In gpg, “decrypting” a signed message without the public key

Question

I know how to use gpg to sign messages or to verify signed messages from others. But I recently noticed that you can "decrypt" a signed message without access to their public key [although you can't verify the signature].

For example, here is a small signed message.

Unlike many signed messages, this message isn't plain-signed. I had thought that without access to the public key for this message, it wouldn't be possible to read it, let alone to verify it.

But if one uses gpg --decrypt on this message, it is able to produce the plaintext version. Alternately, if you use a service like Keybase for gpg, then Keybase is also able to produce the plaintext. What exactly is going on?

A first thought would be that the public key is somehow included in the message, but it appears that this is not true. The fingerprint of the public key is included, though that shouldn't be enough to decrypt the message, right?

sideshowbarker sideshowbarker · Accepted Answer · 2016-12-18T01:28:42

GPG with --sign --armor produces base64-encoded (more precisely Radix-64-encoded) output where the message body is still readable by simply base64-decoding the output.

So I guess another way to put it is that the message is encoded but not encrypted.

To see, run the PGP message in the question through any base64 decoder (e.g., some online one).

Because the message isn’t encrypted but instead only signed, then no key is needed to decrypt it. It’s just a signature and some text wrapped up together. So GPG unwraps it without needing a key.

The word “wrapped” here is just shorthand. Here’s a more detailed explanation:

every PGP message contains a set of what the OpenPGP spec calls “packets”
for --sign output, GPG encodes a message containing (1) a Literal Data Packet that holds the message body and (2) a Signature Packet
when --decrypting --sign output, GPG just decodes it; it decodes the literal-data-packet part to get the (unencrypted) message body and the signature-packet part to get the signature

So recipients only need the key if they want to check the message text against the signature. They don’t need the key to just read the message. They only need GPG or some other implementation of the OpenPGP Message Format standard that understands how to decode the message format.

As far as encryption, there’s no difference between that --signed message and one signed with --clearsign. Neither is encrypted. The only difference otherwise is that for a message signed with --sign, a recipient needs to use GPG to unwrap the text from the signature, while for a message signed with --clearsign, the recipient can see the message text without needing GPG.

In gpg, “decrypting” a signed message without the public key

2 Answers