A-1 was the designation for a code used by the

United States Navy The United States Navy (USN) is the maritime service branch of the United States Armed Forces and one of the eight uniformed services of the United States. It is the largest and most powerful navy in the world, with the estimated tonnage ...

during World War I that replaced the Secret Code of 1887, SIGCODE and another system designed for radio communication. The cryptographic system was developed by Lt. W.W. Smith in the

Office of Naval Operations An office is a space where an organization's employees perform administrative work in order to support and realize objects and goals of the organization. The word "office" may also denote a position within an organization with specific ...

by randomly associating key words with 5 letter patterns.

References

{{Crypto-stub United States Navy World War I Signals intelligence of World War I Military communications of the United States