408 lines
17 KiB
HTML
408 lines
17 KiB
HTML
<!doctype html>
|
|
<html>
|
|
<head>
|
|
<meta charset="utf-8" />
|
|
<meta name="viewport" content="width=device-width, initial-scale=1" />
|
|
<link
|
|
rel="stylesheet"
|
|
href="https://cdnjs.cloudflare.com/ajax/libs/github-markdown-css/5.7.0/github-markdown.min.css"
|
|
/>
|
|
<title>haunt98 posts</title>
|
|
</head>
|
|
<style>
|
|
.markdown-body {
|
|
box-sizing: border-box;
|
|
min-width: 200px;
|
|
max-width: 980px;
|
|
margin: 0 auto;
|
|
padding: 45px;
|
|
font-family:
|
|
Shantell Sans Normal,
|
|
Rec Mono Casual,
|
|
SF Pro,
|
|
Inter,
|
|
sans-serif;
|
|
font-weight: 500;
|
|
}
|
|
|
|
.markdown-body pre {
|
|
font-family:
|
|
Berkeley Mono,
|
|
IBM Plex Mono,
|
|
SF Mono,
|
|
Jetbrains Mono,
|
|
monospace;
|
|
}
|
|
|
|
@media (max-width: 767px) {
|
|
.markdown-body {
|
|
padding: 15px;
|
|
}
|
|
}
|
|
</style>
|
|
<body class="markdown-body">
|
|
<h2>
|
|
<a href="index.html"><code>~</code></a>
|
|
</h2>
|
|
<div class="markdown-heading">
|
|
<h1 class="heading-element">Real World Crypto 101</h1>
|
|
<a
|
|
id="user-content-real-world-crypto-101"
|
|
class="anchor"
|
|
aria-label="Permalink: Real World Crypto 101"
|
|
href="#real-world-crypto-101"
|
|
><span aria-hidden="true" class="octicon octicon-link"></span
|
|
></a>
|
|
</div>
|
|
<p>
|
|
My notes when reading
|
|
<a
|
|
href="https://www.manning.com/books/real-world-cryptography"
|
|
rel="nofollow"
|
|
>Real-World Cryptography</a
|
|
>
|
|
</p>
|
|
<div class="markdown-heading">
|
|
<h2 class="heading-element"><strong>Hash</strong> function</h2>
|
|
<a
|
|
id="user-content-hash-function"
|
|
class="anchor"
|
|
aria-label="Permalink: Hash function"
|
|
href="#hash-function"
|
|
><span aria-hidden="true" class="octicon octicon-link"></span
|
|
></a>
|
|
</div>
|
|
<p>... convert from input to digest</p>
|
|
<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>
|
|
<div class="markdown-heading">
|
|
<h2 class="heading-element">
|
|
<strong>MAC</strong> aka Message Authentication Code
|
|
</h2>
|
|
<a
|
|
id="user-content-mac-aka-message-authentication-code"
|
|
class="anchor"
|
|
aria-label="Permalink: MAC aka Message Authentication Code"
|
|
href="#mac-aka-message-authentication-code"
|
|
><span aria-hidden="true" class="octicon octicon-link"></span
|
|
></a>
|
|
</div>
|
|
<p>... produce from key, message to authentication tag.</p>
|
|
<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>
|
|
<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">->></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">->></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 -> 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, probably
|
|
return error the moment the bytes differ -> attacker recreate byte by
|
|
byte by measuring how long -> timing attacks
|
|
</li>
|
|
</ul>
|
|
<p>Constant time comparison:</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"><</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] -> 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">->></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">-->></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">->></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">-->></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">->></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">-->></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>
|
|
<div class="markdown-heading">
|
|
<h2 class="heading-element">AES (Advanced Encryption Standard)</h2>
|
|
<a
|
|
id="user-content-aes-advanced-encryption-standard"
|
|
class="anchor"
|
|
aria-label="Permalink: AES (Advanced Encryption Standard)"
|
|
href="#aes-advanced-encryption-standard"
|
|
><span aria-hidden="true" class="octicon octicon-link"></span
|
|
></a>
|
|
</div>
|
|
<p>
|
|
Currently (2023) the world using AES-128 which take a key 128 bits == 16
|
|
bytes
|
|
</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
|
|
<strong>block cipher</strong>. AES is deterministic so we can encrypt and
|
|
decrypt.
