openssh unix dev - Feb 2015 - Does ssh-keygen really allow 521 bit ECDSA key generation?

Hello!

I found strange sentence in ssh-keygen man page. There is may be a misprint.

*-b* *bits*Specifies the number of bits in the key to create. For RSA keys,
the minimum size is 768 bits and the default is 2048 bits. Generally, 2048
bits is considered sufficient. DSA keys must be exactly 1024 bits as
specified by FIPS 186-2. For ECDSA keys, the *-b* flag determines the key
length by selecting from one of three elliptic curve sizes: 256, 384 or 521
bits. Attempting to use bit lengths other than these three values for ECDSA
keys will fail. Ed25519 keys have a fixed length and the *-b* flag will be
ignored.

Regards,
Eugene Bright.

Eugene Bright <hexumg at gmail.com> on Wed, 2015/02/25
16:12:
> Hello!
> 
> I found strange sentence in ssh-keygen man page. There is may be a
misprint.
You are referring the fact that 521 is not the power of base 2?
Looks like this is valid nevertheless.

% ssh-keygen -t ecdsa -b 512
Invalid ECDSA key length - valid lengths are 256, 384 or 521 bits
% ssh-keygen -t ecdsa -b 521
Generating public/private ecdsa key pair.
[...]

Wikipedia adds a note about this as well:

http://en.wikipedia.org/wiki/Elliptic_curve_cryptography#cite_note-25
-- 
main(a){char*c=/*    Schoene Gruesse                        
*/"B?IJj;MEH"
"CX:;",b;for(a/*    Chris           get my mail address:   
*/=0;b=c[a++];)
putchar(b-1/(/*               gcc -o sig sig.c && ./sig   
*/b/42*2-3)*42);}
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>>>>> "EB" == Eugene Bright <hexumg at gmail.com>
writes:
EB> I found strange sentence in ssh-keygen man page. There is may be a
misprint.

No, that is correct.

They couldn't find a good prime slightly under 512 bits, so chose the
Mersenne prime 2^521 - 1.

-JimC
-- 
James Cloos <cloos at jhcloos.com>         OpenPGP: 0x997A9F17ED7DAEA6