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Fix generation of one-bit-short RSA keys.

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I carefully tested commit 801ab68ea's rewrite of invent_firstbits in
every way I could think of to ensure that I really was generating two
values whose product was at least 'minproduct'. But unfortunately the
value of 'minproduct' itself was off by a factor of two, which made
the entire system pointless!
This commit is contained in:
Simon Tatham 2019-03-20 11:50:45 +00:00
parent 6ce6fe5712
commit 582284fa6c
1 changed files with 3 additions and 3 deletions
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6
sshprime.c
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@ -382,7 +382,7 @@ static inline unsigned firstbits_b_min(
unsigned a, unsigned lo, unsigned hi, unsigned min_separation)
{
/* To get a large enough product, b must be at least this much */
unsigned b_min = (lo*lo + a - 1) / a;
unsigned b_min = (2*lo*lo + a - 1) / a;
/* Now enforce a<b, optionally with minimum separation */
if (b_min < a + min_separation)
b_min = a + min_separation;
@ -397,7 +397,7 @@ void invent_firstbits(unsigned *one, unsigned *two, unsigned min_separation)
/*
* We'll pick 12 initial bits (number selected at random) for each
* prime, not counting the leading 1. So we want to return two
* values in the range [2^12,2^13) whose product is at least 2^24.
* values in the range [2^12,2^13) whose product is at least 2^25.
*
* Strategy: count up all the viable pairs, then select a random
* number in that range and use it to pick a pair.
@ -405,7 +405,7 @@ void invent_firstbits(unsigned *one, unsigned *two, unsigned min_separation)
* To keep things simple, we'll ensure a < b, and randomly swap
* them at the end.
*/
const unsigned lo = 1<<12, hi = 1<<13, minproduct = lo*lo;
const unsigned lo = 1<<12, hi = 1<<13, minproduct = 2*lo*lo;
unsigned a, b;
/*