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Replace random_byte() with random_read().

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This is in preparation for a PRNG revamp which will want to have a
well defined boundary for any given request-for-randomness, so that it
can destroy the evidence afterwards. So no more looping round calling
random_byte() and then stopping when we feel like it: now you say up
front how many random bytes you want, and call random_read() which
gives you that many in one go.

Most of the call sites that had to be fixed are fairly mechanical, and
quite a few ended up more concise afterwards. A few became more
cumbersome, such as mp_random_bits, in which the new API doesn't let
me load the random bytes directly into the target integer without
triggering undefined behaviour, so instead I have to allocate a
separate temporary buffer.

The _most_ interesting call site was in the PKCS#1 v1.5 padding code
in sshrsa.c (used in SSH-1), in which you need a stream of _nonzero_
random bytes. The previous code just looped on random_byte, retrying
if it got a zero. Now I'm doing a much more interesting thing with an
mpint, essentially scaling a binary fraction repeatedly to extract a
number in the range [0,255) and then adding 1 to it.
This commit is contained in:
Simon Tatham
2019-01-22 19:43:27 +00:00
parent 76aa3f6f7a
commit 628e794832
21 changed files with 108 additions and 101 deletions
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5
ssh1bpp.c
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@ -295,7 +295,7 @@ static PktOut *ssh1_bpp_new_pktout(int pkt_type)
static void ssh1_bpp_format_packet(struct ssh1_bpp_state *s, PktOut *pkt)
{
int pad, biglen, i, pktoffs;
int pad, biglen, pktoffs;
uint32_t crc;
int len;
@ -329,8 +329,7 @@ static void ssh1_bpp_format_packet(struct ssh1_bpp_state *s, PktOut *pkt)
pktoffs = 8 - pad;
biglen = len + pad; /* len(padding+type+data+CRC) */
for (i = pktoffs; i < 4+8; i++)
pkt->data[i] = random_byte();
random_read(pkt->data + pktoffs, 4+8 - pktoffs);
crc = crc32_ssh1(
make_ptrlen(pkt->data + pktoffs + 4, biglen - 4)); /* all ex len */
PUT_32BIT(pkt->data + pktoffs + 4 + biglen - 4, crc);