Network Working Group B. Lloyd Request for Comments: Draft Telebit W A Simpson CSCS October 1991 The PPP Authentication Protocols Status of this Memo This proposal is the product of the Point-to-Point Protocol Working Group of the Internet Engineering Task Force (IETF). Comments on this memo should be submitted to the IETF Working Group at ietf- ppp@ucdavis.edu. Distribution of this memo is unlimited. Abstract The Point-to-Point Protocol (PPP) [1] provides a standard method of encapsulating Network Layer protocol information over point-to-point links. PPP also defines an extensible Link Control Protocol, and provides for Authentication before allowing Network Layer protocols to transmit over the link. This document defines two protocols for Authentication: the simple Password Authentication Protocol, and the Challenge Handshake Authentication Protocol. Lloyd & Simpson FORMFEED[Page i] RFC DRAFT PPP Authentication October 1991 1. Introduction PPP has three main components: 1. A method for encapsulating datagrams over serial links. PPP uses HDLC as a basis for encapsulating datagrams over point- to-point links. At this time, PPP specifies the use of asynchronous or synchronous duplex circuits, either dedicated or circuit switched. 2. A Link Control Protocol (LCP) for establishing, configuring, and testing the data-link connection. 3. A family of Network Control Protocols (NCPs) for establishing and configuring different network-layer protocols. PPP is designed to allow the simultaneous use of multiple network- layer protocols. In order to establish communications over a point-to-point link, the originating PPP would first send LCP packets to configure and test the data link. After the link has been established, PPP provides for an optional Authentication phase before proceeding to the Network- Layer Protocol phase. If an implementation requires that the peer authenticate with some specific authentication protocol, then it must negotiate the use of that authentication protocol during Link Establishment phase. These authentication protocols are intended for use primarily by hosts and routers that connect via switched circuits or dial-up lines to a PPP network server. The server can then use the identification of the connecting host or router in the selection of options for network layer negotiations. When failing authentication, the server should terminate the connection. 1.1. 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 Lloyd & Simpson FORMFEED[Page 1] RFC DRAFT PPP Authentication October 1991 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 should 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 none-the-less be prepared to interoperate with another implementation which does include the option. 1.2. Terminology This document frequently uses the following terms: authenticator The end of the link requiring the authentication. peer The other end of the point-to-point link. silently discard This means the implementation MUST discard the packet without further processing. However, for diagnosis of problems, 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. 2. Management of Keys Distribution and management of Passwords and other Key values .... EDITOR'S NOTE: We need some text for this section. Lloyd & Simpson FORMFEED[Page 2] RFC DRAFT PPP Authentication October 1991 3. Password Authentication Protocol The Password Authentication Protocol (PAP) may be used to verify the identity of the other end of the link. After the Link Establishment phase is complete, an Id/Password pair is repeatedly sent by the peer to the authenticator until authentication is acknowledged or the connection is terminated. Note that PAP is not a strong authentication method. Passwords are passed over the circuit in the clear and there is no protection from playback or repeated trial and error attacks. It is strongly recommended that any implementations which negotiate an Authentication-Protocol Configuration Option offer another authentication method prior to PAP. EDITOR'S NOTE: Some would change "strongly recommended" to REQUIRED. This authentication method is most likely used where the plaintext password must be available to simulate a login at a remote host. In such use, the method is no less secure than the normal user login at the remote host. Lloyd & Simpson FORMFEED[Page 3] RFC DRAFT PPP Authentication October 1991 3.1. Configuration Option Format A summary of the Authentication-Protocol Configuration Option format to negotiate the Password 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 3 Length 4 Authentication-Protocol c023 (hex) for Password Authentication Protocol. Data There is no Data field. Lloyd & Simpson FORMFEED[Page 4] RFC DRAFT PPP Authentication October 1991 3.2. Packet Format Exactly one Password Authentication Protocol packet is encapsulated in the Information field of PPP Data Link Layer frames where the protocol field indicates type hex c023 (Password Authentication Protocol). A summary of the PAP 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 PAP packet. PAP Codes are assigned as follows: 1 Authenticate-Req 2 Authenticate-Ack 3 Authenticate-Nak Identifier The Identifier field is one octet and aids in matching requests and replies. Length The Length field is two octets and indicates the length of the PAP 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. Lloyd & Simpson FORMFEED[Page 5] RFC DRAFT PPP Authentication October 1991 3.3. Authenticate-Req Description The Authenticate-Req packet is used to begin the Password Authentication Protocol. An implementation having sent a LCP Configure-Ack packet with an Authentication-Protocol Configuration Option specifying the Password Authentication Protocol must send an Authenticate-Req packet during the Authentication phase. The Authenticate-Req packet must be repeated until a valid reply packet is received, or an optional