Hi Dirk & Ken, I'm working on a similar problem... automated decryption "in the field" and what I have come to is this:
Encrypt the message with a symmetric algorithm, adding salt and a hash/checksum to ensure validity. Then, taking that result and signing with a private key. In the field - the signature is validated with the matching public key, then the symmetric algorithm decrypts the message. While it is possible that an attacker might unravel the shared keys used in the symmetric encryption, this is not so much our concern as is the authenticity of the message when received. The combination of private key signature plus hash checksum should do that. Our solution needs to be "hands off" automated, which basically precludes the idea of using passphrases (which would not stay secure in our organization anyway.) A determined attacker could get into the source code and tease out the symmetric key, but that would only show them the contents of the message, which, if they have the hardware, they can get anyway by copying the hard drive after the message is decrypted - and as stated above, this is of much less concern than a spoofed message getting automatically accepted. When I studied cryptography at Uni in the 1980s, they taught that private/public key encryption was a more or less interchangeable affair - the only difference between a private key and a public key is the manner in which they are handled. As such, I am a little disappointed in the GnuPGP implementation that doesn't allow encryption with the private key to serve as authentication and obscurity of the message - our private key will be obscured, but obviously not secured since attackers may have control of the standard computer system it is contained in. As things are, I am left to use a layer of symmetric encryption to obscure the message, no more secure in the end than using the private key to encrypt (since the symmetric key is in the devices in the wild), but much more hassle. Unless I'm missing something? Also, thus far I have decided that it's easier to do symmetric encryption with libgcrypt rather than mess with pgp... next week I'll be looking into how to implement the signature with the private key - maybe that's also practical to do in libgcrypt instead of gpgme?
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