AaltoTLS is an implementation of the SSL/TLS network protocol written completely using C# and standard cryptographic libraries of .NET where relevant. It is designed to run fully on open source Mono implementation as well, working around possible bugs and incompatibilities if present. Focus of the project has been mainly on clean layered structure of the code and easy extensibility, not so much on performance or memory use. This is because the project has originally been part of a research project. So benchmarking might be required before using it in production environments.
All code is released under the following MIT/X11 license unless otherwise stated:
Copyright (C) 2010-2011 Aalto University Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
Compiling the source code
There are two methods supported for compiling the source code. The preferred one is using NAnt, in which running command "nant" in the AaltoTLS folder should compile all source files to AaltoTLS-debug folder. If you wish to compile a release build without the debugging information, it can be done running the command "nant release" and it will compile the files to AaltoTLS-release folder.
In development the MonoDevelop 2.6 IDE is used as well, and all the necessary project files should be included. In this case opening the AaltoTLS.sln in MonoDevelop should open the solution and "Build All" should build all the source files into their respective folders.
Supported TLS features
Protocol support: SSLv3.0, TLSv1.0, TLSv1.1, TLSv1.2 Key exchange algorithms: RSA, DHE, ECDHE Signature algorithms: RSA, DSS, ECDSA Encryption algorithms: RC4, DES-CBC, 3DES-CBC, AES-CBC, AES-GCM, SEED, Camellia, ARIA-CBC, ARIA-GCM MAC algorithms: SSLv3, MD5, SHA-1, SHA-256, SHA-384, SHA-512 Pseudorandom functions: SSLv3, TLSv1, TLSv1.2 (SHA-256, SHA-384, SHA-512)
Future work to do
Important work to do:
- Logging system that doesn't output to stdout
- Cipher suite selection based on security features
Less important features:
- TLS 1.2 signature_algorithms extension support
- Session resume using the SessionID of ClientHello
- Renegotiation support as described in RFC 5746
Clean up to be done:
- TLSSession and HandshakeSession both need cleaning up
- Better API for TLSSession required, like TcpClient
Resolved library issues:
RSACryptoServiceProvider.SignData doesn't accept MD5SHA1CryptoServiceProvider, because it doesn't have an OID. If RSA signatures is to be supported in SSL 3.0, TLS 1.0 and TLS 1.1, it needs to be done manually. That's why SignatureAlgorithmRSA depends on BigInteger and is not included in the base cipher suite plugin. The TLS 1.2 version of it doesn't depend on BigInteger at all.
Support for ECC cipher suites was done using the ECDiffieHellmanCng and ECDsaCng classes that are provided by .NET 3.5 when run on Windows 7, Windows Vista SP1 or later or Windows XP SP3. They are not available on older versions of Windows or on other operating systems using Mono, therefore they are excluded from the NAnt build file to cause as little trouble as possible. It's also worth to note that using ECDiffieHellmanCng with TLS 1.2 is impossible, because the API requires including premaster secret as part of the HMAC input. It's a stupid API, but there's not much that can be done. SSLv3, TLSv1.0 and TLSv1.1 are currently supported.
Things to note:
ARCFour cipher is separated into its own plugin, because it includes references to RC4, which in turn is trademarked by RSA Security. Including this cipher is highly recommended, since many servers only support RC4 ciphers for their fast speed and simple implementation.
Diffie-Hellman and RSA signing depend on BigInteger implementation, which is not available in .NET before the version 4.0. Since by the time of writing this .NET 4.0 is not very widely used, a modified version of Microsoft BigInteger implementation that should be almost identical with .NET 4.0 BigInteger is included in the plugin. The BigInteger implementation is released under Microsoft Public License, which is a free software license but not compatible with GNU GPL.
Camellia cipher is separated into its own plugin, because it includes MIT licensed CamelliaEngine class copyrighted by The Legion Of The Bouncy Castle. The license is quite liberal, but because of copyright issues it is better to keep it as a separate plugin. Camellia is quite widely used and even preferred by some servers, so including this plugin is recommended.
ARIA cipher is separated into its own plugin, because it includes a MIT licensed ARIAEngine with different copyright. ARIA is not a very common cipher to be used with TLS and it is probably only used inside South Korea, so this should not be an issue. It is included because the plugin is a very good sample plugin of how to easily add CBC and GCM mode cipher suites with a crypto library where only ECB is available.
When running on Mono and not on real .NET, validating the certificate chain is currently not possible, because Mono doesn't implement the X509 certificate chain validation. Therefore it will always fail when run on Mono but will work when run on .NET.
TLS 1.2 is fully supported, but the SignatureAlgorithms extension is not supported, which means that all signatures are signed using SHA-1 hash. This should not be an issue, since SHA-1 is still considered secure, but in the future it might be good to upgrade support for stronger hash algorithms.
Deflate compression is supported by .NET in DeflateStream class, but this class is not useful for our purposes. TLS requires that the deflate compression can compress single packets as a continuous stream, but flush each packet separately in the middle and send it through the wire. In Microsoft implementation of DeflateStream the Flush() method is not implemented at all and therefore we would have to compress each packet separately. This results in considerable overhead and makes deflate perform badly in most use cases.
Server key generation
Example key generation using openssl:
RSA keys (from 384 bits to 16384 bits in increments of 8 bits, 2048 used):
openssl genrsa -out rsakey.pem 2048 openssl req -new -x509 -days 365 -subj '/C=FI/ST=Uusimaa/L=Espoo/O=Aalto University/CN=localhost' -key rsakey.pem -out rsaca.pem openssl x509 -in rsaca.pem -outform DER -out rsaca.der ./PEM2XML.exe rsakey.pem rsakey.xml
DSA keys (from 512 bits to 1024 bits in increments of 64 bits, 1024 used):
openssl dsaparam -noout -out dsakey.pem -genkey 1024 openssl req -new -x509 -days 365 -subj '/C=FI/ST=Uusimaa/L=Espoo/O=Aalto University/CN=localhost' -key dsakey.pem -out dsaca.pem openssl x509 -in dsaca.pem -outform DER -out dsaca.der ./PEM2XML.exe dsakey.pem dsakey.xml
ECDSAkeys (either secp256r1, secp384r1 or secp521r1, secp256r1 used):
openssl ecparam -noout -out ecdsakey.pem -genkey -name secp256r1 openssl req -new -x509 -days 365 -subj '/C=FI/ST=Uusimaa/L=Espoo/O=Aalto University/CN=localhost' -key ecdsakey.pem -out ecdsaca.pem openssl x509 -in ecdsaca.pem -outform DER -out ecdsaca.der ./PEM2CNG.exe ecdsakey.pem ecdsakey.cng
Starting the server
./TLSServer.exe rsaca.der rsakey.xml ./TLSServer.exe dsaca.der dsakey.xml ./TLSServer.exe ecdsaca.der ecdsakey.cng
If you want to decrypt the resulting traffic using Wireshark, then go to Preferences->Protocols->SSL and modify the "RSA keys list" to include:
This should make Wireshark decrypt all connections that use RSA key exchange method. The DH or DHE sessions cannot be decrypted, because Wireshark is unable to obtain the pre-master secret from DH parameters, which is the point of perfect forward secrecy provided by DH.
After all this, connecting to "https://localhost:4433" with any browser should work, except that it is giving a warning about the certificate not being signed by a known root CA, this is normal. If you do have a certificate signed by a known root CA and with intermediate certificates, you can start TLSServer with:
./TLSServer.exe server.cer intermediate.cer key.xml