Secure Remote Password
========================================

The library contains an implementation of the :wikipedia:`SRP-6a
password based key exchange protocol
<Secure_remote_password_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. Later, the client and server can
perform an SRP exchange, which results in a shared key.

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".

.. warning::

     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
     carefully protected.

.. cpp:function:: BigInt generate_srp6_verifier( \
          const std::string& identifier, \
          const std::string& password, \
          const std::vector<byte>& 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.

.. cpp:function:: std::string srp6_group_identifier( \
            const BigInt& N, const BigInt& g)

.. cpp:class:: SRP6_Server_Session

   .. cpp:function:: 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 (which must match

   .. cpp:function:: SymmetricKey step2(const BigInt& A)

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

.. cpp:function:: 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<byte>& 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).