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comment How to prove that a function F isn't a pseudo random function
Imagine someone making calls to $F'$ for many choices of $x$. If all the internal calls to $F$ are globally unique, then the outputs will look random. That's essentially the definition of $F$ being a PRF -- distinct inputs yield independently random outputs. In that case, $F'$ would be secure. If it is possible to choose some $x$'s to $F'$ that cause a repeated input to $F$, then you would notice duplicated blocks in the outputs. That's a very non-random thing to happen, so $F'$ would not be a PRF.
Jan
22
comment Which cryptography technique does not increase the size of the plain data?
You can get around the problem of CPA impossibility using deterministic nonce-based encryption, if there is some public but non-repeating value that you can use as a nonce (e.g., timestamp, hard disk block number)
Jan
18
revised Could we share a secret key using the Birthday problem?
added 83 characters in body
Jan
18
answered Could we share a secret key using the Birthday problem?
Dec
9
comment Santa Claus' secret permutation
This is a special case of secure multi-party computation. Any general-purpose MPC protocol can do this, it only suffices to write the permutation-sampling process as a boolean/arithmetic circuit.
Dec
4
reviewed Approve What type of encryption scheme can prevent cheating in this coin flip protocol?
Dec
3
reviewed Approve Given $g,g^t$ in a cyclic group of order $pq$, is it hard to compute $g^{t^{-1}}$?
Nov
30
reviewed Approve Signature schemes for underpowered devices (8bit microcontroller)
Nov
30
comment What Does This Symbol Mean? (Hardcore Predicates for One-Way Functions)
SEJPM: \displaystyle should do it.
Nov
30
answered May I use Random Oracle for Inversed Look-up?
Nov
29
comment Reusing same source for single-source randomness extractor
In this case $ext(S_i,\cdot)$ is a linear function of the bits of its input $X$. With enough of them you are bound to have some be linearly dependent, which leads to clear correlations/artifacts in the output.
Nov
21
awarded  Nice Answer
Nov
21
comment For AES CBC, can I encrypt the IV with AES ECB and the same key and include it with the message?
There's nothing wrong in principle with a secret IV. The problem is that CBC mode incorporates the IV in a particular way, with a particular key and particular block cipher. You try to use the same IV, same block cipher, and same key for an independent purpose, and that's where things can go wrong. Because of all the re-use, you basically have to re-analyze the whole construction as a new block cipher mode.
Nov
21
revised Simulation-based proofs and universal composability proofs
added 146 characters in body
Nov
20
answered For AES CBC, can I encrypt the IV with AES ECB and the same key and include it with the message?
Nov
20
answered Simulation-based proofs and universal composability proofs
Nov
16
comment Are all encryption tools made equal?
Regardless of how well it implements the legitimate things in that list, the whole system is severely tarnished by the fact that "XOR" is provided as an option for encryption. That's like seeing an uncooked potato on the menu at a restaurant.
Nov
16
revised Help with pseudorandom functions
added 1 character in body
Nov
15
reviewed No Action Needed Grouping in BGN
Nov
15
reviewed No Action Needed Relationship between exponent and modulus in RSA (as RSA properties as listed in X.509)