static void scalarProduct(long[] output, long[] in, long scalar) {
    for (int i = 0; i < LIMB_CNT; i++) {
      output[i] = in[i] * scalar;
    }
  }
Multiply a number by a scalar: output = in * scalar
java_src/src/main/java/com/google/crypto/tink/internal/Field25519.java