Simple standalone tool(s) for Bitcoin message signatures, currently creating and verifying signatures in the format of the the reference Bitcoin client ("bitcoin-qt"). It is derived from the code of bitcoin-qt.
I may add support for other signature formats. Please open an issue if you're interested in these.
These tools don't protect in-memory data as well as bitcoin-qt does (from being reused after free, or being paged-out.) This is for simplicity, and because I don't know enough to feel comfortable saying I've taken precautions for you.
Remember that arguments passed on a process' command line are visible to local users in most UNIX systems, so make sure you trust all local users enough, or adjust the programs to take arguments another way. This is especially relevant for creating signatures!
The only library this depends on is OpenSSL. On Ubuntu or similar systems, the following should be sufficient to install dependencies and build:
sudo apt-get install build-essential libssl-dev
make
The bitcoin-qt documentation may be helpful in installing dependencies for your system. You don't need the databases, Boost, or miniupnp, only OpenSSL and the GNU toolchain.
Please see the scripts in tests/ for examples of how to use the tools from shell scripts.