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variable signing precompute table

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make ECMULT_GEN_PREC_BITS configurable

ecmult_static_context.h: add compile time config assertion (#3) - Prevents accidentally using a file which was generated with a
different configuration.

README: mention valgrind issue

With --with-ecmult-gen-precision=8, valgrind needs a max stack size
adjustment to not run into a stack switching heuristic:

http://valgrind.org/docs/manual/manual-core.html

> -max-stackframe= [default: 2000000]
> The maximum size of a stack frame. If the stack pointer moves by more than this amount then Valgrind will assume that the program is switching to a different stack.
You may need to use this option if your program has large stack-allocated arrays.

basic-config: undef ECMULT_WINDOW_SIZE before (re-)defining it
This commit is contained in:
djb
2015-10-18 10:35:16 +02:00
committed by douglasbakkum
parent b4bff99028
commit dcb2e3b3ff
8 changed files with 91 additions and 39 deletions
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23
src/ecmult_gen.h
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@@ -10,20 +10,27 @@
#include "scalar.h"
#include "group.h"
#if ECMULT_GEN_PREC_BITS != 2 && ECMULT_GEN_PREC_BITS != 4 && ECMULT_GEN_PREC_BITS != 8
# error "Set ECMULT_GEN_PREC_BITS to 2, 4 or 8."
#endif
#define ECMULT_GEN_PREC_B ECMULT_GEN_PREC_BITS
#define ECMULT_GEN_PREC_G (1 << ECMULT_GEN_PREC_B)
#define ECMULT_GEN_PREC_N (256 / ECMULT_GEN_PREC_B)
typedef struct {
/* For accelerating the computation of a*G:
* To harden against timing attacks, use the following mechanism:
* * Break up the multiplicand into groups of 4 bits, called n_0, n_1, n_2, ..., n_63.
* * Compute sum(n_i * 16^i * G + U_i, i=0..63), where:
* * U_i = U * 2^i (for i=0..62)
* * U_i = U * (1-2^63) (for i=63)
* where U is a point with no known corresponding scalar. Note that sum(U_i, i=0..63) = 0.
* For each i, and each of the 16 possible values of n_i, (n_i * 16^i * G + U_i) is
* precomputed (call it prec(i, n_i)). The formula now becomes sum(prec(i, n_i), i=0..63).
* * Break up the multiplicand into groups of PREC_B bits, called n_0, n_1, n_2, ..., n_(PREC_N-1).
* * Compute sum(n_i * (PREC_G)^i * G + U_i, i=0 ... PREC_N-1), where:
* * U_i = U * 2^i, for i=0 ... PREC_N-2
* * U_i = U * (1-2^(PREC_N-1)), for i=PREC_N-1
* where U is a point with no known corresponding scalar. Note that sum(U_i, i=0 ... PREC_N-1) = 0.
* For each i, and each of the PREC_G possible values of n_i, (n_i * (PREC_G)^i * G + U_i) is
* precomputed (call it prec(i, n_i)). The formula now becomes sum(prec(i, n_i), i=0 ... PREC_N-1).
* None of the resulting prec group elements have a known scalar, and neither do any of
* the intermediate sums while computing a*G.
*/
secp256k1_ge_storage (*prec)[64][16]; /* prec[j][i] = 16^j * i * G + U_i */
secp256k1_ge_storage (*prec)[ECMULT_GEN_PREC_N][ECMULT_GEN_PREC_G]; /* prec[j][i] = (PREC_G)^j * i * G + U_i */
secp256k1_scalar blind;
secp256k1_gej initial;
} secp256k1_ecmult_gen_context;