What is required to decrypt an encrypted file?
KeyCzar is a cryptosystem on its own. It defines a container format, with a header, IV, ciphertext and possibly a (symmetric) signature. The keys it uses are also not just keys, they contain additional information. These "keys" may even contain multiple keys, i.e. an additional one for the signature creation.

To decrypt KeyCzar encrypted files you need a program that is compatible with KeyCzar. Many libraries can be downloaded from the KeyCzar GitHub site. Alternatively you can implement your own library given the specifications hosted on the same site. KeyCzar can be used from the command line as well.

In general you need to understand the protocol (and protocol version) that is being used to encrypt/decrypt files. Just specifying that something is encrypted using AES-256 doesn't say much at all.



Note that KeyCzar also contains methods for asymmetric encryption. You should however first be able to trust the public key of the receiver for that to be useful.