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libsecp256k1/src/util.h  view on Meta::CPAN

#endif

/* ceil(x/y) for integers x > 0 and y > 0. Here, / denotes rational division. */
#define CEIL_DIV(x, y) (1 + ((x) - 1) / (y))

#define ROUND_TO_ALIGN(size) (CEIL_DIV(size, ALIGNMENT) * ALIGNMENT)

/* Macro for restrict, when available and not in a VERIFY build. */
#if defined(SECP256K1_BUILD) && defined(VERIFY)
# define SECP256K1_RESTRICT
#else
# if (!defined(__STDC_VERSION__) || (__STDC_VERSION__ < 199901L) )
#  if SECP256K1_GNUC_PREREQ(3,0)
#   define SECP256K1_RESTRICT __restrict__
#  elif (defined(_MSC_VER) && _MSC_VER >= 1400)
#   define SECP256K1_RESTRICT __restrict
#  else
#   define SECP256K1_RESTRICT
#  endif
# else
#  define SECP256K1_RESTRICT restrict
# endif
#endif

#if defined(__GNUC__)
# define SECP256K1_GNUC_EXT __extension__
#else
# define SECP256K1_GNUC_EXT
#endif

/* Zero memory if flag == 1. Flag must be 0 or 1. Constant time. */
static SECP256K1_INLINE void secp256k1_memczero(void *s, size_t len, int flag) {
    unsigned char *p = (unsigned char *)s;
    /* Access flag with a volatile-qualified lvalue.
       This prevents clang from figuring out (after inlining) that flag can
       take only be 0 or 1, which leads to variable time code. */
    volatile int vflag = flag;
    unsigned char mask = -(unsigned char) vflag;
    while (len) {
        *p &= ~mask;
        p++;
        len--;
    }
}

/* Cleanses memory to prevent leaking sensitive info. Won't be optimized out. */
static SECP256K1_INLINE void secp256k1_memclear(void *ptr, size_t len) {
#if defined(_MSC_VER)
    /* SecureZeroMemory is guaranteed not to be optimized out by MSVC. */
    SecureZeroMemory(ptr, len);
#elif defined(__GNUC__)
    /* We use a memory barrier that scares the compiler away from optimizing out the memset.
     *
     * Quoting Adam Langley <agl@google.com> in commit ad1907fe73334d6c696c8539646c21b11178f20f
     * in BoringSSL (ISC License):
     *    As best as we can tell, this is sufficient to break any optimisations that
     *    might try to eliminate "superfluous" memsets.
     * This method is used in memzero_explicit() the Linux kernel, too. Its advantage is that it
     * is pretty efficient, because the compiler can still implement the memset() efficently,
     * just not remove it entirely. See "Dead Store Elimination (Still) Considered Harmful" by
     * Yang et al. (USENIX Security 2017) for more background.
     */
    memset(ptr, 0, len);
    __asm__ __volatile__("" : : "r"(ptr) : "memory");
#else
    void *(*volatile const volatile_memset)(void *, int, size_t) = memset;
    volatile_memset(ptr, 0, len);
#endif
#ifdef VERIFY
    SECP256K1_CHECKMEM_UNDEFINE(ptr, len);
#endif
}

/** Semantics like memcmp. Variable-time.
 *
 * We use this to avoid possible compiler bugs with memcmp, e.g.
 * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95189
 */
static SECP256K1_INLINE int secp256k1_memcmp_var(const void *s1, const void *s2, size_t n) {
    const unsigned char *p1 = s1, *p2 = s2;
    size_t i;

    for (i = 0; i < n; i++) {
        int diff = p1[i] - p2[i];
        if (diff != 0) {
            return diff;
        }
    }
    return 0;
}

/* Return 1 if all elements of array s are 0 and otherwise return 0.
 * Constant-time. */
static SECP256K1_INLINE int secp256k1_is_zero_array(const unsigned char *s, size_t len) {
    unsigned char acc = 0;
    int ret;
    size_t i;

    for (i = 0; i < len; i++) {
        acc |= s[i];
    }
    ret = (acc == 0);
    /* acc may contain secret values. Try to explicitly clear it. */
    secp256k1_memclear(&acc, sizeof(acc));
    return ret;
}

/** If flag is true, set *r equal to *a; otherwise leave it. Constant-time.  Both *r and *a must be initialized and non-negative.*/
static SECP256K1_INLINE void secp256k1_int_cmov(int *r, const int *a, int flag) {
    unsigned int mask0, mask1, r_masked, a_masked;
    /* Access flag with a volatile-qualified lvalue.
       This prevents clang from figuring out (after inlining) that flag can
       take only be 0 or 1, which leads to variable time code. */
    volatile int vflag = flag;

    /* Casting a negative int to unsigned and back to int is implementation defined behavior */
    VERIFY_CHECK(*r >= 0 && *a >= 0);

    mask0 = (unsigned int)vflag + ~0u;
    mask1 = ~mask0;
    r_masked = ((unsigned int)*r & mask0);