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diff --git a/blog/gnupg/index.org b/blog/gnupg/index.org new file mode 100644 index 0000000..59e12e7 --- /dev/null +++ b/blog/gnupg/index.org @@ -0,0 +1,297 @@ +#+title: GNU Privacy Guard (GPG) +#+date: 2022-07-14 +#+description: Learn how to create a PGP key with GNU Privacy Guard (GPG). +#+filetags: :privacy: + +* The History of GPG +[[https://gnupg.org/][GNU Privacy Guard]], 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 +[[https://www.rfc-editor.org/rfc/rfc4880][RFC 4880]]. + +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 +[[https://en.wikipedia.org/wiki/Werner_Koch][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: + +#+begin_src 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) +#+end_src + +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." ([[https://www.di.ens.fr/~pnguyen/pub_Ng04.htm][Source]]) +- GPG versions prior to 1.4.2.1 contain a false positive signature + verification bug. + ([[https://lists.gnupg.%20org/pipermail/gnupg-announce/2006q1/000211.html][Source]]) +- GPG versions prior to 1.4.2.2 cannot detect injection of unsigned + data. ( + [[https://lists.gnupg.org/pipermail/gnupg-announce/2006q1/000218.html][Source]]) +- 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. ([[https://lwn.net/Articles/727179/][Source]]) +- The [[https://en.wikipedia.org/wiki/ROCA_vulnerability][ROCA + Vulnerability]] affects RSA keys generated by YubiKey 4 tokens. + ([[https://crocs.fi.%20muni.cz/_media/public/papers/nemec_roca_ccs17_preprint.pdf][Source]]) +- The [[https://en.wikipedia.org/wiki/SigSpoof][SigSpoof Attack]] allows + an attacker to spoof digital signatures. + ([[https://arstechnica.%20com/information-technology/2018/06/decades-old-pgp-bug-allowed-hackers-to-spoof-just-about-anyones-signature/][Source]]) +- Libgcrypt 1.9.0 contains a severe flaw related to a heap buffer + overflow, fixed in Libgcrypt 1.9.1 + ([[https://web.archive.%20org/web/20210221012505/https://www.theregister.com/2021/01/29/severe_libgcrypt_bug/][Source]]) + +*** 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: + +- [[https://gnupg.org][GnuPG]] for Linux (depending on distro) +- [[https://gpg4win.org][Gpg4win]] for Windows +- [[https://gpgtools.org][GPGTools]] 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: + +#+begin_src sh +sudo dnf install gpg +#+end_src + +Once installed, a user can create a new key pair with the following +command(s): + +#+begin_src sh +gpg --full-generate-key +#+end_src + +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: + +#+begin_src 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] +#+end_src + +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: + +#+begin_src sh +gpg --encrypt --output example_file.txt.gpg --recipient \ +recipient@example.com example_file.txt +#+end_src + +Once received, the recipient can decrypt the file with the following +command: + +#+begin_src sh +gpg --decrypt --output example_file.txt example_file.txt.gpg +#+end_src + +** 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 [[https://git-scm.com][Git]], the developer +simply needs to alter the =git commit= command to include the =-S= flag. +Here's an example: + +#+begin_src sh +git commit -S -m "my commit message" +#+end_src + +As an expansion of the example above, Git users can configure their +environment with a default key to use by adding their GPG signature: + +#+begin_src sh +git config --global user.signingkey XXXXXXXXXXXXXXXX +#+end_src + +If you're not sure what your signature is, you can find it titled =sig= +in the output of this command: + +#+begin_src sh +gpg --list-signatures +#+end_src + +** 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: + +#+begin_src 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] +#+end_src + +*** 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: + +- [[https://pgp.mit.edu][pgp.mit.edu]] +- [[https://keys.openpgp.org][keys.openpgp.org]] |