Make secp256k1_scalar_get_bits support 32-bit reads

Author: sipa

src/scalar_4x64_impl.h

@@ -41,18 +41,19 @@ SECP256K1_INLINE static void secp256k1_scalar_set_int(secp256k1_scalar *r, unsig
 }
 
 SECP256K1_INLINE static unsigned int secp256k1_scalar_get_bits(const secp256k1_scalar *a, unsigned int offset, unsigned int count) {
+    VERIFY_CHECK(count > 0 && count <= 32);
     VERIFY_CHECK((offset + count - 1) >> 6 == offset >> 6);
-    return (a->d[offset >> 6] >> (offset & 0x3F)) & ((((uint64_t)1) << count) - 1);
+    return (a->d[offset >> 6] >> (offset & 0x3F)) & (0xFFFFFFFF >> (32 - count));
 }
 
 SECP256K1_INLINE static unsigned int secp256k1_scalar_get_bits_var(const secp256k1_scalar *a, unsigned int offset, unsigned int count) {
-    VERIFY_CHECK(count < 32);
+    VERIFY_CHECK(count > 0 && count <= 32);
     VERIFY_CHECK(offset + count <= 256);
     if ((offset + count - 1) >> 6 == offset >> 6) {
         return secp256k1_scalar_get_bits(a, offset, count);
     } else {
         VERIFY_CHECK((offset >> 6) + 1 < 4);
-        return ((a->d[offset >> 6] >> (offset & 0x3F)) | (a->d[(offset >> 6) + 1] << (64 - (offset & 0x3F)))) & ((((uint64_t)1) << count) - 1);
+        return ((a->d[offset >> 6] >> (offset & 0x3F)) | (a->d[(offset >> 6) + 1] << (64 - (offset & 0x3F)))) & (0xFFFFFFFF >> (32 - count));
     }
 }
 

src/scalar_8x32_impl.h

@@ -59,18 +59,19 @@ SECP256K1_INLINE static void secp256k1_scalar_set_int(secp256k1_scalar *r, unsig
 }
 
 SECP256K1_INLINE static unsigned int secp256k1_scalar_get_bits(const secp256k1_scalar *a, unsigned int offset, unsigned int count) {
+    VERIFY_CHECK(count > 0 && count <= 32);
     VERIFY_CHECK((offset + count - 1) >> 5 == offset >> 5);
-    return (a->d[offset >> 5] >> (offset & 0x1F)) & ((1 << count) - 1);
+    return (a->d[offset >> 5] >> (offset & 0x1F)) & (0xFFFFFFFF >> (32 - count));
 }
 
 SECP256K1_INLINE static unsigned int secp256k1_scalar_get_bits_var(const secp256k1_scalar *a, unsigned int offset, unsigned int count) {
-    VERIFY_CHECK(count < 32);
+    VERIFY_CHECK(count > 0 && count <= 32);
     VERIFY_CHECK(offset + count <= 256);
     if ((offset + count - 1) >> 5 == offset >> 5) {
         return secp256k1_scalar_get_bits(a, offset, count);
     } else {
         VERIFY_CHECK((offset >> 5) + 1 < 8);
-        return ((a->d[offset >> 5] >> (offset & 0x1F)) | (a->d[(offset >> 5) + 1] << (32 - (offset & 0x1F)))) & ((((uint32_t)1) << count) - 1);
+        return ((a->d[offset >> 5] >> (offset & 0x1F)) | (a->d[(offset >> 5) + 1] << (32 - (offset & 0x1F)))) & (0xFFFFFFFF >> (32 - count));
     }
 }
 

src/scalar_low_impl.h

@@ -19,10 +19,12 @@ SECP256K1_INLINE static void secp256k1_scalar_clear(secp256k1_scalar *r) { *r =
 SECP256K1_INLINE static void secp256k1_scalar_set_int(secp256k1_scalar *r, unsigned int v) { *r = v; }
 
 SECP256K1_INLINE static unsigned int secp256k1_scalar_get_bits(const secp256k1_scalar *a, unsigned int offset, unsigned int count) {
-    if (offset < 32)
-        return ((*a >> offset) & ((((uint32_t)1) << count) - 1));
-    else
+    VERIFY_CHECK(count > 0 && count <= 32);
+    if (offset < 32) {
+        return (*a >> offset) & (0xFFFFFFFF >> (32 - count));
+    } else {
         return 0;
+    }
 }
 
 SECP256K1_INLINE static unsigned int secp256k1_scalar_get_bits_var(const secp256k1_scalar *a, unsigned int offset, unsigned int count) {