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Bugfix: clear the exceeding destination digits.

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Currently, the fp_sqr_comba_* functions do not fully clear the destination
number, but only overwrites the digits they care about. Eg: if
you call a comba4, it will overwrite the first 8 digits and leave
the others unchanged.

On the other hand, fp_mul_comba_* functions do *not* check incoming
unused digits (relying on the guarantee that they must be zero),
so they will happily compute the wrong result if those digits
are not empty. Testcase for a 32-bit system:

   char buf[64];
   fp_int num, num2, d;

   memset(buf, 0xFF, sizeof(buf);
   fp_read_unsigned_bin(&num, buf);
   fp_set(&d, 1);

   fp_sqr_comba_3(&d, &num);
   // now num is { 0x1, 0x0, 0x0, 0x0, 0x0, 0x0,
   //              0xFFFFFFFF, 0xFFFFFFFF ... }
   // only first 6 digits have been written, but even
   // if num.used is correctly set to 6, this can trigger
   // bugs.

   // Create a number larger than 6 digits
   fp_2expt(&num2, 8*32+4);

   fp_mul_comba_8(&num, &num2, &num2);
   // wrong result has been computed, because the first 8
   // digits of num have been read and multiplied
   // even if num->used == 6, relying on the fact that
   // they should be zero.
This commit is contained in:
Giovanni Bajo 2011-09-20 11:02:53 +02:00
parent da5fa59f2c
commit 50f587c36f
2 changed files with 4 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

3
src/generators/comba_sqr_gen.c
View File

@ -90,8 +90,9 @@ printf(
" B->used = %d;\n"
" B->sign = FP_ZPOS;\n"
" memcpy(B->dp, b, %d * sizeof(fp_digit));\n"
" memset(B->dp + %d, 0, (FP_SIZE - %d) * sizeof(fp_digit));\n"
" fp_clamp(B);\n"
"}\n#endif\n\n\n", N+N, N+N);
"}\n#endif\n\n\n", N+N, N+N, N+N, N+N);
return 0;
}

3
src/generators/comba_sqr_smallgen.c
View File

@ -95,8 +95,9 @@ printf(
" B->used = %d;\n"
" B->sign = FP_ZPOS;\n"
" memcpy(B->dp, b, %d * sizeof(fp_digit));\n"
" memset(B->dp + %d, 0, (FP_SIZE - %d) * sizeof(fp_digit));\n"
" fp_clamp(B);\n"
" break;\n\n", N+N, N+N);
" break;\n\n", N+N, N+N, N+N, N+N);
}
printf("}\n\n}\n");