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diff --git a/blog/2022-07-14-gnupg.org b/blog/2022-07-14-gnupg.org new file mode 100644 index 0000000..66822f0 --- /dev/null +++ b/blog/2022-07-14-gnupg.org @@ -0,0 +1,305 @@ ++++ +date = 2022-07-14 +title = "GNU Privacy Guard (GPG)" +description = "An overview of the GPG encryption program and its uses." +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](/blog/aes-encryption/) (2018) +- [Cryptography Basics](/blog/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. + org/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. + muni.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. + com/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. + org/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) |