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<h2>
<a href="index.html"><code>~</code></a>
</h2>
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<h1>
<a
id="user-content-real-world-crypto-101"
class="anchor"
aria-hidden="true"
tabindex="-1"
href="#real-world-crypto-101"
><span aria-hidden="true" class="octicon octicon-link"></span></a
>Real World Crypto 101
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</h1>
<p>
My notes when reading
<a
href="https://www.manning.com/books/real-world-cryptography"
rel="nofollow"
>Real-World Cryptography</a
>
</p>
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<h2>
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<a
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id="user-content-hash-function-convert-from-input-to-digest"
class="anchor"
aria-hidden="true"
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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<ul>
<li>Pre-image resistance: Given digest, can not find input</li>
<li>
Second pre-image resistance: Given input, digest, can not find another
input produce same digest. Small change to input make digest big change.
</li>
<li>Collision resistance: Can not find 2 input produce same digest.</li>
</ul>
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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"
class="anchor"
aria-hidden="true"
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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<strong>MAC</strong> aka Message Authentication Code produce from key,
message to authentication tag.
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</h2>
<ul>
<li>A send B message with MAC (generate from message and A key).</li>
<li>
B double check message with MAC (generate from receive message and B
key).
</li>
<li>A and B use same key.</li>
</ul>
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<div class="highlight highlight-source-mermaid">
<pre><span class="pl-k">sequenceDiagram</span>
<span class="pl-k">participant</span> <span class="pl-ent">alice</span>
<span class="pl-k">participant</span> <span class="pl-ent">bob</span>
<span class="pl-ent">alice </span><span class="pl-k">-&gt;&gt;</span> <span class="pl-ent">bob</span><span class="pl-k">:</span> <span class="pl-s">send alice, mac(secret_key_alice, alice)</span>
<span class="pl-ent">bob </span><span class="pl-k">-&gt;&gt;</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>
</div>
<ul>
<li>
Prevent forgery: without secret_key, can not generate MAC even if
knowing <strong>a lot of</strong> alice and mac(secret_key, alice),
</li>
<li>Prevent collisions: keep MAC long enough (256-bit),</li>
<li>
Replay attacks: send transaction 2 times with perfectly MAC and u know
why -&gt; instead of mac(secret_key, alice), use
<strong>counter</strong> as mac(secret_key, counter, alice).
</li>
<li>
Verify must be done in <strong>constant time</strong>: if not, probaly
return error the moment the bytes differ -&gt; attacker recreate byte by
byte by measuring how long -&gt; timing attacks
</li>
</ul>
<p>Constant time comparision:</p>
<div class="highlight highlight-source-go">
<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">&lt;</span> <span class="pl-en">len</span>(<span class="pl-s1">x</span>); <span class="pl-s1">i</span><span class="pl-c1">++</span> {
<span class="pl-c">// Use XOR instead of compare x[i] == y[i]</span>
<span class="pl-c">// If x[i] == y[i] -&gt; XOR is 1</span>
<span class="pl-c">// Otherwise XOR is 0</span>
<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>]
}
<span class="pl-c">// v == 1 means all XOR is 1 means x == y</span></pre>
</div>
<p>Use for:</p>
<ul>
<li>
Integrity: because MAC ensure no one can tamper with message without
noticing
</li>
</ul>
<div class="highlight highlight-source-mermaid">
<pre><span class="pl-k">sequenceDiagram</span>
<span class="pl-k">participant</span> <span class="pl-ent">alice</span>
<span class="pl-k">participant</span> <span class="pl-ent">bob</span>
<span class="pl-ent">alice </span><span class="pl-k">-&gt;&gt;</span> <span class="pl-ent">bob</span><span class="pl-k">:</span> <span class="pl-s">send username, password</span>
<span class="pl-ent">bob </span><span class="pl-k">--&gt;&gt;</span> <span class="pl-ent">alice</span><span class="pl-k">:</span> <span class="pl-s">return alice|mac(secret_key, alice)</span>
<span class="pl-ent">alice </span><span class="pl-k">-&gt;&gt;</span> <span class="pl-ent">bob</span><span class="pl-k">:</span> <span class="pl-s">send alice|mac(secret_key, alice)</span>
<span class="pl-ent">bob </span><span class="pl-k">--&gt;&gt;</span> <span class="pl-ent">alice</span><span class="pl-k">:</span> <span class="pl-s">return OK</span>
<span class="pl-ent">alice </span><span class="pl-k">-&gt;&gt;</span> <span class="pl-ent">bob</span><span class="pl-k">:</span> <span class="pl-s">send bob|mac(secret_key, alice)</span>
<span class="pl-ent">bob </span><span class="pl-k">--&gt;&gt;</span> <span class="pl-ent">alice</span><span class="pl-k">:</span> <span class="pl-s">return ERROR</span></pre>
</div>
<p><strong>HMAC</strong> is MAC using hash</p>
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<h2>
<a
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id="user-content-aes-advanced-encryption-standard"
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class="anchor"
aria-hidden="true"
tabindex="-1"
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href="#aes-advanced-encryption-standard"
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><span aria-hidden="true" class="octicon octicon-link"></span></a
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>AES (Advanced Encryption Standard)
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</h2>
<p>
Currently (2023) the world using AES-128 which take a key 128 bits == 16
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bytes
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</p>
<ul>
<li>Take a variable-length key</li>
<li>Take plaintext of 128 bits</li>
<li>Give ciphertext of 128 bits</li>
</ul>
<p>
AES is kind of cipher, handle fixed-size plaintext so we called
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<strong>block cipher</strong>. AES is deterministic so we can encrypt and
decrypt.
