Network Working Group B Lloyd Request for Comments: Draft L&A W A Simpson Daydreamer December 1991 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-ppp@ucdavis.edu mailing list. 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, which allows negotiation of an Authentication Protocol for authenticating its peer 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 [Page i] DRAFT PPP Authentication December 1991 1. Introduction PPP has three main components: 1. A method for encapsulating datagrams over serial links. 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. 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 the Establishment phase. 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 protocol using the Authentication-Protocol Configuration Option 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. Specification 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 Lloyd & Simpson [Page 1] DRAFT PPP Authentication December 1991 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. 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 peer agrees to the authentication protocol to be used in the Configure-Ack during Link Establishment phase. 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. Lloyd & Simpson [Page 2] DRAFT PPP Authentication December 1991 2. Password Authentication Protocol The Password Authentication Protocol (PAP) may be used by the peer to verify its identity. 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. PAP is not a strong authentication method. Passwords are sent over the circuit "in the clear", and there is no protection from playback or repeated trial and error attacks. The peer is in control of the frequency and timing of the attempts. 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 usual user login at the remote host. Any implementations which include a stronger authentication method (such as CHAP, described below) MUST offer to negotiate that method prior to PAP. Lloyd & Simpson [Page 3] DRAFT PPP Authentication December 1991 2.1. 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 [Page 4] DRAFT PPP Authentication December 1991 2.2. 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 [Page 5] DRAFT PPP Authentication December 1991 2.2.1. Authenticate-Req Description The Authenticate-Req packet is used to begin the Password Authentication Protocol. The link peer MUST transmit a PAP packet with the Code field set to 1 (Authenticate-Req) during the Authentication phase. The Authenticate-Req packet must be repeated until a valid reply packet is received, or an optional retry counter expires. The authenticator 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 authenticator 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 MUST be changed each time an Authenticate-Req packet is issued. Lloyd & Simpson [Page 6] DRAFT PPP Authentication December 1991 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 [Page 7] DRAFT PPP Authentication December 1991 2.2.2. 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). If the Peer-ID/Password pair received in a Authenticate-Req is not recognizable or acceptable, then the authenticator SHOULD transmit a PAP packet with the Code field set to 3 (Authenticate-Nak), and take action to terminate the link. 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. The ASCII message should not be NUL or CR/LF terminated. Lloyd & Simpson [Page 8] DRAFT PPP Authentication December 1991 3. Challenge Handshake Authentication Protocol The Challenge Handshake Authentication Protocol (CHAP) may be used to verify the identity of the peer. After the Link Establishment phase is complete, the authenticator sends a challenge to the peer, which includes a randomly generated "challenge" value. The peer responds with a special calculated value called a "message digest". The authenticator checks the response value against its own calculation of the expected digest value. This digest depends upon a "secret" known only to the authenticator and that peer. The digest value is calculated over a stream of octets consisting of the packet identifier, followed by (concatenated with) the secret, followed by (concatenated with) the challenge value. The message digest algorithm is chosen such that it is computationally infeasable to determine the secret when the identifier, the challenge and the digest are known. CHAP provides protection against playback attack through the use of an incrementally changing identifier and a randomly varying challenge value. It is expected that the algorithm used to generate the challenge value will have a periodicity which is not a multiple of the identifier period. This authentication method is most likely used where the same secret is easily maintained at both ends of the link. Lloyd & Simpson [Page 9] DRAFT PPP Authentication December 1991 3.1. 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 | Message ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 3 Length 6 Authentication-Protocol c223 (hex) for Challenge Handshake Authentication Protocol. Digest The Digest field is one octet and indicates the message digest calculation 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 Message The Message field is zero or more octets. The ASCII message Lloyd & Simpson [Page 10] DRAFT PPP Authentication December 1991 should not be NUL or CR/LF terminated. The field is terminated by the Length field. This optional field is implementation dependent. For example, it MAY indicate the location for a callback, such as a phone number or name. Lloyd & Simpson [Page 11] DRAFT PPP Authentication December 1991 3.2. 