SASL: Simple Authentication and Security Layer

7.6

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SASL: Simple Authentication and Security Layer

This library provides implementations of some SASL (RFC 4422) mechanisms. Currently only client support is implemented; future versions of this library may add server support.

1 SASL Introduction

This library implements non-trivial (multi-message) authentication mechanisms using SASL protocol contexts. Since in general SASL is embedded as a sub-protocol of some application protocol, this library does not handle I/O directly. Instead, the user is responsible for transferring messages between the SASL context and the application protocol.

Note that some SASL authentication mechanisms leave the communication of authentication success or failure to the application layer. Even when a mechanism normally communicates the result of authentication, some applications choose to convey failure at the application layer.

The following is a sketch of a typical embedding of SASL in an application protocol:

server → client: ... Hello. I understand the following SASL mechanisms: SCRAM-SHA-1 and CRAM-MD5
client → server: I choose SCRAM-SHA-1. My initial SASL message is “<initial SCRAM-SHA-1 message>”.
server → client: My SASL response is “<SCRAM-SHA-1 response>”.
client → server: My final SASL message is “final <SCRAM-SHA-1 message>”.
server → client: My final SASL response is “<final SCRAM-SHA-1 response>”.
client → server: Great! Let’s get to work....

In particular, the application layer advertises and selects SASL mechanisms, embeds SASL messages using some application-specific framing, and resumes after authentication is complete.

2 SASL Protocol Contexts

(require sasl)

package: sasl-lib

procedure
(sasl-ctx? v) → boolean?
v : any/c

Returns #t if v is a SASL protocol context, #f otherwise.

procedure
(sasl-next-message ctx) → (or/c string? bytes?)
ctx : sasl-ctx?

Returns the next outgoing message to be sent. Subsequent calls to sasl-next-message return the same message until the outgoing message is updated after a call to sasl-receive-message.

This function may be called only when (sasl-state ctx) is 'send/receive or 'send/done; otherwise, an exception is raised.

procedure
(sasl-receive-message ctx message) → void?
ctx : sasl-ctx?
message : (or/c string? bytes?)

Update the SASL context with a newly received message.

If message represents progress or success, then ctx is updated and subseqent calls to sasl-next-message return a new message (or fail, if the protocol is done).

If message indicates authentication failure or if message is ill-formed or invalid, an exception is raised and ctx enters a permanent error state (see sasl-state). The user must take appropriate action after either kind of failure. For example, upon authentication failure the client might close the connection and try again with different credentials.

This function may be called only when (sasl-state ctx) is 'receive or 'send/receive; otherwise, an exception is raised.

procedure
(sasl-state ctx)
→ (or/c 'receive 'send/receive 'send/done 'done 'error)
ctx : sasl-ctx?

Returns a symbol indicating the state that ctx is in with respect to its protocol. The number of states is due to the following factors: the initial SASL message may be sent from the client or the server (depending on the mechanism); the final SASL message mey be sent from the client or the server (depending on the mechanism); and the SASL context doesn’t know whether an outgoing message has been forwarded to the application layer and sent.

The possible states consist of the following:

'receive: the protocol starts with ctx receiving a message
'send/receive: send the current outgoing message (sasl-next-message) if it hasn’t already been sent, then receive
'send/done: send the current outgoing message (sasl-next-message) if it hasn’t already been sent, then the SASL protocol is done
'done: the SASL protocol ended with the last received message
'error: a fatal error occurred

3 SASLprep

(require sasl/saslprep)

package: sasl-lib

procedure
(saslprep s
[ #:allow-unassigned? allow-unassigned?]) → string?
s : string?
allow-unassigned? : boolean? = #f

Implements the SASLprep (RFC 4013) algorithm for preparing user names and passwords for comparison, hashing, etc.

In general, the mechanism implementations in this library call saslprep on their arguments when appropriate.

4 SCRAM Authentication

(require sasl/scram)

package: sasl-lib

This module implements the SCRAM family of authentication mechanisms, namely SCRAM-SHA-1 and SCRAM-SHA-256. The -PLUS variants, which add channel binding, are not implemented.

The SCRAM protocol family has the following structure:

client → server: initial message with nonce prefix
server → client: reply with complete nonce and PBKDF2 salt and iteration count
client → server: client signature
server → client: authentication result and server signature

In particular: the client sends the first message; authentication success or failure is conveyed at in SASL protocol layer; and the server authenticates itself to the client. Messages are represented as strings.

procedure
(make-scram-client-ctx digest
authentication-id
password
[ #:authorization-id authorization-id])
→ sasl-ctx?
  digest : (or/c 'sha1 'sha256)
  authentication-id : string?
  password : string?
  authorization-id : (or/c string? #f) = #f

Creates a SCRAM protocol context. The digest argument selects between SCRAM-SHA-1 and SCRAM-SHA-256. The authentication-id, password, and (if provided) authorization-id arguments are automatically processed using saslprep.

5 CRAM-MD5 Authentication

(require sasl/cram-md5)

package: sasl-lib

This module implements the CRAM-MD5 authentication mechanism.

The CRAM-MD5 protocol has the following structure:

server → client: challenge
client → server: response

In particular, the server sends the first message, and the server communicates authentication success or failure at the application protocol layer. Messages are represented as strings.

procedure
(make-cram-md5-client-ctx authentication-id
password) → sasl-ctx?
authentication-id : string?
password : string?

Creates a CRAM-MD5 protocol context. The authentication-id and password arguments are automatically processed using saslprep.

6 PLAIN Authentication

(require sasl/plain)

package: sasl-lib

This module implements the PLAIN mechanism.

Since the PLAIN mechanism consists of a single message from the client to the server, it is implemented as a simple procedure rather than a SASL protocol context. The authentication outcome is conveyed at the application protocol layer.

procedure
(plain-client-message authentication-id
password
[ #:authorization-id authorization-id])
→ string?
  authentication-id : string?
  password : string?
  authorization-id : (or/c string? #f) = #f

Constructs a PLAIN client message containig the authentication-id, password, and (if present) authorization-id. The arguments are automatically processed with saslprep.

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1	SASL Introduction
2	SASL Protocol Contexts
3	SASLprep
4	SCRAM Authentication
5	CRAM-MD5 Authentication
6	PLAIN Authentication