// Function for SASL security layer negotiation after
// authorization completes.
//
// Returns null for failure, Base64 encoded string on
// success.
private 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;
try {
len = clientContext.VerifySignature(input, 0, input.Length);
}
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". Therefore verify first byte is value 1
// and the 2nd-4th bytes are value zero since token size is not
// applicable when there is no security layer.
if (len < 4 || // expect 4 bytes
input[0] != 1 || // first value 1
input[1] != 0 || // rest value 0
input[2] != 0 ||
input[3] != 0) {
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. The response is
// payload is identical to the received server payload and the
// "authorization identity" is not supplied as it is unnecessary.
// let MakeSignature figure out length of output
byte[] output = null;
try {
len = clientContext.MakeSignature(input, 0, 4, ref output);
}
catch (Win32Exception) {
// any decrypt failure is an auth failure
return null;
}
// return Base64 encoded string of signed payload
return Convert.ToBase64String(output, 0, len);
}
