Size for ECC keys have changed from 256 to 255

[Annie Yousar]

Baptiste,

usually the bit length of the characteristic /p /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.

/Ann.

Am 2021-09-15 um 20:30 schrieb Baptiste Beauplat via Gnupg-users:
> 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 lyknode at cilg.org | grep pub | cut -d : -f 3
> 256
>
> While now the result is:
>
> $gpg --with-colons -k lyknode at cilg.org | 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:
>
> https://dev.gnupg.org/rCdb7b2c591004868abedbc2c19d3bb2efebf8529d
>
> Best,
>
>
> _______________________________________________
> Gnupg-users mailing list
> Gnupg-users at gnupg.org
> http://lists.gnupg.org/mailman/listinfo/gnupg-users
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.gnupg.org/pipermail/gnupg-users/attachments/20210917/ca3b7f95/attachment.html>