bsrp 1.0

B-first SRP-6a protocol implementation

bsrp

bsrp is a python implementation of the secure remote password protocol (SRP-6a). This library enables a login flow in which the server provides the public value B before the client reveals its own public value A. This enables a little bit cleaner of a login flow. This library is designed for server-side use, with a mock client side library included for testing and reference for frontend use. bsrp has a sister library with the same name implemented in javascript here.

Usage

The following example (taken from the test suite) showcases the flow of data between the client and the server to achieve login authentication.

from bsrp.client import (
    generate_a_pair,
    process_challenge,
    verify_session as client_verify_session,
)
from bsrp.server import (
    generate_b_pair,
    generate_salt_and_verifier,
    verify_session as server_verify_session,
)

identity = "test"
password = "#yoloswag"

# A user is initiated with a set identity and password
salt, verifier = generate_salt_and_verifier(identity, password)

# The user initiates the login process by sending the identity
# to the server. Using the verifier and salt, the server calculates
# and calculates public value B
b, B = generate_b_pair(verifier)

# Using the salt and public value B, the client generates
# message M to prove to the serve that the password is correct.
a, A = generate_a_pair()
M, session_key = process_challenge(identity, password, salt, a, A, B)

# The server then verifies the session and generates an evidence
# key H_AMK for the client to use for mutual authentication.
server_H_AMK = server_verify_session(identity, salt, verifier, A, b, M)

assert server_H_AMK is not None

# The client then calculates its own H_AMK to verify the server
# is legit.
client_H_AMK = client_verify_session(A, M, session_key, server_H_AMK)

assert client_H_AMK is not None

# Authentication success
assert server_H_AMK == client_H_AMK

Functions

Utils

SafetyException: Raised if SRP-6a safety checks fail

_get_srp_prime(): Returns integer value of the 2048-bit SRP prime

Note: you may set your own prime by overriding this method

_get_srp_generator(): Returns integer value of the generator used to generate the SRP prime

_generate_random_bytes(length: int): Returns cryptographically random bytes

_to_bytes(obj: Any): Converts object to bytes

_to_int(obj: Any): Converts object to integer

_pad(obj: Any, length: int): Returns left padded byte-string representation of number

_Hash(*args): Returns hash of concatenated argument objects

_calculate_x(salt: bytes, identity: str, password: str): Returns the calculated user secret parameter x

_calculate_M(generator: int, prime: int, identity: str, salt: bytes, A: int, B: int, session_key: bytes): Returns the calculated evidence message M

Server

MessageException: raised when the message from the client does not match.

generate_salt_and_verifier(identity: str, password: str) Returns a tuple of the salt and verification key

generate_b_pair(verifier: int): Returns private ephemeral b for later use and public value B for the client

verify_session(identity: str, salt: bytes, verifier: int, A: int, b: int,M_client: bytes): Returns None if session is invalid, evidence key if message from client is valid

Client

EvidenceException: raised when server evidence key does not match

generate_a_pair(): Returns tuple of private ephemeral a and public value A

process_challenge(identity: str, password: str, salt: bytes, a: int, A: int, B: int): Returns tuple of message and private strong session key

verify_session(A: int, M: bytes, session_key: bytes, server_H_AMK: bytes): Returns None if session is invalid, evidence key if the server was mutually authenticated