fermat primes and dnssec-keygen bug?

[Miek Gieben]

[ Quoting <spainj at countryday.net> at 04:07 on Mar  7 in "RE: fermat primes an..." ]
> > I would recommend that dnssec-keygen starts ignoring the "-e" parameter that everyone has put in their scripts to prevent exponent 3 keys, who are not getting keys with exponent 4294967296 + 1 (F5)
> 
> > Alternatively, if this is done on purpose, I guess we should all migrate the 64 bit machines :)
> 
> This is interesting, if I correctly understand your point, but it appears that dnssec-keygen computes F5 differently than you do in your example in http://code.google.com/p/go/issues/detail?can=2&start=0&num=100&q=&colspec=ID%20Status%20Stars%20Priority%20Owner%20Reporter%20Summary&groupby=&sort=&id=3161.
> 
> In your example:
> 	pubkey := new(rsa.PublicKey)
> 	pubkey.N = big.NewInt(0)
> 	pubkey.E = 4294967296 + 1
> which results in 32-bit integer overflow.
> 
> In bind-9.9.0/lib/dns/opensslrsa_link.c, starting at line 750:
> 	if (exp == 0) {
> 		/* RSA_F4 0x10001 */
> 		BN_set_bit(e, 0);
> 		BN_set_bit(e, 16);
> 	} else {
> 		/* F5 0x100000001 */
> 		BN_set_bit(e, 0);
> 		BN_set_bit(e, 32);
> 	}

Its not about integer overflow, it's about the fact that F5
does not add to the security, but does use up a lot of CPU cycles.

grtz Miek
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