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CryptoNight v1
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CryptoNight v2
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CryptoNight v3 aka CryptoNightR
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Critique
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Reference
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<h1 id="cryptonight">CryptoNight<a class="headerlink" href="#cryptonight" title="Permanent link">&para;</a></h1>
<blockquote>
<p>CryptoNight is a memory hard hash function</p>
</blockquote>
<h2 id="background">Background<a class="headerlink" href="#background" title="Permanent link">&para;</a></h2>
<p>CryptoNight was originally designed around 2013 as part of the CryptoNote suite.</p>
<p>One design goal was to make it very friendly for the off-the-shelf CPU-s, by employing:</p>
<ul>
<li>native AES encryption </li>
<li>fast 64 bit multipliers</li>
<li>scratchpad fitting exactly the size of the per-core L3 cache on Intel CPUs (about 2MB)</li>
</ul>
<p>More ambitious design goal was to make it inefficiently computable on ASIC-s.
This goal has since failed, as it inevitably happens with "ASIC hard" algorithms.
Efficient CryptoNight ASIC was developed in 2017 by Bitmain. </p>
<p>Monero inherited CryptoNight as its proof of work in 2014.
Since then Monero slightly evolved the algorithm to intentionally break compatibility with released ASIC-s.
Currently Monero implements CryptoNight v2, a third iteration of original CryptoNight (v0, v1, v2).</p>
<h2 id="the-goal-is-to-find-small-enough-hash">The goal is to find small-enough hash<a class="headerlink" href="#the-goal-is-to-find-small-enough-hash" title="Permanent link">&para;</a></h2>
<p>In hashing based PoW algorithms the goal is to find small-enough hash.</p>
<p>Hash is simply an integer (normally, a very large integer).
Most hashing functions result in 256-bit hashes (integers between 0 and 2^256).
This includes Bitcoin's double-SHA-256 and Monero's CryptoNight.</p>
<p>Miner randomly tweaks input data until the hash fits under specified threshold.
The threshold (also a large integer) is established collectively by the network as part of the consensus mechanism.
The PoW is only considered valid (solved) if hash fits under the threshold. </p>
<p>Because hash functions are one-way, it is not possible to analytically calculate input data that would result in a small-enough hash.
The solution must be brute-forced by tweaking the input data and recalculating the hash over and over again.</p>
<p>Miners have a few areas of flexibility regarding input data - most importantly they can iterate with the nonce value.
They also have a power over which transactions are included in the block and how they are put together in a merkle tree. </p>
<h2 id="cryptographic-primitives">Cryptographic primitives<a class="headerlink" href="#cryptographic-primitives" title="Permanent link">&para;</a></h2>
<p>CryptoNight is based on:</p>
<ul>
<li>AES encryption</li>
<li>5 hashing functions, all of which were finalists in NIST SHA-3 competition:<ul>
<li>Keccak (the primary one)</li>
<li>BLAKE</li>
<li>Groestl</li>
<li>JH</li>
<li>Skein</li>
</ul>
</li>
</ul>
<h2 id="input-data">Input data<a class="headerlink" href="#input-data" title="Permanent link">&para;</a></h2>
<p>In Monero the input to hashing function is concatenation of:</p>
<ul>
<li>serialized block header (around 46 bytes; subject to varint representation)</li>
<li>merkle tree root (32 bytes)</li>
<li>number of transactions included in the block (around 1-2 bytes; subject to varint representation)</li>
</ul>
<p>See <a href="https://github.com/monero-project/monero/blob/master/src/cryptonote_basic/cryptonote_format_utils.cpp#L1078">get_block_hashing_blob()</a> function to dig further.</p>
<h2 id="algorithm">Algorithm<a class="headerlink" href="#algorithm" title="Permanent link">&para;</a></h2>
<div class="admonition warning">
<p class="admonition-title">Warning</p>
<p>The article attempts to give reader a high-level understanding of the CryptoNight algorithm.
For implementation details refer to CryptoNote Standard and Monero source code.
See references at the bottom. </p>
</div>
<h3 id="overview">Overview<a class="headerlink" href="#overview" title="Permanent link">&para;</a></h3>
<p>CryptoNight attempts to make memory access a bottleneck for performance ("memory hardness"). It has three steps:</p>
<ol>
<li>Initialize large area of memory with pseudo-random data. This memory is known as the scratchpad.</li>
<li>Perform numerous read/write operations at pseudo-random (but deterministic) addresses on the scratchpad.</li>
<li>Hash the entire scratchpad to produce the resulting value.</li>
</ol>
<h3 id="step-1-scratchpad-initialization">Step 1: scratchpad initialization<a class="headerlink" href="#step-1-scratchpad-initialization" title="Permanent link">&para;</a></h3>
<p>Firstly, the input data is hashed with Keccak-1600. This results in 200 bytes of pseudorandom data (1600 bits == 200 bytes).</p>
<p>These 200 bytes become a seed to generate a larger, 2MB-wide buffer of pseudorandom data,
by applying AES-256 encryption.</p>
<p>The first 0..31 bytes of Keccak-1600 hash are used as AES key.</p>
<p>The encryption is performed on 128 bytes-long payloads until 2MB is ready.
