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DSA key gen: start from 2 when looking for g.

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Having just written a comment about how it was almost inconceivably
improbable that you _wouldn't_ be successful in finding a suitable g
on the very first number you tried, I couldn't help noticing that in
fact my very next DSA key generation test had to try twice. Had I made
a mistake in my probability theory?

No, it turns out: I find g by raising consecutive numbers to the power
(p-1)/q and looking to see if they're not 1, but I start with 1
itself, which along with -1 is the only number that _can't_ work!

Save a bit of pointless effort and iterate up from 2 instead.
This commit is contained in:
Simon Tatham 2020-03-01 08:27:54 +00:00
parent a7f409eb61
commit 8b672835c1
1 changed files with 1 additions and 1 deletions
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sshdssg.c
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@ -64,7 +64,7 @@ int dsa_generate(struct dss_key *key, int bits, ProgressReceiver *prog)
*/
progress_start_phase(prog, phase_g);
mp_int *power = mp_div(p, q); /* this is floor(p/q) == (p-1)/q */
mp_int *h = mp_from_integer(1);
mp_int *h = mp_from_integer(2);
mp_int *g;
while (1) {
progress_report_attempt(prog);