function sha1_ft(t, b, c, d) {
        if (t < 20) {
          return (b & c) | ((~b) & d);
        }
        if (t < 40) {
          return b ^ c ^ d;
        }
        if (t < 60) {
          return (b & c) | (b & d) | (c & d);
        }
        return b ^ c ^ d;
      }
Perform the appropriate triplet combination function for the current
iteration