Cryptography support in OpenStego is pretty straightforward. It directly uses the Cryptography support provided by Sun's JRE which was added starting with version 1.4. Thus OpenStego will need at least JRE 1.4 to run.
javax.crypto.Cipher class is used for cryptography. PBEWithMD5AndDES is the algorithm used for encryption (Password based encryption which uses MD5 hash of the password to generate the key for DES which does actual encryption).
I would like to add support for other algorithms like AES, but I think, as of now, DES support is enough.