https://fedoraproject.org/wiki/Changes/yescrypt_as_default_hashing_method_for_shadow == Summary == Make the yescrypt hashing method the default method used for new user passwords stored in <code>/etc/shadow</code>. == Owner == * Name: [[User:besser82| Björn Esser]] * Email: besser82@xxxxxxxxxxxxxxxxx == Detailed Description == yescrypt is a password-based key derivation function (KDF) and password hashing scheme. It builds upon Colin Percival's scrypt, and is based on NIST-approved primitives. Cryptographic security of yescrypt (collision resistance, preimage and second preimage resistance) is based on that of SHA-256, HMAC, and PBKDF2. Even a catastrophic failure of yescrypt’s computational layers to maintain entropy would not affect yescrypt’s cryptographic properties as long as SHA-256, HMAC, and PBKDF2 remain unbroken. That said, in case SHA-256 is ever broken, yescrypt’s additional processing is likely to neutralize the effect of any such break. By the time of this writing, sha256crypt and sha512crypt, as used commonly today for hashing passwords, remain unbroken, but have some flaws by design: * Both hashing methods effectively only use about 90 bits of salt, although the NIST-recommendation for salt length is >= 128 bits. * Long passwords can create a denial-of-service on the CPU. * Passive observation of execution times can predict password length. * No use of a crytographic key derivation function (KDF). In conclusion we should move to a stronger hashing method for computing the entries in the UNIX shadow file. So why not Argon2? * yescrypt has a dependency not only on RAM, but also on fast on-die local memory, which provides bcrypt-like anti-GPU properties even at very low per-hash RAM sizes (where Argon2 might even lose to bcrypt in terms of GPU attack speed). * yescrypt currently has less low-level parallelism within processing of a block, yet allows for tuning it later, whereas Argon2 has a fixed and currently commonly excessive amount of such parallelism, which may be extracted to speed up e.g. GPU attacks through use of more computing resources per the same total memory size due to each hash computation's memory needs being split between 32 threads (yescrypt currently has four 16-byte lanes that can be processed in parallel within a 64-byte sub-block before running into a likely data dependency for the next sub-block, whereas Argon2 allows for parallel processing of eight 128-byte chunks within a 1 KiB block with only two synchronization points for the entire block, as well as of four 32-byte parts of the 128-byte chunks with only two more synchronization points for the entire 1 KiB block). * yescrypt's cryptographic security is provided by SHA-256, HMAC, and PBKDF2, which are NIST-approved and time-tested (the rest of yescrypt's processing, while most crucial for its offline attack resistance properties, provably does not affect its basic cryptographic hash properties), whereas Argon2 relies on the newer BLAKE2 (either choice is just fine for security, but use of approved algorithms may sometimes be required for compliance) Also see [https://www.openwall.com/yescrypt/ yescrypt - scalable KDF and password hashing scheme], the [https://www.password-hashing.net/submissions/specs/yescrypt-v2.pdf PHC submission paper], [https://lists.openwall.net/phc-discussions/2018/03/13/10 PHC yescrypt vs. Argon2], and [https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=978553 the discussion on the Debian bugtracker]. == Feedback == No feedback, yet. == Benefit to Fedora == yescrypt is the default password hashing scheme on recent ALT Linux, Debian testing, and Kali Linux 2021.1+, so we should adopt it as the default, too. Also, it is already the recommended hashing method in the [https://docs.fedoraproject.org/en-US/fedora-coreos/authentication/#_using_password_authentication Fedora CoreOS documentation]. == Scope == * Proposal owners: Help with integration for yescrypt support in some packages. See Dependencies. * Other developers: Integrate yescrypt support in some packages. See Dependencies. * Release engineering: [https://pagure.io/releng/issue/10150 #10150] * Policies and guidelines: N/A (not needed for this Change) * Trademark approval: N/A (not needed for this Change) * Alignment with Objectives: N/A (not needed for this Change) == Upgrade/compatibility impact == No impact, as password hashes, that have been computed using the former default sha512crypt, will continue to work. == How To Test == * Existing installations: Change your user password and check whether the computed password hash in <code>/etc/shadow</code> starts with <code>$y$</code>, like <code>root:$y$j9T$JEFtZ/…</code>. * Fresh installations: Check whether the password hash(es) for the user(s) created by anaconda in <code>/etc/shadow</code> start(s) with <code>$y$</code>, like <code>root:$y$j9T$JEFtZ/…</code>. == User Experience == No user visible changes, but they can rely on safer hashing for their user passwords. == Dependencies == * anaconda: https://github.com/rhinstaller/anaconda/pull/3431 * authselect: https://github.com/authselect/authselect/pull/253 * libuser: WIP ongoing * shadow-utils: https://src.fedoraproject.org/rpms/shadow-utils/pull-request/10 * pam: Is already capable to use yescrypt. * libxcrypt: Is already capable for computing yescrypt hashes. == Contingency Plan == * Blocks release? Yes Partially revert the changes, that have been applied to anaconda, authselect and shadow-utils, and rebuild those packages. == Documentation == Fedora now uses the yescrypt hashing method for new passwords. There are no visible changes nor impacts to the end-user. Users are recommended to change their existing passwords after upgrading. == Release Notes == Fedora now uses the yescrypt hashing method for new passwords. There are no visible changes nor impacts to the end-user. Users are recommended to change their existing passwords after upgrading. -- Ben Cotton He / Him / His Fedora Program Manager Red Hat TZ=America/Indiana/Indianapolis _______________________________________________ devel mailing list -- devel@xxxxxxxxxxxxxxxxxxxxxxx To unsubscribe send an email to devel-leave@xxxxxxxxxxxxxxxxxxxxxxx Fedora Code of Conduct: https://docs.fedoraproject.org/en-US/project/code-of-conduct/ List Guidelines: https://fedoraproject.org/wiki/Mailing_list_guidelines List Archives: https://lists.fedoraproject.org/archives/list/devel@xxxxxxxxxxxxxxxxxxxxxxx Do not reply to spam on the list, report it: https://pagure.io/fedora-infrastructure
