Tell me more ×
Cryptography Stack Exchange is a question and answer site for software developers, mathematicians and others interested in cryptography. It's 100% free, no registration required.
up vote 1 down vote favorite

I would like to ask for a clear (but maybe not so deep) explanation of what the term "perfect secrecy" means.

As far as I have researched and understood, it has to do with probabilities of assuming that a certain variable will be the key for a certain cipher text. And unless I'm confusing some concepts, the one-time pad is the only cipher known to have perfect secrecy, since no amount of resources would be enough to break it.

My notions of probability are kind of weak, which is why I don't understand most documents that speak of it.

share|improve this question

2 Answers

up vote 1 down vote

Informally,if you intercept a cipher-text from a perfectly secure encryption system, you can find a key that causes that cipher-text to decrypt to any message you want ( of the correct length). So without knowing which key the author actually picked, you never learn anything about the message. This holds even if you try every possible key (because all keys decrypt the cipher-text to a possibly valid message)

share|improve this answer
Note this property can only be satisfied if the key space is at least as large as the plaintext space (therefore all modern ciphers except the OTP automatically fail to provide perfect secrecy) – Thomas Sep 28 '12 at 3:54
Thank you! I have a clear idea of what it is now. So if in a certain transposition cipher, each of the blocks of plaintext are assigned to a completely random key... does this achieve perfect secrecy? It would seem the case since you can't possibly learn anything from it with just the ciphertext... and yet, it's different than the one-time pad because of the lack of xor, so it "can't" have perfect secrecy, right? I apologize if I should have made another question instead. – Emyr Sep 28 '12 at 4:11
@Emyr: In a transposition cipher (at least, as I understand the term), no key can map '1111' to '0000', thus when receiving the 0000 ciphertext we know that 1111 wasn't the plain text. So, this can't be a perfect cipher. – Paŭlo Ebermann♦ Sep 28 '12 at 7:31
up vote 0 down vote

Perfect secrecy, in the simplest terms, is data that is completely, entirely patternless. Yet, inside of it there is still your secret. ArtofTheProblem offers a good simple explanation.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.