INTERNET-DRAFT A. Melnikov, Ed. Intended Category: Standards Track ISODE Limited Expires in six months K. Zeilenga, Ed. Obsoletes: RFC 2222 OpenLDAP Project 15 May 2005 Simple Authentication and Security Layer (SASL) <draft-zeilenga-sasl-rfc2222bis-00.txt> Status of this Memo This document is based upon <draft-ietf-sasl-rfc2222bis-xx.txt>. This document offers alternative text. By submitting this Internet-Draft, I accept the provisions of Section 3 of BCP 78. By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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 and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate 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 Copyright (C) The Internet Society (2005). All Rights Reserved. Please see the Full Copyright section near the end of this document for more information. Abstract Zeilenga SASL [Page 1]
INTERNET-DRAFT draft-zeilenga-sasl-rfc2222bis-00.txt 15 May 2005 The Simple Authentication and Security Layer (SASL) is a framework for providing authentication and data security services in connection-oriented protocols via replaceable mechanisms. It provides a structured interface between protocols and mechanisms. The resulting framework allows new protocols to reuse existing mechanisms and allows old protocols to make use of new mechanisms. The framework also provides a protocol for securing subsequent protocol exchanges within a data security layer. This document describes how a SASL mechanism is structured, describes how protocols include support for SASL, and defines the protocol for carrying a data security layer over a connection. Additionally, this document defines one SASL mechanism, the EXTERNAL mechanism. This document obsoletes RFC 2222. Table of Contents [[Page numbers to be filled in by RFC-Editor]] Status of this Memo Abstract 1. Introduction 1.1. Document Audiences 1.2. Relationship to Other Documents 1.3. Conventions 2. Identity Concepts 3. The Authentication Exchange 3.1. Mechanism Naming 3.2. Mechanism Negotiation 3.3. Request Authentication Exchange 3.4. Challenges and Responses 3.4.1. Authorization Identity String 3.5. Aborting Authentication Exchanges 3.6. Authentication Outcome 3.7. Security Layers 3.8. Multiple Authentications 4. Protocol Requirements 5. Mechanism Requirements 6. Security Considerations 6.1. Active Attacks 6.1.1. Man-in-the-middle Attacks 6.1.2. Replay Attacks 6.1.3. Truncation Attacks 6.2. Passive Attacks 6.3. Re-keying 6.4. Other considerations 7. IANA Considerations Zeilenga SASL [Page 2]
INTERNET-DRAFT draft-zeilenga-sasl-rfc2222bis-00.txt 15 May 2005 8. References 9. Editors' Address 10. Acknowledgments A. The SASL EXTERNAL Mechanism B. Changes since RFC 2222 1. Introduction The Simple Authentication and Security Layer (SASL) is a framework for providing authentication and data security services in connection-oriented protocols via replaceable mechanisms. SASL provides a structured interface between protocols and mechanisms. SASL also provides a protocol for securing subsequent protocol exchanges within a data security layer. The data security layer can provide data integrity, data confidential, and other services. SASL's design is intended to allow new protocols to reuse existing mechanisms without requiring redesign of the mechanisms and allows existing protocols to make use of new mechanisms without redesign of protocols. SASL is conceptually a framework which provides an abstraction layer between protocols and mechanisms as illustrated in the following diagram. SMTP LDAP XMPP Other protocols ... \ | | / \ | | / SASL abstraction layer / | | \ / | | \ EXTERNAL GSSAPI PLAIN Other mechanisms ... It is through the interfaces of this abstraction layer that the framework allows any protocol to utilize any mechanism. While this layer does generally hide the particulars of protocols from mechanisms and the particulars of mechanisms from protocols, this layer does not generally hide the particulars of mechanisms from protocol implementations. For example, different mechanisms require different information to operate, some of them use password-based authentication, some of then require realm information, others make use of Kerberos tickets, certificates, etc.. Also, in order to perform authorization, server implementations generally have to implement identity mapping between authentication identities, whose form is mechanism-specific, and authorization identities, whose form is application protocol specific. Section 2 discusses identity concepts. Zeilenga SASL [Page 3]
INTERNET-DRAFT draft-zeilenga-sasl-rfc2222bis-00.txt 15 May 2005 It is possible to design and implement this framework in ways which do abstract away particulars of similar mechanisms. Such a framework implementation, as well as mechanisms implementations, could be designed not only to be shared by multiple implementations of a particular protocol, but be shared by implementations of multiple protocols. The framework incorporates interfaces with both protocols and mechanisms in which authentication exchanges are carried out. Section 3 discusses SASL authentication exchanges. To use SASL, each protocol (amongst other items) provides a method for identifying which mechanism is to be used, provides a method for exchange of mechanism-specific server-challenges and client-responses, and a method the communicating the outcome of the authentication exchange. Section 4 discusses SASL protocol requirements. Each SASL mechanism defines (amongst other items) a series of server challenges and client responses which provide authentication services and negotiate data security services. Section 5 discusses SASL mechanism requirements. Section 6 discusses security considerations. Section 7 discusses IANA considerations. Appendix A defines the SASL EXTERNAL mechanism. 1.1. Document Audiences This document is written to serve several different audiences: - protocol designers using this specification to support authentication in their protocol, - mechanism designers that define new SASL mechanisms, and - implementors of clients or servers for those protocols which support SASL. While the document organization is intended to allow readers to focus on details relevant to their engineering, readers are encouraged to read and understand all aspects of this document. 1.2. Relationship to other documents This document obsoletes RFC 2222. It replaces all portions of RFC 2222 excepting sections 7.1 (the KERBEROS_IV mechanism), 7.2 (the GSSAPI mechanism), 7.3 (the SKEY mechanism). The KERBEROS_IV and SKEY Zeilenga SASL [Page 4]
