Use PGP public key to cipher with Windows CryptoApi in C++

Question

I don't know if this is possible at all. A pair of RSA keys are generated with GNUPG, and the public key is exported to a file. My program receives such file and then it has to encrypt some data with the public RSA key in that file. The program is written in C/C++ and it won't use external libraries, so all I can use is Windows CryptoApi functions (CryptStringToBinary, CryptDecodeObjectEx, CryptImportPublicKeyInfo, etc).

This method works with a public 2048 bit RSA key in PEM format encoded in base64:

But the key exported with GPG looks quite different:

Although it's a 2048 bit key as well. If I trim the blank lines and the header, CryptStringToBinary succeeds and translates it into a binary format, but CryptDecodeObjectEx fails (GetLastError() returns 0x8009310B). Removing the checksum didn't work either.

I'm a bit lost here so, basically, Is there a way to obtain a RSA public key exported with GPG and use it to encrypt data with Windows CryptoApi?

Thank you.

OpenPGP uses its own format not only for keys, but also for encrypted content. If you're trying to encrypt in a manner such that the recipient will be able to decrypt using GnuPG, you won't be able to do so using the Windows Crypto API. Just to make sure you're aware of that. — Jens Erat
Ok, I suspected it but wasn't really sure. Then, is there any open source C/C++ library I can use to encrypt with OpenPGP public keys? Thank you. — Sekoraiko

Jens Erat Jens Erat · Accepted Answer · 2014-12-11T11:26:50

Converting OpenPGP Keys to PEM

Extracting the RSA public key from an OpenPGP key and conterting it to PEM format is possible. Sysmisc has an article about converting to and from OpenPGP keys in different ways. For the way OpenPGP to PEM, it boils down to:

gpgsm -o  secret-gpg-key.p12 --export-secret-key-p12 0xXXXXXXXX
openssl pkcs12 -in secret-gpg-key.p12 -nokeys -out gpg-certs.pem

OpenPGP has its own Message Exchange Format

Yet a caveat, if you want to encrypt to an OpenPGP user, he will not be able to read any (Open, ...)SSL encrypted information using an OpenPGP implementation like GnuPG. OpenPGP not only uses a different key format, but also another message exchange format.

If you want to send OpenPGP messages, use GPGME to interface GnuPG from C, there might also be other libraries for doing so.

Use PGP public key to cipher with Windows CryptoApi in C++

1 Answers

Converting OpenPGP Keys to PEM

OpenPGP has its own Message Exchange Format