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<!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Random number generators and adapters"><meta name="keywords" content="rust, rustlang, rust-lang, rngs"><title>rand::rngs - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceCodePro-Semibold.ttf.woff2"><link rel="stylesheet" type="text/css" href="../../normalize.css"><link rel="stylesheet" type="text/css" href="../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../../ayu.css" disabled><link rel="stylesheet" type="text/css" href="../../dark.css" disabled><link rel="stylesheet" type="text/css" href="../../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../../storage.js"></script><script src="../../crates.js"></script><script defer src="../../main.js"></script>
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<img src="https://www.rust-lang.org/logos/rust-logo-128x128-blk.png" alt="logo"></div>
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</a><h2 class="location"><a href="#">Module rngs</a></h2><div class="sidebar-elems"><section><div class="block"><ul><li><a href="#modules">Modules</a></li><li><a href="#structs">Structs</a></li></ul></div></section><div id="sidebar-vars" data-name="rngs" data-ty="mod" data-relpath="./"></div><script defer src="./sidebar-items.js"></script></div></nav><main><div class="width-limiter"><div class="sub-container"><a class="sub-logo-container" href="../../rand/index.html">
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<h1 class="fqn"><span class="in-band">Module <a href="../index.html">rand</a>::<wbr><a class="mod" href="#">rngs</a><button id="copy-path" onclick="copy_path(this)" title="Copy item path to clipboard"><img src="../../clipboard.svg" width="19" height="18" alt="Copy item path"></button></span></h1><span class="out-of-band"><a class="srclink" href="../../src/rand/rngs/mod.rs.html#9-119">source</a> · <a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span></div><details class="rustdoc-toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Random number generators and adapters</p>
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<h3 id="background-random-number-generators-rngs"><a href="#background-random-number-generators-rngs">Background: Random number generators (RNGs)</a></h3>
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<p>Computers cannot produce random numbers from nowhere. We classify
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random number generators as follows:</p>
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<ul>
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<li>“True” random number generators (TRNGs) use hard-to-predict data sources
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(e.g. the high-resolution parts of event timings and sensor jitter) to
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harvest random bit-sequences, apply algorithms to remove bias and
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estimate available entropy, then combine these bits into a byte-sequence
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or an entropy pool. This job is usually done by the operating system or
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a hardware generator (HRNG).</li>
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<li>“Pseudo”-random number generators (PRNGs) use algorithms to transform a
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seed into a sequence of pseudo-random numbers. These generators can be
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fast and produce well-distributed unpredictable random numbers (or not).
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They are usually deterministic: given algorithm and seed, the output
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sequence can be reproduced. They have finite period and eventually loop;
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with many algorithms this period is fixed and can be proven sufficiently
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long, while others are chaotic and the period depends on the seed.</li>
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<li>“Cryptographically secure” pseudo-random number generators (CSPRNGs)
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are the sub-set of PRNGs which are secure. Security of the generator
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relies both on hiding the internal state and using a strong algorithm.</li>
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</ul>
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<h3 id="traits-and-functionality"><a href="#traits-and-functionality">Traits and functionality</a></h3>
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<p>All RNGs implement the <a href="../trait.RngCore.html"><code>RngCore</code></a> trait, as a consequence of which the
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<a href="../trait.Rng.html"><code>Rng</code></a> extension trait is automatically implemented. Secure RNGs may
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additionally implement the <a href="../trait.CryptoRng.html"><code>CryptoRng</code></a> trait.</p>
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<p>All PRNGs require a seed to produce their random number sequence. The
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<a href="../trait.SeedableRng.html"><code>SeedableRng</code></a> trait provides three ways of constructing PRNGs:</p>
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<ul>
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<li><code>from_seed</code> accepts a type specific to the PRNG</li>
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<li><code>from_rng</code> allows a PRNG to be seeded from any other RNG</li>
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<li><code>seed_from_u64</code> allows any PRNG to be seeded from a <code>u64</code> insecurely</li>
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<li><code>from_entropy</code> securely seeds a PRNG from fresh entropy</li>
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</ul>
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<p>Use the <a href="../../rand_core/index.html" title="rand_core"><code>rand_core</code></a> crate when implementing your own RNGs.</p>
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<h3 id="our-generators"><a href="#our-generators">Our generators</a></h3>
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<p>This crate provides several random number generators:</p>
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<ul>
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<li><a href="struct.OsRng.html" title="OsRng"><code>OsRng</code></a> is an interface to the operating system’s random number
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source. Typically the operating system uses a CSPRNG with entropy
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provided by a TRNG and some type of on-going re-seeding.</li>
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<li><a href="struct.ThreadRng.html" title="ThreadRng"><code>ThreadRng</code></a>, provided by the <a href="../fn.thread_rng.html"><code>thread_rng</code></a> function, is a handle to a
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thread-local CSPRNG with periodic seeding from <a href="struct.OsRng.html" title="OsRng"><code>OsRng</code></a>. Because this
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is local, it is typically much faster than <a href="struct.OsRng.html" title="OsRng"><code>OsRng</code></a>. It should be
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secure, though the paranoid may prefer <a href="struct.OsRng.html" title="OsRng"><code>OsRng</code></a>.</li>
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<li><a href="struct.StdRng.html" title="StdRng"><code>StdRng</code></a> is a CSPRNG chosen for good performance and trust of security
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(based on reviews, maturity and usage). The current algorithm is ChaCha12,
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which is well established and rigorously analysed.
