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Now that I've removed side-channel leakage from both prime candidate generation (via mp_unsafe_mod_integer) and Miller-Rabin, the probabilistic prime generation system in this code base is now able to get through testsc without it detecting any source of cache or timing side channels. So you should be able to generate an RSA key (in which the primes themselves must be secret) in a more hostile environment than you could previously be confident of. This is a bit counterintuitive, because _obviously_ random prime generation takes a variable amount of time, because it has to keep retrying until an attempt succeeds! But that's OK as long as the attempts are completely independent, because then any timing or cache information leaked by a _failed_ attempt will only tell an attacker about the numbers used in the failed attempt, and those numbers have been thrown away, so it doesn't matter who knows them. It's only important that the _successful_ attempt, from generating the random candidate through to completing its verification as (probably) prime, should be side-channel clean, because that's the attempt whose data is actually going to be turned into a private key that needs to be kept secret. (In particular, this means you have to avoid the old-fashioned strategy of generating successive prime candidates by incrementing a starting value until you find something not divisible by any small prime, because the number of iterations of that method would be a timing leak. Happily, we stopped doing that last year, in commit 08a3547bc54051e: now every candidate integer is generated independently, and if one fails the initial checks, we throw it away and start completely from scratch with a fresh random value.) So the test harness works by repeatedly running the prime generator in one-shot mode until an attempt succeeds, and then resetting the random-number stream to where it was just before the successful attempt. Then we generate the same prime number again, this time with the sclog mechanism turned on - and then, we compare it against the version we previously generated with the same random numbers, to make sure they're the same. This checks that the attempts really _are_ independent, in the sense that the prime generator is a pure function of its random input stream, and doesn't depend on state left over from previous attempts.
This is the README for PuTTY, a free Windows and Unix Telnet and SSH
client.
PuTTY is built using CMake <https://cmake.org/>. To compile in the
simplest way (on any of Linux, Windows or Mac), run these commands in
the source directory:
cmake .
cmake --build .
Documentation (in various formats including Windows Help and Unix
`man' pages) is built from the Halibut (`.but') files in the `doc'
subdirectory using `doc/Makefile'. If you aren't using one of our
source snapshots, you'll need to do this yourself. Halibut can be
found at <https://www.chiark.greenend.org.uk/~sgtatham/halibut/>.
The PuTTY home web site is
https://www.chiark.greenend.org.uk/~sgtatham/putty/
If you want to send bug reports or feature requests, please read the
Feedback section of the web site before doing so. Sending one-line
reports saying `it doesn't work' will waste your time as much as
ours.
See the file LICENCE for the licence conditions.