pysrp

Tom Cocagne <tom.cocagne@gmail.com>

pysrp provides a Python implementation of the Secure Remote Password protocol (SRP).

SRP Overview

SRP is a cryptographically strong authentication protocol for password-based, mutual authentication over an insecure network connection.

Unlike other common challenge-response autentication protocols, such as Kerberos and SSL, SRP does not rely on an external infrastructure of trusted key servers or certificate management. Instead, SRP server applications use verification keys derived from each user's password to determine the authenticity of a network connection.

SRP provides mutual-authentication in that successful authentication requires both sides of the connection to have knowledge of the user's password. If the client side lacks the user's password or the server side lacks the proper verification key, the authentication will fail.

Unlike SSL, SRP does not directly encrypt all data flowing through the authenticated connection. However, successful authentication does result in a cryptographically strong shared key that can be used for symmetric-key encryption.

For a full description of the pysrp package and the SRP protocol, please refer to the pysrp documentation

Note: RFC5054 now provides the de-facto standard for the hashing algorithm used for interoperable SRP implementations. When using pysrp to interact with another SRP implementation, use the srp.rfc5054_enable() method to enable RFC5054 compatibility. Otherwise a pysrp-specific default implementation will be used.

Usage Example

import srp

# Consider enabling RFC5054 compatibility for interoperation with non pysrp SRP-6a implementations
#srp.rfc5054_enable()

# The salt and verifier returned from srp.create_salted_verification_key() should be
# stored on the server.
salt, vkey = srp.create_salted_verification_key( 'testuser', 'testpassword' )

class AuthenticationFailed (Exception):
    pass

# ~~~ Begin Authentication ~~~

usr      = srp.User( 'testuser', 'testpassword' )
uname, A = usr.start_authentication()

# The authentication process can fail at each step from this
# point on. To comply with the SRP protocol, the authentication
# process should be aborted on the first failure.

# Client => Server: username, A
svr      = srp.Verifier( uname, salt, vkey, A )
s,B      = svr.get_challenge()

if s is None or B is None:
    raise AuthenticationFailed()

# Server => Client: s, B
M        = usr.process_challenge( s, B )

if M is None:
    raise AuthenticationFailed()

# Client => Server: M
HAMK     = svr.verify_session( M )

if HAMK is None:
    raise AuthenticationFailed()

# Server => Client: HAMK
usr.verify_session( HAMK )

# At this point the authentication process is complete.

assert usr.authenticated()
assert svr.authenticated()

Installation

$ pip install srp

Implementation

It consists of 3 modules: A pure Python implementation, A ctypes + OpenSSL implementation, and a C extension module. The ctypes & extension modules are approximately 10-20x faster than the pure Python implementation and can take advantage of multiple CPUs. The extension module will be used if available, otherwise the library will fall back to the ctypes implementation followed by the pure Python implementation.

Note: The test_srp.py script prints the performance timings for each combination of hash algorithm and prime number size. This may be of use in deciding which pair of parameters to use in the unlikely event that the defaults are unacceptable.

Installation from source:

   $ python setup.py install

Documentation:

   $ cd srp/doc
   $ sphinx-build -b html . <desired output directory>

Validity & Performance Testing:

   $ python setup.py build
   $ python srp/test_srp.py