Secure Remote Password

The library contains an implementation of the SRP6-a password authenticated key exchange protocol in srp6.h.

A SRP client provides what is called a SRP verifier to the server. This verifier is based on a password, but the password cannot be easily derived from the verifier (however brute force attacks are possible). Later, the client and server can perform an SRP exchange, which results in a shared secret key. This key can be used for mutual authentication and/or encryption.

SRP works in a discrete logarithm group. Special parameter sets for SRP6 are defined, denoted in the library as “modp/srp/<size>”, for example “modp/srp/2048”.


While knowledge of the verifier does not easily allow an attacker to get the raw password, they could still use the verifier to impersonate the server to the client, so verifiers should be protected as carefully as a plaintext password would be.

BigInt generate_srp6_verifier(const std::string &username, const std::string &password, const std::vector<uint8_t> &salt, const std::string &group_id, const std::string &hash_id)

Generates a new verifier using the specified password and salt. This is stored by the server. The salt must also be stored. Later, the given username and password are used to by the client during the key agreement step.

std::string srp6_group_identifier(const BigInt &N, const BigInt &g)
class SRP6_Server_Session
BigInt step1(const BigInt &v, const std::string &group_id, const std::string &hash_id, RandomNumberGenerator &rng)

Takes a verifier (generated by generate_srp6_verifier) along with the group_id, and output a value B which is provided to the client.

SymmetricKey step2(const BigInt &A)

Takes the parameter A generated by srp6_client_agree, and return the shared secret key.

In the event of an impersonation attack (or wrong username/password, etc) no error occurs, but the key returned will be different on the two sides. The two sides must verify each other, for example by using the shared secret to key an HMAC and then exchanging authenticated messages.

std::pair<BigInt, SymmetricKey> srp6_client_agree(const std::string &username, const std::string &password, const std::string &group_id, const std::string &hash_id, const std::vector<uint8_t> &salt, const BigInt &B, RandomNumberGenerator &rng)

The client receives these parameters from the server, except for the username and password which are provided by the user. The parameter B is the output of step1.

The client agreement step outputs a shared symmetric key along with the parameter A which is returned to the server (and allows it the compute the shared key).