Add scalar_set_b32_seckey which does the same as scalar_set_b32 and also returns whether it's a valid secret key

src/scalar.h

@@ -39,6 +39,10 @@ static unsigned int secp256k1_scalar_get_bits_var(const secp256k1_scalar *a, uns
  */
 static void secp256k1_scalar_set_b32(secp256k1_scalar *r, const unsigned char *bin, int *overflow);
 
+/** Set a scalar from a big endian byte array and returns 1 if it is a valid
+ *  seckey and 0 otherwise. */
+static int secp256k1_scalar_set_b32_seckey(secp256k1_scalar *r, const unsigned char *bin);
+
 /** Set a scalar to an unsigned integer. */
 static void secp256k1_scalar_set_int(secp256k1_scalar *r, unsigned int v);
 

src/scalar_impl.h

@@ -55,6 +55,12 @@ static void secp256k1_scalar_order_get_num(secp256k1_num *r) {
 }
 #endif
 
+static int secp256k1_scalar_set_b32_seckey(secp256k1_scalar *r, const unsigned char *bin) {
+    int overflow;
+    secp256k1_scalar_set_b32(r, bin, &overflow);
+    return (!overflow) & (!secp256k1_scalar_is_zero(r));
+}
+
 static void secp256k1_scalar_inverse(secp256k1_scalar *r, const secp256k1_scalar *x) {
 #if defined(EXHAUSTIVE_TEST_ORDER)
     int i;

src/tests.c

@@ -140,6 +140,12 @@ void random_scalar_order(secp256k1_scalar *num) {
     } while(1);
 }
 
+void random_scalar_order_b32(unsigned char *b32) {
+    secp256k1_scalar num;
+    random_scalar_order(&num);
+    secp256k1_scalar_get_b32(b32, &num);
+}
+
 void run_context_tests(int use_prealloc) {
     secp256k1_pubkey pubkey;
     secp256k1_pubkey zero_pubkey;
@@ -1077,11 +1083,31 @@ void scalar_test(void) {
 
 }
 
+void run_scalar_set_b32_seckey_tests(void) {
+    unsigned char b32[32];
+    secp256k1_scalar s1;
+    secp256k1_scalar s2;
+
+    /* Usually set_b32 and set_b32_seckey give the same result */
+    random_scalar_order_b32(b32);
+    secp256k1_scalar_set_b32(&s1, b32, NULL);
+    CHECK(secp256k1_scalar_set_b32_seckey(&s2, b32) == 1);
+    CHECK(secp256k1_scalar_eq(&s1, &s2) == 1);
+
+    memset(b32, 0, sizeof(b32));
+    CHECK(secp256k1_scalar_set_b32_seckey(&s2, b32) == 0);
+    memset(b32, 0xFF, sizeof(b32));
+    CHECK(secp256k1_scalar_set_b32_seckey(&s2, b32) == 0);
+}
+
 void run_scalar_tests(void) {
     int i;
     for (i = 0; i < 128 * count; i++) {
         scalar_test();
     }
+    for (i = 0; i < count; i++) {
+        run_scalar_set_b32_seckey_tests();
+    }
 
     {
         /* (-1)+1 should be zero. */