In military

Military

A military is an organization authorized by its greater society to use lethal force, usually including use of weapons, in defending its country by combating actual or perceived threats. The military may have additional functions of use to its greater society, such as advancing a political agenda e.g...

 terminology, a countersign is a sign, word, or any other signal previously agreed upon and required to be exchanged between a sentry or guard and anybody approaching his or her post. The term usually encompasses both the sign given by the approaching party as well as the sentry's reply. However, in some armies, the countersign is strictly the reply of the sentry to the password given by the person approaching. A well-known sign/countersign is that used by the Allied forces on D-Day

D-Day

D-Day is a term often used in military parlance to denote the day on which a combat attack or operation is to be initiated. "D-Day" often represents a variable, designating the day upon which some significant event will occur or has occurred; see Military designation of days and hours for similar...

 during World War II

World War II

World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

: the challenge/sign was "flash", the password

Password

A password is a secret word or string of characters that is used for authentication, to prove identity or gain access to a resource . The password should be kept secret from those not allowed access....

 "thunder", and the countersign (to challenge the person giving the first code word) "Welcome".

Obfuscation

The signs are often worked into a sentence in some way, nominally to obfuscate which words are key in case there are spies nearby. An example would be the sign "blue" and the countersign "moon". The approacher might say, "I'm feeling blue today... got any ideas of how I could cheer up?" The sentry could reply with, "Well, you could always moon somebody." In actuality it would be easy to figure out what the words are, although this method is better than shouting the words plainly. Presumably this effect is multiplied when the listener doesn't speak the language.

In computer networks

A similar technique is becoming common in computer networking, particularly for banking web sites. For example (as in Bank of America

Bank of America

Bank of America Corporation, an American multinational banking and financial services corporation, is the second largest bank holding company in the United States by assets, and the fourth largest bank in the U.S. by market capitalization. The bank is headquartered in Charlotte, North Carolina...

's "SiteKey" system), a user might enter his or her user-ID, but withhold the corresponding password until the server replied to the user-ID with a previously agreed-upon phrase and/or image. So long as the connection is secure, and the user can choose from a wide range of phrases and/or images in the set-up process, he/she can be confident that he/she is signing on to the legitimate site, rather than to a spoofed one

Phishing

Phishing is a way of attempting to acquire information such as usernames, passwords, and credit card details by masquerading as a trustworthy entity in an electronic communication. Communications purporting to be from popular social web sites, auction sites, online payment processors or IT...

.

In literature

The opening lines of William Shakespeare

William Shakespeare

William Shakespeare was an English poet and playwright, widely regarded as the greatest writer in the English language and the world's pre-eminent dramatist. He is often called England's national poet and the "Bard of Avon"...

's play Hamlet

Hamlet

The Tragical History of Hamlet, Prince of Denmark, or more simply Hamlet, is a tragedy by William Shakespeare, believed to have been written between 1599 and 1601...

are between soldiers on duty are viewed as representing a crude sign, where the line "Long live the King!" was a sign between soldiers:

Bernardo. Who's there?

Francisco. Nay, answer me. Stand and unfold yourself.

Bernardo. Long live the King!

Francisco. Bernardo?

Bernardo. He.