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Nov
4
comment CPA Secure Chosen plaintext scheme
@DW. Hmm, is that a standard concept? I assumed the formula was just plucked from an attempt to formalize the general concept of indistinguishablility under chosen-plaintext attacks.
Nov
4
comment CPA Secure Chosen plaintext scheme
@DW: I think what is confusing (but perhaps I'm the one it confuses?) is that the question attempted to formulate CPA in terms of a PRF rather than a PRP. I implicitly assumed that this was simply a typo or sloppy terminology in the question. Since CPA (unless I'm misunderstanding) stands for "chosen plaintext attack", that seems to imply that we're dealing with a purported cipher scheme (which is by definition supposed to be reversible given a key), and what role would an actual PRF have in that context?
Nov
2
awarded  Citizen Patrol
Nov
2
awarded  Editor
Nov
2
revised CPA Secure Chosen plaintext scheme
TeXify
Nov
1
answered CPA Secure Chosen plaintext scheme
Nov
21
comment A mathematical explanation of the DES encryption system
@Borja: you write "I know this has a mathematical explanation" -- where do you know this from? Whoever told you this, do you have reason to trust them?
Nov
21
comment A mathematical explanation of the DES encryption system
No information about where the S-boxes came from was available when DES was first specified; this generated significant amounts of paranoia. It was later found that the exact choices increased the resistance of DES to differential cryptanalysis, but AFAIK the precise way they were selected is still classified (that is, if the details are even written down anywhere). There may be no more to it, mathematically, than "use this magic table lookup".
Nov
19
awarded  Supporter
Nov
19
comment How does a chosen ciphertext attack work, with a simple example?
Depends on what you consider communication. If the attacker simply steals a tamper-proof decrypting machine that will self-destruct after decrypting $n$ messages without ever revealing the key explicitly, does that count as "communication"? If you can think of any way the attacker can get to know the decrypted messages without "unprotected communication" happening, then no. Otherwise maybe.
Nov
19
comment How does a chosen ciphertext attack work, with a simple example?
The important fact is that the attacker somehow learns what his test message decrypts to. It doesn't need to be anything as "obviously risky" as calling on an unsecured line -- in a more modern setting, we could be thinking of a protocol where an encrypted message contains a nonce that the recipient will later echo back without encryption, or of a setting where the encrypted message contains data that the recipient is supposed to publish on behalf of the sender.
Nov
19
awarded  Teacher
Nov
18
answered How does a chosen ciphertext attack work, with a simple example?