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diff --git a/content/blog/2022-07-14-gnupg.md b/content/blog/2022-07-14-gnupg.md new file mode 100644 index 0000000..9bd2e19 --- /dev/null +++ b/content/blog/2022-07-14-gnupg.md @@ -0,0 +1,314 @@ ++++ +date = 2022-07-14 +title = "GNU Privacy Guard (GPG)" +description = "" +draft = false ++++ + +# The History of GPG + +[GNU Privacy Guard](https://gnupg.org/), also known as GnuPG and GPG, is +a free (\"free\" as in both speech and beer) software that fully +implements the OpenPGP Message Format documented in [RFC +4880](https://www.rfc-editor.org/rfc/rfc4880). + +I won\'t go in-depth on the full history of the software in this post, +but it is important to understand that GPG is not the same as PGP +(Pretty Good Privacy), which is a different implementation of RFC 4880. +However, GPG was designed to interoperate with PGP. + +GPG was originally developed in the late 1990s by [Werner +Koch](https://en.wikipedia.org/wiki/Werner_Koch) and has historically +been funded generously by the German government. + +Now that we have all the high-level info out of the way, let\'s dive +into the different aspects of GPG and its uses. + +# Encryption Algorithms + +GPG supports a wide range of different encryption algorithms, including +public-key, cipher, hash, and compression algorithms. The support for +these algorithms has grown since the adoption of the Libgcrypt library +in the 2.x versions of GPG. + +As you will be able to see below in an example of a full key generation +with the GPG command line tool, GPG recommends the following algorithms +to new users: + +```sh +Please select what kind of key you want: + (1) RSA and RSA + (2) DSA and Elgamal + (3) DSA (sign only) + (4) RSA (sign only) + (9) ECC (sign and encrypt) *default* + (10) ECC (sign only) +``` + +I am not doing an in-depth explanation here in order to keep the focus +on GPG and not encryption algorithms. If you want a deep dive into +cryptography or encryption algorithms, please read my other posts: + +- [AES Encryption](../aes-encryption/) (2018) +- [Cryptography Basics](../cryptography-basics/) (2020) + +## Vulnerabilities + +As of 2022-07-14, there are a few different vulnerabilities associated +with GPG or the libraries it uses: + +- GPG versions 1.0.2--1.2.3 contains a bug where \"as soon as one + (GPG-generated) ElGamal signature of an arbitrary message is + released, one can recover the signer\'s private key in less than a + second on a PC.\" + ([Source](https://www.di.ens.fr/~pnguyen/pub_Ng04.htm)) +- GPG versions prior to 1.4.2.1 contain a false positive signature + verification bug. + ([Source](https://lists.gnupg.%20org/pipermail/gnupg-announce/2006q1/000211.html)) +- GPG versions prior to 1.4.2.2 cannot detect injection of unsigned + data. ( + [Source](https://lists.gnupg.org/pipermail/gnupg-announce/2006q1/000218.html)) +- Libgcrypt, a library used by GPG, contained a bug which enabled full + key recovery for RSA-1024 and some RSA-2048 keys. This was resolved + in a GPG update in 2017. + ([Source](https://lwn.net/Articles/727179/)) +- The [ROCA + Vulnerability](https://en.wikipedia.org/wiki/ROCA_vulnerability) + affects RSA keys generated by YubiKey 4 tokens. + ([Source](https://crocs.fi.%20muni.cz/_media/public/papers/nemec_roca_ccs17_preprint.pdf)) +- The [SigSpoof Attack](https://en.wikipedia.org/wiki/SigSpoof) allows + an attacker to spoof digital signatures. + ([Source](https://arstechnica.%20com/information-technology/2018/06/decades-old-pgp-bug-allowed-hackers-to-spoof-just-about-anyones-signature/)) +- Libgcrypt 1.9.0 contains a severe flaw related to a heap buffer + overflow, fixed in Libgcrypt 1.9.1 + ([Source](https://web.archive.%20org/web/20210221012505/https://www.theregister.com/2021/01/29/severe_libgcrypt_bug/)) + +### Platforms + +Originally developed as a command-line program for \*nix systems, GPG +now has a wealth of front-end applications and libraries available for +end-users. However, the most recommended programs remain the same: + +- [GnuPG](https://gnupg.org) for Linux (depending on distro) +- [Gpg4win](https://gpg4win.org) for Windows +- [GPGTools](https://gpgtools.org) for macOS + +# Creating a Key Pair + +In order to create a GPG key pair, a user would first need to install +GPG on their system. If we\'re assuming that the user is on Fedora +Linux, they would execute the following: + +```sh +sudo dnf install gpg +``` + +Once installed, a user can create a new key pair with the following +command(s): + +```sh +gpg --full-generate-key +``` + +GPG will walk the user through an interactive setup that asks for an +algorithm preference, expiration date, name, and email to associate with +this key. + +See the following example key set-up for a default key generation using +the GnuPG command-line interface: + +```sh +gpg (GnuPG) 2.3.6; Copyright (C) 2021 Free Software Foundation, Inc. +This is free software: you are free to change and redistribute it. +There is NO WARRANTY, to the extent permitted by law. + +Please select what kind of key you want: + (1) RSA and RSA + (2) DSA and Elgamal + (3) DSA (sign only) + (4) RSA (sign only) + (9) ECC (sign and encrypt) *default* + (10) ECC (sign only) + (14) Existing key from card +Your selection? 9 +Please select which elliptic curve you want: + (1) Curve 25519 *default* + (4) NIST P-384 +Your selection? 