| Internet-Draft | Extended SASL | October 2023 |
| Cridland, et al. | Expires 25 April 2024 | [Page] |
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. This document also defines how servers can request fulfillment of extra authentication related tasks, such as two factor authentication and/or password change.¶
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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 confidentiality, 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 that 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.¶
It is possible to design and implement this framework in ways that 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 to be shared by implementations of multiple protocols.¶
Since the publication of [RFC4422] a number of shortcomings in the SASL framework were identified that are addressed in this document. [[Note that the current draft only shows changes from RFC 4422.]]¶
This specification addresses a number of shortfalls in [RFC4422]:¶
Support for Two-factor authentication (2FA) or Multi-factor authentication (MFA) tasks.¶
Support for mandatory password change tasks.¶
Channel Binding advertisement.¶
Shared key derivation after successful authentication.¶
Extensions to SASL documented herein are primarily to allow for better capability discovery, and additional results beyond total success or abject failure.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
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].¶
Example lines prefaced by "C:" are sent by the client and ones prefaced by "S:" by the server. If a single "C:" or "S:" label applies to multiple lines, then the line breaks between those lines are for editorial clarity only, and are not part of the actual protocol exchange.¶
A protocol profile might have a way to indicate that while the SASL exchange was successful, it is insufficient to allow authentication at this time. We call this indicator a "continue outcome message". This can be used to indicate a need to execute extra task(s), for example, that the Client needs to perform a Second Factor Authentication ("2FA") and/or is required to change password (this list is not meant to be exhaustive in any way).¶
These tasks are analogous to a SASL mechanism, but have a number of differences - they may never attempt to negotiate a new authorization identifier, nor a new security layer.¶
Each task has a name ([[need to define syntax and maximum length]]) that SHOULD be registered with IANA.¶
A client MAY choose any one of the offered tasks; if multiple are required, once a task is successfully completed, the server will indicate that another one is required, and so on, until all mandatory tasks are complete.¶
The continue outcome message MAY include SASL mechanism specific additional data, as the successful outcome message does.¶
Finally, the continue outcome message MAY contain a textual data, which contains human-readable data explaining the nature of the step required.¶
Example 1. Continue Required (using XMPP protocol)
S: <continue xmlns='urn:xmpp:sasl:2'>
<additional-data>
SSdtIGJvcmVkIG5vdy4=
</additional-data>
<tasks>
<task>HOTP-EXAMPLE</task>
<task>TOTP-EXAMPLE</task>
</tasks>
<text>This account requires 2FA</text>
</continue>
¶
If any SASL security layer was negotiated, it takes effect after the final octet of the first continue outcome message.¶
Clients respond with a task initiation message, which has a single mandatory parameter containing the selected task name, and MAY contain any task specific data.¶
Each task MUST end either by the server sending a task failure indicator message, if the task failed, a continue outcome message, if the task was completed successfully and the server requests the client to perform a new task, or a successful outcome message, indicating that the task was completed successfully and no further tasks are needed.¶
Example 2. Fictional TOTP task
<!-- Client starts TOTP-EXAMPLE task -->
<next xmlns='urn:xmpp:sasl:2' task='TOTP-EXAMPLE'>
<totp xmlns="urn:totp:example">
SSd2ZSBydW4gb3V0IG9mIGlkZWFzIGhlcmUu
</totp>
</next>
<!-- Server provides needed data to Client -->
<task-data xmlns='urn:xmpp:sasl:2'>
<totp xmlns="urn:totp:example">
94d27acffa2e99a42ba7786162a9e73e7ab17b9d
</totp>
</task-data>
<!-- Client responds with requested TOP data -->
<task-data xmlns='urn:xmpp:sasl:2'>
<totp xmlns="urn:totp:example">
OTRkMjdhY2ZmYTJlOTlhNDJiYTc3ODYxNjJhOWU3M2U3YWIxN2I5ZAo=
</totp>
</task-data>
<!-- Server indicates successful completion of TOTP-EXAMPLE task -->
<success xmlns='urn:xmpp:sasl:2'>
<totp xmlns="urn:totp:example">
SGFkIHlvdSBnb2luZywgdGhlcmUsIGRpZG4ndCBJPw==
</totp>
<authorization-identifier>juliet@montague.example</authorization-identifier>
</success>
¶
Most modern SASL mechanism are expected to support channel bindings [RFC5056].¶
Explicit advertisement of server supported channel binding types improves interoperability and is also necessary to prevent certain MITM attacks.¶
Example 3. Channel Binding Type advertisement in XMPP (XEP-0440).
Note that the <sasl-channel-binding> element is the new channel
binding advertisement.
<stream:features>
<sasl-channel-binding xmlns='urn:xmpp:sasl-cb:0'>
<channel-binding type='tls-server-end-point'/>
<channel-binding type='tls-exporter'/>
</sasl-channel-binding>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>EXTERNAL</mechanism>
<mechanism>SCRAM-SHA-1-PLUS</mechanism>
<mechanism>PLAIN</mechanism>
</mechanisms>
</stream:features>
¶
[[Note that the item number continues the numbering of requirements from Section 4 of RFC 4422.]]¶
[[Alexey: All of the features below are optional (in order to remain backward compatible with RFC 4422). However if any is implemented, all of them MUST be implemented in a protocol. This makes client implementations easier.]]¶
9) Facility to advertise the list of Channel Bindings that the server supports. See Section 4.¶
10) Detail any task negotiation facility that the protocol provides. [[Alexey: does this need to be done before authentication starts, or is it sufficient to discover these at the end of SASL mechanism exchange, when specific extra tasks are requested?]] A protocol MUST specify a facility through which the client may discover, the names of the SASL tasks that the server makes available to the client. This facility is typically provided through the protocol's extensions or capabilities discovery facility (so that the client can be made aware of which tasks might be required to complete authentication before starting any authentication), as well as provided at the end of a SASL authentication exchange.¶
11) Definition of the messages necessary for executing tasks, including the following:¶
A message to initiate execution of a task (see Section 3). This message MUST contain a field for carrying the name of the task selected by the client. This message MUST allow for an optional field for carrying an initial task related data. The specification MUST describe how messages with an empty initial task related data are distinguished from messages with no initial task related data. This field MUST be capable of carrying arbitrary sequences of octets (including zero-length sequences and sequences containing zero-valued octets).¶
Messages to transfer task related data from server and from client (see Section 3). Each of these messages MUST be capable of carrying arbitrary sequences of octets (including zero-length sequences and sequences containing zero-valued octets).¶
A message to indicate the outcome of the task execution and whether or not execution of further tasks is required by the server (see Section 3). This message MUST allow for an optional field for carrying additional data with a successful outcome. 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).¶
[[Note that the number continues the numbering of requirements from Section 5 of RFC 4422.]]¶
6) Optional facility for SASL mechanism shared key derivation after a successful authentication. .¶
[[Ideally need 2 protocols. For example XMPP and IMAP. IMAP can have a new command for tasks.]]¶
TBD¶
IANA is requested to create a new subregistry of "SASL tasks" for registering SASL tasks as specified in this document.¶
The registration template is as follows:¶
Task Name: <need to define syntax!> Task Type: <define the list of types and whether or not it will be extensible> Pointer to specification text: Notes (optional):¶
The registration procedure for the above registry is Expert Review.¶
TBD. Thank you XSF for XEP-0388, XEP-0440 and XEP-scram-upgrade.¶