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<ol class="chapter"><li class="chapter-item expanded "><a href="0_intro.html"><strong aria-hidden="true">1.</strong> Introduction</a></li><li class="chapter-item expanded "><a href="1_installation.html"><strong aria-hidden="true">2.</strong> Installation</a></li><li class="chapter-item expanded "><a href="2_encryption.html" class="active"><strong aria-hidden="true">3.</strong> Encryption</a></li><li class="chapter-item expanded "><a href="3_key_format.html"><strong aria-hidden="true">4.</strong> Key Format</a></li><li class="chapter-item expanded "><a href="4_usage_and_examples.html"><strong aria-hidden="true">5.</strong> Usage & Examples</a></li><li class="chapter-item expanded "><a href="5_rust_library.html"><strong aria-hidden="true">6.</strong> Rust Library</a></li></ol> </div>
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<h1 class="menu-title">Crypt4GH Format</h1>
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<h1 id="encryption-algorithm---crypt4gh"><a class="header" href="#encryption-algorithm---crypt4gh">Encryption Algorithm - Crypt4GH</a></h1>
<h2 id="encryption-format"><a class="header" href="#encryption-format">Encryption Format</a></h2>
<p>A random session key (of 256 bits) is generated to seed a ChaCha20 engine, with Poly1305 authentication mode. For each segment of at most 64kB of data, a nonce is randomly generated and prepended to the segment. Using the two latters, the original file is segmented and each segment is encrypted.</p>
<p>The header is prepended to the encrypted data.</p>
<p>Informally, the header contains, the word <code>crypt4gh</code>, the format version (currently 1), the number of header packets, and the sequence of header packets.</p>
<p>A header packet is a length followed by its content. The content can be a data encryption packet or an edit list packet.</p>
<p>All packets are encrypted using a Curve25519-based encryption.</p>
<p><img src="https://crypt4gh.readthedocs.io/en/latest/_images/encryption.png" alt="Encryption process" /></p>
<h2 id="features"><a class="header" href="#features">Features</a></h2>
<p>The advantages of the format are, among others:</p>
<ul>
<li>Re-encrypting the file for another user requires only to decrypt the header and encrypt it with the user’s public key.</li>
<li>Header packets can be encrypted for multiple recipients.</li>
<li>Re-arranging the file to chunk a portion requires only to decrypt the header, re-encrypt with an edit list, and select the cipher segments surrounding the portion. The file itself is not decrypted and reencrypted.</li>
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