You want to prove to him they are in fact differently-coloured, but nothing else; in particular, you do not want to reveal which one is the red and which is the green ball.
Secrecy mechanisms are used, for the most part, to prevent eavesdroppers from getting one's card number, but most fraud is conducted without eavesdropping.
Digital signatures typically are used to provide nonrepudiation. Two balls and the colour-blind friend[ edit ] This example requires two identical objects with different colours such as two coloured balls. For some security requirements, authentication by itself may be sufficient.
However, digital cryptography generally "flips coins" by relying on a pseudo-random number generatorwhich is akin to a coin with a fixed pattern of heads and tails known only to the coin's owner. Thus, the distinction can be made between data that are confidential i.
This is formalized by showing that every verifier has some simulator that, given only the statement to be proved and no access to the provercan produce a transcript that "looks like" an interaction between the honest prover and the verifier in question.
Such a requirement could be enforced through the use of a digital signature belonging to the data source being bound to the personal information before it is disseminated.
In fact, even a person who was present as an observer at the original experiment would be unconvinced, since Victor and Peggy might have orchestrated the whole "experiment" from start to finish.