GTKCrypto allows you to easily encrypt, sign and compute hash of your data.
$ git clone https://github.com/paolostivanin/GTKCrypto.git
$ cd GTKCrypto
$ mkdir build && cd $_
$ cmake -DCMAKE_INSTALL_PREFIX=/usr ..
$ sudo make install
Don't trust me, trust the code. But if you really want to be sure that I'm not doing things in the wrong way, then you can just encrypt a file using GTKCrypto and write your own decryption program.
This is a relatively quick and easy task to achieve, just be sure to first understand the structure of the encrypted file (spoiler:
enc_file = metadata + encrypted_data + HMAC). More info HERE.
Stable and pre-release versions can be found HERE
This software is licensed under the GNU General Public License version 3 and above.