// Function for SASL security layer negotiation after
// authorization completes.
//
// Returns null for failure, Base64 encoded string on
// success.
private static string?GetSecurityLayerOutgoingBlob(string?challenge, NTAuthentication clientContext)
{
// must have a security layer challenge
if (challenge == null)
{
return(null);
}
// "unwrap" challenge
byte[] input = Convert.FromBase64String(challenge);
int len;
int newOffset;
Span <byte> unwrappedChallenge;
try
{
len = clientContext.Unwrap(input, out newOffset, out _);
unwrappedChallenge = input.AsSpan(newOffset, len);
}
catch (Win32Exception)
{
// any decrypt failure is an auth failure
return(null);
}
// Per RFC 2222 Section 7.2.2:
// the client should then expect the server to issue a
// token in a subsequent challenge. The client passes
// this token to GSS_Unwrap and interprets the first
// octet of cleartext as a bit-mask specifying the
// security layers supported by the server and the
// second through fourth octets as the maximum size
// output_message to send to the server.
// Section 7.2.3
// The security layer and their corresponding bit-masks
// are as follows:
// 1 No security layer
// 2 Integrity protection
// Sender calls GSS_Wrap with conf_flag set to FALSE
// 4 Privacy protection
// Sender calls GSS_Wrap with conf_flag set to TRUE
//
// Exchange 2007 and our client only support
// "No security layer". We verify that the server offers
// option to use no security layer and negotiate that if
// possible.
if (unwrappedChallenge.Length != 4 || (unwrappedChallenge[0] & 1) != 1)
{
return(null);
}
// Continuing with RFC 2222 section 7.2.2:
// The client then constructs data, with the first octet
// containing the bit-mask specifying the selected security
// layer, the second through fourth octets containing in
// network byte order the maximum size output_message the client
// is able to receive, and the remaining octets containing the
// authorization identity.
//
// So now this contructs the "wrapped" response.
// let MakeSignature figure out length of output
byte[]? output = null;
try
{
len = clientContext.Wrap(_saslNoSecurtyLayerToken, ref output, false);
}
catch (Win32Exception)
{
// any encrypt failure is an auth failure
return(null);
}
// return Base64 encoded string of signed payload
return(Convert.ToBase64String(output, 0, len));
}