Description

Tools to implement Secure Remote Password (SRP) authentication

SRP is a secure password-based authentication and key-exchange protocol - a password-authenticated key agreement protocol (PAKE).

This package contains protocol implementation for Python 2 and 3.

You may import it into you applications and use its API or you may use srptools command-line utility (CLI):

CLI usage

Command-line utility requires click package to be installed.

Basic scenario:

> srptools get_user_data_triplet
> srptools server get_private_and_public
> srptools client get_private_and_public
> srptools client get_session_data
> srptools server get_session_data

Help is available:

> srptools --help

API usage

Preliminary step. Agree on communication details:

from srptools import SRPContext

context = SRPContext('alice', 'password123')
username, password_verifier, salt = context.get_user_data_triplet()
prime = context.prime
gen = context.generator

Simplified workflow:

from srptools import SRPContext, SRPServerSession, SRPClientSession

# Receive username from client and generate server public.
server_session = SRPServerSession(SRPContext(username, prime=prime, generator=gen), password_verifier)
server_public = server_session.public

# Receive server public and salt and process them.
client_session = SRPClientSession(SRPContext('alice', 'password123', prime=prime, generator=gen))
client_session.process(server_public, salt)
# Generate client public and session key.
client_public = client_session.public

# Process client public and compare session keys.
server_session.process(client_public, salt)

assert server_session.key == client_session.key

Extended workflow

from srptools import SRPContext, SRPServerSession, SRPClientSession

# Receive username from client and generate server public.
server_session = SRPServerSession(SRPContext(username, prime=prime, generator=gen), password_verifier)
server_public = server_session.public

# Receive server public and salt and process them.
client_session = SRPClientSession(SRPContext('alice', 'password123', prime=prime, generator=gen))
client_session.process(server_public, salt)
# Generate client public and session key proof.
client_public = client_session.public
client_session_key_proof = client_session.key_proof

# Process client public and verify session key proof.
server_session.process(client_public, salt)
assert server_session.verify_proof(client_session_key_proof)
# Generate session key proof hash.
server_session_key_proof_hash = client_session.key_proof_hash

# Verify session key proof hash received from server.
assert client_session.verify_proof(server_session_key_proof_hash)

Usage hints

• srptools.constants contains basic constants which can be used with SRPContext for server and client to agree upon communication details.

• .process() methods of session classes may raise SRPException in certain circumstances. Auth process on such occasions must be stopped.

• .private attribute of session classes may be used to restore sessions:

  server_private = server_session.private
  
  # Restore session on new request.
  server_session = SRPServerSession(context, password_verifier, private=server_private)

• SRPContext is rather flexible, so you can implement some custom server/client session logic with its help.

• Basic values are represented as hex strings but base64 encoded values are also supported:

  server_public = server_session.public_b64
  
  # Receive server public and salt and process them.
  client_session = SRPClientSession(SRPContext('alice', 'password123', prime=prime, generator=gen))
  client_session.process(server_public, salt, base64=True)
  
  # Use srptools.hex_from_b64() to represent base64 value as hex.
  server_public_hex = hex_from_b64(server_public)

Links

• RFC 2945 - The SRP Authentication and Key Exchange System

  https://tools.ietf.org/html/rfc2945

• RFC 5054 - Using the Secure Remote Password (SRP) Protocol for TLS Authentication

  https://tools.ietf.org/html/rfc5054