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/*
* SRP-6a (RFC 5054 compatatible)
* (C) 2011,2012 Jack Lloyd
*
* Botan is released under the Simplified BSD License (see license.txt)
*/
#include <botan/srp6.h>
#include <botan/dl_group.h>
#include <botan/numthry.h>
namespace Botan {
namespace {
BigInt hash_seq(const std::string& hash_id,
size_t pad_to,
const BigInt& in1,
const BigInt& in2)
{
std::unique_ptr<HashFunction> hash_fn(HashFunction::create_or_throw(hash_id));
hash_fn->update(BigInt::encode_1363(in1, pad_to));
hash_fn->update(BigInt::encode_1363(in2, pad_to));
return BigInt::decode(hash_fn->final());
}
BigInt compute_x(const std::string& hash_id,
const std::string& identifier,
const std::string& password,
const std::vector<uint8_t>& salt)
{
std::unique_ptr<HashFunction> hash_fn(HashFunction::create_or_throw(hash_id));
hash_fn->update(identifier);
hash_fn->update(":");
hash_fn->update(password);
secure_vector<uint8_t> inner_h = hash_fn->final();
hash_fn->update(salt);
hash_fn->update(inner_h);
secure_vector<uint8_t> outer_h = hash_fn->final();
return BigInt::decode(outer_h);
}
}
std::string srp6_group_identifier(const BigInt& N, const BigInt& g)
{
/*
This function assumes that only one 'standard' SRP parameter set has
been defined for a particular bitsize. As of this writing that is the case.
*/
try
{
const std::string group_name = "modp/srp/" + std::to_string(N.bits());
DL_Group group(group_name);
if(group.get_p() == N && group.get_g() == g)
return group_name;
throw Exception("Unknown SRP params");
}
catch(...)
{
throw Invalid_Argument("Bad SRP group parameters");
}
}
std::pair<BigInt, SymmetricKey>
srp6_client_agree(const std::string& identifier,
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)
{
DL_Group group(group_id);
const BigInt& g = group.get_g();
const BigInt& p = group.get_p();
const size_t p_bytes = group.get_p().bytes();
if(B <= 0 || B >= p)
throw Exception("Invalid SRP parameter from server");
BigInt k = hash_seq(hash_id, p_bytes, p, g);
BigInt a(rng, 256);
BigInt A = power_mod(g, a, p);
BigInt u = hash_seq(hash_id, p_bytes, A, B);
const BigInt x = compute_x(hash_id, identifier, password, salt);
BigInt S = power_mod((B - (k * power_mod(g, x, p))) % p, (a + (u * x)), p);
SymmetricKey Sk(BigInt::encode_1363(S, p_bytes));
return std::make_pair(A, Sk);
}
BigInt generate_srp6_verifier(const std::string& identifier,
const std::string& password,
const std::vector<uint8_t>& salt,
const std::string& group_id,
const std::string& hash_id)
{
const BigInt x = compute_x(hash_id, identifier, password, salt);
DL_Group group(group_id);
return power_mod(group.get_g(), x, group.get_p());
}
BigInt SRP6_Server_Session::step1(const BigInt& v,
const std::string& group_id,
const std::string& hash_id,
RandomNumberGenerator& rng)
{
DL_Group group(group_id);
const BigInt& g = group.get_g();
const BigInt& p = group.get_p();
m_p_bytes = p.bytes();
m_v = v;
m_b = BigInt(rng, 256);
m_p = p;
m_hash_id = hash_id;
const BigInt k = hash_seq(hash_id, m_p_bytes, p, g);
m_B = (v*k + power_mod(g, m_b, p)) % p;
return m_B;
}
SymmetricKey SRP6_Server_Session::step2(const BigInt& A)
{
if(A <= 0 || A >= m_p)
throw Exception("Invalid SRP parameter from client");
const BigInt u = hash_seq(m_hash_id, m_p_bytes, A, m_B);
const BigInt S = power_mod(A * power_mod(m_v, u, m_p), m_b, m_p);
return BigInt::encode_1363(S, m_p_bytes);
}
}
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