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visits member for 3 years, 3 months
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Oct
3
comment RSA - Calculating d
Use the Extended Euclidean algorithm. See Modular multiplicative inverse
Oct
2
comment Hardness proof?
We can also show that AES-CBC is secure as unauthenticated encryption if AES itself is a secure block cipher.
Oct
1
comment Calculate the RSA private exponent from the CRT parameters
Compute $\phi = (P-1)(Q-1)$ and then the modular multiplicative inverse of $e$ using extended euclidean.
Oct
1
comment finding collision for truncated SHA1 hash output
Is your indentation correct? Don't you need to indent the two lines after the if statement?
Oct
1
comment finding collision for truncated SHA1 hash output
@StuartP.Bentley Doesn't your program search for preimages not collisions? A 40 bit prefix collision costs about as much to find as a 20 bit prefix preimage.
Oct
1
comment Small Encryption Exponent
1) Simply running a factoring software sounds a little dull for a homework question. It's also weird that the moduli for Problem 1 are smaller. If you bothered to setup factoring software for Problem 2 you use it to solve problem 1 as well, without exploiting the same message encryption weakness. 2) the short message and small exponent approach doesn't seem to work.
Oct
1
comment Calculate the RSA private exponent from the CRT parameters
1) Do you know e? Different es result in different ds. 2) How did you end up with knowing all those values but not knowing d?
Oct
1
comment Small Encryption Exponent
If the message is shorter than the length of the modulus divided by the public exponent, you can simply compute the $e$ root, for example via binary search. For real RSA the padding ensures that the message has similar length as the modulus, but for unpadded RSA short messages can happen.
Sep
28
comment Keccak and compression functions
@Dingo13 Blake and Skein are not MD constructions, but they still use a kind of compression function. Compared with a basic compression function, the most significant difference is the addition of a tweak, which is used to uniquely mark each block and to signal the end of the input message.
Sep
28
comment Question about second preimage resistance of hash function combiner
It's pretty obvious that collisions and second pre-images of the concatenation imply the same attack against both hashes and are at least as strong as the stronger. On the other hand, concatenating a pre-image resistant hash with the leading bits of the message is practically bad concerning "it's hard to recover the message from a hash", even if it might still fulfill some definitions of pre-image security.
Sep
28
comment Question about second preimage resistance of hash function combiner
For second preimages I agree fully. For first preimages it depends on the precise definition you're using.
Sep
26
comment Roots of polynomial in Shamir secret sharing
1) Since shares can have a 0 y value they can be roots, in which case the attacker trivially learns some roots. 2) What you describe in your comment has almost nothing to do with SSS, except that both use polynomials in finite fields. 3) Just to be sure, with roots you mean the points at which y=0 and not the coefficients of the polynomial, right?
Sep
26
comment Roots of polynomial in Shamir secret sharing
Why do you want to know that? Unless it's just idle curiosity, I'm pretty sure that you're asking the wrong question.
Sep
24
comment Generation of N bit prime numbers -> what is the actual range?
Personally I'd approximate the $\sqrt{2}$ by 1.5, which results in the simple algorithm of setting the two most significant bits to 1.
Sep
23
comment Is it possible to find out whether a number is greater than another number without knowing the numbers?
Similar to Yao's Millionaires' Problem, except here it's one party knowing both values instead of two parties wanting to compare their values.
Sep
23
comment Deprecation of RSA-SHA-1 in DKIM keys?
The feasible attack against SHA-1 is a collision attack, not even a chosen prefix attack, let alone a second pre-image attack.
Sep
22
comment Is it possible to hand-negotiate an SSL/TLS session?
It's also important to note that even if you managed to pass through binary data, you'd need to be able to perform cryptographic operations in your head, no small feat.
Sep
22
comment Using modulus in one time pad?
1) What would you use instead? It's correct and it's convenient. What more do you want? 2) Nowadays we typically use xor (addition modulo 2) which is fast in software and encryption/decryption are the same function. 3) why do you claim M is a large number? It's often 2 or 256 and very rarely larger than 64 bits or so.
Sep
22
comment Random vs. Fixed Paddings
Are you only talking about paddings used with symmetric block ciphers, as in your examples? Or also about paddings used for RSA/Rabin encryption? These kinds of paddings are quite different and should not be discussed in a single question.
Sep
22
comment Random vs. Fixed Paddings
I don't understand what you want to say. 1) If you're talking about padding oracles (adaptive chosen ciphertext), the proper defense is a MAC. 2) If you're talking about semantic security and known/chosen plaintext attacks, it's the job of IVs/nonces to achieve that. 3) If you're talking about asymmetric encryption (RSA, Rabin) you need randomized padding, but RSA padding is conceptually very different from the block cipher paddings the OP mentioned. RSA padding includes properties which in the symmetric case are handled by MAC and IV instead of the padding.