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 challenges, responses 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 [Page 12] DRAFT PPP Authentication December 1991 3.2.1. Challenge and Response Description The Challenge packet is used to begin the Challenge Handshake Authentication Protocol. The authenticator MUST transmit a CHAP packet with the Code field set to 1 (Challenge). The Challenge packet must be reissued until a valid Response packet is received, or an optional retry counter expires. 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 peer SHOULD expect Challenge packets during the Authentication phase and the Network-Layer Protocol phase. Whenever a Challenge packet is received, the peer MUST transmit a CHAP packet with the Code field set to 2 (Response). Whenever a Response packet is received, the authenticator compares the Response Value with its own calculation of the expected 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 ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Name ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Code 1 for Challenge; 2 for Response. Identifier The Identifier field is one octet. The Identifier field MUST be changed each time a Challenge is issued. Lloyd & Simpson [Page 13] DRAFT PPP Authentication December 1991 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 random stream of octets. The Challenge Value MUST be changed each time a Challenge is issued. The length of the Challenge Value depends upon the method used to generate the octets, and SHOULD be independent of the digest method used. The Response Value is the message digest value (described above). The length of the Response Value depends upon the digest method used. 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. The Name should not be NUL or CR/LF terminated. The field is terminated by the Length field. Since CHAP may be used to authenticate many different systems, the content of the name field(s) 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 [Page 14] DRAFT PPP Authentication December 1991 3.2.2. Success and Failure Description If the Value received in a Response is equal to the expected value, then the implementation MUST transmit a CHAP packet with the Code field set to 3 (Success). If the Value received in a Response is not equal to the expected value, then the implementation SHOULD transmit a CHAP packet with the Code field set to 4 (Failure), and 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 3 for Success; 4 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. The ASCII message should not be NUL or CR/LF terminated. The field is terminated by the Length field. Lloyd & Simpson [Page 15] DRAFT PPP Authentication December 1991 Security Considerations Security issues are the primary topic of this RFC. The LCP state machine may renegotiate the authentication protocol at any time. It is recommended that any counters used for authentication failure not be reset until after successful authentication, or subsequent termination of the failed link. Distribution and management of passwords and other secret values should take place in a secure fashion. This topic is currently undergoing research and experimentation. SNMP Security Protocols [4] should be used where applicable. That document also has an excellent overview of threats to network protocols. References [1] Simpson, W. A., "The Point-to-Point Protocol", RFC in progress. [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 progress. [4] Galvin, J., K. McCloghrie, and J. Davin, "SNMP Security Protocols", RFC in progress. Acknowledgments Some of the text in this document is taken from RFC 1172, by Drew Perkins of Carnegie Mellon University, and by Russ Hobby of the University of California at Davis. Special thanks to Dave Balenson, Steve Crocker, and James Galvin, for their extensive explanations and suggestions. 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 Lloyd & Simpson [Page 16] DRAFT PPP Authentication December 1991 EMail: brian@ray.lloyd.com Author's Address Questions about this memo can also be directed to: William Allen Simpson Daydreamer Computer Systems Consulting Services P O Box 6205 East Lansing, MI 48826-6025 EMail: Bill_Simpson@um.cc.umich.edu Lloyd & Simpson [Page 17] DRAFT PPP Authentication December 1991 Table of Contents 1. Introduction .......................................... 1 1.1 Specification Requirements ...................... 1 1.2 Terminology ..................................... 2 2. Password Authentication Protocol ...................... 3 2.1 Configuration Option Format ..................... 4 2.2 Packet Format ................................... 5 2.2.1 Authenticate-Req ................................ 6 2.2.2 Authenticate-Ack and Authenticate-Nak ........... 8 3. Challenge Handshake Authentication Protocol ........... 9 3.1 Configuration Option Format ..................... 10 3.2 Packet Format ................................... 12 3.2.1 Challenge and Response .......................... 13 3.2.2 Success and Failure ............................. 15 SECURITY CONSIDERATIONS ...................................... 16 REFERENCES ................................................... 16 ACKNOWLEDGEMENTS .......................................... 16 CHAIR'S ADDRESS .............................................. 16 AUTHOR'S ADDRESS ............................................. 17