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<a href="https://gitlab.com/dkg">Daniel Kahn Gillmor</a> created an issue: <a href="https://gitlab.com/gnutls/gnutls/-/issues/1236">#1236</a>
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<p dir="auto">over on the IETF's <a href="https://www.ietf.org/mailman/listinfo/spasm" rel="nofollow noreferrer noopener" target="_blank">LAMPS WG's mailing list</a>, <a href="https://mailarchive.ietf.org/arch/msg/spasm/fy6ilJRnqBaXiZctVyHJELQMmK8/" rel="nofollow noreferrer noopener" target="_blank">David A. Cooper writes</a>:</p>
<pre class="code highlight js-syntax-highlight language-plaintext" lang="plaintext" v-pre="true"><code><span id="LC1" class="line" lang="plaintext">the keyUsage extensions in the certificates are </span>
<span id="LC2" class="line" lang="plaintext">not DER encoded. The keyUsage extensions in the certificates are all </span>
<span id="LC3" class="line" lang="plaintext">encoded in the form "03 03 07 XX 00."</span>
<span id="LC4" class="line" lang="plaintext"></span>
<span id="LC5" class="line" lang="plaintext">It seems that the developer of the encoder mistakenly believed that the </span>
<span id="LC6" class="line" lang="plaintext">bit string should always be 9 bits long in order to correspond to the </span>
<span id="LC7" class="line" lang="plaintext">definition of keyUsage:</span>
<span id="LC8" class="line" lang="plaintext"></span>
<span id="LC9" class="line" lang="plaintext"> KeyUsage ::= BIT STRING {</span>
<span id="LC10" class="line" lang="plaintext"> digitalSignature (0),</span>
<span id="LC11" class="line" lang="plaintext"> nonRepudiation (1), -- recent editions of X.509</span>
<span id="LC12" class="line" lang="plaintext"> have</span>
<span id="LC13" class="line" lang="plaintext"> -- renamed this bit to</span>
<span id="LC14" class="line" lang="plaintext"> contentCommitment</span>
<span id="LC15" class="line" lang="plaintext"> keyEncipherment (2),</span>
<span id="LC16" class="line" lang="plaintext"> dataEncipherment (3),</span>
<span id="LC17" class="line" lang="plaintext"> keyAgreement (4),</span>
<span id="LC18" class="line" lang="plaintext"> keyCertSign (5),</span>
<span id="LC19" class="line" lang="plaintext"> cRLSign (6),</span>
<span id="LC20" class="line" lang="plaintext"> encipherOnly (7),</span>
<span id="LC21" class="line" lang="plaintext"> decipherOnly (8) }</span>
<span id="LC22" class="line" lang="plaintext"></span>
<span id="LC23" class="line" lang="plaintext">However, X.680 and X.690 specify that when encoding a named bit list, </span>
<span id="LC24" class="line" lang="plaintext">such as above, any trailing zeros must be removed. So, a keyUsage </span>
<span id="LC25" class="line" lang="plaintext">extension with just digitalSignature and nonRepudiation set would be </span>
<span id="LC26" class="line" lang="plaintext">encoded as a bit string that is only two bits long ('11') and a keyUsage </span>
<span id="LC27" class="line" lang="plaintext">extension with just keyAgreement set would be encoded as a bit string </span>
<span id="LC28" class="line" lang="plaintext">that is five bits long ('00001').</span>
<span id="LC29" class="line" lang="plaintext"></span>
<span id="LC30" class="line" lang="plaintext">So, the correct encoding for the keyUsage extension would be:</span>
<span id="LC31" class="line" lang="plaintext"></span>
<span id="LC32" class="line" lang="plaintext"> * "03 02 01 06" for the CA certificate</span>
<span id="LC33" class="line" lang="plaintext"> * "03 02 06 C0" for the digital signature certificates</span>
<span id="LC34" class="line" lang="plaintext"> * "03 02 05 20" for the encryption certificates</span>
<span id="LC35" class="line" lang="plaintext"> * "03 02 03 08" for the key agreement certificates</span></code></pre>
<p dir="auto">This concerns <a href="https://datatracker.ietf.org/doc/draft-ietf-lamps-samples/" rel="nofollow noreferrer noopener" target="_blank">draft-ietf-lamps-samples</a>, which contains certificates being generated by <code>certtool</code>.</p>
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