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Is unconditional security and perfect secrecy one and the same thing, i.e a cryptosystem provides unconditional security if and only if it provides perfect secrecy ?

I've wondered about the above and seen that the one-time-pad provides unconditional security in terms of perfect secrecy. Does this hold in general ?

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Yes, in context of encryption the two terms have the same meaning, i.e. even an unbounded adversary cannot succeed in breaking the secrecy of the scheme.

Note that there may be other primitives which also provide unconditional security with respect to some other notions, such as information theoretically secure message authentication codes with respect to unforgeability.

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  • $\begingroup$ They have the same meaning; but are they equavalent ? $\endgroup$ – Shuzheng Dec 23 '14 at 5:50
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    $\begingroup$ @NicolasLykkeIversen: No they are not. Perfect secrecy is a specific term usually used about encryption schemes. It essentially means that the messages are kept secret against a computationally unbounded adversary. Unconditional security is a more general term. It essentially means security against an adversary with unbounded computational power. However, here security can mean many things for many different types of cryptosystems. E.g., for a message authentication code security can imply such things as unforgeability. For MPC it could imply fairness and so on. $\endgroup$ – Guut Boy Dec 23 '14 at 8:24
  • $\begingroup$ So a cryptosystem can be unconditional secure even thus it doesn't achieve perfect secrecy ? $\endgroup$ – Shuzheng Dec 23 '14 at 8:56
  • $\begingroup$ @user111854 That is one way to put. Perfect secrecy is in a sense a special case of unconditional security. However, an unconditionally secure encryption scheme will have perfect secrecy. The point is that there are cryptosystems were secrecy either does not matter, or is not enough to call the system 'secure'. Think of a signature scheme, here secrecy is not something we care about, we just care about unforgeability. $\endgroup$ – Guut Boy Dec 23 '14 at 9:02

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