Cryptography Stack Exchange
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12 answers

Differences between industrial and military cryptography

Industrial and military cryptography should follow the same basic rules, but what does make them different is: Higher key length, protocols unknown to the civilian world, and perhaps unique methods of transferring media Are there other…
R1w
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Calculating RSA private exponent when given public exponent and the modulus factors using extended Euclid

When given $p = 5, q = 11, N = 55$ and $e = 17$, I'm trying to compute the RSA private key $d$. I can calculate $\varphi(N) = 40$, but my lecturer then says to use the extended Euclidean algorithm to compute $d$. That's where I get stuck. Here's my…
DougalMaguire
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What does it mean for a random number generator to be cryptographically secure?

I've never heard a good answer. I'd like to hear details about: What are the criteria that make an RNG cryptographically secure? Why must your RNG be cryptographically secure? I.e., what are the consequences if it is not? Examples of secure and…
agotsis
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Simply put, what does “perfect secrecy” mean?

I would like to ask for a clear (but maybe not so deep) explanation of what the term "perfect secrecy" means. As far as I have researched and understood, it has to do with probabilities of assuming that a certain variable will be the key for a…
Emyr
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6 answers

How to check that you got the right key when brute forcing an encryption?

How do you know when you have the right key when brute-forcing? Let's say that they test the right key. They then have to check that the decrypted text makes sense. To do so, they can test whether there is a word in the dictionary. But then let's…
ChiseledAbs
42
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4 answers

Purpose of outer key in HMAC

From what I know, the HMAC constructions has two strength: It's resistant to length extensions Since the key is consumed before the message, the attacker does not know the initial state, preventing simple collision attacks. But the simple…
CodesInChaos
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Are there any known collisions for the SHA (1 & 2) family of hash functions?

Are there any known collisions for the hash functions SHA-1, SHA-224, SHA-256, SHA-384, and SHA-512? By that, I mean are there known values of $a$ and $b$ where $F(a) = F(b)$ and $a ≠ b$?
Pacerier
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What are preimage resistance and collision resistance, and how can the lack thereof be exploited?

What is "preimage resistance", and how can the lack thereof be exploited? How is this different from collision resistance, and are there any known preimage attacks that would be considered feasible?
John Gietzen
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4 answers

After 20 years of AES, what are the retrospective changes that should have been made?

I realize that this could be very opinion based, but I feel that there should be some solid information on AES at this point that could be referenced. After 20 years, I expect that there should be a "woulda, coulda, shoulda" list somewhere. For…
b degnan
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Elliptic curve ed25519 vs ed448 - Differences

Other than key size, What are some differences between the Elliptic curve ed25519 and ed448?
Nathan Aw
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4 answers

How can hashes be unique if they are limited in number?

I'm curious, how can for example SHA-256 be unique if there are only a limited number of them?! For clarification: how many MD5 hashes are there? $16^{32}$ MD5 hashes can be produced. $16^{64}$ SHA-256 hashes can be produced. while there are…
M D P
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What's the difference between RSA and Diffie-Hellman?

I've been reading the same thing on a lot of websites: RSA is for communication using the public and private key for both the server and client, whereas Diffie-Hellman is just for exchanging the same secret key that will then be used for both…
user3407319
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What is difference between PRG, PRF, and PRP

Until what I have gotten is: A PRG is generator is a part of PRF that produces pseudo-random values for the function. PRF is semantically secure and has no worries of being invertible. Fine, then where is PRP used? What is PRP, where it comes to,…
Khunshan Ahmad
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In RSA, how does the CPU deal with this huge modulus (8192 bits)?

Whilst I understand how the RSA algorithm works, I don't understand how the CPU operates when it needs to use the mod function with a huge number $n$, for example. $n = 8192$ bits; $c = m^e \mod n$; In essence my question is how does the CPU deal…
artless-person
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Galois fields in cryptography

I don't really understand Galois fields, but I've noticed they're used a lot in crypto. I tried to read into them, but quickly got lost in the mess of heiroglyphs and alien terms. I understand they're sets of the form $GF(p^n)$ for $n\geq1$ where…
Polynomial
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