All Questions

5k views

Which one of the Block Cipher modes is the best?

I have two questions regarding the Block Cipher Modes: Which one of the modes is considered the best? I know CBC has a problem of IV since the next block of the plain text is XORed with the result ...
363 views

Is it safe to store initial counter value for AES-CTR alongside with ciphertext?

The initial counter value can actually be thought of as a sort of IV, which I assume is safe to transmit with the ciphertext, but I really want to be sure that this is indeed safe.
370 views

Is there a secret sharing scheme which allows delegation/re-sharing without reconstructing the original secret?

EDIT: Ilmari Karonen's answer below well not exactly what I want, gives a very good idea of what I am trying to accomplish. Are there any known secret sharing schemes that allow new parties to be ...
554 views

Why does an algorithm need a keyspace?

Why does an algorithm need a keyspace? I thought the person encrypting would define his/her own key.
183 views

Algorithm for computing square roots in $GF(2^n)$

Short question: is there an algorithm for efficiently computing square roots in $\mathbb{F}_{2^n}$?
258 views

Can we say that if $P=NP$ there is no CPA secure public key encryption?

I've learned that public key encryption is based on the problem of Discrete Log (as regard to group theory) which believed to be hard. But, can we say that it doesn't matter on which problem our ...
1k views

Why can't the IV be predictable when its said it doesn't need to be a secret?

I heard multiple times not to reuse the same IV and IV should be random but doesn't need to be secret. I also heard if the IV is something like sequential numbers or something predictable I should ...
2k views

Calculating the amount of zero bits to be appended to the message

From FIPS 180-3 Suppose that the length of the message, M, is l bits. Append the bit “1” to the end of the message, followed ...
643 views

A patched SHA1 attempt for password verification

This is a classical scenario: For our web based authentication platform we have a username and a password field. User enters these values and presses Log In. The browser gathers the user data, ...
375 views

Why does CTR mode XOR the plaintext into the output of the block cipher rather than XORing the plaintext into the input of the block cipher?

As I understand it, CTR mode essentially turns a block cipher into a stream cipher like so: ...
1k views

NIST temporarily closed — will that have a negative impact on the future of cryptography?

For those who didn't notice yet, in the USA, the National Institute of Standards and Technology(NIST) has been shut down temporarily because of the poor fiscal situation of the USA. The text at their ...
351 views

Is this a structural weakness of Feistel networks?

I'm doing a lot of reading about Feistel networks. Something occurred to me a bit ago that I hadn't realized previously, namely that in any Feistel construction there are bits of the plaintext that ...
502 views

P = NP and current cryptographic systems

I've recently heard some people claiming that if the fact that P = NP is proven, most (all?) the current cryptographic algorithm considered secure like RSA will be unusable in secure systems. My ...
3k views

Export from US of crypto software with key-size > 56 bits still needs permission?

Elsewhere someone stated that he couldn't publish his crypto software on the Internet, because US Export Regulations require approval, if the key size is greater than 56 bits. I know that several ...
329 views

Why do we need Hash by key?

(i'm just trying to find what am I missing...) Assuming John have a clear text message , he can create a regular hash ( like md5 , or sha256) and then encrypt the ...