Provides the implementations of the SASL SRP mechanism, as specified in draft-burdis-cat-srp-sasl.

## Package overview

The Secure Remote Password ( SRP) is a password-based, zero-knowledge, authentication and key-exchange protocol developed by Thomas Wu. It has good performance, is not plaintext-equivalent and maintains perfect forward secrecy. It provides authentication (optionally mutual authentication) and the negotiation of a session key.

The mechanism described in the IETF Draft, and implemented in this library, is based on the SRP-6 protocol, described in "SRP Protocol Design" and "SRP: The Open Source Password Authentication Standard". SRP-6 is an improved version of the original SRP protocol (also called SRP-3) described in RFC-2945.

## Protocol description overview

In this mechanism, the following exchange occurs when a new session is negotiated between the client and the server. It will also occur when the client requests re-use of the parameters of a previous session and either the server does not support such re-use or no longer considers the previous session to be valid:

Where:

- U: is the authentication identity (username),
- I: is the authorisation identity (userid),
- sid: is the identifier of a previous session whose parameters the client wishes to re-use,
- cn: is the client's nonce used in deriving a new shared context key from the shared context key of the previous session,
- 0x00: is an octet indicating that the previous session parameters will NOT be re-used,
- N: is the safe prime modulus,
- g: is the generator,
- s: is the user's password salt,
- B: is the server's ephemeral public key,
- L: is the options list indicating available security services,
- A: is the client's ephemeral public key,
- M1: is the client's evidence that the shared key K is known,
- o: is the options list indicating chosen security services,
- cIV: is the client's initial vector for the chosen encryption algorithm,
- M2: is the server's evidence that the shared key K is known,
- sIV: is the server's initial vector for the chosen encryption algorithm,
- sid: is the identifier the server gives to this session for possible later re-use of the negotiated parameters,
- ttl: is the time period for which this session's parameters may be re-usable.

On the other hand, the following exchange occurs when the client requests that the parameters negotiated in a previous session be re-used in this session, but with a newly derived shared context key, and the server agrees:

Where:

- 0xFF: is an octet indicating that the previous session parameters will be re-used,
- sn: is the server's nonce used in deriving a new shared context key from the shared context key of the previous session.