocol key, octet string) -> (octet string) PRF+(key, shared-info) -> pseudo-random( key, 1 || shared-info ) || pseudo-random( key, 2 || shared-info ) || pseudo-random( key, 3 || shared-info ) || ... Here the counter value 1, 2, 3, and so on are encoded as a one-octet integer. The pseudo-random() operation is specified by the enctype of the protocol key. PRF+() uses the counter to generate enough bits as needed by the random-to-key() function for the encryption type specified for the resulting key; unneeded bits are removed from the tail.

Since KDCs ignore unknown options, a client requiring anonymous communication needs to make sure that the returned ticket is actually anonymous. This is because a KDC that does not understand the anonymous option would not return an anonymous ticket. By using the mechanism defined in this specification, the client does not reveal the client's identity to the server but the client identity may be revealed to the KDC of the server principal (when the server principal is in a different realm than that of the client), and any KDC on the cross-realm authentication path. The Kerberos client MUST verify the ticket being used is indeed anonymous before communicating with the server, otherwise, the client's identity may be revealed unintentionally. In cases where specific server principals must not have access to the client's identity (for example, an anonymous poll service), the KDC can define server-principal-specific policy that ensures any normal service ticket can NEVER be issued to any of these server principals. If the KDC that issued an anonymous ticket were to maintain records of the association of identities to an anonymous ticket, then someone obtaining such records could breach the anonymity. Additionally, the implementations of most (for now all) KDC's respond to requests at the time that they are received. Traffic analysis on the connection to the KDC will allow an attacker to match client identities to anonymous tickets issued. Because there are plaintext parts of the tickets that are exposed on the wire, such matching by a third-party observer is relatively straightforward. A service that is authenticated by the anonymous principals may be able to infer the identity of the client by examining and linking quasi-static protocol information such as the IP address from which a request is received, or by linking multiple uses of the same anonymous ticket. Two mechanisms, the FAST facility with the hide-client-names option in and the Kerberos5 starttls option , protect the client identity so that an attacker would never be able to observe the client identity sent to the KDC. Transport or network layer security between the client and the server will help prevent tracking of a particular ticket to link a ticket to a user. In addition, clients can limit how often a ticket is reused to minimize ticket linking. The client's real identity is not revealed when the client is authenticated as the anonymous principal. Application servers MAY reject the authentication in order to, for example, prevent information disclosure or as part of Denial of Service (DoS) prevention. Application servers MUST avoid accepting anonymous credentials in situations where they must record the client's identity; for example, when there must be an audit trail.

JK Jaganathan helped editing early revisions of this document. Clifford Neuman contributed the core notions of this document. Ken Raeburn reviewed the document and provided suggestions for improvements. Martin Rex wrote the text for GSS-API considerations. Nicolas Williams reviewed the GSS-API considerations section and suggested ideas for improvements. Sam Hartman and Nicolas Williams were great champions of this work. Miguel Garcia and Phillip Hallam-Baker reviewed the document and provided helpful suggestions. In addition, the following individuals made significant contributions: Jeffrey Altman, Tom Yu, Chaskiel M Grundman, Love Hornquist Astrand, Jeffrey Hutzelman, and Olga Kornievskaia.

This document defines a new 'anonymous' Kerberos well-known name and a new 'anonymous' Kerberos well-known realm based on . IANA has added these two values to the Kerberos naming registries that are created in .