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Anonymity is derived from the greek word ανωνυμία, meaning without a name or name-less. In colloquial use, the term typically refers to a person, and often means that the personal identity, or personally identifiable information of that person is not known.
More strictly, and in reference to an arbitrary element (e.g. a human, an object, a computer), within a well-defined set (called the "anonymity set"), "anonymity" of that element refers to the property of that element of not being identifiable within this set. If it is not identifiable, then the element is said to be "anonymous".
An example: Suppose that only Alice, Bob, and Carol have the keys to a bank safe and that, one day, the contents of the safe are missing (without the lock being violated). Without any additional information, we do not know for sure whether it was Alice, Bob or Carol that opened the safe; the perpetrator remains anonymous. In particular, each of the elements in {Alice, Bob, Carol} has a 1/3 chance of being the perpetrator. However, as long as none of them has been identified as being the perpetrator with 100% certaintly, we can say that the perpetrator remains anonymous.
Anonymity is not an absolute. That is, the degree of anonymity one enjoys may vary. In the above example, if Carol has a plausible alibi for the time of the perpetration, then we may deduce that it must have been either Alice or Bob who opened the safe. That is, the probability of the elements {Alice, Bob, Carol} of being the perpetrator is now 1/2, 1/2, and 0 respectively. This clearly amounts to a reduction of the perpetrator's anonymity (i.e. although the perpetrator still remains anonymous, it is now more likely than before that (s)he is either Alice or Bob).
The term "anonymous message" typically refers to message (which is, for example, transmitted over some form of network) that does not carry any information about its sender and its intended recipient. It is therefore unclear if multiple such messages have been sent by the same sender or if they have the same intended recipient.
Sometimes it is desired that a person can establish a long-term relationship (such as a reputation) with some other entity, without his/her personal identity being disclosed to that entity. In this case, it may be useful for the person to establish a unique identifier, called a pseudonym, with the other entity. Examples of pseudonyms are nicknames, credit card numbers, student numbers, bank account numbers, IP addresses. A pseudonym enables the other entity to link different messages from the same person and, thereby, the maintenance of a long-term relationship. Although typically pseudonyms do not contain personally identifying information, communication that is based on pseudonyms is often not classified as "anonymous", but as "pseudonymous" instead. Indeed, in some contexts, anonymity and pseudonymity are separate concepts.
However, in other contexts what matters is that both anonymity and pseudonymity are concepts that are, among other things, concerned with hiding a person's legal identity. In such contexts people may not distinguish between anonymity and pseudonymity.
The problem of determining whether or not the identity of a communication partner is the same as one previously encountered is the problem of authentication. |
