Infinity isn't currently needed here, but correctly handling it is a
little more safe against future changes.
Update docs for it to make it clear that it is not constant time in A
(the input point). It never was constant time in Q (and would be a little
complicated to make constant time in A).
If it was later made constant time in A, infinity support would be easy
to preserve, e.g. by running it on a dummy value and cmoving infinity into
the output.
37dba329c6cb0f7a4228a11dc26aa3a342a3a5d0 Remove unnecessary sign variable from wnaf_const (Jonas Nick)
6bb0b77e158fc2f9e56e4b65b08bcb660d4c588b Fix test_constant_wnaf for -1 and add a test for it. (Jonas Nick)
Pull request description:
There currently is a single branch in the `ecmul_const` function that is not being exercised by the tests. This branch is unreachable and therefore I'm suggesting to remove it.
For your convenience the paper the wnaf algorithm can be found [here (The Width-w NAF Method Provides Small Memory and Fast Elliptic Scalar Multiplications Secure against Side Channel Attacks)](http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.563.1267&rep=rep1&type=pdf). Similarly, unless I'm missing something important, I don't see how their algorithm needs to consider `sign(u[i-1])` unless `d` can be negative - which doesn't make much sense to me either.
ACKs for top commit:
real-or-random:
ACK 37dba329c6cb0f7a4228a11dc26aa3a342a3a5d0 I verified the correctness of the change and claimed invariant by manual inspection. I tested the code, both with 32bit and 64bit scalars.
Tree-SHA512: 9db45f76bd881d00a81923b6d2ae1c3e0f49a82a5d55347f01e1ce4e924d9a3bf55483a0697f25039c327e33edca6796ba3205c068d9f2f99aa5d655e46b15be
ECMULT_CONST_TABLE_GET_GE was branching on its secret input.
Also makes secp256k1_gej_double_var implemented as a wrapper
on secp256k1_gej_double_nonzero instead of the other way
around. This wasn't a constant time bug but it was fragile
and could easily become one in the future if the double_var
algorithm is changed.
248f046 Make sure we're not using an uninitialized variable in secp256k1_wnaf_const(...) (practicalswift)
Pull request description:
Make sure we're not using an uninitialized variable in `secp256k1_wnaf_const(...)`:
```
In file included from src/secp256k1.c:15:0,
from src/tests.c:17:
src/ecmult_const_impl.h: In function ‘secp256k1_wnaf_const’:
src/ecmult_const_impl.h:117:20: warning: ‘u’ may be used uninitialized in this function [-Wmaybe-uninitialized]
wnaf[word] = u * global_sign;
^
```
**Note to reviewers:** Perhaps an `assert(…);` is a bit drastic. What would be a more graceful way to handle this? :-)
Tree-SHA512: 536cd7cc5b87a84fbaac578cecbba81b8d82e4672a30a2db9a674b82856132e79b0158a6a88609bc24942ebdbf1fcd2c4399a4c31ab0654b88ace9c0e6f1eaf3
Identifiers starting with an underscore and followed immediately by a capital letter are reserved by the C++ standard.
The only header guards not fixed are those in the headers auto-generated from java.
If you compile without ./configure --enable-exhaustive-tests=no,
this will create a binary ./exhaustive_tests which will execute
every function possible on a group of small order obtained by
moving to a twist of our curve and locating a generator of small
order.
Currently defaults to order 13, though by changing some #ifdefs
you can get a couple other ones. (Currently 199, which will take
forever to run, and 14, which won't work because it's composite.)
TODO exhaustive tests for the various modules
Designed with clear separation of the wNAF conversion, precomputation
and exponentiation (since the precomp at least we will probably want
to separate in the API for users who reuse points a lot.
Future work:
- actually separate precomp in the API
- do multiexp rather than single exponentiation
