Network Working Group J. Myers Internet Draft April 2001 Document: draft-myers-saslrev-01.txt Simple Authentication and Security Layer (SASL) Status of this Memo This document is an Internet Draft and is in full conformance with all provisions of Section 10 of RFC 2026. 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 ``work in progress''. The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. To learn the current status of any Internet-Draft, please check the 1id-abstracts.txt listing contained in the Internet-Drafts Shadow Directories on ds.internic.net, nic.nordu.net, ftp.isi.edu, or munnari.oz.au. A revised version of this draft document will be submitted to the RFC editor as a Draft Standard for the Internet Community. Discussion and suggestions for improvement are requested. Distribution of this draft is unlimited. J. Myers [Page i] Internet DRAFT SASL April 25, 2001 1. Abstract SASL provides a method for adding authentication support with an optional security layer to connection-based protocols. It also describes a structure for authentication mechanisms. The result is an abstraction layer between protocols and authentication mechanisms such that any SASL-compatible authentication mechanism can be used with any SASL-compatible protocol. This document describes how a SASL authentication mechanism is structured, how a protocol adds support for SASL, defines the protocol for carrying a security layer over a connection, and defines the EXTERNAL SASL authentication mechanism. 2. Organization of this document 2.1. How to read this document This document is written to serve two different audiences, protocol designers using this specification to support authentication in their protocol, and implementors of clients or servers for those protocols using this specification. The sections "Overview", "Authentication Mechanisms", "Protocol Profile Requirements", "Specific Issues", and "Security Considerations" cover issues that protocol designers need to understand and address in profiling this specification for use in a specific protocol. Implementors of a protocol using this specification need the protocol-specific profiling information in addition to the information in this document. 2.2. Conventions used in this document In examples, "C:" and "S:" indicate lines sent by the client and server respectively. The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY" in this document are to be interpreted as defined in "Key words for use in RFCs to Indicate Requirement Levels" [KEYWORDS]. 3. Overview The Simple Authentication and Security Layer (SASL) is a method for adding authentication support to connection-based protocols. The SASL specification has three layers, as indicated in the diagram J. Myers [Page 2] Internet DRAFT SASL April 25, 2001 below. At the top, a protocol definition using SASL specifies a profile, including a command for identifying and authenticating a user to a server and for optionally negotiating a security layer for subsequent protocol interactions. At the bottom, a SASL mechanism definition specifies an authentication mechanism. The SASL framework, specified by this document, constrains the behavior of protocol profiles and mechanisms, separating protocol from mechanism and defining how they interact. SMTP Protocol LDAP Protocol Etc Profile Profile . . . \----- | -----/ \ | / SASL framework / | \ /----- | -----\ EXTERNAL DIGEST-MD5 Etc SASL mechanism SASL mechanism . . . This separation between the definition of protocols and the definition of authentication mechanisms is crucial. It permits an authentication mechanism to be defined once, making it usable by any SASL protocol profiles. In many implementations, the same SASL mechanism code is used for multiple protocols. 4. Authentication mechanisms SASL mechanisms are named by strings, from 1 to 20 characters in length, consisting of upper-case letters, digits, hyphens, and/or underscores. SASL mechanism names must be registered with the IANA. IETF Standards Track documents may override this registration requirement by reserving a portion of the SASL mechanism namespace for their own use; the GSSAPI mechanism specification [SASL-GSSAPI] does this. Procedures for registering new SASL mechanisms are given in the section "Registration procedures". The "sasl-mech" rule below defines the syntax of a SASL mechanism name. This uses the augmented Backus-Naur Form (BNF) notation as specified in [ABNF] and the ABNF core rules as specified in Appendix A of the ABNF specification [ABNF]. sasl-mech = 1*20mech-char mech-char = %x41-5A / DIGIT / "-" / "_" ; mech names restricted to uppercase letters, ; digits, "-" and "_" J. Myers [Page 3] Internet DRAFT SASL April 25, 2001 4.1. Authentication protocol exchange A SASL mechanism is responsible for conducting an authentication protocol exchange. This consists of a series of server challenges and client responses, the contents of which are specific to and defined by the mechanism. To the protocol, the challenges and responses are opaque binary tokens of arbitrary length. The protocol's profile then specifies how these binary tokens are then encoded for transfer over the connection. After receiving an authentication command or any client response, a server mechanism may issue a challenge, indicate failure, or indicate completion. The server mechanism MAY return additional data with a completion indication. The protocol's profile specifies how each of these is then represented over the connection. After receiving a challenge, a client mechanism may issue a response or abort the exchange. The protocol's profile specifies how each of these is then represented over the connection. During the authentication protocol exchange, the mechanism performs authentication, transmits an authorization identity (frequently known as a userid) from the client to server, and negotiates the use of a mechanism-specific security layer. If the use of a security layer is agreed upon, then the mechanism must also define or negotiate the maximum security layer buffer size that each side is able to receive. 