1 +Please specify how long the key should be valid. + 0 = key does not expire + <n> = key expires in n days + <n>w = key expires in n weeks + <n>m = key expires in n months + <n>y = key expires in n years +Key is valid for? (0) 0 +Key does not expire at all +Is this correct? (y/N) y + +GnuPG needs to construct a user ID to identify your key. + +Real name: John Doe +Email address: johndoe@example.com +Comment: test key +You selected this USER-ID: + "John Doe (test key) <johndoe@example.com>" + +Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? O +We need to generate a lot of random bytes. It is a good idea to perform +some other action (type on the keyboard, move the mouse, utilize the +disks) during the prime generation; this gives the random number +generator a better chance to gain enough entropy. +We need to generate a lot of random bytes. It is a good idea to perform +some other action (type on the keyboard, move the mouse, utilize the +disks) during the prime generation; this gives the random number +generator a better chance to gain enough entropy. +gpg: revocation certificate stored as 'example.rev' +public and secret key created and signed. + +pub ed25519 2022-07-14 [SC] + E955B7700FFC11EF51C2BA1FE096AACDD4C32E9C +uid John Doe (test key) <johndoe@example.com> +sub cv25519 2022-07-14 [E] +``` + +Please note that GUI apps may differ slightly from the GPG command-line +interface. + +# Common Usage + +As noted in RFC 4880, the general functions of OpenPGP are as follows: + +- digital signatures +- encryption +- compression +- Radix-64 conversion +- key management and certificate services + +From this, you can probably gather that the main use of GPG is for +encrypting data and/or signing the data with a key. The purpose of +encrypting data with GPG is to ensure that no one except the intended +recipient(s) can access the data. + +Let\'s explore some specific GPG use-cases. + +## Email + +One of the more popular uses of GPG is to sign and/or encrypt emails. +With the use of a GPG keypair, you can encrypt a message, its subject, +and even the attachments within. + +The first process, regarding the signing of a message without any +encryption, is generally used to provide assurance that an email is +truly coming from the sender that the message claims. When I send an +email, and it\'s signed with my public key, the recipient(s) of the +message can verify that the message was signed with my personal key. + +The second process, regarding the actual encryption of the message and +its contents, works by using a combination of the sender\'s keys and the +recipient\'s keys. This process may vary slightly by implementation, but +it most commonly uses asymmetric cryptography, also known as public-key +cryptography. In this version of encryption, the sender\'s private key +to sign the message and a combination of the sender\'s keys and the +recipient\'s public key to encrypt the message. + +If two people each have their own private keys and exchange their public +keys, they can send encrypted messages back and forth with GPG. This is +also possible with symmetric cryptography, but the process differs since +there are no key pairs. + +Implementation of email encryption varies greatly between email clients, +so you will need to reference your email client\'s documentation to +ensure you are setting it up correctly for that specific client. + +## File Encryption + +As noted in the section above regarding emails, GPG enables users to be +able to send a message to each other if they are both set-up with GPG +keys. In this example, I am going to show how a user could send a file +called `example_file.txt` to another user via the +recipient\'s email. + +The sender would find the file they want to send and execute the +following command: + +```sh +gpg --encrypt --output example_file.txt.gpg --recipient \ +recipient@example.com example_file.txt +``` + +Once received, the recipient can decrypt the file with the following +command: + +```sh +gpg --decrypt --output example_file.txt example_file.txt.gpg +``` + +## Ownership Signatures + +One important aspect of GPG, especially for developers, is the ability +to sign data without encrypting it. For example, developers often sign +code changes when they commit the changes back to a central repository, +in order to display ownership of who made the changes. This allows other +users to look at a code change and determine that the change was valid. + +In order to do this using [Git](https://git-scm.com), the developer +simply needs to alter the `git commit` command to include the +`-S` flag. Here\'s an example: + +```sh +git commit -S -m "my commit message" +``` + +As an expansion of the example above, Git users can configure their +environment with a default key to use by adding their GPG signature: + +```sh +git config --global user.signingkey XXXXXXXXXXXXXXXX +``` + +If you\'re not sure what your signature is, you can find it titled +`sig` in the output of this command: + +```sh +gpg --list-signatures +``` + +## File Integrity + +When a person generates a signature for data, they are allowing users +the ability to verify the signature on that data in the future to ensure +the data has not been corrupted. This is most common with software +applications hosted on the internet - developers provide signatures so +that users can verify a website was not hijacked and download links +replaced with dangerous software. + +In order to verify signed data, a user needs to have: + +1. The signed data +2. A signature file +3. The public GPG key of the signer + +Once the signer\'s public key is imported on the user\'s system, and +they have the data and signature, they can verify the data with the +following commands: + +```sh +# If the signature is attached to the data +gpg --verify [signature-file] + +# If the signature is detached as a separate file from the data +gpg --verify [signature-file] [original-file] +``` + +### Finding Public Keys + +In order to use GPG with others, a user needs to know the other user(s) +keys. This is easy to do if the user knows the other user(s) in person, +but may be hard if the relationship is strictly digital. Luckily, there +are a few options. The first option is to look at a user\'s web page or +social pages if they have them. + +Otherwise, the best option is to use a keyserver, such as: + +- [pgp.mit.edu](https://pgp.mit.edu) +- [keys.openpgp.org](https://keys.openpgp.org) |