scalar: use newly introduced secp256k1_{read,write}_be64 helpers (4x64 impl.)

src/util.h

@@ -353,4 +353,28 @@ SECP256K1_INLINE static void secp256k1_write_be32(unsigned char* p, uint32_t x)
     p[0] = x >> 24;
 }
 
+/* Read a uint64_t in big endian */
+SECP256K1_INLINE static uint64_t secp256k1_read_be64(const unsigned char* p) {
+    return (uint64_t)p[0] << 56 |
+           (uint64_t)p[1] << 48 |
+           (uint64_t)p[2] << 40 |
+           (uint64_t)p[3] << 32 |
+           (uint64_t)p[4] << 24 |
+           (uint64_t)p[5] << 16 |
+           (uint64_t)p[6] << 8  |
+           (uint64_t)p[7];
+}
+
+/* Write a uint64_t in big endian */
+SECP256K1_INLINE static void secp256k1_write_be64(unsigned char* p, uint64_t x) {
+    p[7] = x;
+    p[6] = x >>  8;
+    p[5] = x >> 16;
+    p[4] = x >> 24;
+    p[3] = x >> 32;
+    p[2] = x >> 40;
+    p[1] = x >> 48;
+    p[0] = x >> 56;
+}
+
 #endif /* SECP256K1_UTIL_H */