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<a href="https://gitlab.com/nmav">Nikos Mavrogiannopoulos</a>
commented:
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<p dir="auto"><a href="https://gitlab.com/tomato42" data-user="258094" data-reference-type="user" data-container="body" data-placement="bottom" class="gfm gfm-project_member" title="Hubert Kario (@mention me if you need reply)">@tomato42</a> what you raised in private email regarding the text of rfc5246:</p>
<pre class="code highlight js-syntax-highlight plaintext" lang="plaintext" v-pre="true"><code><span id="LC1" class="line" lang="plaintext"> record_overflow</span>
<span id="LC2" class="line" lang="plaintext"> A TLSCiphertext record was received that had a length more than</span>
<span id="LC3" class="line" lang="plaintext"> 2^14+2048 bytes, or a record decrypted to a TLSCompressed record</span>
<span id="LC4" class="line" lang="plaintext"> with more than 2^14+1024 bytes. This message is always fatal and</span>
<span id="LC5" class="line" lang="plaintext"> should never be observed in communication between proper</span>
<span id="LC6" class="line" lang="plaintext"> implementations (except when messages were corrupted in the</span>
<span id="LC7" class="line" lang="plaintext"> network).</span></code></pre>
<p dir="auto">This is a description of the alert, with some informal? way how to apply it. The actual behavior the RFC mandates is:</p>
<pre class="code highlight js-syntax-highlight plaintext" lang="plaintext" v-pre="true"><code><span id="LC1" class="line" lang="plaintext"></span>
<span id="LC2" class="line" lang="plaintext"> length</span>
<span id="LC3" class="line" lang="plaintext"> The length (in bytes) of the following TLSPlaintext.fragment. The</span>
<span id="LC4" class="line" lang="plaintext"> length MUST NOT exceed 2^14.</span></code></pre>
<p dir="auto">That's what gnutls enforces, and it does it by calculating in <code>_gnutls_epoch_set_keys</code> what's the maximum overhead required by this particular ciphersuite. It does not use the number 1024, as this looks like an arbitrary max limit. This change relaxes the strict calculation of the ciphersuite overhead for the cbc ciphersuites, by allowing 256 bytes of additional data as in TLS1.3. That's in accordance with the guidance but smaller than 1024.</p>
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