Simple standalone tool(s) for Bitcoin message signatures, currently only verifying of signatures from the reference Bitcoin client ("bitcoin-qt"). It is derived from the code of bitcoin-qt.
I may add support for creation of signatures or other signature formats. Please open an issue if you're interested in these.
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 tests/verifymessage for examples of how to use the verifymessage tool from a shell script.
These tools may not protect in-memory data as well as bitcoin-qt does (from leaking after free, or being paged-out.) For now I don't think this is a problem since there are no secrets being handled.
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.