4.2. Proxy authentication The identity derived from the client's authentication credentials is known as the "authentication identity". With any mechanism, transmitting an authorization identity of the empty string directs the server to derive an authorization identity from the client's authentication identity. If the authorization identity transmitted during the authentication protocol exchange is not the empty string, this is typically referred to as "proxy authentication". This feature permits agents such as proxy servers to authenticate using their own credentials, yet request the access privileges of the identity for which they are proxying. As a client might not have the same information as the server, clients SHOULD NOT themselves try to derive authorization identities from auhentication identities when clients could instead transmit an authorization identity of the empty string. J. Myers [Page 4] Internet DRAFT SASL April 25, 2001 4.3. Security layers If use of a security layer is negotiated by the authentication protocol exchange, the security layer is applied to all subsequent data sent over the connection. The security layer takes effect immediately following the last response of the authentication exchange for data sent by the client and the completion indication for data sent by the server. Once the security layer is in effect, the protocol stream is processed by the security layer into buffers of security encoded data. Each buffer of security encoded data is transferred over the connection as a stream of octets prepended with a four octet field in network byte order that represents the length of the following buffer. The length of the security encoded data buffer must be no larger than the maximum size that was either defined in the mechanism specification or negotiated by the other side during the authentication protocol exchange. 5. Protocol profile requirements In order to use this specification, a protocol definition MUST supply the following information: 1. A service name, to be selected from the IANA registry of "service" elements for the GSSAPI host-based service name form. [GSSAPI] This service name is made available to the authentication mechanism. The registry is available at the URL "http://www.isi.edu/in-notes/iana/assignments/gssapi-service-names" 2. A definition of the command to initiate the authentication protocol exchange. This command must have as a parameter the name of the mechanism being selected by the client. The command SHOULD have an optional parameter giving an initial response. This optional parameter allows the client to avoid a round trip when using a mechanism which is defined to have the client send data first. When this initial response is sent by the client and the selected mechanism is defined to have the server start with an initial challenge, the command fails. See section 6.1 of this document for further information. 3. A definition of the method by which the authentication protocol exchange is carried out, including how the challenges and responses are encoded, how the server indicates completion or J. Myers [Page 5] Internet DRAFT SASL April 25, 2001 failure of the exchange, how the client aborts an exchange, and how the exchange method interacts with any line length limits in the protocol. The command SHOULD have a method for the server to include an optional challenge with a success notification. This allows the server to avoid a round trip when using a mechanism which is defined to have the server send additional data along with the indication of successful completion. See section 6.2 of this document for further information. 4. Identification of the octet where any negotiated security layer starts to take effect, in both directions. 5. A specification of how the authorization identity passed from the client to the server is to be interpreted. In addition, a protocol definition SHOULD specify a mechanism through which a client may obtain the names of the SASL mechanisms available to it. This is typically done through the protocol's extensions or capabilities mechanism. 6. Specific issues 6.1. Client sends data first Some mechanisms specify that the first data sent in the authentication protocol exchange is from the client to the server. If a protocol's profile permits the command which initiates an authentication protocol exchange to contain an initial client response, this parameter SHOULD be used with such mechanisms. If the initial client response parameter is not given, or if a protocol's profile does not permit the command which initiates an authentication protocol exchange to contain an initial client response, then the server issues a challenge with no data. The client's response to this challenge is then used as the initial client response. (The server then proceeds to send the next challenge, indicates completion, or indicates failure.) 