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<a href="struct.StdRng.html" title="StdRng"><code>StdRng</code></a> provides the algorithm used by <a href="struct.ThreadRng.html" title="ThreadRng"><code>ThreadRng</code></a> but without
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periodic reseeding.</li>
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<li>[<code>SmallRng</code>] is an <strong>insecure</strong> PRNG designed to be fast, simple, require
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little memory, and have good output quality.</li>
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</ul>
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<p>The algorithms selected for <a href="struct.StdRng.html" title="StdRng"><code>StdRng</code></a> and [<code>SmallRng</code>] may change in any
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release and may be platform-dependent, therefore they should be considered
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<strong>not reproducible</strong>.</p>
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<h3 id="additional-generators"><a href="#additional-generators">Additional generators</a></h3>
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<p><strong>TRNGs</strong>: The <a href="https://crates.io/crates/rdrand"><code>rdrand</code></a> crate provides an interface to the RDRAND and
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RDSEED instructions available in modern Intel and AMD CPUs.
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The <a href="https://crates.io/crates/rand_jitter"><code>rand_jitter</code></a> crate provides a user-space implementation of
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entropy harvesting from CPU timer jitter, but is very slow and has
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<a href="https://github.com/rust-random/rand/issues/699">security issues</a>.</p>
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<p><strong>PRNGs</strong>: Several companion crates are available, providing individual or
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families of PRNG algorithms. These provide the implementations behind
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<a href="struct.StdRng.html" title="StdRng"><code>StdRng</code></a> and [<code>SmallRng</code>] but can also be used directly, indeed <em>should</em>
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be used directly when <strong>reproducibility</strong> matters.
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Some suggestions are: <a href="https://crates.io/crates/rand_chacha"><code>rand_chacha</code></a>, <a href="https://crates.io/crates/rand_pcg"><code>rand_pcg</code></a>, <a href="https://crates.io/crates/rand_xoshiro"><code>rand_xoshiro</code></a>.
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A full list can be found by searching for crates with the <a href="https://crates.io/keywords/rng"><code>rng</code> tag</a>.</p>
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</div></details><h2 id="modules" class="small-section-header"><a href="#modules">Modules</a></h2>
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<div class="item-table"><div class="item-row"><div class="item-left module-item"><a class="mod" href="adapter/index.html" title="rand::rngs::adapter mod">adapter</a></div><div class="item-right docblock-short"><p>Wrappers / adapters forming RNGs</p>
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</div></div><div class="item-row"><div class="item-left module-item"><a class="mod" href="mock/index.html" title="rand::rngs::mock mod">mock</a></div><div class="item-right docblock-short"><p>Mock random number generator</p>
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</div></div></div><h2 id="structs" class="small-section-header"><a href="#structs">Structs</a></h2>
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<div class="item-table"><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.OsRng.html" title="rand::rngs::OsRng struct">OsRng</a></div><div class="item-right docblock-short"><p>A random number generator that retrieves randomness from the
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operating system.</p>
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</div></div><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.StdRng.html" title="rand::rngs::StdRng struct">StdRng</a></div><div class="item-right docblock-short"><p>The standard RNG. The PRNG algorithm in <code>StdRng</code> is chosen to be efficient
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on the current platform, to be statistically strong and unpredictable
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(meaning a cryptographically secure PRNG).</p>
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</div></div><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.ThreadRng.html" title="rand::rngs::ThreadRng struct">ThreadRng</a></div><div class="item-right docblock-short"><p>A reference to the thread-local generator</p>
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