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<p>Baptiste,</p>
<p>usually the bit length of the characteristic <i>p </i>of the
finite field, where all the coordinate computationa are made, is
taken as the key length. And whereas this bit length of the prime
for the NIST curve P-256 is 256 , it is 255 for Curve25519.
Therefore the key length value is correct, despite the public key
has a bitlength of 256.</p>
<p>/Ann.<br>
</p>
<div class="moz-cite-prefix">Am 2021-09-15 um 20:30 schrieb Baptiste
Beauplat via Gnupg-users:<br>
</div>
<blockquote type="cite"
cite="mid:20210915183025.4znxqwshi46annjc@libra.beauplat.fr">
<pre class="moz-quote-pre" wrap="">
Hi,
I noticed that the key size reported by gpg --with-colons for ECC keys
(ed25519) have changed from 256 to 255.
For instance, on my key I previously got:
$gpg --with-colons -k <a class="moz-txt-link-abbreviated" href="mailto:lyknode@cilg.org">lyknode@cilg.org</a> | grep pub | cut -d : -f 3
256
While now the result is:
$gpg --with-colons -k <a class="moz-txt-link-abbreviated" href="mailto:lyknode@cilg.org">lyknode@cilg.org</a> | grep pub | cut -d : -f 3
255
I was wondering if that's a bug, since from what I understand (certainly
way to little) the public key size of ed25519 is fixed at 256 and the
value 255 is only used in the prime number calculation (2^255 - 19).
Note: The changed comes from the following commit in libgcrypt:
<a class="moz-txt-link-freetext" href="https://dev.gnupg.org/rCdb7b2c591004868abedbc2c19d3bb2efebf8529d">https://dev.gnupg.org/rCdb7b2c591004868abedbc2c19d3bb2efebf8529d</a>
Best,
</pre>
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