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Use natural BigNum sizes for SRP hash computations

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TrinityCore/AzerothCore's UpdateBigNumbers uses BN_num_bytes (natural
size without padding) when hashing values for u and M1. Our code was
using fixed 32-byte padding which produces different hashes when any
value (salt, A, B, N) has leading zeros in big-endian representation.
This commit is contained in:
Kelsi 2026-02-05 12:59:48 -08:00
parent 45fc9996e2
commit 933b50eab5
1 changed files with 25 additions and 21 deletions
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46
src/auth/srp.cpp
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@ -113,12 +113,13 @@ void SRP::computeSessionKey() {
LOG_DEBUG("Computing session key");
// u = H(A | B) - scrambling parameter
std::vector<uint8_t> A_bytes = A.toArray(true, 32); // 32 bytes, little-endian
std::vector<uint8_t> B_bytes = B.toArray(true, 32); // 32 bytes, little-endian
// Use natural BigNum sizes to match TrinityCore's UpdateBigNumbers behavior
std::vector<uint8_t> A_bytes_u = A.toArray(true);
std::vector<uint8_t> B_bytes_u = B.toArray(true);
std::vector<uint8_t> AB;
AB.insert(AB.end(), A_bytes.begin(), A_bytes.end());
AB.insert(AB.end(), B_bytes.begin(), B_bytes.end());
AB.insert(AB.end(), A_bytes_u.begin(), A_bytes_u.end());
AB.insert(AB.end(), B_bytes_u.begin(), B_bytes_u.end());
std::vector<uint8_t> u_bytes = Crypto::sha1(AB);
u = BigNum(u_bytes, true);
@ -176,8 +177,9 @@ void SRP::computeProofs(const std::string& username) {
std::string upperUser = username;
std::transform(upperUser.begin(), upperUser.end(), upperUser.begin(), ::toupper);
// Compute H(N) and H(g)
std::vector<uint8_t> N_bytes = N.toArray(true, 32); // 32 bytes (256-bit prime)
// Compute H(N) and H(g) using natural BigNum sizes
// This matches TrinityCore/AzerothCore's UpdateBigNumbers behavior
std::vector<uint8_t> N_bytes = N.toArray(true);
std::vector<uint8_t> g_bytes = g.toArray(true);
std::vector<uint8_t> N_hash = Crypto::sha1(N_bytes);
@ -192,31 +194,33 @@ void SRP::computeProofs(const std::string& username) {
// Compute H(username)
std::vector<uint8_t> user_hash = Crypto::sha1(upperUser);
// Get A, B, and salt as byte arrays
std::vector<uint8_t> A_bytes = A.toArray(true, 32);
std::vector<uint8_t> B_bytes = B.toArray(true, 32);
std::vector<uint8_t> s_bytes = s.toArray(true, 32);
// Get A, B, and salt as byte arrays — natural sizes for hash inputs
std::vector<uint8_t> A_bytes = A.toArray(true);
std::vector<uint8_t> B_bytes = B.toArray(true);
std::vector<uint8_t> s_bytes = s.toArray(true);
// M1 = H( H(N)^H(g) | H(I) | s | A | B | K )
std::vector<uint8_t> M1_input;
M1_input.insert(M1_input.end(), Ng_xor.begin(), Ng_xor.end()); // 20 bytes
M1_input.insert(M1_input.end(), user_hash.begin(), user_hash.end()); // 20 bytes
M1_input.insert(M1_input.end(), s_bytes.begin(), s_bytes.end()); // 32 bytes
M1_input.insert(M1_input.end(), A_bytes.begin(), A_bytes.end()); // 32 bytes
M1_input.insert(M1_input.end(), B_bytes.begin(), B_bytes.end()); // 32 bytes
M1_input.insert(M1_input.end(), K.begin(), K.end()); // 40 bytes
M1_input.insert(M1_input.end(), Ng_xor.begin(), Ng_xor.end());
M1_input.insert(M1_input.end(), user_hash.begin(), user_hash.end());
M1_input.insert(M1_input.end(), s_bytes.begin(), s_bytes.end());
M1_input.insert(M1_input.end(), A_bytes.begin(), A_bytes.end());
M1_input.insert(M1_input.end(), B_bytes.begin(), B_bytes.end());
M1_input.insert(M1_input.end(), K.begin(), K.end());
M1 = Crypto::sha1(M1_input); // 20 bytes
M1 = Crypto::sha1(M1_input);
LOG_DEBUG("Client proof M1 calculated (", M1.size(), " bytes)");
LOG_DEBUG(" M1 hash input sizes: Ng_xor=20 user=20 s=", s_bytes.size(),
" A=", A_bytes.size(), " B=", B_bytes.size(), " K=", K.size());
// M2 = H( A | M1 | K )
std::vector<uint8_t> M2_input;
M2_input.insert(M2_input.end(), A_bytes.begin(), A_bytes.end()); // 32 bytes
M2_input.insert(M2_input.end(), M1.begin(), M1.end()); // 20 bytes
M2_input.insert(M2_input.end(), K.begin(), K.end()); // 40 bytes
M2_input.insert(M2_input.end(), A_bytes.begin(), A_bytes.end());
M2_input.insert(M2_input.end(), M1.begin(), M1.end());
M2_input.insert(M2_input.end(), K.begin(), K.end());
M2 = Crypto::sha1(M2_input); // 20 bytes
M2 = Crypto::sha1(M2_input);
LOG_DEBUG("Expected server proof M2 calculated (", M2.size(), " bytes)");
}