6.2. Server returns success with additional data Some mechanisms may specify that additional data be sent to the client along with an indication of successful completion of the exchange. This data would, for example, authenticate the server to the client. J. Myers [Page 6] Internet DRAFT SASL April 25, 2001 If a protocol's profile does not permit this additional data to be returned with a success indication, then the server issues the data as a server challenge, without an indication of successful completion. The client then responds with no data. After receiving this empty response, the server then indicates successful completion (with no additional data). 6.3. Multiple authentications Unless otherwise stated by the protocol's profile, only one successful SASL negotiation may occur in a protocol session. In this case, once an authentication protocol exchange has successfully completed, further attempts to initiate an authentication protocol exchange fail. In the case that a profile explicitly permits multiple successful SASL negotiations to occur, then in no case may multiple security layers be simultaneously in effect. If a security layer is in effect and a subsequent SASL negotiation selects no security layer, the original security layer remains in effect. If a security layer is in effect and a subsequent SASL negotiation selects a second security layer, then the second security layer replaces the first. 7. Registration procedures Registration of a SASL mechanism is done by filling in the template in section 7.4 and sending it in to iana@iana.org. IANA has the right to reject obviously bogus registrations, but will perform no review of claims made in the registration form. There is no naming convention for SASL mechanisms; any name that conforms to the syntax of a SASL mechanism name can be registered. While the registration procedures do not require it, authors of SASL mechanisms are encouraged to seek community review and comment whenever that is feasible. Authors may seek community review by posting a specification of their proposed mechanism as an internet- draft. SASL mechanisms intended for widespread use should be standardized through the normal IETF process, when appropriate. 7.1. Comments on SASL mechanism registrations Comments on registered SASL mechanisms should first be sent to the "owner" of the mechanism. Submitters of comments may, after a reasonable attempt to contact the owner, request IANA to attach their comment to the SASL mechanism registration itself. If IANA approves of this the comment will be made accessible in conjunction with the SASL mechanism registration itself. J. Myers [Page 7] Internet DRAFT SASL April 25, 2001 7.2. Location of registered SASL mechanism list SASL mechanism registrations are available at the URL "http://www.isi.edu/in-notes/iana/assignments/sasl-mechanisms" The SASL mechanism description and other supporting material may also be published as an Informational RFC by sending it to "rfc-editor@rfc-editor.org" (please follow the instructions to RFC authors [RFC-INSTRUCTIONS]). 7.3. Change control Once a SASL mechanism registration has been published by IANA, the author may request a change to its definition. The change request follows the same procedure as the registration request. The owner of a SASL mechanism may pass responsibility for the SASL mechanism to another person or agency by informing IANA; this can be done without discussion or review. The IESG may reassign responsibility for a SASL mechanism. The most common case of this will be to enable changes to be made to mechanisms where the author of the registration has died, moved out of contact or is otherwise unable to make changes that are important to the community. SASL mechanism registrations may not be deleted; mechanisms which are no longer believed appropriate for use can be declared OBSOLETE by a change to their "intended use" field; such SASL mechanisms will be clearly marked in the lists published by IANA. The IESG is considered to be the owner of all SASL mechanisms which are on the IETF standards track. 7.4. Registration template To: iana@isi.edu Subject: Registration of SASL mechanism X SASL mechanism name: Security considerations: Published specification (optional, recommended): Person & email address to contact for further information: Intended usage: J. Myers [Page 8] Internet DRAFT SASL April 25, 2001 (One of COMMON, LIMITED USE or OBSOLETE) Owner/Change controller: (Any other information that the author deems interesting may be added below this line.) 8. The external mechanism The mechanism name associated with external authentication is "EXTERNAL". The client sends an initial response with the UTF-8 encoding of the authorization identity. The server uses information, external to SASL, to determine whether the client is authorized to authenticate as the authorization identity. If the client is so authorized, the server indicates successful completion of the authentication exchange; otherwise the server indicates failure. The system providing this external information may be, for example, IPsec or TLS. If the client sends the empty string as the authorization identity (thus requesting the authorization identity be derived from the client's authentication credentials), the authorization identity is to be derived from authentication credentials which exist in the system which is providing the external authentication. 