INTERNET-DRAFT draft-zeilenga-sasl-rfc2222bis-00.txt 15 May 2005 mechanisms are now viewed as obsolete and their specifications provided in RFC 2222 are Historic. The GSSAPI mechanism is now separately specified [SASL-GSSAPI]. Appendix B provides a summary of changes since RFC 2222. 1.3. Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119]. Character names in this document use the notation for code points and names from the Unicode Standard [Unicode]. For example, the letter "a" may be represented as either <U+0061> or <LATIN SMALL LETTER A>. Note: a glossary of terms used in Unicode can be found in [Glossary]. Information on the Unicode character encoding model can be found in [CharModel]. In examples, "C:" and "S:" indicate lines of data to be sent by the client and server respectively. Lines have been wrapped for improved readability. 2. Identity Concepts In practice, authentication and authorization may involve multiple identities, possible in different forms (simple username, Kerberos principal, X.500 Distinguished Name, etc.), possibly with different representations (e.g.: ABNF-described UTF-8 encoded Unicode character string, BER-encoded Distinguished Name). While technical specifications often prescribe both the identity form and representation used on the network, different identity forms and/or representations may (and often are) used within implementations. How identities of different forms relate to each other is, generally, a local matter. Additionally, the forms and representations used within an implementation is a local matter. However, conceptually, SASL framework involves two identities: 1) an identity associated with the authentication credentials (termed the authentication identity), and 2) an identity to act as (termed the authorization identity). The client provides its credentials and, optionally, a character string representing the requested authorization identity as part of the SASL exchange. When this string is omitted or empty, the client Zeilenga SASL [Page 5]
INTERNET-DRAFT draft-zeilenga-sasl-rfc2222bis-00.txt 15 May 2005 is requesting to act as the identity associated with the credentials (e.g., the user is requesting to act as the authentication identity). The server is responsible for verifying the client's credentials and verifying that the client is allowed to act as the authorization identity. A SASL exchange fails if either (or both) of these verifications fails. (The SASL exchange may fail for other reasons, such as service authorization failure.) SASL mechanism specifications describe the credential form(s) (e.g., X.509 certificates, Kerberos tickets, simple username/password) used to authenticate the client, including (where appropriate) the syntax and semantics of associated identities. SASL protocol specifications describe the identity form(s) used in authorization and, in particular, prescribe the syntax and semantics of the authorization identity character string to be transferred by mechanisms. However, the precise form(s) of the authentication identities (used within the server in its verifications, or otherwise) and the precise form(s) of the authorization identities (used in making authorization decisions, or otherwise) is beyond the scope of the SASL and this specification. In some circumstances, the precise identity forms used in some context outside of the SASL exchange may be dictated by other specifications. For instance, an identity assumption authorization (proxy authorization) policy specification may dictate how authentication and authorization identities are represented in the policy statement. 3. The Authentication Exchange Each authentication exchange consists of a message from the client to the server requesting authentication via a particular mechanism, followed by pairs of challenges from servers and responses from clients, followed by message from the server indicating the outcome of the authentication exchange. (Note: exchanges may also be aborted as discussed in Section 3.5.) The following illustration provides a high-level overview of an authentication exchange. C: Request authentication exchange S: Initial challenge C: Initial response <additional challenge/response messages> S: Outcome of authentication exchange If the outcome is successful and a data layer was negotiated, this Zeilenga SASL [Page 6]
INTERNET-DRAFT draft-zeilenga-sasl-rfc2222bis-00.txt 15 May 2005 layer is then installed. Protocols may allow this data to be carried in the request message. This applies as well to the following illustrations. Some mechanisms specify that the first data sent in the authentication exchange is from the client to the server. Protocols may provide an optional initial response field in the request message to carry this data. Where the mechanism specifies the first data data sent in the exchange is from the client to the server, the protocol provides an optional initial response field, and the client uses this field, the exchange is shorten by one round-trip: C: Request authentication exchange + Initial response <additional challenge/response messages> S: Outcome of authentication exchange Where the mechanism specifies the first data sent in the exchange is from the client to the server and this field is unavailable or unused, the client request is followed by an empty challenge. C: Request authentication exchange S: Empty Challenge C: Initial Response <additional challenge/response messages> S: Outcome of authentication exchange Should a client include an initial response in its request where the mechanism does not allow the client to send data first, the authentication exchange fails. Some mechanisms specify that the server is to send additional data to the client along when indicating a successful outcome. Protocols may provide an optional additional data field in the outcome message to carry this data. Where the mechanism specifies the server is to return additional data with the successful outcome, the protocol provides an optional additional data field in the outcome message, and the server uses this field, the exchange is shorten by one round-trip: C: Request authentication exchange S: Initial challenge C: Initial response <additional challenge/response messages> S: Outcome of authentication exchange with additional data with success Where the mechanism specifies the server is to return additional data to the client with a successful outcome and and this field is unavailable or unused, the additional data is sent as a challenge Zeilenga SASL [Page 7]
