# Twisted Conch: SSH in Python with Twisted

- Paul Swartz z3p@twistedmatrix.com

## Introduction

Although it is a newcomer on the Secure Shell stage, Twisted Conch has quickly caught up with the two most popular free *nix implementations and the most popular free Windows implementation in terms of functionality. This rapid development time, as well as the stability and other advantages, owes much to Python and the Twisted networking framework.

## Other implementations (servers)

Other than Conch, there are three popular server implementations. OpenSSH works with versions 1 and 2 of the protocol, and is the most popular on *nix systems. FSecure is more popular on Windows servers, and also works with both versions. LSH is newer, and implements version 2. All three are written in C, with LSH having some supporting Scheme code to generate C files.

## Other implementations (clients)

On *nix, the SSH clients are provided by the server implementations (OpenSSH and LSH). On Windows, there are a couple of separate clients. PuTTY is the most popular and supports Telnet along with SSH1 and 2. TeraTerm recently incorporated SSH into the core: before it had been an extension module. MindTerm is the only implementation in this list to be written in a language other than C. It runs as a Java applet, allowing SSH to run on any computer with a JVM.

## Why Twisted?

Why is Twisted ideal for this type of project? Firstly, it is an asynchronous library, meaning there are no worries about threading or concurrency issues. This means more developer time can be devoted to making the code work well, rather than just work. Second, Python lends itself to this kind of development: the code is easy to read and easy to write. Third, the Twisted library is high-level, so developers do not need to worry about select loops or callbacks. Twisted handles all of that and allows developers to concentrate on the code.

## No forking/threads

Unlike OpenSSH, the Conch server does not fork a process for each incoming connection. Instead, it uses the Twisted reactor to multiplex the connections. The only fork done is to execute a process such as a shell, but running a shell is not necessary, in which case the entire protocol would be run in-process. One of the initial features of the server was an in-process Python interpreter which allowed a user to interact with the server as it was running. (It is currently disabled for security reasons.) Threads are only used to interface with synchronous libraries, such as PyPAM (Pluggable Authentication Modules support) or PyME (GPGME support). By not using forks or threads, the time it takes for the Conch server to start an SSH session is roughly half of the time it takes for OpenSSH. However, this does require that code in Conch be non-blocking, which is an obstacle for programmers not used to that style.

## Security - No Pointers

OpenSSH, LSH, and PuTTY are all written in C. Many security holes are a result of problems with unsafe pointer usage, which is a large problem in C code. Many other security holes result from related issues, such as buffer overflows, off-by-one errors on arrays, and memory allocation/deallocation bugs. Python is pointer-safe, and so is not vulnerable to this class of hole. This also means that no arbitrary data from over-the-wire is ever run, meaning control always stays with the Conch server.

## Security - High Level

Being written in Python provides more security than just pointer safety. The strong builtin library that comes with Python (including powerful data types like the list and dictionary) means that fewer wheels need to be reinvented. This limits the potential to make mistakes in implementation. Exceptions are another powerful tool. They centralize error handling, rather than the mix of methods that the C libraries use. All errors are caught and dealt with: this might mean that the server stops accepting connections, but it never compromises security.

## Security - Not Root

Also, Conch does not need to run as root. In the default server, root privileges are used for two things: to bind to ports < 1024, and to fork a process as a different user. If neither of these are needed, the server need not run as root at all. Even if they are, the server is only running as root for those small sections. The rest of the time, it runs under the effective user and group ID of the user who started the server. This limits the amount of damage that could be inflicted in the event of a compromise.

## Interfacing with other software

OpenSSH can interact with subsystems such as SFTP only by executing a process to handle it. Not only is forking a process expensive, it limits the interaction to a generic bitstream, which leaves developers to determine how to interact with their users. Conch can run in the same process as other Python software, and is easily integrated with other Twisted servers. This allows for things like secure remote administration of a Twisted web server, encrypted communication to a Reality MUD, or secure remote object access using Perspective Broker. This saves the hassle and expense of forking, and allows Python developers to interact with Conch the way they know best: with Python.

## Speed

No one can deny that compiled C is faster than Python. Some part of Conch use C (PyCrypto, TGMP) to speed frequent operations, but the majority of the code is in Python. The client suffers the most from this because of the time it takes to start the interpreter. Work is being done to speed up the client by caching connections. This does not eliminate the interpreter start-up cost, but it removes the cost of negotiating a new connection. This effort is similar to FSH (also in Python) but interacts more nicely with the SSH protocol. Psyco helps as well, offering a speedup of roughly 2x - 5x.

## Age

As I said in the introduction, Conch is still a newcomer on the Secure Shell stage (The first commit for Conch was July 15, 2002.) Although Python solves a large class of holes, it is probable that other security holes are in the code. Until a full audit is conducted of Twisted and of Conch, it should not be used for security-critical systems.

## Applications with Conch

One of the applications for Conch is with Reality, a MUD framework using Twisted. Conch makes it easy to allow secure connections to the MUD in addition or even in place of a standard Telnet connection. As problems such as character theft become more prevalent on the Internet, a secure interface becomes more important.

More generally, work is being done on Insults, a replacement for libraries like Curses and S-Lang. It allows developers to write GUI code that interacts well with Conch and other Twisted software. Although it is in the initial stages of development, it shows much promise for the future.

## Future Directions

There are several different directions for Conch to move in. One of the most interesting is system for generalized authentication forwarding. This would allow all authentication to be performed on a host that the user controls, which would help to stop vulnerabilities such as timing attacks. Second is more work with applications. Insults is becoming more powerful, and it will be interesting to see what it can be used for. Also important are auditing of the code and increasing the speed. These will make the code more useful in general, as well as improving security. Other ideas include direct support for SFTP/SCP, support for a key agent, and interfacing with Twisted Names to support DNSSEC.

## Conclusion

Although it is new, Conch is a working implementation of the Secure Shell protocol. It is robust enough to serve as both the client and server on systems I and others use daily.