PGP stealth by Henry Hastur has the stego support for pgp2 formats and RSA. (Aside from stripping boiler plate Hal Finney had observed that you have to
make sure the RSA encryption part doesnt narrow down which key it could be
addressed to.  (A message m > user A's n public value could not be addressed
to A (as m is computed mod n, it is always < n)).

Its C code, quite old and not really maintained but perhaps you could use it
for comparison or ideas.

http://www.cypherspace.org/adam/stealth/

Adam

On Mon, Sep 02, 2013 at 11:04:05AM -0700, Romana Machado wrote:

 I've decided to upgrade my project, Stego, conceived as  an

 easy-to-use, near-universally available, maximally browser compliant,

 message PGP encrypted, steganography web app, to encode JPEGs, the most

 universal image format today (in cell phone cameras, and all over the

 web). Which means I have to decipher information-dense papers, pick a

 suitable algorithm, and code it up in client-side Javascript. Which

 greatly increases the workload, but I expect I'll be a better engineer

 for it. It also means that I'll be reusing none of the original code.

 Fortunately there are a few open source Javascript JPEG libraries. I'm

 writing to ask for help with picking the stego algorithm, hoping that

 someone here has a knowledgable opinion.

 Romana Machado

 310-940-7888