/// <summary>
/// Transcript-Hash(M1, M2, ... Mn) = Hash(M1 || M2 || ... || Mn)
/// </summary>
/// <param name="messages"></param>
/// <returns></returns>
public BitString TranscriptHash(params BitString[] messages)
{
var concatenatedMessages = BitString.Empty();
foreach (var message in messages)
{
concatenatedMessages = concatenatedMessages.ConcatenateBits(message);
}
return(_sha.HashMessage(concatenatedMessages).Digest);
}
public TlsKdfV13HandshakeSecretResult GetDerivedHandshakeSecret(BitString diffieHellmanSharedSecret, BitString salt,
BitString clientHello, BitString serverHello)
{
var handshakeSecret = _hkdf.Extract(salt, diffieHellmanSharedSecret);
var clientHandshakeTrafficSecret = DeriveSecret(handshakeSecret, "c hs traffic", clientHello, serverHello);
var serverHandshakeTrafficSecret = DeriveSecret(handshakeSecret, "s hs traffic", clientHello, serverHello);
var derivedHandshakeSecret = DeriveSecret(handshakeSecret, "derived", BitString.Empty());
return(new TlsKdfV13HandshakeSecretResult()
{
HandshakeSecret = handshakeSecret,
ClientHandshakeTrafficSecret = clientHandshakeTrafficSecret,
ServerHandshakeTrafficSecret = serverHandshakeTrafficSecret,
DerivedHandshakeSecret = derivedHandshakeSecret,
});
}
public TlsKdfV13EarlySecretResult GetDerivedEarlySecret(bool isExternalPsk, BitString preSharedKey, BitString clientHello)
{
var earlySecret = _hkdf.Extract(_bitstringHashLengthBits, preSharedKey);
var binderKey = DeriveSecret(earlySecret, isExternalPsk ? "ext binder" : "res binder", BitString.Empty());
var clientEarlyTrafficSecret = DeriveSecret(earlySecret, "c e traffic", clientHello);
var earlyExporterMasterSecret = DeriveSecret(earlySecret, "e exp master", clientHello);
var derivedEarlySecret = DeriveSecret(earlySecret, "derived", BitString.Empty());
return(new TlsKdfV13EarlySecretResult()
{
EarlySecret = earlySecret,
BinderKey = binderKey,
ClientEarlyTrafficSecret = clientEarlyTrafficSecret,
EarlyExporterMasterSecret = earlyExporterMasterSecret,
DerivedEarlySecret = derivedEarlySecret,
});
}
/// <summary>
/// HKDF-Expand-Label(Secret, Label, Context, Length) =
/// HKDF-Expand(Secret, HkdfLabel, Length)
///
/// The Hash function used by Transcript-Hash and HKDF is the cipher
/// suite hash algorithm. Hash.length is its output length in bytes.
/// Messages is the concatenation of the indicated handshake messages,
/// including the handshake message type and length fields, but not
/// including record layer headers. Note that in some cases a zero-
/// length Context (indicated by "") is passed to HKDF-Expand-Label. The
/// labels specified in this document are all ASCII strings and do not
/// include a trailing NUL byte.
/// </summary>
/// <param name="secret">The secret.</param>
/// <param name="label">The label of the intermediate value.</param>
/// <param name="context">The context of the KDF step.</param>
/// <param name="hashLengthBytes">Hash output length in bytes.</param>
/// <returns>HKDF expanded bitstring based on the secret and built other info.</returns>
public BitString ExpandLabel(BitString secret, string label, BitString context, int hashLengthBytes)
{
const int maxLabelContextSizes = 255;
var expandedLabel = Encoding.ASCII.GetBytes("tls13 " + label);
if (label.Length > maxLabelContextSizes)
{
throw new ArgumentOutOfRangeException($"{nameof(label)} exceeds max labels length of {maxLabelContextSizes}");
}
if (context.BitLength.CeilingDivide(maxLabelContextSizes) > maxLabelContextSizes)
{
throw new ArgumentOutOfRangeException($"{nameof(context)} exceeds max labels length of {maxLabelContextSizes}");
}
var otherInfo = BitString.Empty()
.ConcatenateBits(BitString.To16BitString((short)hashLengthBytes))
.ConcatenateBits(BitString.To8BitString((byte)expandedLabel.Length))
.ConcatenateBits(new BitString(expandedLabel))
.ConcatenateBits(BitString.To8BitString((byte)(context.BitLength / BitString.BITSINBYTE)))
.ConcatenateBits(context);
return(_hkdf.Expand(secret, otherInfo, hashLengthBytes));
}
