private static unsafe byte[] CreateSessionData(byte[] sessionSecret, SrpUserCredential user)
{
byte[] initializationVector = SaltGenerator.Create(16);
int len = sessionSecret.Length;
int blockLen = (len + 15) & ~15; //Add 15, then round down. (Effectively rounds up to the nearest 128 bit boundary).
byte[] dataToEncrypt = new byte[blockLen];
sessionSecret.CopyTo(dataToEncrypt, 0);
//fill the remainder of the block with random bits
if (len != blockLen)
{
SaltGenerator.Create(blockLen - len).CopyTo(dataToEncrypt, len);
}
byte[] ticket = new byte[1 + 16 + 8 + 16 + 2 + blockLen + 32];
using (Aes aes = Cipher.CreateAes())
{
aes.Key = user.ServerEncryptionkey;
aes.IV = initializationVector;
aes.Mode = CipherMode.CBC;
aes.Padding = PaddingMode.None;
ICryptoTransform decrypt = aes.CreateEncryptor();
byte[] encryptedData = decrypt.TransformFinalBlock(dataToEncrypt, 0, dataToEncrypt.Length);
fixed(byte *lp = ticket)
{
using (BinaryStreamPointerWrapper stream = new BinaryStreamPointerWrapper(lp, ticket.Length))
{
stream.Write((byte)1);
stream.Write((short)len);
stream.Write(user.ServerKeyName);
stream.Write(DateTime.UtcNow);
stream.Write(initializationVector);
stream.Write(encryptedData);
stream.Write(HMAC <Sha256Digest> .Compute(user.ServerHMACKey, ticket, 0, ticket.Length - 32));
}
}
}
return(ticket);
}
public byte[] ComputeClientSignature(byte[] authMessage)
{
byte[] result = new byte[m_clientSignature.GetMacSize()];
if (Monitor.TryEnter(m_clientSignature))
{
try
{
m_clientSignature.BlockUpdate(authMessage, 0, authMessage.Length);
m_clientSignature.DoFinal(result, 0);
}
finally
{
Monitor.Exit(m_clientSignature);
}
}
else
{
return(HMAC.Compute(Scram.CreateDigest(HashMethod), StoredKey, authMessage));
}
return(result);
}
/// <summary>
/// Attempts to load the session resume ticket.
/// </summary>
/// <param name="ticket">The serialized data for the key</param>
/// <param name="user">The user's credentials so the proper encryption key can be used</param>
/// <param name="sessionSecret">the session secret decoded if successful. null otherwise</param>
/// <returns>
/// True if the ticket is authentic
/// </returns>
unsafe static bool TryLoadTicket(byte[] ticket, SrpUserCredential user, out byte[] sessionSecret)
{
sessionSecret = null;
//Ticket Structure:
// byte TicketVersion = 1
// int16 Username Length
// int16 Session Key Length
// Guid ServerKeyName
// DateTime AuthenticationTime
// byte[16] IV
// byte[] Username
// byte[] Encrypted Session Data (Round [Session Key Length] up to a 128 bit number to get the length of the encrypted data)
// byte[32] HMAC (Sha2-256)
if (ticket == null || ticket.Length < 1 + 16 + 8 + 16 + 2 + 32)
return(false);
fixed(byte *lp = ticket)
{
var stream = new BinaryStreamPointerWrapper(lp, ticket.Length);
if (stream.ReadUInt8() != 1)
{
return(false);
}
int sessionKeyLength = stream.ReadUInt16();
if (sessionKeyLength < 0 || sessionKeyLength > 1024) //Max session key is 8192 SRP.
{
return(false);
}
int encryptedDataLength = (sessionKeyLength + 15) & ~15; //Add 15, then round down. (Effecitvely rounds up to the nearest 128 bit boundary).
if (ticket.Length != 1 + 2 + 16 + 8 + 16 + encryptedDataLength + 32)
{
return(false);
}
if (!user.ServerKeyName.SecureEquals(stream.ReadGuid()))
{
return(false);
}
long maxTicketAge = DateTime.UtcNow.Ticks + TimeSpan.TicksPerMinute * 10; //Allows for time syncing issues that might move the server's time back by a few minutes.
long minTicketAge = maxTicketAge - TimeSpan.TicksPerDay;
long issueTime = stream.ReadInt64();
if (issueTime < minTicketAge || issueTime > maxTicketAge)
{
return(false);
}
byte[] initializationVector = stream.ReadBytes(16);
//Verify the signature if everything else looks good.
//This is last because it is the most computationally complex.
//This limits denial of service attackes.
byte[] hmac = HMAC <Sha256Digest> .Compute(user.ServerHMACKey, ticket, 0, ticket.Length - 32);
if (!hmac.SecureEquals(ticket, ticket.Length - 32, 32))
{
return(false);
}
byte[] encryptedData = stream.ReadBytes(encryptedDataLength);
var aes = new RijndaelManaged();
aes.Key = user.ServerEncryptionkey;
aes.IV = initializationVector;
aes.Mode = CipherMode.CBC;
aes.Padding = PaddingMode.None;
var decrypt = aes.CreateDecryptor();
sessionSecret = decrypt.TransformFinalBlock(encryptedData, 0, encryptedData.Length);
return(true);
}
}
internal static byte[] ComputeHMAC(HashMethod hashMethod, byte[] key, byte[] message)
{
return(HMAC.Compute(CreateDigest(hashMethod), key, message));
}