counter expires. An implementation receiving a Configure-Ack with said Configuration Option should expect the peer to send an Authenticate-Req packet. Upon reception of an Authenticate-Req packet, some type of Authenticate reply (described below) MUST be returned. Note: Because the reply might be lost, the protocol MUST allow repeated Authenticate-Req packets after completing the Authentication phase. To prevent discovery of alternative Identities and Passwords, any Authenticate-Req packets received during the Network-Layer Protocol phase MUST return the same reply returned when the Authentication phase completed. Any Authenticate-Req packets received during any other phase MUST be silently discarded. A summary of the Authenticate-Req 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Peer-ID Length| Peer-Id ... +-+-+-+-+-+-+-+-+-+-+-+-+ | Passwd-Length | Password ... +-+-+-+-+-+-+-+-+-+-+-+-+-+ Code 1 for Authenticate-Req. Identifier The Identifier field is one octet and aids in matching requests and replies. The Identifier field SHOULD be changed each time an Lloyd & Simpson FORMFEED[Page 6] RFC DRAFT PPP Authentication October 1991 Authenticate-Req packet is transmitted. Peer-ID-Length The Peer-ID-Length field is one octet and indicates the length of the Peer-ID field Peer-ID The Peer-ID field is zero or more octets and indicates the name of the peer to be authenticated. Passwd-Length The Passwd-Length field is one octet and indicates the length of the Password field. Password The Password field is zero or more octets and indicates the password to be used for authentication. Lloyd & Simpson FORMFEED[Page 7] RFC DRAFT PPP Authentication October 1991 3.4. Authenticate-Ack and Authenticate-Nak Description If the Peer-ID/Password pair received in an Authenticate-Req is both recognizable and acceptable, then the authenticator MUST transmit a PAP packet with the Code field set to 2 (Authenticate- Ack), the Identifier field copied from the received Authenticate- Req packet, and the Message field optionally filled with an ASCII message. If the Peer-ID/Password pair received in a Authenticate-Req is not recognizable or acceptable, then the authenticator MUST transmit a PAP packet with the Code field set to 3 (Authenticate-Nak), the Identifier field copied from the received Authenticate-Req packet, and the Message field optionally filled with an ASCII message. A summary of the Authenticate-Ack and Authenticate-Nak 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Msg-Length | Message ... +-+-+-+-+-+-+-+-+-+-+-+-+- Code 2 for Authenticate-Ack; 3 for Authenticate-Nak. 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 Authenticate-Req which caused this reply. Msg-Length The Msg-Length field is one octet and indicates the length of the Message field. Message The Message field is zero or more octets and indicates an ASCII Lloyd & Simpson FORMFEED[Page 8] RFC DRAFT PPP Authentication October 1991 message. Lloyd & Simpson FORMFEED[Page 9] RFC DRAFT PPP Authentication October 1991 4. Challenge Handshake Authentication Protocol The Challenge Handshake Authentication Protocol (CHAP) may be used to verify the identity of the other end of the link. After the Link Establishment phase is complete, an Id is sent by the peer to the authenticator, which returns a randomly generated "challenge" value. The peer responds with a challenge checksum of the challenge value paired with a local secret. The authenticator then checks the "response" value against the checksum value of its challenge combined with its copy of the secret. The CHAP authentication method provides protection against playback attack, and substitutions in circuit-switched connections after the initial link establishment. This authentication method is most likely used where the same secret is easily maintained at both ends of the link. Lloyd & Simpson FORMFEED[Page 10] RFC DRAFT PPP Authentication October 1991 4.1. Configuration Option Format A summary of the Authentication-Protocol Configuration Option format to negotiate the Challenge Handshake 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Digest | Callback | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 3 Length 5 Authentication-Protocol c223 (hex) for Challenge Handshake Authentication Protocol. Digest The Digest field is one octet and indicates the challenge method to be used. The most up-to-date values of the CHAP Digest field are specified in the most recent "Assigned Numbers" RFC [2]. Current values are assigned as follows: 0-4 unused (reserved) 5 MD5 [3] Callback The Callback field is one octet containing a flag which indicates that the authenticator will hang up and call back after successful authentication. Current values are assigned as follows: 0 no callback 1 callback Lloyd & Simpson FORMFEED[Page 11] RFC DRAFT PPP Authentication October 1991 4.2. Packet Format Exactly one Challenge Handshake Authentication Protocol packet is encapsulated in the Information field of PPP Data Link Layer frames where the protocol field indicates type hex c223 (Challenge Handshake Authentication Protocol). A summary of the CHAP 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 CHAP packet. CHAP Codes are assigned as follows: 1 Challenge 2 Response 3 Success 4 Failure Identifier The Identifier field is one octet and aids in matching requests and replies. Length The Length field is two octets and indicates the length of the CHAP 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. Lloyd & Simpson FORMFEED[Page 12] RFC DRAFT PPP Authentication October 1991 4.3. Challenge and Response Description The Challenge packet is used to begin the Challenge Handshake Authentication Protocol. An implementation having sent a LCP Configure-Ack packet with an Authentication-Protocol Configuration Option specifying the Challenge Handshake Authentication Protocol must expect a Challenge packet during