Verify digital signature without key public key in signature

Question

When receiving emails, I sometimes see the following:

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1

I found an integer overflow in PHP, in the conversation of dates to "Julian Day Count" function.

The commit, with a PoC can be found here: https://github.com/MegaManSec/php-src/commit/a538d2f5605798422f2746636 ecdc300f8ebcaa1
Use CVE-2015-1353.

CVE assignment team, MITRE CVE Numbering Authority M/S M300 202 Burlington
Road, Bedford, MA 01730 USA [ PGP key available through
http://cve.mitre.org/cve/request_id.html ]

-----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.14 (SunOS)

iQEcBAEBAgAGBQJUw/LpAAoJEKllVAevmvmsiYoH/Ag+n/8x+blfJyccUhjt39bp
nRxsuZw2MIx7COJcoufIxeycu2YGnm1O9RxZBA9lKmVYjb0wjMi9yHogcWkT8UOo 
i93ARMw7V6UFp1nV+2Kv5BgVh2EcwEBXyDkKkcaN2l68Dm3nRoApMB4i4m7G67BC 
K2T1L4eq1orQCNaR7n4hup2155pHZbLqZQRMAYn5EGQPr/+zPjgq+PQKes631US5 
SXrnthRKOAfZk9QVIlxf5t1JfVvC3Cta0HgETTaXJ8TbqEAA5AXw8hl+RmhWgFnI 
Cto73LM+iiA1tyxDXdXnfdRqV/uxmqK+FCgO6asyCuT7EYOpBZ74Wmq1z+RVJRc=
=VLR4 
-----END PGP SIGNATURE-----

From what I understand a digital signature works in the following way:

1. The content of the message is hashed to provide a fixed length output of the entire content.
2. The hash which is generated is then encrypted with the senders private key (PS: I thought public key was used to encrypt things, different context?)
3. The process above creates a digital signature which is transmitted with the original message
4. The receiver uses the public key to decrypt the ciphertext to return the hash and then hashes the message them-self to see if its the same

The problem I cant understand is the public key part, in the example above from Mitre, there is no public key sent for me to decrypt the signature. So how do I go about verifying this signature? I can see that they have provided a link "PGP key available throught" but this link contains the key ID. What is this key ID and is it what I need to find the public key?

Answer 1

Finding CVE's PGP/KEY

This article explains how to search for a GPG key, and specifically the CVE site's public key. There are instructions on how to import it there, and I've provided basic importation instructions below.

Typical GPG/PGP

In PGP/GPG when a user generates a key there is a key ID also associated with it. You'll notice, in the link that you provided, that the last 4 bytes of the fingerprint are also the key ID.

Generally, the user will upload their public key to a key server. The key ID is what the key server uses to look up keys within its database. Other users can search the key server for the key ID associated with the data they're trying to decrypt or verify. GPG uses your keychain to perform all of its operations. On a Linux box you can install GPG.

Debian:

sudo apt-get install pgpgpg

You can then import keys into your keychain with a given key ID from a keyserver:

gpg --recv-keys 0x<key_id> --keyserver 'http://key-server.org'

Or if you have the keyfile:

gpg --import <keyfile>

There are a few ways that a document can be signed. One way by creating a detached signature file. In this case you can do the following:

gpg --verify file-1.2.3.tar.sig file-1.2.3.tar

However, in your case you have both the message and the signature in the same file. You can tell because you have both -----BEGIN PGP SIGNED MESSAGE----- and -----BEGIN PGP SIGNATURE----- within the message. In this case you can just do the following:

gpg --output your-file.txt --decrypt my-file.gpg

---

For each of these verify commands GPG will attempt to look in your keychain for the key used to sign the message. If it can't find it then it will print an error message.

You mentioned that you see this in emails. If you're using Thunderbird there is an extension called Enigmail that will allow you to set keyservers, import GPG/PGP keys, and encrypt/decrypt/verify email signatures for you.

---

To answer your questions on how public keys and private keys relate I would read through Thomas Pornin's Answer here.

Answer 2

I dont have much inside into pgp or gpg, but i can tell you how this would work with simple rsa.

If you want to sign something you encrypt it with your private key, anyone who gets this message can decrypt it with your public key and thus knows that someone who has the private key has sent this message (hopefully only you have the private key).

If both had rsa keys you could encrypt it with your private key and then encrypt it with the public key of the person you want to send the message to. The receiver then decrypt the message accordingly and you have a message that only one person can open and only one person could have sent.

As far as i understand pgp and gpg they hash the messages with a secret key and then they encrypt this key with RSA.

For your problems with the keys: there are public key servers ( eg. MIT: https://pgp.mit.edu/)