INTERNET-DRAFT draft-zeilenga-sasl-rfc2222bis-00.txt 15 May 2005 whose response is empty. After receiving this response, the server then indicates the successful outcome. C: Request authentication exchange S: Initial challenge C: Initial response <additional challenge/response messages> S: Additional data challenge C: Empty Response S: Outcome of authentication exchange Where mechanisms specify the first data sent in the exchange is from the client to the server and additional data is to sent to the client along with indicating a successful outcome, and the protocol provides fields support both, the exchange can be shorted by two round-trips: C: Request authentication exchange + Initial response <additional challenge/response messages> S: Outcome of authentication exchange with additional data with success instead of: C: Request authentication exchange S: Empty Challenge C: Initial Response <additional challenge/response messages> S: Additional data challenge C: Empty Response S: Outcome of authentication exchange 3.1. Mechanism Naming SASL mechanisms are named by character strings, from 1 to 20 characters in length, consisting of ASCII [ASCII] uppercase letters, digits, hyphens, and/or underscores. In the following Augmented Backus-Naur Form (ABNF) [ABNF] grammar, the <sasl-mech> production defines the syntax of a SASL mechanism name. sasl-mech = 1*20mech-char mech-char = UPPER-ALPHA / DIGIT / HYPHEN / UNDERSCORE ; mech-char is restricted to A-Z (uppercase only), 0-9, -, and _ ; from ASCII character set. UPPER-ALPHA = %x41-5A ; A-Z (uppercase only) DIGIT = %x30-39 ; 0-9 Zeilenga SASL [Page 8]
INTERNET-DRAFT draft-zeilenga-sasl-rfc2222bis-00.txt 15 May 2005 HYPHEN = %x2D ; hyphen (-) UNDERSCORE = %x5F ; underscore (_) SASL mechanisms names are registered as discussed in Section 7.1. 3.2. Mechanism Negotiation Mechanism negotiation is protocol-specific. Commonly, a protocol will specify that the server advertises supported and available mechanisms to the client via some facility provided by the protocol and the client will then use select the "best" mechanism from this which its supports and finds suitable. It is noted that the mechanism negotiation is not protected by the subsequent authentication exchange and hence is subject to downgrade attacks if not protected by other means. To detect downgrade attacks, a protocol may allow the client to discover available mechanism subsequent to the authentication exchange (and, hence, protected by any data security layer provided by mechanism) so that downgrade attacks can be detected. 3.3. Request Authentication Exchange The authentication exchange is initiated by the client by requesting authentication via a mechanism it specifies. The client sends a message that contains the name of the mechanism to the server. The particulars of the message are protocol specific. It is noted that the name of the mechanism is not protected by the mechanism, and hence subject to alteration by an attacker if not integrity protected by other means. Where the mechanism is defined to allow the client to send data first, and the protocol's request message includes an optional initial response field, the client may include the response to the initial challenge in the authentication request message. 3.4. Challenges and Responses The authentication exchange involves pairs of server-challenges and client-responses, the particulars of which are mechanism specific. These challenges and responses are enclosed in protocol messages, the particulars of which are protocol specific. Zeilenga SASL [Page 9]
INTERNET-DRAFT draft-zeilenga-sasl-rfc2222bis-00.txt 15 May 2005 Through these challenges and responses, the mechanism may: - authenticate the client to the server, - authenticate the server to the client, - transfer an authorization identity string, - negotiate a security layer, and - provide other services. The negotiation of the security layer may involve negotiation of the security services to be provided in the layer, how these services will be provided, and negotiation of a maximum cipher-text buffer size each size is able to receive in the layer (see Section 3.6). After receiving an authentication request or any client response, the server may issue a challenge, abort the exchange, or indicate the outcome of an exchange. After receiving a challenge, a client mechanism may issue a response or abort the exchange. 3.4.1. Authorization Identity String The authorization identity string is a sequence of zero or more Unicode [Unicode] characters, excluding the NUL (U+0000) character, representing the identity to act as. If the authorization identity string is absent, the client is requesting to act as the identity the server associates with the client's credentials. An empty string is equivalent to an absent authorization identity. Non-empty authorization identity string indicates the client wishes to act as the identity represented by the string. In this case, the form of identity represented by the string, as well as the precise syntax and semantics of the string, is protocol specific. While character encoding schema used to transfer the authorization identity string in the authentication exchange is mechanism specific, mechanisms are expected to be capable of carrying the entire Unicode repertoire (with the exception of the NUL character). 3.5. Aborting Authentication Exchanges A client or server may desire to abort an authentication exchange it is unwilling or unable to continue (or enter into). A client may abort the authentication exchange by sending a message, the particulars of which are mechanism specific, to the server, indicating the exchange is aborted. The server may be required by the Zeilenga SASL [Page 10]
