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Add support for using Diffie-Hellman with short exponents (sshdh.c

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contains a reference to a paper on the subject). Reduces time taken
for DH group exchange to the point where it's viable to enable it
all the time, so I have. :-)

[originally from svn r991]
This commit is contained in:
Simon Tatham
2001-03-10 11:04:07 +00:00
parent 0c8635beda
commit d823077f18
3 changed files with 59 additions and 29 deletions
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40
sshdh.c
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@ -75,8 +75,19 @@ void dh_cleanup(void) {
/*
* DH stage 1: invent a number x between 1 and q, and compute e =
* g^x mod p. Return e.
*
* If `nbits' is greater than zero, it is used as an upper limit
* for the number of bits in x. This is safe provided that (a) you
* use twice as many bits in x as the number of bits you expect to
* use in your session key, and (b) the DH group is a safe prime
* (which SSH demands that it must be).
*
* P. C. van Oorschot, M. J. Wiener
* "On Diffie-Hellman Key Agreement with Short Exponents".
* Advances in Cryptology: Proceedings of Eurocrypt '96
* Springer-Verlag, May 1996.
*/
Bignum dh_create_e(void) {
Bignum dh_create_e(int nbits) {
int i;
int nbytes;
@ -91,10 +102,25 @@ Bignum dh_create_e(void) {
* with qmask.
*/
if (x) freebn(x);
ssh1_write_bignum(buf, qmask);
for (i = 2; i < nbytes; i++)
buf[i] &= random_byte();
ssh1_read_bignum(buf, &x);
if (nbits == 0 || nbits > ssh1_bignum_bitcount(qmask)) {
ssh1_write_bignum(buf, qmask);
for (i = 2; i < nbytes; i++)
buf[i] &= random_byte();
ssh1_read_bignum(buf, &x);
} else {
int b, nb;
x = bn_power_2(nbits);
nb = 0;
for (i = 0; i < nbits; i++) {
if (nb == 0) {
nb = 8;
b = random_byte();
}
bignum_set_bit(x, i, b & 1);
b >>= 1;
nb--;
}
}
} while (bignum_cmp(x, One) <= 0 || bignum_cmp(x, q) >= 0);
/*
@ -109,5 +135,7 @@ Bignum dh_create_e(void) {
* DH stage 2: given a number f, compute K = f^x mod p.
*/
Bignum dh_find_K(Bignum f) {
return modpow(f, x, p);
Bignum ret;
ret = modpow(f, x, p);
return ret;
}