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yescrypt - scalable KDF and password hashing scheme

yescrypt is a password-based key derivation function (KDF) and password hashing scheme. It builds upon Colin Percival's scrypt. This implementation is able to compute native yescrypt hashes as well as classic scrypt.

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The source code of yescrypt may be browsed via CVSweb.

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Why yescrypt?

Like it or not, password authentication remains relevant (including as one of several authentication factors), password hash database leaks happen, the leaks are not always detected and fully dealt with right away, and even once they are many users' same or similar passwords reused elsewhere remain exposed. To mitigate these risks (as well as those present in other scenarios where password-based key derivation or password hashing is relevant), computationally expensive (bcrypt, PBKDF2, etc.) and more recently also memory-hard (scrypt, Argon2, etc.) password hashing schemes have been introduced. Unfortunately, at high target throughput and/or low target latency their memory usage is unreasonably low, up to the point where they're not obviously better than the much older bcrypt (considering attackers with pre-existing hardware). This is a primary drawback that yescrypt addresses.

Most notable for large-scale deployments is yescrypt's optional initialization and reuse of a large lookup table, typically occupying at least tens of gigabytes of RAM and essentially forming a site-specific ROM. This limits attackers' use of pre-existing hardware such as botnet nodes.

yescrypt's other changes from scrypt additionally slow down GPUs and to a lesser extent FPGAs and ASICs even when its memory usage is low and even when there's no ROM, and provide extra knobs and built-in features.

Technically, yescrypt is the most scalable password hashing scheme so far, providing near-optimal security from offline password cracking across the whole range from kilobytes to terabytes and beyond. However, the price for this is complexity, and we recognize that complexity is a major drawback of any software. Thus, at this time we focus on large-scale deployments, where the added complexity is relatively small compared to the total complexity of the authentication service setup. For smaller deployments, bcrypt with its simplicity and existing library support is a reasonable short-term choice (although we're making progress towards more efficient FPGA attacks on bcrypt under a separate project). We might introduce a cut-down yescrypt-lite later or/and yescrypt might become part of standard or popular libraries, making it more suitable for smaller deployments as well.

Please check out our presentation slides on yescrypt. (Some of the detail on the last few slides pertains to yescrypt 0.9.x and is no longer valid for yescrypt 1.0+, but overall this slide deck still applies.)

Performance

Please see the PERFORMANCE file inside the yescrypt distribution for example setup and benchmarks relevant to the mass user authentication use case.

The test system is a server (kindly provided by Packet) with dual Xeon Gold 5120 CPUs (2.2 GHz, turbo to up to 3.2 GHz) and 384 GiB RAM (12x DDR4-2400 ECC Reg). These CPUs have 14 cores and 6 memory channels each, for a total of 28 physical cores, 56 logical CPUs (HT is enabled), and 12 memory channels.

Some highlights: initialization of a 368 GiB ROM takes 22 seconds (to be done on server bootup), and while using the ROM we're able to compute over 21k, over 10k, or around 1200 hashes per second with per-hash RAM usage of 1.4375 MiB, 2.875 MiB, or 23 MiB, respectively.

When not using a ROM, we're able to compute over 21k, over 10k, or around 1200 hashes per second with per-hash RAM usage of 2 MiB, 4 MiB, or 32 MiB, respectively.

Comparison to scrypt and Argon2

yescrypt's advantages:

yescrypt's drawbacks:

Other observations:

A note on cryptocoins

Although multiple CPU mining focused cryptocoins already use yescrypt 0.5'ish as their proof-of-work (PoW) scheme, we currently do not recommend yescrypt 1.0+ for use by cryptocoins. If you feel like starting a new coin or forking one, for now please use the exact same older revision of yescrypt that other coins already use (this is also easier for you to do, through forking another coin's existing codebase). We might introduce a special mode optimized for the PoW use case later.

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