2023-08-23 17:09:04 +00:00
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<h2>
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<a href="index.html"><code>~</code></a>
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</h2>
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<h1>
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<a
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id="user-content-real-world-crypto-101"
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class="anchor"
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aria-hidden="true"
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2023-08-30 03:28:39 +00:00
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tabindex="-1"
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2023-08-23 17:09:04 +00:00
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href="#real-world-crypto-101"
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><span aria-hidden="true" class="octicon octicon-link"></span></a
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>Real World Crypto 101
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</h1>
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<p>
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My notes when reading
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<a
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href="https://www.manning.com/books/real-world-cryptography"
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rel="nofollow"
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>Real-World Cryptography</a
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>
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</p>
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2023-09-10 18:03:48 +00:00
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<h2>
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<a
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id="user-content-hash-function-convert-from-input-to-digest"
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class="anchor"
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aria-hidden="true"
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tabindex="-1"
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href="#hash-function-convert-from-input-to-digest"
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><span aria-hidden="true" class="octicon octicon-link"></span
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></a>
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<strong>Hash</strong> function convert from input to digest
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</h2>
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2023-08-23 17:09:04 +00:00
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<ul>
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<li>Pre-image resistance: Given digest, can not find input</li>
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<li>
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Second pre-image resistance: Given input, digest, can not find another
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input produce same digest. Small change to input make digest big change.
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</li>
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<li>Collision resistance: Can not find 2 input produce same digest.</li>
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</ul>
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2023-09-10 18:03:48 +00:00
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<h2>
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<a
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id="user-content-mac-aka-message-authentication-code-produce-from-key-message-to-authentication-tag"
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class="anchor"
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aria-hidden="true"
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tabindex="-1"
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href="#mac-aka-message-authentication-code-produce-from-key-message-to-authentication-tag"
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><span aria-hidden="true" class="octicon octicon-link"></span
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></a>
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2023-08-23 17:09:04 +00:00
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<strong>MAC</strong> aka Message Authentication Code produce from key,
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2023-09-10 18:03:48 +00:00
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message to authentication tag.
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</h2>
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2023-08-24 04:44:53 +00:00
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<ul>
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<li>A send B message with MAC (generate from message and A key).</li>
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<li>
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B double check message with MAC (generate from receive message and B
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key).
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</li>
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<li>A and B use same key.</li>
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</ul>
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2023-09-10 18:03:48 +00:00
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<div class="highlight highlight-source-mermaid">
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<pre><span class="pl-k">sequenceDiagram</span>
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<span class="pl-k">participant</span> <span class="pl-ent">alice</span>
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<span class="pl-k">participant</span> <span class="pl-ent">bob</span>
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<span class="pl-ent">alice </span><span class="pl-k">->></span> <span class="pl-ent">bob</span><span class="pl-k">:</span> <span class="pl-s">send alice, mac(secret_key_alice, alice)</span>
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<span class="pl-ent">bob </span><span class="pl-k">->></span> <span class="pl-ent">bob</span><span class="pl-k">:</span> <span class="pl-s">compare mac(secret_key_alice, alice) with mac(secret_key_bob, alice)</span></pre>
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</div>
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<ul>
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<li>
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Prevent forgery: without secret_key, can not generate MAC even if
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knowing <strong>a lot of</strong> alice and mac(secret_key, alice),
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</li>
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<li>Prevent collisions: keep MAC long enough (256-bit),</li>
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<li>
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Replay attacks: send transaction 2 times with perfectly MAC and u know
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why -> instead of mac(secret_key, alice), use
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<strong>counter</strong> as mac(secret_key, counter, alice).
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</li>
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<li>
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Verify must be done in <strong>constant time</strong>: if not, probaly
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return error the moment the bytes differ -> attacker recreate byte by
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byte by measuring how long -> timing attacks
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</li>
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</ul>
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<p>Constant time comparision:</p>
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<div class="highlight highlight-source-go">
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<pre><span class="pl-k">for</span> <span class="pl-s1">i</span> <span class="pl-c1">:=</span> <span class="pl-c1">0</span>; <span class="pl-s1">i</span> <span class="pl-c1"><</span> <span class="pl-en">len</span>(<span class="pl-s1">x</span>); <span class="pl-s1">i</span><span class="pl-c1">++</span> {
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<span class="pl-c">// Use XOR instead of compare x[i] == y[i]</span>
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<span class="pl-c">// If x[i] == y[i] -> XOR is 1</span>
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<span class="pl-c">// Otherwise XOR is 0</span>
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<span class="pl-s1">v</span> <span class="pl-c1">|=</span> <span class="pl-s1">x</span>[<span class="pl-s1">i</span>] <span class="pl-c1">^</span> <span class="pl-s1">y</span>[<span class="pl-s1">i</span>]
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}
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<span class="pl-c">// v == 1 means all XOR is 1 means x == y</span></pre>
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</div>
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<p>Use for:</p>
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<ul>
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<li>
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Integrity: because MAC ensure no one can tamper with message without
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noticing
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</li>
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</ul>
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<div class="highlight highlight-source-mermaid">
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<pre><span class="pl-k">sequenceDiagram</span>
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<span class="pl-k">participant</span> <span class="pl-ent">alice</span>
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<span class="pl-k">participant</span> <span class="pl-ent">bob</span>
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<span class="pl-ent">alice </span><span class="pl-k">->></span> <span class="pl-ent">bob</span><span class="pl-k">:</span> <span class="pl-s">send username, password</span>
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<span class="pl-ent">bob </span><span class="pl-k">-->></span> <span class="pl-ent">alice</span><span class="pl-k">:</span> <span class="pl-s">return alice|mac(secret_key, alice)</span>
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<span class="pl-ent">alice </span><span class="pl-k">->></span> <span class="pl-ent">bob</span><span class="pl-k">:</span> <span class="pl-s">send alice|mac(secret_key, alice)</span>
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<span class="pl-ent">bob </span><span class="pl-k">-->></span> <span class="pl-ent">alice</span><span class="pl-k">:</span> <span class="pl-s">return OK</span>
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<span class="pl-ent">alice </span><span class="pl-k">->></span> <span class="pl-ent">bob</span><span class="pl-k">:</span> <span class="pl-s">send bob|mac(secret_key, alice)</span>
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<span class="pl-ent">bob </span><span class="pl-k">-->></span> <span class="pl-ent">alice</span><span class="pl-k">:</span> <span class="pl-s">return ERROR</span></pre>
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</div>
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<p><strong>HMAC</strong> is MAC using hash</p>
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2023-08-23 17:09:04 +00:00
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<div>
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Feel free to ask me via
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<a href="mailto:hauvipapro+posts@gmail.com">email</a> or
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<a rel="me" href="https://hachyderm.io/@haunguyen">Mastodon</a>.
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<br />Source code is available on
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<a href="https://github.com/haunt98/posts-go">GitHub</a>
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<a href="https://codeberg.org/yoshie/posts-go">Codeberg</a>
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<a href="https://git.sr.ht/~youngyoshie/posts-go">sourcehut</a>
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<a href="https://gitea.treehouse.systems/yoshie/posts-go">Treehouse</a>
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<a href="https://gitlab.com/youngyoshie/posts-go">GitLab</a>
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