Function CalculateScalarOffset

src/blindpsbt.cpp:315–347

Compute the scalar offset used for the final blinder computation value * asset_blinder + value_blinder FIXME this method should be in libsecp, as should `ComputeAndAddToScalarOffset`

// value * asset_blinder + value_blinder
// FIXME this method should be in libsecp, as should `ComputeAndAddToScalarOffset`
bool CalculateScalarOffset(uint256& out, CAmount value, const uint256& asset_blinder, const uint256& value_blinder)
{
    // If the asset_blinder is 0, then the equation resolves to just the value_blinder
    if (asset_blinder.IsNull()) {
        out = value_blinder;
        return true;
    }

    out = asset_blinder;
    uint256 val;
    // tweak_mul expects a 32 byte, big endian tweak.
    // We need to pack the 8 byte CAmount into a uint256 with the correct padding, so start it at 24 bytes from the front
    WriteBE64(val.begin() + 24, value);
    if (value > 0) {
        if (secp256k1_ec_seckey_tweak_mul(secp256k1_blind_context, out.begin(), val.begin()) != 1) return false;
    } else {
        out = value_blinder;
        return true;
    }
    if (!value_blinder.IsNull()) {
        uint256 value_negated = value_blinder;
        if (secp256k1_ec_seckey_negate(secp256k1_blind_context, value_negated.begin()) != 1) {
            return false;
        }
        // Special-case zero, which would otherwise cause `secp256k1_ec_seckey_tweak_add` to fail
        if (value_negated == out) {
            out = uint256{};
            return true;
        }
        if (secp256k1_ec_seckey_tweak_add(secp256k1_blind_context, out.begin(), value_blinder.begin()) != 1) return false;
    }
    return true;
}

// Computes a scalar offset and adds it to another existing one
bool ComputeAndAddToScalarOffset(uint256& a, CAmount value, const uint256& asset_blinder, const uint256& value_blinder)