Network Working Group L J Blunk J R Vollbrecht Internet Draft Merit expires in six months March 1994 PPP Kerberos Authentication Protocol (KAP) Status of this Memo This document is the product of the Point-to-Point Protocol Extensions Working Group of the Internet Engineering Task Force (IETF). Comments should be submitted to the ietf-ppp@merit.edu mailing list. Distribution of this memo is unlimited. This document is an Internet Draft. Internet Drafts are working documents of the Internet Engineering Task Force (IETF), its Areas, and its Working Groups. Note that other groups may also distribute working documents as Internet Drafts. Internet Drafts are draft documents valid for a maximum of six months. Internet Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as a ``working draft'' or ``work in progress.'' Please check the 1id-abstracts.txt listing contained in the internet-drafts Shadow Directories on nic.ddn.mil, ds.internic.net, ftp.isi.edu, nic.nordu.net, or munnari.oz.au to learn the current status of any Internet Draft. Abstract The Point-to-Point Protocol (PPP) [1] provides a standard method for transporting multi-protocol datagrams over point-to-point links. PPP also defines an extensible Link Control Protocol, which allows negotiation of an Authentication Protocol for authenticating its peer before allowing Network Layer protocols to transmit over the link. This document defines the Kerberos Authentication Protocol. Blunk & Vollbrecht expires in six months [Page i] DRAFT PPP KAP March 1994 1. Introduction In order to establish communications over a point-to-point link, each end of the PPP link must first send LCP packets to configure the data link during Link Establishment phase. After the link has been established, PPP provides for an optional Authentication phase before proceeding to the Network-Layer Protocol phase. By default, authentication is not mandatory. If authentication of the link is desired, an implementation MUST specify the Authentication-Protocol Configuration Option during Link Establishment phase. These authentication protocols are intended for use primarily by hosts and routers that connect to a PPP network server via switched circuits or dial-up lines, but might be applied to dedicated links as well. The server can use the identification of the connecting host or router in the selection of options for network layer negotiations. This document defines the PPP KAP authentication protocol. The Link Establishment and Authentication phases, and the Authentication- Protocol Configuration Option, are defined in The Point-to-Point Protocol (PPP) [1]. 1.1. Specification of Requirements In this document, several words are used to signify the requirements of the specification. These words are often capitalized. MUST This word, or the adjective "required", means that the definition is an absolute requirement of the specification. MUST NOT This phrase means that the definition is an absolute prohibition of the specification. SHOULD This word, or the adjective "recommended", means that there may exist valid reasons in particular circumstances to ignore this item, but the full implications must be understood and carefully weighed before choosing a different course. MAY This word, or the adjective "optional", means that this item is one of an allowed set of alternatives. An implementation which does not include this option MUST be prepared to interoperate with another implementation which does include the option. Blunk & Vollbrecht expires in six months [Page 1] DRAFT PPP KAP March 1994 1.2. Terminology This document frequently uses the following terms: authenticator The end of the link requiring the authentication. The authenticator specifies the authentication protocol to be used in the Configure-Request during Link Establishment phase. peer The other end of the point-to-point link; the end which is being authenticated by the authenticator. silently discard This means the implementation discards the packet without further processing. The implementation SHOULD provide the capability of logging the error, including the contents of the silently discarded packet, and SHOULD record the event in a statistics counter. Blunk & Vollbrecht expires in six months [Page 2] DRAFT PPP KAP March 1994 2. Kerberos Authentication Protocol The Kerberos Authentication Protocol (KAP) is used to verify the identity of a peer using the Kerberos Authentication System. Normally, Kerberos authentication requires that the peer be on a fully-connnected network. Given the nature of PPP, it is unlikely that the peer being authenticated will have such connectivity. Given this constraint, the KAP protocol is designed to operate over the PPP link prior to any Network Protocols having been negotiated. It is assumed that the authenticator has network connectivity to the Kerberos server(s) to be used for authentication. 1. After the Link Establishment phase is complete, the authenticator sends a request for the identity of the the peer. 2. The peer replys with a Kerberos principal to be used by the authenticator to retrieve Kerberos credentials. The peer does not specify the service principal identity. However, it is recommended that the authenticator use ppp-kap as its primay name so that the peer may check this value in the credentials. This gives the peer some assurance that it is authenticating to a service authorized to provide PPP access. The peer will not likely need to know or verify the instance name of the authenticator. 