The first payload are Keccak-1600 bytes 66..191.
The next payload is encryption result of the previous payload.</p>
<p>Each 128-byte payload is actually encrypted 10 times.</p>
<p>The details are a bit more nuanced, see "Scratchpad Initialization" in <a href="https://cryptonote.org/cns/cns008.txt">CryptoNote Standard</a>. </p>
<h3 id="step-2-memory-hard-loop">Step 2: memory-hard loop<a class="headerlink" href="#step-2-memory-hard-loop" title="Permanent link">&para;</a></h3>
<p>The second step is basically 524288 iterations of a simple stateful algorithm.</p>
<p>Each algorithm iteration reads from and writes back to the scratchpad,
at pseudorandom-but-deterministic locations.</p>
<p>Critically, next iteration depends on the state prepared by previous iterations.
It is not possible to directly calculate state of future iterations.</p>
<p>The specific operations include AES, XOR, 8byte_mul, 8byte_add - operations that are CPU-friendly (highly optimized on modern CPU-s).</p>
<p>The goal here is to make memory latency the bottleneck in attempt to close the gap between potential ASIC-s and general purpose CPU-s.</p>
<h3 id="step-3-hashing">Step 3: hashing<a class="headerlink" href="#step-3-hashing" title="Permanent link">&para;</a></h3>
<p>The final step (simplifying) is to:</p>
<ul>
<li>combine original Keccak-1600 output with the whole scratchpad</li>
<li>pick the hashing algorithm based on 2 low-order bits of the result<ul>
<li>0=BLAKE-256</li>
<li>1=Groestl-256</li>
<li>2=JH-256 </li>
<li>3=Skein-256</li>
</ul>
</li>
<li>hash the result with selected function</li>
</ul>
<p>The resulting 256-bit hash is the final output of CryptoNight algorithm.</p>
<h2 id="monero-specific-modifications">Monero specific modifications<a class="headerlink" href="#monero-specific-modifications" title="Permanent link">&para;</a></h2>
<h3 id="cryptonight-v0">CryptoNight v0<a class="headerlink" href="#cryptonight-v0" title="Permanent link">&para;</a></h3>
<p>This is how Monero community refers to original implementation of CryptoNight.</p>
<h3 id="cryptonight-v1">CryptoNight v1<a class="headerlink" href="#cryptonight-v1" title="Permanent link">&para;</a></h3>
<p>See the <a href="https://github.com/monero-project/monero/pull/3253/files">source code diff</a>. </p>
<h3 id="cryptonight-v2">CryptoNight v2<a class="headerlink" href="#cryptonight-v2" title="Permanent link">&para;</a></h3>
<p>See the <a href="https://github.com/SChernykh/xmr-stak-cpu/blob/master/README.md">rationale</a> and the <a href="https://github.com/monero-project/monero/commit/5fd83c13fbf8dc304909345e60a853c15b0de1e5#diff-7000dc02c792439471da62856f839d62">source code diff</a>.</p>
<h3 id="cryptonight-v3-aka-cryptonightr">CryptoNight v3 aka CryptoNightR<a class="headerlink" href="#cryptonight-v3-aka-cryptonightr" title="Permanent link">&para;</a></h3>
<p>See the <a href="https://github.com/monero-project/monero/pull/5126">rationale</a> and the <a href="https://github.com/monero-project/monero/pull/5126/files">source code diff</a>.</p>
<h2 id="critique">Critique<a class="headerlink" href="#critique" title="Permanent link">&para;</a></h2>
<ul>
<li>CryptoNight hash is relatively expensive to verify. This poses a risk of DoS-ing nodes with incorrect proofs to process. See <a href="/proof-of-work/what-is-pow/#strong-asymmetry">strong asymmetry</a> requirement. </li>
<li>The hash function was designed from scratch with limited peer review. While CryptoNight is composed of proven and peer-reviewed primitives, combining secure primitives doesn't necessarily result in a secure cryptosystem.</li>
<li>CryptoNight ultimately failed to prevent ASIC-s.</li>
<li>Complexity of CryptoNight kills competition in ASIC manufacturing.</li>
</ul>
<p>CryptoNight proof of work remains one of the most controversial aspect of Monero.</p>
<h2 id="reference">Reference<a class="headerlink" href="#reference" title="Permanent link">&para;</a></h2>
<ul>
<li><a href="https://cryptonote.org/cns/cns008.txt">CryptoNight hash function</a> description in the CryptoNote Standard</li>
<li><a href="https://github.com/monero-project/monero/blob/master/src/crypto/slow-hash.c">CryptoNight v2 source code</a><ul>
<li>The entry point is <code>cn_slow_hash()</code> function. Manually removing support and optimizations for multiple architectures should help you understand the actual code. </li>
</ul>
</li>
<li>"Egalitarian Proof of Work" chapter in <a href="https://downloads.getmonero.org/whitepaper_annotated.pdf">CryptoNote whitepaper</a> </li>
<li><a href="https://da-data.blogspot.com/2014/08/minting-money-with-monero-and-cpu.html">First days of Monero mining</a> by dr David Andersen</li>
<li>Some <a href="https://github.com/monero-project/monero/tree/master/tests/hash">test vectors</a> in Monero source code </li>
</ul>
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