Cryptography/Goals of Cryptography
Crytography is the science of keeping pieces of information secret, i.e. of designing technical systems such that an observer can infer as few as possible - optimally none - information from observing the system. The motivation for this is that the owner of the system wants to prevent the observer from taking advantage (e.g. monetary, influential, emotional) of the possible intelligence.
This secrecy or hiding is achieved by removing contextual information from the system's observable state and/or behaviour, without which the observer cannot gain intelligence about the system.
The term is very often used in conjunction in the context of message exchange between two entities, but of course not restricted to this case.
Hiding System State Alone
It may be advantageous for an ATM machine to hide the information as to how much cash is still available in the machine. It may e.g. only disclose the information that no more bank notes are available from it to the holder of a valid debit card.
Hiding Communication Content
Two companies doing business with each other may not wish to disclose the information on pricing of their products to third parties tapping into their communications.
Hiding the Fact of Communicating
Well known entities with well known fields of activity may wish to hide the fact that they are communicating at all since an observer aware of their fields of activity may already from the fact of some communication happening, be able to infer information.
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