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<a href="https://gitlab.com/hanno">Hanno Böck</a> created an issue: <a href="https://gitlab.com/gnutls/gnutls/-/issues/1747">#1747</a>
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<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">ECDSA private key values should be stored in a bytestring of fixed-length with the size of the modulus (see RFC 5915 [1]). This also mitigates possible sidechannels that may leak information about leading zeros when loading a private key (see also [2]).</p>
<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">It appears gnutls encodes the private d value with an additional zero if the upmost bit of the d value is set (~1/2 of keys). It encodes d with a byte too small if the top 9 bits are zero. (happens in 1/512 of keys).</p>
<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">To test, generate a large number of ecdsa keys:</p>
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<pre class="code highlight js-syntax-highlight language-plaintext" v-pre="true" style='display: block; font-size: 14px; color: #3a383f; line-height: 1.6em; overflow-x: auto; border-radius: .25rem; position: relative; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; font-variant-ligatures: none; word-break: break-all; word-wrap: break-word; background-color: #fbfafd; margin: 0 0 1rem; padding: 12px; border: 1px solid #dcdcde;'><code style='font-size: inherit; color: inherit; word-wrap: normal; word-break: keep-all; background-color: inherit; border-radius: .25rem; white-space: pre; margin-top: 0; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; font-variant-ligatures: none; overflow-wrap: normal; padding: unset;'><span id="LC1" class="line" lang="plaintext" style="margin-top: 0;">for x in $(seq 1 1000); do certtool --generate-privkey --outfile $x.key --key-type ecdsa --outder; done</span></code></pre>
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<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">Given the fixed-size encoding, all private keys with the same curve should have the same size, but they differ. Some are 121 bytes (correctly encoded), some 122 bytes, some (few) 120 bytes.</p>
<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">Here's a python script that checks if ECDSA's d has the correct encoding size:
<a href="https://github.com/hannob/tlshelpers/blob/main/checkkeyenc" rel="nofollow noreferrer noopener" target="_blank" style="margin-top: 0;">https://github.com/hannob/tlshelpers/blob/main/checkkeyenc</a></p>
<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">Given this can lead to a timing sidechannel leaking (very limited) information about the private key, one may see this as a security issue. However, the risk is very low and probably not practically relevant, as the information that can be leaked is extremely limited.</p>
<p dir="auto" style="color: #3a383f; margin: 0;" align="initial">[1] <a href="https://www.rfc-editor.org/rfc/rfc5915.html#section-3" rel="nofollow noreferrer noopener" target="_blank" style="margin-top: 0;">https://www.rfc-editor.org/rfc/rfc5915.html#section-3</a>
[2] <a href="https://seclists.org/oss-sec/2025/q4/38" rel="nofollow noreferrer noopener" target="_blank">https://seclists.org/oss-sec/2025/q4/38</a></p>
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