INTERNET-DRAFT draft-zeilenga-sasl-rfc2222bis-00.txt 15 May 2005 protocol to return a message in response to the client's abort message. Likewise, a server may abort the authentication exchange by sending a message, the particulars of which are mechanism specific, to the client, indicating the exchange is aborted. 3.6. Authentication Outcome At the conclusion of the authentication exchange, the server sends a message, the particulars of which are protocol specific, to the client indicating the outcome of the exchange. The outcome is not successful if - the authentication exchange failed for any reason, - the clients credentials could not be verified, - the server cannot associate an identity with the client's credentials, - the client-provided authorization identity string is malformed, - the identity associated with the client's credentials are not authorized to act as the requested authorization identity, - the negotiated security layer (or lack thereof) is not suitable, or - the server is not willing to provide service to the client for any reason. The protocol may include a field in this optional additional data. If the outcome is successful and a security layer was negotiated, this layer is then installed. If the outcome is unsuccessful, or a security layer was not negotiated, any existing security is left in place. 3.7. Security Layers SASL mechanisms may offer a wide range of services in security layers. Typical services include data integrity and data confidentiality. SASL mechanisms which do not provide a security layer are treated as negotiating no security layer. If use of a security layer is negotiated in the authentication protocol exchange, the layer is installed by the server after indicating the outcome of the authentication exchange and installed by the client upon receipt the outcome indication. In both cases, the layer is installed before transfer of further protocol data. The precise position that the layer takes effect in the protocol data Zeilenga SASL [Page 11]
INTERNET-DRAFT draft-zeilenga-sasl-rfc2222bis-00.txt 15 May 2005 stream is protocol specific. Once the security layer is in effect in the protocol data stream, it remains in effect until either a subsequently negotiated security layer is installed, or the underlying transport connection is closed. When in effect, the security layer processes protocol data into buffers of protected data. If at any time the security layer is unable to continue producing buffers protecting protocol data, the underlying transport connection MUST be closed. If the security layer is not able to decode a received buffer, the underlying connection MUST be closed. In both cases the underlying transport connection SHOULD be closed gracefully. Each buffer of protected data is transferred over the underlying transport connection as a sequence of octets prepended with a four octet field in network byte order that represents the length of the buffer. The length of the protected data buffer MUST be no larger than the maximum size the other side expects. Upon the receipt of a length field whose value is greater than maximum size, the receiver SHOULD close the connection, as this might be a sign of an attack. The maximum size for each side expects is fixed by the mechanism, either through negotiation or by its specification. 3.8. Multiple Authentications Unless explicitly permitted in the protocol (as stated in the protocol's technical specification), only one successful SASL authentication exchange may occur in a protocol session. In this case, once an authentication exchange has successfully completed, further attempts to initiate an authentication exchange fail. Where multiple successful SASL authentication exchanges are permitted in the protocol, then in no case may multiple SASL security layers be simultaneously 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. If a security layer is in effect and a subsequent SASL negotiation selects no security layer, the original security layer remains in effect. Where multiple successful SASL negotiations are permitted in the protocol, the effect of a failed SASL authentication exchange upon the previously established authentication and authorization state is protocol specific. The protocol's technical specification should be consulted to determine whether the previous authentication and authorization state remains in force, or changed to an anonymous Zeilenga SASL [Page 12]
INTERNET-DRAFT draft-zeilenga-sasl-rfc2222bis-00.txt 15 May 2005 state, or otherwise effected. Regardless of the protocol-specific effect upon previously established authentication and authorization state, the previously negotiated security layer remains in effect. 4. Protocol Requirements In order for a protocol to offer SASL services, its specification 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 [RFC2743]. 2) Detail any mechanism negotiation facility the protocol provides (see Section 3.2). A protocol SHOULD specify a facility through which the client may discover, both before initiation of the SASL exchange and after installing security layers negotiated by the exchange, the names of the SASL mechanisms the server makes available to the client. The latter is important to allow the client to detect downgrade attacks. This facility is typically provided through the protocol's extensions or capabilities discovery facility. 3) Definition of the messages necessary for authentication exchange, including: a) A message to initiate the authentication exchange (see Section 3.3). This message MUST contain a field for carrying the name of the mechanism selected by the client. This message SHOULD contain an optional field for carrying an initial response. If the message is defined with this field, the specification MUST describe how messages with an empty initial response are distinguished from messages with no initial response. This field MUST be capable of carrying arbitrary sequences of octets (including zero length sequences and sequences containing zero-valued octets). b) Messages to transfer a server challenges and client responses. (see Section 3.4). Each of these messages MUST be capable of carrying arbitrary sequences of octets (including zero length sequences and Zeilenga SASL [Page 13]
INTERNET-DRAFT draft-zeilenga-sasl-rfc2222bis-00.txt 15 May 2005 sequences containing zero-valued octets). c) A message to indicate the outcome of the authentication exchange (see Section 3.6). This message SHOULD contain an optional field for carrying additional data with a successful outcome. If the message is defined with this field, the specification MUST describe how messages with an empty additional data are distinguished from messages with no additional data. This field MUST be capable of carrying arbitrary sequences of octets (including zero length sequences and sequences containing zero-valued octets). 4) Prescribe the syntax and semantics of non-empty authorization identity strings (see Section 3.4.1). Specifications are encouraged to prescribe use existing authorization identity forms as well as existing string representations, such as simple user names [RFC4013]. Where the specification does not precisely prescribe how identities in SASL relate to identities used elsewhere in the protocol, for instance in access control policy statements, it may be appropriate for the protocol to provide a facility by which the client may discover information (such as the representation of the authentication identity used in making access control decisions) about established identities for these uses. 5) Detail any facility the protocol provide allow the client and/or server to abort authentication exchange (see Section 3.5). Protocols which support multiple authentications typically allow a client to abort an on-going authentication exchange by initiating a new authentication exchange. Protocols which do not support multiple authentications may require the client to close the connection and start over to abort an on-going authentication exchange. Protocols typically allow the server to abort on-going authentication exchanges by returning a non-successful outcome message. 6) Identify precisely where newly negotiated security layers starts to take effect, in both directions (see Section 3.7). Zeilenga SASL [Page 14]
