/// <summary>
/// Creates the Password Authentication Key based on the SRP protocol
/// </summary>
/// <param name="userID"> Username of CognitoUser</param>
/// <param name="userPassword">Password of CognitoUser</param>
/// <param name="poolName">PoolName of CognitoUserPool (part of poolID after "_")</param>
/// <param name="Aa">Returned from TupleAa</param>
/// <param name="B">BigInteger SRPB from AWS ChallengeParameters</param>
/// <param name="salt">BigInteger salt from AWS ChallengeParameters</param>
/// <returns>Returns the password authentication key for the SRP protocol</returns>
public static byte[] GetPasswordAuthenticationKey(string userID,
string userPassword,
string poolName,
Tuple <BigInteger, BigInteger> Aa,
BigInteger B,
BigInteger salt)
{
// Authenticate the password
// u = H(A, B)
byte[] contentBytes = CognitoAuthHelper.CombineBytes(new [] { Aa.Item1.ToBigEndianByteArray(), B.ToBigEndianByteArray() });
byte[] digest = CognitoAuthHelper.Sha256.ComputeHash(contentBytes);
BigInteger u = BigIntegerExtensions.FromUnsignedBigEndian(digest);
if (u.Equals(BigInteger.Zero))
{
throw new ArgumentException("Hash of A and B cannot be zero.");
}
// x = H(salt | H(poolName | userId | ":" | password))
byte[] userIdContent = CognitoAuthHelper.CombineBytes(new byte[][] { Encoding.UTF8.GetBytes(poolName), Encoding.UTF8.GetBytes(userID),
Encoding.UTF8.GetBytes(":"), Encoding.UTF8.GetBytes(userPassword) });
byte[] userIdHash = CognitoAuthHelper.Sha256.ComputeHash(userIdContent);
byte[] xBytes = CognitoAuthHelper.CombineBytes(new byte[][] { salt.ToBigEndianByteArray(), userIdHash });
byte[] xDigest = CognitoAuthHelper.Sha256.ComputeHash(xBytes);
BigInteger x = BigIntegerExtensions.FromUnsignedBigEndian(xDigest);
// Use HKDF to get final password authentication key
var first = (B - k * BigInteger.ModPow(g, x, N)).TrueMod(N);
var second = BigInteger.ModPow(first, Aa.Item2 + u * x, N);
HkdfSha256 hkdfSha256 = new HkdfSha256(u.ToBigEndianByteArray(), second.ToBigEndianByteArray());
return(hkdfSha256.Expand(Encoding.UTF8.GetBytes(DerivedKeyInfo), DerivedKeySizeBytes));
}
/// <summary>
/// Generates the claim for authenticating a user through the SRP protocol
/// </summary>
/// <param name="username"> Username of CognitoUser</param>
/// <param name="password"> Password of CognitoUser</param>
/// <param name="poolName"> PoolName of CognitoUserPool (from poolID: <region>_<poolName>)</param>
/// <param name="tupleAa"> TupleAa from CreateAaTuple</param>
/// <param name="saltString"> salt provided in ChallengeParameters from Cognito </param>
/// <param name="srpbString"> srpb provided in ChallengeParameters from Cognito</param>
/// <param name="secretBlockBase64">secret block provided in ChallengeParameters from Cognito</param>
/// <param name="formattedTimestamp">En-US Culture of Current Time</param>
/// <returns>Returns the claim for authenticating the given user</returns>
public static byte[] AuthenticateUser(
string username,
string password,
string poolName,
Tuple <BigInteger, BigInteger> tupleAa,
string saltString,
string srpbString,
string secretBlockBase64,
string formattedTimestamp)
{
var B = BigIntegerExtensions.FromUnsignedLittleEndianHex(srpbString);
if (B.TrueMod(N).Equals(BigInteger.Zero))
{
throw new ArgumentException("B mod N cannot be zero.", nameof(srpbString));
}
var secretBlockBytes = Convert.FromBase64String(secretBlockBase64);
var salt = BigIntegerExtensions.FromUnsignedLittleEndianHex(saltString);
// Need to generate the key to hash the response based on our A and what AWS sent back
var key = GetPasswordAuthenticationKey(username, password, poolName, tupleAa, B, salt);
// HMAC our data with key (HKDF(S)) (the shared secret)
var contentBytes = CognitoAuthHelper.CombineBytes(new [] { Encoding.UTF8.GetBytes(poolName), Encoding.UTF8.GetBytes(username),
secretBlockBytes, Encoding.UTF8.GetBytes(formattedTimestamp) });
using (var hashAlgorithm = new HMACSHA256(key))
{
return(hashAlgorithm.ComputeHash(contentBytes));
}
}
/// <summary>
/// Generates a DeviceSecretVerifierConfigType object based on a CognitoDevice's Key, Group Key, and Password
/// </summary>
/// <param name="deviceGroupKey">The Group Key of the CognitoDevice</param>
/// <param name="devicePass">A random password for the CognitoDevice (used in the future for logging in via this device)</param>