|
|
</p>
|
|
<div class="markdown-heading">
|
|
<h2 class="heading-element">AES-CBC (Cipher Block Chaining)</h2>
|
|
<a
|
|
id="user-content-aes-cbc-cipher-block-chaining"
|
|
class="anchor"
|
|
aria-label="Permalink: AES-CBC (Cipher Block Chaining)"
|
|
href="#aes-cbc-cipher-block-chaining"
|
|
><span aria-hidden="true" class="octicon octicon-link"></span
|
|
></a>
|
|
</div>
|
|
<p>
|
|
What if text you want to encrypt is 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 -> 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 (See
|
|
<a href="https://words.filippo.io/the-ecb-penguin/" rel="nofollow"
|
|
>The ECB penguin</a
|
|
>).
|
|
</p>
|
|
<p>CBC = deterministic block cipher + IV (initialization vector)</p>
|
|
<p>AES-CBC encrypt:</p>
|
|
<ul>
|
|
<li>IV XOR first plaintext -> AES encrypt -> first 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 -> XOR IV -> 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 -> BEAST attack (Browser Exploit Against SSL/TLS).
|
|
</p>
|
|
<div class="markdown-heading">
|
|
<h2 class="heading-element">
|
|
AEAD (Authenticated Encryption with Associated Data)
|
|
</h2>
|
|
<a
|
|
id="user-content-aead-authenticated-encryption-with-associated-data"
|
|
class="anchor"
|
|
aria-label="Permalink: AEAD (Authenticated Encryption with Associated Data)"
|
|
href="#aead-authenticated-encryption-with-associated-data"
|
|
><span aria-hidden="true" class="octicon octicon-link"></span
|
|
></a>
|
|
</div>
|
|
<p>
|
|
Because AES-CBC requires IV which shows public -> attacker can change
|
|
IV -> lack of authenticity -> use AES-CBC-HMAC or AEAD.
|
|
</p>
|
|
<p>AEAD provides a way to authenticate <strong>associated data</strong>.</p>
|
|
<div class="markdown-heading">
|
|
<h2 class="heading-element">AES-GCM (Galois/Counter Mode)</h2>
|
|
<a
|
|
id="user-content-aes-gcm-galoiscounter-mode"
|
|
class="anchor"
|
|
aria-label="Permalink: AES-GCM (Galois/Counter Mode)"
|
|
href="#aes-gcm-galoiscounter-mode"
|
|
><span aria-hidden="true" class="octicon octicon-link"></span
|
|
></a>
|
|
</div>
|
|
<p>... is one of AEAD implementation.</p>
|
|
<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 -> 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>
|
|
<ul>
|
|
<li>No need padding.</li>
|
|
<li>Ciphertext is same length as plaintext.</li>
|
|
</ul>
|
|
<p>GMAC is MAC with GHASH. GHASH resembles CBC mode.</p>
|
|
<div class="markdown-heading">
|
|
<h2 class="heading-element">ChaCha20-Poly1305</h2>
|
|
<a
|
|
id="user-content-chacha20-poly1305"
|
|
class="anchor"
|
|
aria-label="Permalink: ChaCha20-Poly1305"
|
|
href="#chacha20-poly1305"
|
|
><span aria-hidden="true" class="octicon octicon-link"></span
|
|
></a>
|
|
</div>
|
|
<p>... is one of AEAD implementation.</p>
|
|
<p>ChaCha20-Poly1305 = ChaCha20 stream cipher + Poly1305 MAC</p>
|
|
<div class="markdown-heading">
|
|
<h2 class="heading-element">Key exchange</h2>
|
|
<a
|
|
id="user-content-key-exchange"
|
|
class="anchor"
|
|
aria-label="Permalink: Key exchange"
|
|
href="#key-exchange"
|
|
><span aria-hidden="true" class="octicon octicon-link"></span
|
|
></a>
|
|
</div>
|
|
<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">->></span> <span class="pl-ent">alice</span><span class="pl-k">:</span> <span class="pl-s">generate key pair: public_key, secret_key</span>
|
|
<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">generate key pair: public_key, secret_key</span>
|
|
<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 public_key</span>
|
|
<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">send public_key</span>
|
|
<span class="pl-ent">alice </span><span class="pl-k">->></span> <span class="pl-ent">alice</span><span class="pl-k">:</span> <span class="pl-s">generate shared_secret(secret_key, bob_public_key)</span>
|
|
<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">generate shared_secret(secret_key, alice_public_key)</span></pre>
|
|
</div>
|
|
<p>
|
|
Prevent MITM (Man In The Middle) passive attack. If attacker can intercept
|
|
public_key then it's over.