INTERNET-DRAFT draft-zeilenga-sasl-rfc2222bis-00.txt 15 May 2005 Typically, specifications require security layer to start taking effect, in data being sent by the server, on the first octet following the outcome message and, in data being sent by the client, on the first octet sent after receipt of the outcome message. 7) If the protocol supports other layered security services, such as Transport Layer Security (TLS) [RFC2246], the specification MUST prescribe the order in which security layers are applied to protocol data. For instance, where a protocol supports both TLS and SASL security layers, the specification could prescribe any of the following: a) SASL security layer is always applied first to data being sent and, hence, applied last to received data, b) SASL security layer is always applied last to data being sent and, hence, applied first to received data, c) Layers are applied in the order in which they were installed, d) Layers are applied in the reverse order in which they were installed, or e) Both TLS and SASL security layers cannot be installed. 8) Indicate whether the protocol supports multiple authentications (see Section 3.8). If so, the protocol MUST detail the effect a failed SASL authentication exchange will have upon previously established authentication and authorization state. Protocol specifications SHOULD avoid stating implementation requirements which would hinder replacement of applicable mechanisms. In general, protocol specification SHOULD be mechanism neutral. There are a number reasonable exceptions to this recommendation, including: - detailing how credentials (which are mechanism-specific) are managed in the protocol, - detailing how authentication identities (which are mechanism-specific) and authorization identities (which are protocol-specific) relate to each other, and - detailing which mechanisms are applicable to the protocol. 5. Mechanism Requirements SASL mechanism specifications MUST supply the following information: 1) The name of the mechanism (see Section 3.1). This name MUST be registered as discussed in Section 8.1. Zeilenga SASL [Page 15]
INTERNET-DRAFT draft-zeilenga-sasl-rfc2222bis-00.txt 15 May 2005 2) A definition of the server-challenges and client-responses of the authentication exchange, as well as: a) An indication whether the client is expected to send data first. If so, when the client does not send data first, the initial challenge MUST be specified as being an empty challenge. b) An indication whether the server is expected to provide additional data when indicating a successful outcome. If so, if the server sends the additional data as a challenge, the specification MUST indicate the response to this challenge is an empty response. SASL mechanisms SHOULD be designed to minimize the number of round challenges and responses necessary to complete the exchange. 3) An indication of whether the mechanism is capable of transferring authorization identity strings (see Section 3.4.1). While some legacy mechanisms are incapable of transmitting an authorization identity (which means that for these mechanisms the authorization identity is always the empty string), newly defined mechanisms SHOULD be capable of transferring authorization identity strings. The mechanism SHOULD NOT be capable of transferring both no authorization identity string and an empty authorization identity. Mechanisms which are capable of transferring an authorization identity string MUST be capable of transferring arbitrary non-empty sequences of Unicode characters, excluding those which contain the NUL (U+0000) character. Mechanisms SHOULD use the UTF-8 [RFC3629] transformation format. The specification MUST detail how any Unicode code points special to the mechanism which might appear in the authorization identity string are escaped to avoid ambiguity during decoding of the authorization identity string. Typically, mechanisms which have special characters require these special characters to be escaped or encoded in the character string (after encoding it a particular Unicode transformation format) using a data encoding scheme such as Base64 [RFC3548]. 4) The specification MUST detail whether or not the mechanism offers a security layer. If the mechanism does, the specification MUST detail the security and other services offered in the layer as well as how these services are to be implemented. 5) If the underlying cryptographic technology used by a mechanism supports data integrity, then the mechanism specification MUST Zeilenga SASL [Page 16]
INTERNET-DRAFT draft-zeilenga-sasl-rfc2222bis-00.txt 15 May 2005 integrity protect the transmission of an authorization identity and the negotiation of the security layer. SASL mechanisms SHOULD be protocol neutral. SASL mechanisms SHOULD reuse existing credential and identity forms, as well as associated syntaxes and semantics. SASL mechanisms SHOULD use UTF-8 transformation format [RFC3629] for encoding Unicode [Unicode] code points for transfer. In order to avoid interoperability problems due to differing normalizations, when a mechanism calls for character data is to be used as input to a cryptographic and/or comparison function, the specification MUST detail precisely how and where (client or server) the character data is to be prepared, including all normalizations, for input into the function to ensure proper operation. For simple user names and/or passwords in authentication credentials, SASLprep [RFC4013] (a profile of the StringPrep [RFC3454] preparation algorithm), SHOULD be specified as the preparation algorithm. The mechanism SHOULD NOT use the authorization identity string in generation of any long-term cryptographic keys or hashes as there is no requirement that the authorization identity string be be non-canonical. Long-term, here, means a term longer than the duration of the authentication exchange in which they were generated in. That is, as different clients (of the same or different protocol) may provide different authorization identity strings which are semantically equivalent, use of authorization identity strings in generation of cryptographic keys and hashes will likely lead to interoperability and other problems. 