challenge

Challenge may refer to:

• Voter challenging or caging, a method of challenging the registration status of voters
• Euphemism [婉转的说法] for disability
• Peremptory challenge, a dismissal [when someone is removed from their job] of potential jurors [陪审员] from jury duty
• Challenge (rhetoric [修辞]), a dare or a motivational impetus [刺激] to action

Sports

• challenge (competition), when a challenger requests to compete against a champion with the title at stake [作为赌注]
• challenge match, a type of exhibition game [表演赛] not part of a wider tournament [联赛; 锦标赛] or series
• video referee request, made by a competitor to review the decision of an on-field official
• challenge (NFL), when a head coach requests the referees review a play

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A challenge coin is a small coin or medallion, bearing an organization's insignia [象征] or emblem [象征] and carried by the organization's members. 柯林斯词典说insignia和emblem是同义词。

The Roman Empire rewarded soldiers by presenting them with coins to recognize their achievements. Prior to actual Challenge Coins being minted, soldiers who acted bravely in battle would be rewarded by comrades or superiors by buying that individual a drink. They would give that soldier a coin to buy the drink but more commonly, they would make a spectacle by slapping it down loudly on the bar or presenting them a coin in an informal group setting. Receiving a coin from an officer was generally a considerably more valuable coin and rarely presented.

Challenge coins were also known as "Portrait Medals" during the Renaissance, and were often used to commemorate specific events involving royalty, nobility, or other types of well-to-do individuals. The medals would be given as gifts or awards, and people also exchanged them with friends and associates. The most common format was for one side to depict the patron while the other showed something that represented that individual's family, house [royal family or dynasty ], lineage, and/or seal [印章].

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In computer security, challenge–response authentication is a family of protocols in which one party presents a question ("challenge") and another party must provide a valid answer ("response") to be authenticated. The simplest example of a challenge–response protocol is password authentication, where the challenge is asking for the password and the valid response is the correct password.

Clearly an adversary who can eavesdrop on a password authentication can then authenticate itself in the same way. One solution is to issue multiple passwords, each of them marked with an identifier. The verifier can ask for any of the passwords, and the prover must have that correct password for that identifier. Assuming that the passwords are chosen independently, an adversary who intercepts one challenge–response message pair has no clues to help with a different challenge at a different time.

For example, when other communications security methods are unavailable, the U.S. military uses the AKAC-1553 TRIAD numeral cipher to authenticate and encrypt some communications. TRIAD includes a list of three-letter challenge codes, which the verifier is supposed to choose randomly from, and random three-letter responses to them. For added security, each set of codes is only valid for a particular time period which is ordinarily 24 hours.

A more interesting challenge–response technique works as follows. Say, Bob is controlling access to some resource. Alice comes along seeking entry. Bob issues a challenge, perhaps "52w72y". Alice must respond with the one string of characters which "fits" the challenge Bob issued. The "fit" is determined by an algorithm "known" to Bob and Alice. (The correct response might be as simple as "63x83z" (each character of response one more than that of challenge), but in the real world, the "rules" would be much more complex.) Bob issues a different challenge each time, and thus knowing a previous correct response (even if it is not "hidden" by the means of communication used between Alice and Bob) is of no use.

六级/考研单词: dismiss, jury, rhetoric, dare, impetus, exhibit, tournament, referee, bearing, mint, comrade, superior, spectacle, loud, seldom, medal, renaissance, commemorate, noble, depict, patron, dynasty, seal, compute, protocol, valid, password, adverse, issue, multiple, militant, random, thereby