Cryptography Weekly Newsletter
Cryptography Weekly Newsletter

Top new questions this week:

Is there a null HMAC?

I came up with this question while unit testing a function that includes an HMAC. What is the expected behavior of an HMAC with a well-defined key and a set of data with no elements in it? Is there ...

asked by JohnDvorak 7 votes
answered by Henrick Hellström 13 votes

How does hash speed vary based on string length?

Surprisingly I have not been able to find an answer to this question on google. If I have a function that is based on any of the popular hashing algorithms used for password generation, by what ...

asked by SLC 6 votes
answered by fgrieu 7 votes

How to design a deliberately weak PRNG for experimentation?

I'm looking for a PRNG that is deliberately weak, that is, follows a set of rules stochasticly. For example: Encounters a set of bits that is a multiple of X, starts exhibiting bias, or starts to ...

asked by Trevor Frinstein 4 votes
answered by sh1 1 vote

Is reusing keys for CBC and CBC-MAC secure when using encrypt-then-MAC?

I know reusing keys is a bad idea, especially for CBC and CBC-MAC, but the example for why on Wikipedia seemed to use encrypt-and-MAC. It seems obvious that the last block of encryption will be equal ...

mac cbc key-reuse cbc-mac  
asked by user1193112 4 votes
answered by otus 4 votes

Common Modulus Attack not reproducible

I want to calculate a simple example of the RSA common modulus attack. However, the result is not correct and I do not find my mistake. p=$29, q=37, n=p*q = 1073, \phi(n) = 1008, e1 = 5, e2 = 11$ ...

asked by null 4 votes
answered by Meysam Ghahramani 2 votes

How to prove that a function F isn't a pseudo random function

Let $F$ be a length-preserving pseudorandom function. For the following constructions of a keyed function $F' : \{0, 1\}^n \times \{0, 1\}^{nāˆ’1} \to \{0, 1\}^{2n}$, state whether $F'$ is a ...

asked by Gabriele Saturni 4 votes

RSA private key finding method

Bob uses RSA with $p=83$ and $q=103$. His public key is $e=445$. Alice claims that she can calculate Bob's private key without using the definition $d=e^{āˆ’1}\bmod Ļ†(n)$? But how?

rsa attack  
asked by Saab 3 votes
answered by fgrieu 6 votes

Greatest hits from previous weeks:

Is there an existing cryptography algorithm / method that both encrypts AND compresses text?

So I'm trying to find a method of encryption that not only obfuscates text, but also compresses the result. For example, if I encrypted ninechars, the ideal result would be less than nine characters. ...

encryption compression encoding  
asked by goatmeal 2 votes
answered by Ilmari Karonen 12 votes

What is the difference between MAC and HMAC?

In reference to this question, what are the "stronger security properties" that HMAC provides over MAC. I got that MAC requires an IV whereas HMAC doesn't. I also understood that MAC may reveal ...

mac hmac  
asked by TheRookierLearner 10 votes
answered by Seth 19 votes

Can you answer these?

What happens to NORX if the same (key, nonce, header data) triple is reused

As I understand it, reusing a (key, nonce, header data) triple in NORX leaks the XOR of the initial common prefix (but this is zero, so only its length is leaked) the XOR of the first distinct ...

authenticated-encryption norx  
asked by Demetri 3 votes

Attribute based access control and role based, online bookstore example

I am asked to answer a question that says: One online book-store wants to grand access to clients regarding their subscription. There are 3 types of subscriptions A , B and C. Customers get access ...

asked by Aereal 1 vote

Breaking Vigenere via Kasiski/Babbage method? (non-programmatic)

I'm really hoping someone can explain to me what is going on in the second major component of this form of analysis. If you wish, for convenience, here's a page that already has some ciphered text ...

cryptanalysis vigenere  
asked by smitty_werbermanjensen 1 vote
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