draft SASL base spec
John Gardiner Myers <jgmyers@netscape.com> Wed, 21 March 2001 03:36 UTC
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Subject: draft SASL base spec
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Network Working Group J. Myers Internet Draft March 2001 Document: draft-myers-saslrev-XX.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''. 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 March 19, 2001 1. Abstract This document describes a method for adding authentication support to connection-based protocols. To use this specification, a protocol includes a command for identifying and authenticating a user to a server and for optionally negotiating protection of subsequent protocol interactions. If its use is negotiated, a security layer is inserted between the protocol and the connection. This document describes how a protocol specifies such a command, defines several mechanisms for use by the command, and defines the protocol used for carrying a negotiated security layer over the connection. 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]. 2.3. Examples Examples in this document are for the IMAP profile [IMAP4] of this specification. The base64 encoding of challenges and responses, as well as the "+ " preceding the responses are part of the IMAP4 profile, not part of the SASL specification itself. J. Myers [Page 2] Internet DRAFT SASL March 19, 2001 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 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. Procedures for registering new SASL mechanisms are given in the section "Registration procedures" 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. J. Myers [Page 3] Internet DRAFT SASL March 19, 2001 After receiving an authentication command or any client response, a server mechanism may issue a challenge, indicate failure, or indicate completion. 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 authorization identity transmitted during the authentication protocol exchange may be different than the identity derived from the client's authentication credentials. When the two are different, this is typically referred to as "proxy authentication". The identity derived from the client's authentication credentials is known as the "authentication identity". 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. 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. 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 J. Myers [Page 4] Internet DRAFT SASL March 19, 2001 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. 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 failure of the exchange, how the client aborts an exchange, and how the exchange method interacts with any line length limits in the protocol. 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. 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 J. Myers [Page 5] Internet DRAFT SASL March 19, 2001 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 server challenge 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. If a protocol's profile does not permit this server challenge to be returned with a success indication, then the server issues the 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. 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@isi.edu. IANA has the right to reject obviously bogus registrations, but will perform no review of clams 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. J. Myers [Page 6] Internet DRAFT SASL March 19, 2001 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. 7.2. Location of registered SASL mechanism list SASL mechanism registrations will be posted in the anonymous FTP directory "ftp://ftp.isi.edu/in-notes/iana/assignments/sasl-mechanisms/" and all registered SASL mechanisms will be listed in the periodically issued "Assigned Numbers" RFC [currently RFC-1700]. The SASL mechanism description and other supporting material may also be published as an Informational RFC by sending it to "rfc-editor@isi.edu" (please follow the instructions to RFC authors [RFC-1543]). 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. J. Myers [Page 7] Internet DRAFT SASL March 19, 2001 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: (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 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. J. Myers [Page 8] Internet DRAFT SASL March 19, 2001 9. References [IMAP4] Crispin, M., "Internet Message Access Protocol - Version 4", RFC 1730, University of Washington, December 1994. [GSSAPI] Linn, J., "Generic Security Service Application Program Interface, Version 2", RFC 2078, January 1997 [KEYWORDS] Bradner, "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997 [RFC-1543] Postel, J., "Instructions to RFC Authors", RFC 1543, October 1993 [SASL-GSSAPI] Myers, J., "SASL GSSAPI mechanisms", draft-ietf-cat- sasl-gssapi-XX.txt, September 2000 [SASL-OTP] Newman, C., "The One-Time-Password SASL Mechanism", RFC 2444, October 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. J. Myers [Page 9] Internet DRAFT SASL March 19, 2001 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. 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 J. Myers [Page 10] Internet DRAFT SASL March 19, 2001 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" 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. J. Myers [Page 11] Internet DRAFT SASL March 19, 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 2.3. Examples .................................................... 2 3. Overview .................................................... 3 4. Authentication mechanisms ................................... 3 4.1. Authentication protocol exchange ............................ 3 4.2. Proxy authentication ........................................ 4 4.3. Security layers ............................................. 4 5. Protocol profile requirements ............................... 5 6. Specific issues ............................................. 5 6.1. Client sends data first ..................................... 5 6.2. Server returns success with additional data ................. 6 6.3. Multiple authentications .................................... 6 7. Registration procedures ..................................... 6 7.1. Comments on SASL mechanism registrations .................... 7 7.2. Location of registered SASL mechanism list .................. 7 7.3. Change control .............................................. 7 7.4. Registration template ....................................... 8 8. The external mechanism ...................................... 8 9. References .................................................. 9 10. Security considerations ..................................... 9 11. Author's address ............................................ 10 Appendix A. Relation of SASL to transport security ................ 10 Appendix B. IANA considerations ................................... 11 Appendix C. Changes since RFC 2222 ................................ 11 J. Myers [Page ii]
- draft SASL base spec John Gardiner Myers
- Re: draft SASL base spec Kurt D. Zeilenga
- Re: draft SASL base spec John Gardiner Myers