SASL Authentication and SCRAM

SCRAM-SHA-1 is a SASL mechanism improving on DIGEST-MD5. Its main benefits are in offering both a method to salt and hash the password in storage and in transit. This page aims to give a short introduction on how to implement it in a client.

Note: There is an update of SCRAM-SHA-1/SCRAM-SHA-1-PLUS: "Salted Challenge Response Authentication Mechanism (SCRAM) SASL and GSS-API Mechanisms"

 RFC7677: SCRAM-SHA-256 and SCRAM-SHA-256-PLUS Simple Authentication and Security Layer (SASL) Mechanisms

Overview

The basic overview of how this mechanism works is:

1. The client sends the username it wants to authenticate as.
2. The server sends back the salt for that user and the number of iterations (either by generating them or looking them up in its database for the given username).
3. The client hashes the password with the given salt for the given number of iterations.
4. The client sends the result back.
5. The server does a variation of the hashing and sends it result back to the client, so the client can also verify that the server had the password/a hash of the password.

The cryptographic algorithms needed are SHA-1, HMAC with SHA-1 and PBKDF2 with SHA-1. It is advised to find libraries to use these algorithms instead of implementing them from scratch.

In detail

1. First normalize the password (using SASLprep), this will be normalizedPassword. This is to ensure the UTF8 encoding can't contain variations of the same password.
2. Pick a random string (for example 32 hex encoded bytes). This will be clientNonce.
3. The initialMessage is:

   "n=" .. username .. ",r=" .. clientNonce

4. The client prepends the GS2 header ("n,,") to the initialMessage and base64-encodes the result. It sends this as its first message:

   <auth xmlns="urn:ietf:params:xml:ns:xmpp-sasl" mechanism="SCRAM-SHA-1">
       biwsbj1yb21lbyxyPTZkNDQyYjVkOWU1MWE3NDBmMzY5ZTNkY2VjZjMxNzhl
   </auth>
   

5. The server responds with a challenge. The data of the challenge is base64 encoded:

   <challenge xmlns="urn:ietf:params:xml:ns:xmpp-sasl">
       cj02ZDQ0MmI1ZDllNTFhNzQwZjM2OWUzZGNlY2YzMTc4ZWMxMmIzOTg1YmJkNGE4ZTZmODE0YjQyMmFiNzY2NTczLHM9UVNYQ1IrUTZzZWs4YmY5MixpPTQwOTY=
   </challenge>
   

6. The client base64 decodes it:

   r=6d442b5d9e51a740f369e3dcecf3178ec12b3985bbd4a8e6f814b422ab766573,s=QSXCR+Q6sek8bf92,i=4096
   

7. The client parses this:
   • r = This is the serverNonce. The client MUST ensure that it starts with the clientNonce it sent in its initial message.
   • s = This is the salt, base64 encoded (yes, this is base64-encoded twice!)
   • i = This is the number of iterations, i.
8. The client computes:

   clientFinalMessageBare = "c=biws,r=" .. serverNonce
   saltedPassword = PBKDF2-SHA-1(normalizedPassword, salt, i)
   clientKey = HMAC-SHA-1(saltedPassword, "Client Key")
   storedKey = SHA-1(clientKey)
   authMessage = initialMessage .. "," .. serverFirstMessage .. "," .. clientFinalMessageBare
   clientSignature = HMAC-SHA-1(storedKey, authMessage)
   clientProof = clientKey XOR clientSignature
   serverKey = HMAC-SHA-1(saltedPassword, "Server Key")
   serverSignature = HMAC-SHA-1(serverKey, authMessage)
   clientFinalMessage = clientFinalMessageBare .. ",p=" .. base64(clientProof)
   

9. The client base64 encodes the clientFinalMessage and sends it as a response:

   <response xmlns="urn:ietf:params:xml:ns:xmpp-sasl">
       Yz1iaXdzLHI9NmQ0NDJiNWQ5ZTUxYTc0MGYzNjllM2RjZWNmMzE3OGVjMTJiMzk4NWJiZDRhOGU2ZjgxNGI0MjJhYjc2NjU3MyxwPXlxbTcyWWxmc2hFTmpQUjFYeGFucG5IUVA4bz0=
   </response>
   

10. If everything went well, you'll get a <success> response from the server:

    <success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
        dj1wTk5ERlZFUXh1WHhDb1NFaVc4R0VaKzFSU289
    </success>
    

    Base64 decoded this contains:

    v=pNNDFVEQxuXxCoSEiW8GEZ+1RSo=
    

11. The client MUST make sure the value of v is the base64 encoding of the serverSignature (yes, this value is also base64-encoded twice).

Extras

This is the basic version of the algorithm. You can extend it to do:

• Channel binding. This mixes in some information from the TLS connection to the procedure to prevent MitM attacks.
• Hashed storage. If the server always sends the same salt and i values, then the client can store only clientKey, instead of the user's password. This is more secure (as the client doesn't need to store the password, just a hard to reverse salt) and faster, as the client doesn't need to do all the hashing every time.
• Possibly, also adding SCRAM-SHA-256 (currently in draft) (though server support seems non-existent).

Common pitfalls

• Don't assume anything about the length of the nonces or salt (though if you generate them, make sure they are long enough and cryptographically random).
• The salt is base64 encoded and can contain any data (embedded NULs).
• Not using SASLprep may work fine for people using ASCII passwords, but it may completely break logging in for people using other scripts.
• The initialMessage part of the authMessage does not include the GS2 header (in most situations, this is "n,,").

Test vectors

Here is a complete example:

Username: user

Password: pencil

Client generates the random nonce fyko+d2lbbFgONRv9qkxdawL

Initial message: n,,n=user,r=fyko+d2lbbFgONRv9qkxdawL

Server generates the random nonce 3rfcNHYJY1ZVvWVs7j

Server replies: r=fyko+d2lbbFgONRv9qkxdawL3rfcNHYJY1ZVvWVs7j,s=QSXCR+Q6sek8bf92,i=4096

The salt (hex): 4125c247e43ab1e93c6dff76

Client final message bare: c=biws,r=fyko+d2lbbFgONRv9qkxdawL3rfcNHYJY1ZVvWVs7j

Salted password (hex): 1d96ee3a529b5a5f9e47c01f229a2cb8a6e15f7d

Client key (hex): e234c47bf6c36696dd6d852b99aaa2ba26555728

Stored key (hex): e9d94660c39d65c38fbad91c358f14da0eef2bd6

Auth message: n=user,r=fyko+d2lbbFgONRv9qkxdawL,r=fyko+d2lbbFgONRv9qkxdawL3rfcNHYJY1ZVvWVs7j,s=QSXCR+Q6sek8bf92,i=4096,c=biws,r=fyko+d2lbbFgONRv9qkxdawL3rfcNHYJY1ZVvWVs7j

Client signature (hex): 5d7138c486b0bfabdf49e3e2da8bd6e5c79db613

Client proof (hex): bf45fcbf7073d93d022466c94321745fe1c8e13b

Server key (hex): 0fe09258b3ac852ba502cc62ba903eaacdbf7d31

Server signature (hex): ae617da6a57c4bbb2e0286568dae1d251905b0a4

Client final message: c=biws,r=fyko+d2lbbFgONRv9qkxdawL3rfcNHYJY1ZVvWVs7j,p=v0X8v3Bz2T0CJGbJQyF0X+HI4Ts=

Server final message: v=rmF9pqV8S7suAoZWja4dJRkFsKQ=

Server's server signature (hex): ae617da6a57c4bbb2e0286568dae1d251905b0a4