the Authentication phase. An implementation receiving a Configure-Ack with said Configuration Option should transmit a CHAP packet with the Code field set to 1 (Challenge), the Identifier field set, the Value field filled with a randomly generated value, and the Name field filled with the identification of the authenticator. A Challenge packet MAY also be transmitted at any time during the Network-Layer Protocol phase to ensure that the connection has not been altered. The random value MUST be different each time a challenge is issued. Whenever a Challenge packet is received, the implementation MUST transmit a CHAP packet with the Code field set to 2 (Response), the Identifier field copied from the received Challenge packet, and the Value field filled with the challenge checksum of the Challenge Value followed by the local copy of the secret which corresponds to the Name transmitted in the initial Identity packet. Whenever a Response packet is received, the implementation calculates the challenge checksum of its Challenge Value followed by its copy of the secret which corresponds to the Name received in the initial Identity packet, and compares this checksum with the Response Value. Based on this comparison, the authenticator sends a Success or Failure packet (described below). 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 ... (up to 255 octets) +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- | Name ... (remainder of Length) +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- Lloyd & Simpson FORMFEED[Page 13] RFC DRAFT PPP Authentication October 1991 Code 1 for Challenge; 2 for Response. 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 packet which caused this reply. Value The Value field is two or more octets, depending on the digest method negotiated. It contains the values used in the challenge handshake. The most significant octet is transmitted first. Name The Name field is one or more octets representing the identification of the system transmitting the packet. There are no limitations on the content of this field. However, the use of standard ASCII character strings are encouraged. Since CHAP may be used to authenticate many different systems the content of the name field may be used as a key to locate the proper secret in a database of secrets. This also makes it possible to support more than one name/secret pair per system. Lloyd & Simpson FORMFEED[Page 14] RFC DRAFT PPP Authentication October 1991 4.4. Success and Failure Description If the Value received in a Response is acceptable, then the implementation MUST transmit a CHAP packet with the Code field set to 3 (Success), the Identifier field copied from the received Response packet, and the Message field optionally filled with an ASCII message. If the Value received in a Response is not acceptable, then the implementation MAY transmit a CHAP packet with the Code field set to 4 (Failure), the Identifier field copied from the received Response packet, and the Message field optionally filled with an ASCII message. 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 4 for Success; 5 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 Response which caused this reply. Message The Message field is zero or more octets and indicates an ASCII message. Lloyd & Simpson FORMFEED[Page 15] RFC DRAFT PPP Authentication October 1991 References [1] Perkins, D., "The Point-to-Point Protocol for the Transmission of Multi-Protocol of Datagrams Over Point-to- Point Links", RFC 1171, July, 1990. [2] Reynolds, J., and J. Postel, "Assigned Numbers", RFC 1060, USC/Information Sciences Institute, March 1990. [3] Rivest, R., and S. Dusse, "The MD5 Message-Digest Algorithm", RFC in publication Security Considerations Security issues are the primary topic of this RFC. Acknowledgments Some of the text in this document is taken from previous documents produced by the Point-to-Point Protocol Working Group of the Internet Engineering Task Force (IETF), formerly chaired by Drew Perkins of Carnegie Mellon University, by Russ Hobby of the University of California at Davis, and by Steve Knowles of FTP Software. Chair's Address The working group can be contacted via the current chair: Brian Lloyd Lloyd & Associates 3420 Sudbury Road Cameron Park, California 95682 Phone: (916) 676-1147 EMail: brian@ray.lloyd.com Author's Address Questions about this memo can also be directed to: William Allen Simpson Computer Systems Consulting Services Lloyd & Simpson FORMFEED[Page 16] RFC DRAFT PPP Authentication October 1991 P O Box 6205 East Lansing, MI 48826-6025 EMail: Bill_Simpson@um.cc.umich.edu Lloyd & Simpson FORMFEED[Page 17] RFC DRAFT PPP Authentication October 1991 TTaabbllee ooff CCoonntteennttss 1. Introduction .......................................... 1 1.1 Requirements .................................... 1 1.2 Terminology ..................................... 2 2. Management of Keys .................................... 2 3. Password Authentication Protocol ...................... 3 3.1 Configuration Option Format ..................... 4 3.2 Packet Format ................................... 5 3.3 Authenticate-Req ................................ 6 3.4 Authenticate-Ack and Authenticate-Nak ........... 8 4. Challenge Handshake Authentication Protocol ........... 10 4.1 Configuration Option Format ..................... 11 4.2 Packet Format ................................... 12 4.3 Challenge and Response .......................... 13 4.4 Success and Failure .......................... 15 REFERENCES ................................................ 16 SECURITY CONSIDERATIONS ...................................... 16 ACKNOWLEDGEMENTS ............................................. 16 CHAIR'S ADDRESS .............................................. 16 AUTHOR'S ADDRESS ............................................. 16 Lloyd & Simpson FORMFEED[Page ii]