encryption - How to do PGP in Python (generate keys, encrypt/decrypt)

Question

I'm making a program in Python to be distributed to windows users via an installer.

The program needs to be able to download a file every day encrypted with the user's public key and then decrypt it.

So I need to find a Python library that will let me generate public and private PGP keys, and also decrypt files encrypted with the public key.

Is this something pyCrypto will do (documentation is nebulous)? Are there other pure Python libraries? How about a standalone command line tool in any language?

All I saw so far was GNUPG but installing that on Windows does stuff to the registry and throws dll's everywhere, and then I have to worry about whether the user already has this installed, how to backup their existing keyrings, etc. I'd rather just have a python library or command line tool and mange the keys myself.

Update: pyME might work but it doesn't seem to be compatible with Python 2.4 which I have to use.

Answer 1

You don't need PyCrypto or PyMe, fine though those packages may be - you will have all kinds of problems building under Windows. Instead, why not avoid the rabbit-holes and do what I did? Use gnupg 1.4.9. You don't need to do a full installation on end-user machines - just gpg.exe and iconv.dll from the distribution are sufficient, and you just need to have them somewhere in the path or accessed from your Python code using a full pathname. No changes to the registry are needed, and everything (executables and data files) can be confined to a single folder if you want.

There's a module GPG.py which was originally written by Andrew Kuchling, improved by Richard Jones and improved further by Steve Traugott. It's available here, but as-is it's not suitable for Windows because it uses os.fork(). Although originally part of PyCrypto, it is completely independent of the other parts of PyCrypto and needs only gpg.exe/iconv.dll in order to work.

I have a version (gnupg.py) derived from Traugott's GPG.py, which uses the subprocess module. It works fine under Windows, at least for my purposes - I use it to do the following:

• Key management - generation, listing, export etc.
• Import keys from an external source (e.g. public keys received from a partner company)
• Encrypt and decrypt data
• Sign and verify signatures

The module I've got is not ideal to show right now, because it includes some other stuff which shouldn't be there - which means I can't release it as-is at the moment. At some point, perhaps in the next couple of weeks, I hope to be able to tidy it up, add some more unit tests (I don't have any unit tests for sign/verify, for example) and release it (either under the original PyCrypto licence or a similar commercial-friendly license). If you can't wait, go with Traugott's module and modify it yourself - it wasn't too much work to make it work with the subprocess module.

This approach was a lot less painful than the others (e.g. SWIG-based solutions, or solutions which require building with MinGW/MSYS), which I considered and experimented with. I've used the same (gpg.exe/iconv.dll) approach with systems written in other languages, e.g. C#, with equally painless results.

P.S. It works with Python 2.4 as well as Python 2.5 and later. Not tested with other versions, though I don't foresee any problems.