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ecp: fix implementation-defined behaviour

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The comment about optimizing compiler's likely behaviour has been
checked with GCC and Clang on x86-64, 32-bit Arm and 64-bit Arm.

Signed-off-by: Manuel Pégourié-Gonnard <[email protected]>
This commit is contained in:
Manuel Pégourié-Gonnard
2026-05-18 10:45:11 +02:00
parent ec70994ba9
commit 6b4b14ae71
1 changed files with 10 additions and 1 deletions
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library/ecp_curves.c
+10 -1
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@@ -5018,7 +5018,16 @@ int mbedtls_ecp_mod_p192_raw(mbedtls_mpi_uint *Np, size_t Nn)
static inline int8_t extract_carry(int64_t cur)
{
return (int8_t) (cur >> 32);
/*
* Extract the signed carry without relying on `cur >> 32` for negative `cur`,
* since signed right shift is implementation-defined. Shift as uint64_t, then
* sign-extend explicitly.
*
* Modern optimizing compilers will likely generate the same code
* as the less portable (int8_t) (cur >> 32) anyway.
*/
uint32_t hi = (uint32_t) (((uint64_t) cur) >> 32);
return (int8_t) ((int64_t) hi - ((int64_t) (hi >> 31) << 32));
}
#define ADD(j) cur += A(j)