8.1 Formal syntax The following syntax specification uses the augmented Backus-Naur Form (BNF) notation as specified in [ABNF]. This uses the ABNF core rules as specified in Appendix A of the ABNF specification [ABNF]. The "initial-response" rule below defines the initial response sent from client to server. initial-response = *UTF8 UTF8 = %x01-7F / UTF8-2 / UTF8-3 / UTF8-4 UTF8-loworder = %x80-BF UTF8-2 = %xC1-DF UTF8-loworder ; Disallow overlong sequences beginning with 0xC0. UTF8-3 = (%xE0 %xA0-BF UTF8-loworder) / (%xE1-EC 2UTF8-loworder) / (%xED %x80-9F UTF8-loworder) / (%xEE 2UTF8-loworder) / J. Myers [Page 9] Internet DRAFT SASL April 25, 2001 (%xEF %x80-BE UTF8-loworder) / (%xEF %xBF %x80-BD) ; Disallow overlong sequences beginning with 0xE0, ; disallow encoded surrogate characters, and ; disallow U+FFFE, U+FFFF. UTF8-4 = (%xF0 %x90-BF 2UTF8-loworder) / (%xF1-F7 3UTF8-loworder) ; Disallow overlong sequences beginning with 0xF0. 8.2 Example The following is an example of an EXTERNAL authentication in the SMTP protocol [SMTP-AUTH]. In this example, the client is proxy authenticating, sending the authorization id "fred". The server has determined the client's identity through IPsec and has a security policy that permits that identity to proxy authenticate as any other identity. To the protocol profile, the four octet sequence "fred" is an opaque binary blob. The SASL protocol profile for SMTP specifies that server challenges and client responses are encoded in BASE64; the BASE64 encoding of "fred" is "ZnJlZA==". S: 220 smtp.example.com ESMTP server ready C: EHLO jgm.example.com S: 250-smtp.example.com S: 250 AUTH DIGEST-MD5 EXTERNAL C: AUTH EXTERNAL ZnJlZA== S: 235 Authentication successful. 9. References [ABNF] Crocker, Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, November 1997 [GSSAPI] Linn, "Generic Security Service Application Program Interface, Version 2, Update 1", RFC 2743, January 2000 [KEYWORDS] Bradner, "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997 [RFC-INSTRUCTIONS] Postel, Reynolds, "Instructions to RFC Authors", RFC 2223, October 1997 [SASL-GSSAPI] Myers, "SASL GSSAPI mechanisms", draft-ietf-cat-sasl- gssapi-XX.txt, September 2000 [SASL-OTP] Newman, "The One-Time-Password SASL Mechanism", RFC 2444, J. Myers [Page 10] Internet DRAFT SASL April 25, 2001 October 1998 [SMTP-AUTH] Myers, "SMTP Service Extension for Authentication", RFC 2554, March 1999 [UTF-8] Yergeau, "UTF-8, a transformation format of ISO 10646", RFC 2279, Janyary 1998 10. Security considerations Security issues are discussed throughout this memo. The mechanisms that support integrity protection are designed such that the negotiation of the security layer and authorization identity is integrity protected. When the client selects a security layer with at least integrity protection, this protects against an active attacker hijacking the connection and modifying the authentication exchange to negotiate a plaintext connection. When a server or client supports multiple authentication mechanisms, each of which has a different security strength, it is possible for an active attacker to cause a party to use the least secure mechanism supported. To protect against this sort of attack, a client or server which supports mechanisms of different strengths should have a configurable minimum strength that it will use. It is not sufficient for this minimum strength check to only be on the server, since an active attacker can change which mechanisms the client sees as being supported, causing the client to send authentication credentials for its weakest supported mechanism. The client's selection of a SASL mechanism is done in the clear and may be modified by an active attacker. It is important for any new SASL mechanisms to be designed such that an active attacker cannot obtain an authentication with weaker security properties by modifying the SASL mechanism name and/or the challenges and responses. Any protocol interactions prior to authentication are performed in the clear and may be modified by an active attacker. In the case where a client selects integrity protection, it is important that any security-sensitive protocol negotiations be performed after authentication is complete. Protocols should be designed such that negotiations performed prior to authentication should be either ignored or revalidated once authentication is complete. The EXTERNAL mechanism provides no security protection; it is vulnerable to spoofing by either client or server, active attack, and eavesdropping. It should only be used when external security mechanisms are present and have sufficient strength. J. Myers [Page 11] Internet DRAFT SASL April 25, 2001 11. Author's address John G. Myers Netscape Communications 501 E. Middlefield Road Mail Stop SCA 15:201 Mountain View, CA 94043-4042 Email: jgmyers@netscape.com Appendix A. Relation of SASL to transport security Questions have been raised about the relationship between SASL and various services (such as IPsec and TLS) which provide a secured connection. Two of the key features of SASL are: 1. The separation of the authorization identity from the identity in the client's credentials. This permits agents such as proxy servers to authenticate using their own credentials, yet request the access privileges of the identity for which they are proxying. 