Add a is_zero() helper

src/libsodium/include/sodium/utils.h

@@ -40,6 +40,9 @@ int sodium_compare(const unsigned char *b1_, const unsigned char *b2_,
                    size_t len)
             __attribute__ ((warn_unused_result));
 
+SODIUM_EXPORT
+int sodium_is_zero(const unsigned char *n, const size_t nlen);
+
 SODIUM_EXPORT
 void sodium_increment(unsigned char *n, const size_t nlen);
 

src/libsodium/sodium/utils.c

@@ -154,6 +154,31 @@ sodium_compare(const unsigned char *b1_, const unsigned char *b2_, size_t len)
     return (int) (gt + gt + eq) - 1;
 }
 
+int
+sodium_is_zero(const unsigned char *n, const size_t nlen)
+{
+    size_t        i;
+    unsigned char c = 0U;
+
+    for (i = (size_t) 0U; i < nlen; i++) {
+        c |= n[i];
+    }
+    return ((c - 1U) >> 8) & 1U;
+}
+
+void
+sodium_increment(unsigned char *n, const size_t nlen)
+{
+    size_t       i;
+    unsigned int c = 1U << 8;
+
+    for (i = (size_t) 0U; i < nlen; i++) {
+        c >>= 8;
+        c += n[i];
+        n[i] = (unsigned char) c;
+    }
+}
+
 /* Derived from original code by CodesInChaos */
 char *
 sodium_bin2hex(char * const hex, const size_t hex_maxlen,
@@ -568,16 +593,3 @@ sodium_mprotect_readwrite(void *ptr)
 {
     return _sodium_mprotect(ptr, _mprotect_readwrite);
 }
-
-void
-sodium_increment(unsigned char *n, const size_t nlen)
-{
-    size_t       i;
-    unsigned int c = 1U << 8;
-
-    for (i = (size_t) 0U; i < nlen; i++) {
-        c >>= 8;
-        c += n[i];
-        n[i] = (unsigned char) c;
-    }
-}

test/default/sodium_utils.c

@@ -105,5 +105,13 @@ int main(void)
                    (unsigned int) bin_len);
         }
     }
+    memset(buf1, 0, sizeof buf1);
+    if (sodium_is_zero(buf1, sizeof buf1) != 1) {
+        printf("sodium_is_zero() failed\n");
+    }
+    buf1[randombytes_uniform(sizeof buf1)]++;
+    if (sodium_is_zero(buf1, sizeof buf1) != 0) {
+        printf("sodium_is_zero() failed\n");
+    }
     return 0;
 }