Unknown to the Japanese, the U.S. Navy, led by the Communication Security Section of the Office of Naval Communications, had for several years enjoyed some success with penetrating Japanese communication ciphers and codes. By March 1942, the U.S. was able to decipher up to 15% of the IJN's "Ro" or "Naval Codebook D" code (called "JN-25B" by the U.S.), which was used by the IJN for approximately half of its communications. By the end of April, the U.S. was reading up to 85% of the signals broadcast in the "Ro" code.