csrp-gmp

csrp-gmp is a minimal C implementation of the Secure Remote Password protocol, originally written by Tom Cocagne to depend on OpenSSL, ported to LibGMP and highly improved by est31. The project is intended for direct inclusion into utilizing programs. It's only dependency is LibGMP.

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 Kereros 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.

Various notes

As LibGMP doesn't ship with a cryptographically strong PRNG, strong PRNGs provided (and seeded) by the OS are used instead. If you are on a non-windows platform, make sure that the file /dev/urandom exists and can be read by the application. Support for custom seeding has been removed.

The call srp_user_new has a new parameter, username_for_verifier, allowing to use different usernames for verifier and srp login. Also, srp_user_start_authentication and srp_verifier_new have new parameters to specify a and b values.

Also, some cleanups were done regarding types: lengths are now size_t, and the const qualifier has been removed at many places where its usage was misleading.

Added option for srp_create_salted_verification_key call to specify a salt. You can now specify bytes_b for srp_verifier_new, bytes_a for srp_user_start_authentication, allowing full control over the login process, which is good for unit tests. csrp-gmp itself uses this additional flexibility to automatically test for RFC 5054 compatibility inside test_srp.c.

We ship with OpenSSL's implementation of the SHA256 and SHA-1 hash algorithms. Support for other hash algoritms was dropped (but re-introducing is fairly easy, just copy from an OpenSSL source distribution).

Development

Clang format

You can invoke clang-format like:

clang-format-3.7 -i *.c *.h

You should do this before doing any commit to the code. Best is you put it into a commit hook, like when you execute these bash instructions:

cat << "EOF" > .git/hooks/pre-commit
#!/bin/bash
for f in `git diff-index --cached --name-only HEAD | grep -E "\.(c|h)$"` ; do
	clang-format-3.7 -i "${f}"
	git add "${f}"
done
EOF
chmod +x .git/hooks/pre-commit

Quick compile helper

If you want to compile this and get running fast, this is the command you can use on a recent Ubuntu (~15.04) box on a 64 bit arch:

cc -L/usr/lib/x86_64-linux-gnu/ -I/usr/include/ -lgmp -I. srp.c test_srp.c -Isha sha/sha*.c -lgmp -o srp-test

For other distributions/operating systems, you might have to adjust the paths.

Usage Example

For an usage example, see test_srp.c