6. Security Considerations Security issues are discussed throughout this memo. Many existing SASL mechanisms do not provide adequate protection against passive attacks, let alone active attacks, against the authentication exchange. Many existing SASL mechanisms do not offer any security layers. It is hoped that future SASL mechanisms will provide strong protection against passive and active attacks in the authentication exchange, as well as security layers with strong basis data security features (e.g., data integrity and data confidentiality) services. It is also hoped that future mechanisms will provide more advance data security services like re-keying (see Section 6.1). Zeilenga SASL [Page 17]
INTERNET-DRAFT draft-zeilenga-sasl-rfc2222bis-00.txt 15 May 2005 Regardless, the SASL framework is suspectable to downgrade attacks. Section 6.1 offers a variety of approaches for preventing or detecting these attacks. In some cases, it is appropriate to use data integrity protective services (e.g., TLS) external to SASL to protect against downgrade attacks in SASL. This is especially true when the mechanisms available to the client do not themselves offer adequate integrity or confidentiality protection of the authentication exchange and/or protocol data. 6.1. Active Attacks 6.1.1. Man-in-the-middle Attacks 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 plain-text connection. In this case, 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. Negotiation of the SASL mechanism, initiation of the SASL exchange, and portions of the SASL authentication exchange itself are security-sensitive negotiations. 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. In order to detect downgrade attacks to the least (or less) secure mechanism supported, the client may discover the SASL mechanisms the server makes available both before the SASL authentication exchange and after the negotiated SASL security layer (with at least integrity protection) has been installed through the protocol's mechanism discovery facility. If the client finds that the integrity protected list (the list obtained after the security layer was installed) contains a stronger mechanism than those in the previously obtained list, the client should assume the previously obtained list was modified by an attacker. Zeilenga SASL [Page 18]
INTERNET-DRAFT draft-zeilenga-sasl-rfc2222bis-00.txt 15 May 2005 The client's initiation of the SASL exchange, including the the 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. When use of a security layer is negotiated by the authentication protocol exchange, the receiver should handle gracefully any protected data buffer larger than the defined/negotiated maximal size. In particular, it must not blindly allocate the amount of memory specified in the buffer size field, as this might cause the "out of memory" condition. If the receiver detects a large block, it SHOULD close the connection. Distributed server implementations need to be careful in how they trust other parties. In particular, authentication secrets should only be disclosed to other parties that are trusted to manage and use those secrets in manner acceptable to disclosing party. Applications using SASL assume that SASL security layers providing data confidentiality are secure even when an attacker chooses the text to be protected by the security layer. Similarly applications assume that the SASL security layer is secure even if the attacker can manipulate the cipher-text output of the security layer. New SASL of new SASL mechanisms SHOULD meet these assumptions. 6.1.2. Replay Attacks Some mechanisms may be subject to replay attacks unless protected by external data security services (e.g., TLS). 6.1.3. Truncation Attacks Most existing SASL security layers to not, themselves, offer protection against truncation attack. In a truncation attacks, the active attacker causes the protocol session to be closed, causing a truncation of the possibly integrity protected data stream that leads to behavior of one or both the protocol peers that inappropriate benefits the attacker. Truncation attacks are fairly easy to defend against in connection-oriented application-level protocols. A protocol can defend against these attacks simply by ensuring that each information exchange has a clear final result and that each protocol session has a graceful closure mechanism, and that these are integrity protected. Zeilenga SASL [Page 19]
INTERNET-DRAFT draft-zeilenga-sasl-rfc2222bis-00.txt 15 May 2005 6.2. Passive Attacks Many mechanisms are subject to various passive attacks, including simple eavesdropping of unprotected credential information in mechanisms, such as PLAIN, to online and offline dictionary attacks. 6.3. Re-keying The secure or administratively permitted lifetimes of SASL mechanisms' security layers are finite. Cryptographic keys weaken as they are used and as time passes; the more time and/or cipher-text that a cryptanalyst has after the first use of the a key, the easier it is for the cryptanalyst to mount attacks on the key. Administrative limits on security layers lifetime may take the form of time limits expressed in X.509 certificates, Kerberos V tickets, or in directories, and are often desired. In practice one likely effect of administrative security layers lifetime limits is that applications may find that security layers stop working in the middle of application protocol operation, such as, perhaps, during large data transfers. As the result of this the connection will be closed (see Section 3.7), which will result in unpleasant user experience. Re-keying (key renegotiation process) is a way of addressing the weakening of cryptographic keys. SASL framework does not itself provide for re-keying. SASL mechanisms may. Designers of future SASL mechanisms should consider providing re-keying services. Applications that wish to re-key SASL security layers where the mechanism does not provide for re-keying should reauthenticate the same IDs and replace the expired or soon-to-expire security layers. This approach requires support for reauthentication in the application protocols (see Section 3.8). 6.5. Other Considerations Unicode security considerations [UTR36] apply to authorization identity strings, and well as UTF-8 [RFC3629] security considerations where UTF-8 is used. SASLprep [RFC4013] and StringPrep [RFC3454] security considerations also apply where used. Protocol designers and implementors should understand the security considerations of mechanisms so they may select mechanisms which are applicable to their needs. Multi-level negotiation of security features is prone to downgrade Zeilenga SASL [Page 20]