|
|
</p>
|
|
<div class="markdown-heading">
|
|
<h2 class="heading-element">Diffie-Hellman</h2>
|
|
<a
|
|
id="user-content-diffie-hellman"
|
|
class="anchor"
|
|
aria-label="Permalink: Diffie-Hellman"
|
|
href="#diffie-hellman"
|
|
><span aria-hidden="true" class="octicon octicon-link"></span
|
|
></a>
|
|
</div>
|
|
<p>... is key exchange algorithm.</p>
|
|
<ul>
|
|
<li>
|
|
Alice and Bob agree on prime number <code>p</code> and generator
|
|
<code>g</code>.
|
|
</li>
|
|
<li>
|
|
Alice generate secret <code>a</code> and public
|
|
<code>A = g^a mod p</code>.
|
|
</li>
|
|
<li>
|
|
Bob generate secret <code>b</code> and public
|
|
<code>B = g^b mod p</code>.
|
|
</li>
|
|
<li>
|
|
Alice and Bob exchange <code>A</code> and <code>B</code>
|
|
<ul>
|
|
<li>
|
|
Same secret <code>A^b mod p == B^a mod p == g^(ab) mod p</code>
|
|
</li>
|
|
</ul>
|
|
</li>
|
|
</ul>
|
|
<p>Diffie-Hellman is based on group theory.</p>
|
|
<div class="markdown-heading">
|
|
<h2 class="heading-element">Elliptic Curve Diffie-Hellman (ECDH)</h2>
|
|
<a
|
|
id="user-content-elliptic-curve-diffie-hellman-ecdh"
|
|
class="anchor"
|
|
aria-label="Permalink: Elliptic Curve Diffie-Hellman (ECDH)"
|
|
href="#elliptic-curve-diffie-hellman-ecdh"
|
|
><span aria-hidden="true" class="octicon octicon-link"></span
|
|
></a>
|
|
</div>
|
|
<p>Instead of prime number, use elliptic curve.</p>
|
|
<ul>
|
|
<li>
|
|
Alice and Bob agree on elliptic curve <code>E</code> and generator
|
|
<code>G</code>.
|
|
</li>
|
|
<li>
|
|
Alice generate secret <code>a</code> and public <code>A = [a]G</code>.
|
|
</li>
|
|
<li>
|
|
Bob generate secret <code>b</code> and public <code>B = [b]G</code>.
|
|
</li>
|
|
<li>
|
|
Alice and Bob exchange <code>A</code> and <code>B</code>
|
|
<ul>
|
|
<li>Same secret <code>[a]B == [b]A == [ab]G</code></li>
|
|
</ul>
|
|
</li>
|
|
</ul>
|
|
|
|
<div>
|
|
Feel free to ask me via
|
|
<a href="mailto:hauvipapro+posts@gmail.com">email</a> or
|
|
<a rel="me" href="https://hachyderm.io/@haunguyen">Mastodon</a>.
|
|
<br />Source code is available on
|
|
<a href="https://github.com/haunt98/posts-go">GitHub</a>
|
|
<a href="https://codeberg.org/yoshie/posts-go">Codeberg</a>
|
|
<a href="https://git.sr.ht/~youngyoshie/posts-go">sourcehut</a>
|
|
<a href="https://gitea.treehouse.systems/yoshie/posts-go">Treehouse</a>
|
|
<a href="https://gitlab.com/youngyoshie/posts-go">GitLab</a>
|
|
</div>
|
|
</body>
|
|
</html>
|