Fix potential side-channel timing attack issue (#1375)

* Fix potential side-channel timing attack issue

* Eliminate the extra allocation

* Remove serverHash variable

---------

Co-authored-by: Rob Hague <rob.hague00@gmail.com>

src/Renci.SshNet/Security/Cryptography/CipherDigitalSignature.cs

@@ -40,7 +40,11 @@ namespace Renci.SshNet.Security.Cryptography
             var encryptedSignature = _cipher.Decrypt(signature);
             var hashData = Hash(input);
             var expected = DerEncode(hashData);
-            return expected.IsEqualTo(encryptedSignature);
+#if NETSTANDARD2_1_OR_GREATER || NETCOREAPP2_1_OR_GREATER
+            return System.Security.Cryptography.CryptographicOperations.FixedTimeEquals(expected, encryptedSignature);
+#else
+            return Chaos.NaCl.CryptoBytes.ConstantTimeEquals(expected, encryptedSignature);
+#endif
         }
 
         /// <summary>

src/Renci.SshNet/Session.cs

@@ -1291,11 +1291,11 @@ namespace Renci.SshNet
             if (_serverMac != null && _serverEtm)
             {
                 var clientHash = _serverMac.ComputeHash(data, 0, data.Length - serverMacLength);
-                var serverHash = data.Take(data.Length - serverMacLength, serverMacLength);
-
-                // TODO Add IsEqualTo overload that takes left+right index and number of bytes to compare.
-                // TODO That way we can eliminate the extra allocation of the Take above.
-                if (!serverHash.IsEqualTo(clientHash))
+#if NETSTANDARD2_1_OR_GREATER || NETCOREAPP2_1_OR_GREATER
+                if (!CryptographicOperations.FixedTimeEquals(clientHash, new ReadOnlySpan<byte>(data, data.Length - serverMacLength, serverMacLength)))
+#else
+                if (!Security.Chaos.NaCl.CryptoBytes.ConstantTimeEquals(clientHash, 0, data, data.Length - serverMacLength, serverMacLength))
+#endif
                 {
                     throw new SshConnectionException("MAC error", DisconnectReason.MacError);
                 }
@@ -1319,11 +1319,11 @@ namespace Renci.SshNet
             if (_serverMac != null && !_serverEtm)
             {
                 var clientHash = _serverMac.ComputeHash(data, 0, data.Length - serverMacLength);
-                var serverHash = data.Take(data.Length - serverMacLength, serverMacLength);
-
-                // TODO Add IsEqualTo overload that takes left+right index and number of bytes to compare.
-                // TODO That way we can eliminate the extra allocation of the Take above.
-                if (!serverHash.IsEqualTo(clientHash))
+#if NETSTANDARD2_1_OR_GREATER || NETCOREAPP2_1_OR_GREATER
+                if (!CryptographicOperations.FixedTimeEquals(clientHash, new ReadOnlySpan<byte>(data, data.Length - serverMacLength, serverMacLength)))
+#else
+                if (!Security.Chaos.NaCl.CryptoBytes.ConstantTimeEquals(clientHash, 0, data, data.Length - serverMacLength, serverMacLength))
+#endif
                 {
                     throw new SshConnectionException("MAC error", DisconnectReason.MacError);
                 }