2. Upon successful completion of an authentication exchange, the server knows the authorization identity the client wishes to use. This allows servers to move to a "user is authenticated" state in the protocol. These features are extremely important to some application protocols, yet Transport Security services do not always provide them. To define SASL mechanisms based on these services would be a very messy task, as the framing of these services would be redundant with the framing of SASL and some method of providing these important SASL features would have to be devised. Sometimes it is desired to enable within an existing connection the use of a security service which does not fit the SASL model. (TLS is an example of such a service.) This can be done by adding a command, for example "STARTTLS", to the protocol. Such a command is outside the scope of SASL, and should be different from the command which starts a SASL authentication protocol exchange. In certain situations, it is reasonable to use SASL underneath one of these Transport Security services. The transport service would secure the connection, either service would authenticate the client, and SASL would negotiate the authorization identity. The SASL negotiation would be what moves the protocol from "unauthenticated" J. Myers [Page 12] Internet DRAFT SASL April 25, 2001 to "authenticated" state. The "EXTERNAL" SASL mechanism is explicitly intended to handle the case where the transport service secures the connection and authenticates the client and SASL negotiates the authorization identity. When using SASL underneath a sufficiently strong Transport Security service, a SASL security layer would most likely be redundant. The client and server would thus probably want to negotiate off the use of a SASL security layer. Appendix B. IANA considerations The IANA is directed to modify the SASL mechanisms registry as follows: 1. Change the "Intended usage" of the KERBEROS_V4 and SKEY mechanism registrations to OBSOLETE. 2. Change the "Published specification" of the EXTERNAL mechanism to this document. Appendix C. Changes since RFC 2222 The GSSAPI mechanism was removed. It is now specified in a separate document [SASL-GSSAPI]. The "KERBEROS_V4" mechanism defined in RFC 2222 is obsolete and has been removed. The "SKEY" mechanism described in RFC 2222 is obsolete and has been removed. It has been replaced by the OTP mechanism [SASL-OTP]. The overview has been substantially reorganized and clarified. The word "must" in the first paragraph of the "Protocol profile requirements" section was changed to "MUST". Specified that protocol profiles SHOULD provide a way for clients to discover available SASL mechanisms. Specified that the authorization identity in the EXTERNAL mechanism is encoded in UTF-8. Added a statement that a protocol profile SHOULD allow challenge data to be sent with a success indication. Added a security consideration for the EXTERNAL mechansim. J. Myers [Page 13] Internet DRAFT SASL April 25, 2001 Clarified sections concerning success with addtional data. J. Myers [Page 14] Internet DRAFT SASL April 25, 2001 Table of Contents Status of this Memo ............................................... i 1. Abstract .................................................... 2 2. Organization of this document ............................... 2 2.1. How to read this document ................................... 2 2.2. Conventions used in this document ........................... 2 3. Overview .................................................... 2 4. Authentication mechanisms ................................... 3 4.1. Authentication protocol exchange ............................ 4 4.2. Proxy authentication ........................................ 4 4.3. Security layers ............................................. 5 5. Protocol profile requirements ............................... 5 6. Specific issues ............................................. 6 6.1. Client sends data first ..................................... 6 6.2. Server returns success with additional data ................. 6 6.3. Multiple authentications .................................... 7 7. Registration procedures ..................................... 7 7.1. Comments on SASL mechanism registrations .................... 7 7.2. Location of registered SASL mechanism list .................. 8 7.3. Change control .............................................. 8 7.4. Registration template ....................................... 8 8. The external mechanism ...................................... 9 8.1 Formal syntax ............................................... 9 8.2 Example ..................................................... 10 9. References .................................................. 10 10. Security considerations ..................................... 11 11. Author's address ............................................ 12 Appendix A. Relation of SASL to transport security ................ 12 Appendix B. IANA considerations ................................... 13 Appendix C. Changes since RFC 2222 ................................ 13 J. Myers [Page ii]