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</p>
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<h2>
<a
id="user-content-aes-cbc-cipher-block-chaining"
class="anchor"
aria-hidden="true"
tabindex="-1"
href="#aes-cbc-cipher-block-chaining"
><span aria-hidden="true" class="octicon octicon-link"></span></a
>AES-CBC (Cipher Block Chaining)
</h2>
<p>
What if text you want to encrypt longer than 128 bytes ? We add
<strong>padding</strong> for text to become multi block which has 128
bytes, then encrypt each block.
</p>
<p>
Adding padding bytes is easy, remove it after decrypt is hard. How do you
know which is padding bytes you add if you use random bytes ?
</p>
<p>
Just use <strong>PKCS#7 padding</strong>. Example AES-128 use block of 16
bytes but only have 9 bytes, should add 7 bytes padding. Just fill all
padding bytes with padding length aka value <code>07</code>.
</p>
<div class="highlight highlight-text-adblock">
<pre>XX XX XX XX XX XX XX XX XX 07 07 07 07 07 07 07</pre>
</div>
<p>
So to know how much padding bytes should we remove -&gt; read last bytes
(<code>07</code>) to know the length to remove trailing padding bytes.
</p>
<p>
The problem with naive way to split text, add padding bytes then encrypt
each block using AES-128 is repeated text. Because it leaks information if
text is made up from many repeated text (ECB penguin).
</p>
<p>CBC = deterministic block cipher + IV (initialization vector)</p>
<p>AES-CBC encrypt:</p>
<ul>
<li>
IV XOR first plaintext -&gt; AES encrypt -&gt; first ciphertext.
ciphertext.
</li>
<li>Use first ciphertext as IV to second ciphertext and so on.</li>
</ul>
<p>AES-CBC decrypt:</p>
<ul>
<li>AES decrypt first ciphertext -&gt; XOR IV -&gt; first plaintext.</li>
<li>Use first ciphertext as IV to second block and so on.</li>
</ul>
<p>Because IV, same plaintext can encrypt to different ciphertext.</p>
<p>
<strong>WARNING</strong> If IV become predictable, AES-CBC become
deterministic -&gt; BEAST attack (Browser Exploit Against SSL/TLS).
</p>
<h2>
<a
id="user-content-aead-authenticated-encryption-with-associated-data"
class="anchor"
aria-hidden="true"
tabindex="-1"
href="#aead-authenticated-encryption-with-associated-data"
><span aria-hidden="true" class="octicon octicon-link"></span></a
>AEAD (Authenticated Encryption with Associated Data)
</h2>
<p>
Because AES-CBC requires IV which shows public -&gt; attacker can change
IV -&gt; lack of authenticity -&gt; use AES-CBC-HMAC or AEAD.
</p>
<p>AEAD provides a way to authenticate <strong>associated data</strong>.</p>
<h2>
<a
id="user-content-aes-gcm-galoiscounter-mode-aead"
class="anchor"
aria-hidden="true"
tabindex="-1"
href="#aes-gcm-galoiscounter-mode-aead"
><span aria-hidden="true" class="octicon octicon-link"></span></a
>AES-GCM (Galois/Counter Mode) AEAD
</h2>
<p>AES-GCM = AES-CTR (Counter) + GMAC message authentication code</p>
<p>AES-CTR encrypt:</p>
<ul>
<li>Create nonce 12 bytes (same purpose as IV).</li>
<li>Concatenate nonce with counter 4 bytes: 1, 2, 3, ...</li>
<li>
Encrypt AES from concatenated none with counter to
<strong>keystream</strong>.
</li>
<li>XOR keystream with plaintext -&gt; ciphertext.</li>
</ul>
<p>
Limit is counter only up to 4 bytes so only handle plaintext of 2^32 - 1
blocks of 16 bytes aka 69 GBs.
</p>
<p>
AES-CTR no need padding because if keystream is longer than plaintext, it
is truncated to plaintext length before XOR.
</p>
<p>This is stream cipher, differ from block cipher.</p>
<p>GMAC is MAC with GHASH. GHASH resembles CBC mode.</p>
<h2>
<a
id="user-content-chacha20-poly1305-aed"
class="anchor"
aria-hidden="true"
tabindex="-1"
href="#chacha20-poly1305-aed"
><span aria-hidden="true" class="octicon octicon-link"></span></a
>ChaCha20-Poly1305 AED
</h2>
<p>ChaCha20-Poly1305 = ChaCha20 stream cipher + Poly1305 MAC</p>
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