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If I want to encrypt a text content file using a symmetric algorithm like AES, is there a standard format gpg will expect, where it can detect a symmetric key (encrypted with recipient gpg public key RSA) in my output? E.g. should the encrypted key be a fixed length field at the beginning of my output?

I am not able to use a suite gpg tools or command line. I am restricted to manually using a subset of cryptography APIs supported in a legacy system.

From reading up at e.g. https://futureboy.us/pgp.html#Symmetric, https://www.gnupg.org/gph/en/manual/x110.html, https://www.rfc-editor.org/rfc/rfc4880 I see that this approach seems to be in use.

But I am struggling to find any reference that would help me manually construct an appropriately structured "package".

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If you encrypt a text file with the GPG utility, it randomly chooses a symmetric key, encrypts the file using that key, then encrypts the symmetric key using an asymmetric algorithm, and embeds the encrypted symmetric key in the file. There is a format that GPG uses and expects, but it is not easy to work with.

There is an RFC that describes the OpenPGP file format. It's not fun reading, but with a lot of hex dumps and a lot of time with the GPG source, I did figure out the basics of a GPG-encrypted file.

I did take the openssh project and carve out the sftp piece to build a package that encrypts the files when it puts them on the remote server and decrypts them when it reads from the remote server. The format of the files is GPG-compatible, without using the GPG utility. You might find some useful code in there for generating GPG-readable files, mostly in the iron subdirectory. IronCoreLabs/ironssh GitHub project

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  • $\begingroup$ Thanks for your reply @Bob Wall. I will take a look at that GPG-readable related code as you suggest. $\endgroup$
    – MikeRoger
    Dec 5, 2018 at 9:00
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    $\begingroup$ @MikeRoger, it's been quite a while since I looked at it, but you can contact me via GitHub if you have any questions - might be able to dredge up some memories of the file format and some of its quirks and foibles. $\endgroup$
    – Bob Wall
    Dec 5, 2018 at 17:54

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