A cryptoperiod is the time span during which a specific

cryptographic key A key in cryptography is a piece of information, usually a string of numbers or letters that are stored in a file, which, when processed through a cryptographic algorithm In mathematics and computer science, an algorithm () is a finite sequenc ...

is authorized for use. Common government guidelines range from 1 to 3 years for

asymmetric cryptography Public-key cryptography, or asymmetric cryptography, is the field of cryptographic systems that use pairs of related keys. Each key pair consists of a public key and a corresponding private key. Key pairs are generated with cryptographic alg ...

, and 1 day to 7 days for symmetric

cipher In cryptography, a cipher (or cypher) is an algorithm for performing encryption or decryption—a series of well-defined steps that can be followed as a procedure. An alternative, less common term is ''encipherment''. To encipher or encode i ...

traffic keys. Factors to consider include the strength of the underlying encryption algorithm, key length, the likelihood of compromise through a security breach and the availability of mechanisms of revoking keys. In traditional cryptographic practice, keys were changed at regular intervals, typically at the same time each day. The code word for a key change, in NSA parlance, is HJ or Hotel Juliet in the

NATO phonetic alphabet The International Radiotelephony Spelling Alphabet or simply the Radiotelephony Spelling Alphabet, commonly known as the NATO phonetic alphabet, is the most widely used set of clear-code words for communicating the letters of the Latin/Roman ...

. When cryptographic devices began to be used in large scale, those who had to update the key had to set a specific time to synchronize the re-key. This was accomplished at the hour (H) the Julian (J) Date changed, among crypto-accountants, managers and users the jargon "HJ" became the accepted term meaning it was time to change the crypto-key.

NESTOR in Vietnam

During the

Vietnam War The Vietnam War (1 November 1955 – 30 April 1975) was an armed conflict in Vietnam, Laos, and Cambodia fought between North Vietnam (Democratic Republic of Vietnam) and South Vietnam (Republic of Vietnam) and their allies. North Vietnam w ...

, the United States issued its forces a series of

secure voice Secure voice (alternatively secure speech or ciphony) is a term in cryptography for the encryption of voice communication over a range of communication types such as radio, telephone or Voice over IP, IP. History The implementation of voice en ...

encryption equipment code-named NESTOR. According to a U.S. Army history: "To maintain compatibility, key changes had to occur simultaneously in all units. The time chosen for this change was midnight, tactically the worst possible time because the greatest number of enemy contacts occurred from 2200 to 0200. Moreover, where several units shared the same keying device, having to move at night to change key settings was inconvenient and dangerous and added to the reasons for not using the equipment. Later the time of the daily NESTOR key change was moved to 0600."Vietnam Studies—Division-Level Communications 1962-1973
, Lieutenant General Charles R. Myer, U.S. Department of the Army, 1982, Chapter 8

References

*''Conduct Unbecoming,'' Randy Shilts, Macmillan, 2005, p. 117, describes HJ key change procedure Key management {{crypto-stub