3. The authenticator then uses the Kerberos Authentication Service Protocol to request the Kerberos credentials to be used by the peer. These credentials are then forwarded to the peer together with a challenge. The challenge is used to prevent replay attacks in the case a ticket may have been captured. 4. The peer decrypts these credentials to obtain a ticket for the ppp-kap service and a session key. The session key is used to encrypt the challenge. The peer then sends the ticket together with the response to the challenge back to the authenticator. 5. The authenticator then decrypts the ticket and checks the response to its challenge. If these are valid, the authentication is acknowledged with a success message; otherwise the authenticator replies with a failure response. 6. At random intervals, the authenticator may send a challenge to the peer. The peer generates a response to the challenge by encrypting it in the session key and forwards this back to the authenticator. Likewise, the peer may send a challenge to the authenticator. This assures the identity of the authenticator to the peer. Blunk & Vollbrecht expires in six months [Page 3] DRAFT PPP KAP March 1994 Advantages KAP provides an authentication mechanism which does not require passing a user's password in the clear. It also allows PPP authentication to leverage off of existing Kerberos authentication servers. The authenticator is not the repository for any secrets. This is a significant advantage when there are many peers. The peer need only know its own secret and the primary name of the service requesting authentication. This authentication can be mutual. The ability of the peer to successfully decrypt the Kerberos credentials and generate a response to the challenge is the assurance to the authenticator that the peer is authentic. The ability of the authenticator to encrypt a response to a challenge using the session key is the assurance to the peer that the authenticator is also authentic. Disadvantages KAP depends upon encryption technology which may be subject to export controls. Blunk & Vollbrecht expires in six months [Page 4] DRAFT PPP KAP March 1994 2.1. Configuration Option Format A summary of the Authentication-Protocol Configuration Option format to negotiate the Kerberos Authentication Protocol is shown below. The fields are transmitted from left to right. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Authentication-Protocol | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Kerberos Type | +-+-+-+-+-+-+-+-+ Type 3 Length 5 Authentication-Protocol ???? for Kerberos Authentication Protocol Kerberos Type 0-3 Unused 4 Kerberos V4 5 Kerberos V5 Blunk & Vollbrecht expires in six months [Page 5] DRAFT PPP KAP March 1994 2.2. Packet Format Exactly one Kerberos Authentication Protocol packet is encapsulated in the Information field of a PPP Data Link Layer frame where the protocol field indicates type hex ???? (Kerberos Authentication Protocol). A summary of the KAP packet format is shown below. The fields are transmitted from left to right. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Data ... +-+-+-+-+ Code The Code field is one octet and identifies the type of KAP packet. KAP Codes are assigned as follows: 1 Initiate 2 Principal 3 Credentials 4 Ticket 5 Success 6 Failure 7 Challenge 8 Response Identifier The Identifier field is one octet and aids in matching replies with requests and responses to challenges. Length The Length field is two octets and indicates the length of the KAP packet including the Code, Identifier, Length and Data fields. Octets outside the range of the Length field should be treated as Data Link Layer padding and should be ignored on reception. Data The Data field is zero or more octets. The format of the Data field is determined by the Code field. Blunk & Vollbrecht expires in six months [Page 6] DRAFT PPP KAP March 1994 2.2.1. Initiate Description The Initiate packet is used to begin the Kerberos Authentication Protocol. The authenticator MUST transmit a KAP packet with the Code field set to 1 (Initiate). Additional Initiate packets MUST be sent until a valid Response packet is received, or an optional retry counter expires. A summary of the Initiate packet format is shown below. The fields are transmitted from left to right. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Code 1 Identifier The Identifier field is one octet. The Identifier field MUST be changed each time an Initiate is sent. Blunk & Vollbrecht expires in six months [Page 7] DRAFT PPP KAP March 1994 2.2.2. Principal Description The Principal packet is used to identify the peer to the authenticator. It is sent in response to an Initiate packet. The peer MUST transmit a KAP packet with the Code field set to 2 (Principal). The identifier of the Principal packet MUST match the indentifier in the Initiate packet. The authenticator will respond to the Principal packet with either a Credentials packet or a Failure packet. A summary of the Principal packet format is shown below. The fields are transmitted from left to right. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Name-Size | Name ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Instance-Size | Instance ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Realm-Size | Realm ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Code 2 Identifier The Identifier field is one octet. The Principal Identifier MUST be copied from the Identifier field of the Initiate which caused the Response. Principal fields The Name, Instance, and Realm fields are used to identify the Kerberos Principal. Blunk & Vollbrecht expires in six months [Page 8] DRAFT PPP KAP March 1994 2.2.3. Credentials Description The Credentials packet is used to supply the peer with the needed credentials for authentication. It is sent in response to a Principal packet and contains a ticket for the authenticator and a session key. The authenticator MUST transmit a KAP packet with the Code field set to 3 (Credentials). Additional Credentials packets MUST be sent until a valid Response packet is received, or an optional retry counter expires. A summary of the Credentials packet format is shown below. The fields are transmitted from left to right. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cred-Size | Credentials ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Chal-Size | Challenge ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Code 3 Identifier The Identifier field is one octet. The Identifier field MUST be changed each time a Credentials is sent. Credentials The Credentials field is copied from the KRB_AS_REP packet received by the authenticator from the Kerberos server. It takes the form: { T_(peer,authenticator), K_(peer, authenticator) }K_peer Challenge The Challenge Value is a variable stream of octets. The Challenge Value MUST be changed each time a Credentials is sent. Blunk & Vollbrecht expires in six months [Page 9] DRAFT PPP KAP March 1994 2.2.4. Ticket Description The Ticket packet is sent by the peer to the authenticator as verification of its authenticity. It is sent in response to a Credentials packet and contains a ticket for the authenticator and a response to the authenticator's challenge. The response for the challenge is generated by encrypting the challenge with the session key. The peer MUST transmit a KAP packet with the Code field set to 4 (Ticket). A summary of the Ticket packet format is shown below. The fields are transmitted from left to right. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Ticket-Size | Ticket ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Response-Size | Response ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Code 4 Identifier The Identifier field is one octet. The Identifier field MUST match the Indentifier field of the Credentials packet that it is sent in response to. Ticket The Ticket field is copied from the Credentials data received from the authenticator. It takes the form: T_(peer,authenticator) Response The response field is generated by encrypting the challenge from the Credentials packets using the Session key. Blunk & Vollbrecht expires in six months [Page 10] DRAFT PPP KAP March 1994 2.2.5. Success and Failure Description If the fields received in a Ticket match the expected values AND the response to the challenge is correct, then the implementation MUST transmit a KAP packet with the Code field set to 5 (Success). If the fields received in a Ticket do NOT match the expected values OR the response to the challenge does NOT match the expected value, then the implementation MUST transmit a KAP packet with the Code field set to 6 (Failure), and SHOULD take action to terminate the link. A summary of the Success and Failure packet format is shown below. The fields are transmitted from left to right. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Message ... +-+-+-+-+-+-+-+-+-+-+-+-+- Code 5 for Success; 6 for Failure. Identifier The Identifier field is one octet and aids in matching requests and replies. The Identifier field MUST be copied from the Identifier field of the Principal or Ticket packet which caused this reply. Message The Message field is zero or more octets, and its contents are implementation dependent. It is intended to be human readable, and MUST NOT affect operation of the protocol. It is recommended that the message contain displayable ASCII characters 32 through 126 decimal. Mechanisms for extension to other character sets are the topic of future research. The size is determined from the Length field. Blunk & Vollbrecht expires in six months [Page 11] DRAFT PPP KAP March 1994 2.2.6. Challenge and Response Description The Challenge packet is used to periodically authenticate one peer to the other. The peer may use the Challenge to mutually authenticate the authenticator. The Challenge must be transmitted as a KAP packet with the Code field set to 7 (Challenge). A Challenge packet MAY be transmitted at any time during the Network-Layer Protocol phase to ensure that the connection has not been altered. The peer or authenticator SHOULD expect Challenge packets during the Authentication phase and the Network-Layer Protocol phase. Whenever a Challenge packet is received, a KAP packet with the Code field set to 8 (Response) must be transmitted. Whenever a Response packet is received, the Response Value is compared with the expected value. Based on this comparison, a Success or Failure packet (described above) must be sent. Implementation Note: Because the Success might be lost, the authenticator MUST allow repeated Response packets after completing the Authentication phase. To prevent discovery of alternative Names and Secrets, any Response packets received having the current Challenge Identifier MUST return the same reply Code returned when the Authentication phase completed (the message portion MAY be different). Any Response packets received during any other phase MUST be silently discarded. When the Failure is lost, and the authenticator terminates the link, the LCP Terminate-Request and Terminate-Ack provide an alternative indication that authentication failed. Blunk & Vollbrecht expires in six months [Page 12] DRAFT PPP KAP March 1994 A summary of the Challenge and Response packet format is shown below. The fields are transmitted from left to right. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Value-Size | Value ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Code 7 for Challenge; 8 for Response. Identifier The Identifier field is one octet. The Identifier field MUST be changed each time a Challenge is sent. The Response Identifier MUST be copied from the Identifier field of the Challenge which caused the Response. Value-Size This field is one octet and indicates the length of the Value field. Value The Value field is one or more octets. The most significant octet is transmitted first. The Challenge Value is a variable stream of octets. The Challenge Value MUST be changed each time a Challenge is sent. The length of the Challenge Value depends upon the method used to generate the octets. The Response Value is the DES encryption of the Challenge Value. The Session Key is used to perform the encryption on the Challenge. The length of the Response Value depends upon the length of the Challenge. Blunk & Vollbrecht expires in six months [Page 13] DRAFT PPP KAP March 1994 Security Considerations Security issues are the primary topic of this RFC. The interaction of the authentication protocols within PPP are highly implementation dependent. This is indicated by the use of SHOULD throughout the document. For example, upon failure of authentication, some implementations do not terminate the link. Instead, the implementation limits the kind of traffic in the Network-Layer Protocols to a filtered subset, which in turn allows the user opportunity to update secrets or send mail to the network administrator indicating a problem. There is no provision for re-tries of failed authentication. However, the LCP state machine can renegotiate the authentication protocol at any time, thus allowing a new attempt. It is recommended that any counters used for authentication failure not be reset until after successful authentication, or subsequent termination of the failed link. There is no requirement that authentication be full duplex or that the same protocol be used in both directions. It is perfectly acceptable for different protocols to be used in each direction. This will, of course, depend on the specific protocols negotiated. In practice, within or associated with each PPP server, there is a database which associates "user" names with authentication information ("secrets"). It is not anticipated that a particular named user would be authenticated by multiple methods. This would make the user vulnerable to attacks which negotiate the least secure method from among a set (such as PAP rather than KAP). Instead, for each named user there should be an indication of exactly one method used to authenticate that user name. If a user needs to make use of different authentication methods under different circumstances, then distinct user names SHOULD be employed, each of which identifies exactly one authentication method. Blunk & Vollbrecht expires in six months [Page 14] DRAFT PPP KAP March 1994 References [1] Simpson, W. A., "The Point-to-Point Protocol (PPP)", work in progress. [2] Reynolds, J., and J. Postel, "Assigned Numbers", RFC 1340, USC/Information Sciences Institute, July 1992. Acknowledgments Thanks to Bill Simpson for his initial work on this document. Chair's Address The working group can be contacted via the current chair: Fred Baker Advanced Computer Communications 315 Bollay Drive Santa Barbara, California, 93111 EMail: fbaker@acc.com Author's Address Questions about this memo can also be directed to: Larry J Blunk John R Vollbrecht EMail: ljb@merit.edu EMail: jrv@merit.edu Blunk & Vollbrecht expires in six months [Page 15] DRAFT PPP KAP March 1994 Table of Contents 1. Introduction .......................................... 1 1.1 Specification of Requirements ................... 1 1.2 Terminology ..................................... 2 2. Kerberos Authentication Protocol ...................... 3 2.1 Configuration Option Format ..................... 5 2.2 Packet Format ................................... 6 2.2.1 Initiate ........................................ 7 2.2.2 Principal ....................................... 8 2.2.3 Credentials ..................................... 9 2.2.4 Ticket .......................................... 10 2.2.5 Success and Failure ............................. 11 2.2.6 Challenge and Response .......................... 12 SECURITY CONSIDERATIONS ...................................... 14 REFERENCES ................................................... 15 ACKNOWLEDGEMENTS ............................................. 15 CHAIR'S ADDRESS .............................................. 15 AUTHOR'S ADDRESS ............................................. 15