INTERNET-DRAFT draft-zeilenga-sasl-rfc2222bis-00.txt 15 May 2005 attack. Protocol designers should avoid offering higher level negotiation of security features in protocols (e.g., above SASL mechanism negotiation) and mechanism designers should avoid lower level negotiation of security features in mechanisms (e.g., below SASL mechanism negotiation). 7. IANA Considerations 7.1. SASL Mechanism Registry SASL mechanism registry is maintained by IANA. The registry is currently available at <http://www.iana.org/assignments/sasl-mechanisms>. 7.1.1. Registration Procedure Registration of a SASL mechanism is requested by filling in the following template: Subject: Registration of SASL mechanism X Family of SASL mechanisms: (YES or NO) SASL mechanism name (or prefix for the family): Security considerations: Published specification (recommended): Person & email address to contact for further information: Intended usage: (One of COMMON, LIMITED USE or OBSOLETE) Owner/Change controller: Note: (Any other information that the author deems interesting may be added here .) and sending it via electronic mail 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. IANA will register new values on a First Come First Served basis, as defined in BCP 64 [RFC2434]. There is no naming convention for SASL mechanisms; any name that conforms to the syntax of a SASL mechanism name can be registered. Zeilenga SASL [Page 21]
INTERNET-DRAFT draft-zeilenga-sasl-rfc2222bis-00.txt 15 May 2005 However an IETF Standards Track document may reserve a portion of the SASL mechanism namespace ("family of SASL mechanisms") for its own use, amending the registration rules for that portion of the namespace. Each family of SASL mechanisms MUST be identified by a prefix. While the registration procedures do not require expert review, 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. 7.1.2. Comments on SASL Mechanism Registrations Comments on registered SASL mechanisms should first be sent to the "owner" of the mechanism and/or to the SASL WG mailing list. Submitters of comments may, after a reasonable attempt to contact the owner, request IANA to attach their comment to the SASL mechanism registration itself by sending mail to <iana@iana.org>. At IANA sole discretion, IANA may attach the comment to the registration SASL mechanism. 7.1.3. 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 usage" 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 Zeilenga SASL [Page 22]
INTERNET-DRAFT draft-zeilenga-sasl-rfc2222bis-00.txt 15 May 2005 are on the IETF standards track. 7.2. Registration Changes It is requested that IANA updates 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 as indicated below: Subject: Updated Registration of SASL mechanism EXTERNAL Family of SASL mechanisms: NO SASL mechanism name: EXTERNAL Security considerations: See RFC XXXX, section 9. Published specification (optional, recommended): RFC XXXX Person & email address to contact for further information: Alexey Melnikov <Alexey.Melnikov@isode.com> Intended usage: COMMON Owner/Change controller: IESG <iesg@ietf.org> Note: Updates existing entry for EXTERNAL 8. References [[Note to the RFC Editor: please replace the citation tags used in referencing Internet-Drafts with tags of the form RFCnnnn where possible.]] 8.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14 (also RFC 2119), March 1997. [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26 (also RFC 2434), October 1998. [RFC2743] Linn, J., "Generic Security Service Application Program Interface, Version 2, Update 1", RFC 2743, January 2000. [RFC3454] Hoffman, P. and M. Blanchet, "Preparation of Internationalized Strings ('stringprep')", RFC 3454, December 2002. Zeilenga SASL [Page 23]
INTERNET-DRAFT draft-zeilenga-sasl-rfc2222bis-00.txt 15 May 2005 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC 3629 (also STD 63), November 2003. [RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names and Passwords", RFC 4013, February 2005. [ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", draft-crocker-abnf-rfc2234bis, a work in progress. [ASCII] Coded Character Set--7-bit American Standard Code for Information Interchange, ANSI X3.4-1986. [Unicode] The Unicode Consortium, "The Unicode Standard, Version 3.2.0" is defined by "The Unicode Standard, Version 3.0" (Reading, MA, Addison-Wesley, 2000. ISBN 0-201-61633-5), as amended by the "Unicode Standard Annex #27: Unicode 3.1" (http://www.unicode.org/reports/tr27/) and by the "Unicode Standard Annex #28: Unicode 3.2" (http://www.unicode.org/reports/tr28/). [CharModel] Whistler, K. and M. Davis, "Unicode Technical Report #17, Character Encoding Model", UTR17, <http://www.unicode.org/unicode/reports/tr17/>, August 2000. [Glossary] The Unicode Consortium, "Unicode Glossary", <http://www.unicode.org/glossary/>. 8.2. Informative References [RFC2244] Newman, C. and J. Myers, "ACAP -- Application Configuration Access Protocol", RFC 2244, November 1997. [RFC2246] Dierks, T. and, C. Allen, "The TLS Protocol Version 1.0", RFC 2246, January 1999. [RFC3548] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 3548, July 2003. [RFC2401] Kent, S., and R. Atkinson, "Security Architecture for the Internet Protocol", RFC 2401, November 1998. [SASL-GSSAPI] Melnikov, A. (Editor), "SASL GSSAPI mechanisms", draft-ietf-sasl-gssapi-XX.txt, a work in progress. [UTR36] Davis, M., "(Draft) Unicode Technical Report #36, Character Encoding Model", UTR17, Zeilenga SASL [Page 24]