/// <param name="username">The username of the CognitoDevice user</param>
/// <returns></returns>
public static DeviceSecretVerifierConfigType GenerateDeviceVerifier(string deviceGroupKey, string devicePass, string username)
{
Random r = new Random();
byte[] userIdContent = CognitoAuthHelper.CombineBytes(
Encoding.UTF8.GetBytes(deviceGroupKey),
Encoding.UTF8.GetBytes(username),
Encoding.UTF8.GetBytes(":"),
Encoding.UTF8.GetBytes(devicePass)
);
byte[] userIdHash = CognitoAuthHelper.Sha256.ComputeHash(userIdContent);
byte[] saltBytes = new byte[16];
RandomNumberGenerator.Create().GetBytes(saltBytes);
// setting the initial byte to 0-127 to avoid negative salt or password verifier error
saltBytes[0] = (byte)r.Next(sbyte.MaxValue);
byte[] xBytes = CognitoAuthHelper.CombineBytes(saltBytes, userIdHash);
byte[] xDigest = CognitoAuthHelper.Sha256.ComputeHash(xBytes);
BigInteger x = BigIntegerExtensions.FromUnsignedBigEndian(xDigest);
var v = BigInteger.ModPow(g, x, N);
byte[] vBytes = v.ToBigEndianByteArray();
return(new DeviceSecretVerifierConfigType
{
PasswordVerifier = Convert.ToBase64String(vBytes),
Salt = Convert.ToBase64String(saltBytes)
});
}
/// <summary>
/// Internal method for generating k for the input key material to HKDF
/// </summary>
/// <returns>Returns the BigInteger k value for the HKDF protocol's ikm</returns>
private static BigInteger CreateKForGeneratingIkm()
{
var content = CognitoAuthHelper.CombineBytes(new[] { N.ToBigEndianByteArray(), g.ToBigEndianByteArray() });
var messageDigest = CognitoAuthHelper.Sha256.ComputeHash(content);
return(BigIntegerExtensions.FromBigEndian(messageDigest));
}
static AuthenticationHelper()
{
// generate k for the input key material to HKDF
var content = CognitoAuthHelper.CombineBytes(new[] { N.ToBigEndianByteArray(), g.ToBigEndianByteArray() });
var messageDigest = CognitoAuthHelper.Sha256.ComputeHash(content);
k = BigIntegerExtensions.FromUnsignedBigEndian(messageDigest);
}
/// <summary>
/// Create a cryptographically secure random BigInteger
/// </summary>
/// <returns></returns>
public static BigInteger CreateBigIntegerRandom()
{
var b = new byte[SecretKeySizeBytes];
using (var cryptoRandom = RandomNumberGenerator.Create())
{
cryptoRandom.GetBytes(b);
}
return(BigIntegerExtensions.FromUnsignedBigEndian(b));
}
/// <summary>
/// Creates the Device Password Authentication Key based on the SRP protocol
/// </summary>
/// <param name="username"> Username of Cognito User</param>
/// <param name="devicePass">Password of CognitoDevice</param>
/// <param name="deviceGroup">GroupKey of CognitoDevice</param>
/// <param name="Aa">Returned from TupleAa</param>
/// <param name="B">BigInteger SRPB from AWS ChallengeParameters</param>
/// <param name="salt">BigInteger salt from AWS ChallengeParameters</param>
/// <returns>Returns the password authentication key for the SRP protocol</returns>
public static byte[] GetDeviceAuthenticationKey(
string username,
string devicePass,
string deviceGroup,
Tuple <BigInteger, BigInteger> Aa,
BigInteger B,
BigInteger salt)
{
// Authenticate the password
// u = H(A, B)
byte[] contentBytes = CognitoAuthHelper.CombineBytes(Aa.Item1.ToBigEndianByteArray(), B.ToBigEndianByteArray());
byte[] digest = CognitoAuthHelper.Sha256.ComputeHash(contentBytes);
BigInteger u = BigIntegerExtensions.FromUnsignedBigEndian(digest);
if (u.Equals(BigInteger.Zero))
{
throw new ArgumentException("Hash of A and B cannot be zero.");
}
// x = H(salt | H(deviceGroupKey | deviceKey | ":" | devicePassword))
byte[] deviceContent = CognitoAuthHelper.CombineBytes(Encoding.UTF8.GetBytes(deviceGroup), Encoding.UTF8.GetBytes(username),
Encoding.UTF8.GetBytes(":"), Encoding.UTF8.GetBytes(devicePass));
byte[] deviceHash = CognitoAuthHelper.Sha256.ComputeHash(deviceContent);
byte[] xBytes = CognitoAuthHelper.CombineBytes(salt.ToBigEndianByteArray(), deviceHash);
byte[] xDigest = CognitoAuthHelper.Sha256.ComputeHash(xBytes);
BigInteger x = BigIntegerExtensions.FromUnsignedBigEndian(xDigest);
var gX = BigInteger.ModPow(g, x, N);
// Use HKDF to get final password authentication key
var intValue2 = (B - k * gX).TrueMod(N);
var s_value = BigInteger.ModPow(intValue2, Aa.Item2 + u * x, N);
HkdfSha256 hkdfSha256 = new HkdfSha256(u.ToBigEndianByteArray(), s_value.ToBigEndianByteArray());
return(hkdfSha256.Expand(Encoding.UTF8.GetBytes(DerivedKeyInfo), DerivedKeySizeBytes));
}