INTERNET-DRAFT draft-zeilenga-sasl-rfc2222bis-00.txt 15 May 2005 <http://www.unicode.org/unicode/reports/tr36/>, February 2005. 9. Editors' Address Alexey Melnikov Isode Limited 5 Castle Business Village 36 Station Road Hampton, Middlesex, TW12 2BX, United Kingdom Email: Alexey.Melnikov@isode.com URI: http://www.melnikov.ca/ Kurt D. Zeilenga OpenLDAP Foundation Email: Kurt@OpenLDAP.org 10. Acknowledgments This document is a revision of RFC 2222 written by John Myers. This revision is a product of the IETF Simple Authentication and Security Layer (SASL) Working Group. The following individuals contributed significantly to this revision: Abhijit Menon-Sen, Hallvard B Furuseth, Jeffrey Hutzelman, John Myers, Luke Howard, Magnus Nystrom, Nicolas Williams, Peter Saint-Andre, RL 'Bob' Morgan, Rob Siemborski, Sam Hartman, Simon Josefsson, Tim Alsop, and Tony Hansen. Appendix A. The SASL EXTERNAL Mechanism This appendix is normative. The EXTERNAL mechanism allows a client to request the server use credentials established by means external to the mechanism to authenticate the client. The external means may be, for instance, IP Security [RFC2401] or TLS [RFC2246] services. In absence of some apriori agreement between the client and the server, the client cannot make any assumption as to what external means the server has used to Zeilenga SASL [Page 25]
INTERNET-DRAFT draft-zeilenga-sasl-rfc2222bis-00.txt 15 May 2005 obtain the client's credentials, nor make an assumption as to the form of credentials. For example, the client cannot assume the server will use the credentials the client has established via TLS. A.1. EXTERNAL Technical Specification The name of this mechanism is "EXTERNAL". The mechanism does not provide a security layer. The mechanism is capable of transferring an authorization identity string. If empty, the client is requesting to act as the identity the server has associated with the client's credentials. If non-empty, the client is requesting to act as the identity represented by the string. The client is expected to send data first in the authentication exchange. Where the client does not provide an initial response data in its request to initiate the authentication exchange, the server is to respond to the request with an empty initial challenge and then the client is to provide its initial response. The client sends the initial response containing the UTF-8 [RFC3629] encoding of the requested authorization identity string. This response is non-empty when the client is requesting to act as identity represented by the (non-empty) string. This response is empty when the client is requesting to act as the identity the server associated with its authentication credentials. The syntax of the initial response is specified as a value of the <extern-initial-resp> production detailed below using the Augmented Backus-Naur Form (ABNF) [RFC2234] notation. external-initial-resp = authz-id-string authz-id-string = *( UTF8-char-no-nul ) UTF8-char-no-nul = UTF8-1-no-nul / UTF8-2 / UTF8-3 / UTF8-4 UTF8-1-no-nul = %x01-7F where the <UTF8-2>, <UTF8-3>, and <UTF8-4> productions are as defined in [RFC3629]. There are no additional challenges and responses. Hence, the server is to return the outcome of the authentication exchange. The exchange fails if - the client has not established its credentials via external means, Zeilenga SASL [Page 26]
INTERNET-DRAFT draft-zeilenga-sasl-rfc2222bis-00.txt 15 May 2005 - the client's credentials are inadequate, - The client provided an empty authorization identity string and the server unwilling or unable to associate an authorization identity with the clients credentials, - The client provided a non-empty authorization identity string which is invalid per the syntax requirements of the applicable application protocol specification, - The client provided a non-empty authorization identity string representing an identity which the client is not allowed to act as, or - the server is unwilling or unable to provide service to the client for any other reason. Otherwise the exchange is successful. When indicating a successful outcome, additional data is not provided. A.2. SASL EXTERNAL Examples This section provides examples of EXTERNAL authentication exchanges. The examples are intended to help the readers under the above text. The examples are not definitive. The Application Configuration Access Protocol (ACAP) [RFC2244] is used in the examples. The first example shows use of EXTERNAL with an empty authorization identity. In this example, the initial response is not sent the client's request to initiate authentication exchange. S: * ACAP (SASL "DIGEST-MD5") C: a001 STARTTLS S: a001 OK "Begin TLS negotiation now" <TLS negotiation, further commands are under TLS layer> S: * ACAP (SASL "DIGEST-MD5" "EXTERNAL") C: a002 AUTHENTICATE "EXTERNAL" S: + "" C: + "" S: a002 OK "Authenticated" In second example shows use of EXTERNAL with an authorization identity of "fred@example.com". In this example, the initial response is sent with the clients request to initiate the authentication exchange. This saves a round-trip. S: * ACAP (SASL "DIGEST-MD5") C: a001 STARTTLS S: a001 OK "Begin TLS negotiation now" <TLS negotiation, further commands are under TLS layer> S: * ACAP (SASL "DIGEST-MD5" "EXTERNAL") Zeilenga SASL [Page 27]
INTERNET-DRAFT draft-zeilenga-sasl-rfc2222bis-00.txt 15 May 2005 C: a002 AUTHENTICATE "EXTERNAL" {16+} C: fred@example.com S: a002 NO "Cannot assume requested authorization identity" A.3. Security Considerations 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 adequate security services have been established. Appendix B. Changes since RFC 2222 This appendix is non-normative. The material in RFC 2222 was significantly rewritten in the production of this document. RFC 2222, by not stating the authorization identity string was a string of Unicode characters, let alone character data, implied the authorization identity string was a string of octets. - The authorization identity string is now defined as a string of Unicode characters. The NUL (U+0000) is prohibited. While protocol specifications are responsible for defining the authorization identity form, as well as the Unicode string syntax and related semantics, mechanism specifications are responsible for defining how the Unicode string is carried in the authentication exchange. The following technical change was made to the EXTERNAL mechanism: - The authorization identity string is to be UTF-8 encoded. It is noted that protocol and mechanism specification requirements have been significant tightened. Existing protocol and mechanism specifications will need to be updated to meet these requirements. Intellectual Property Rights The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in Zeilenga SASL [Page 28]
INTERNET-DRAFT draft-zeilenga-sasl-rfc2222bis-00.txt 15 May 2005 this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Full Copyright Copyright (C) The Internet Society (2005). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Zeilenga SASL [Page 29]