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1d
comment One-time-pad with multiplication
It sounds like that has nothing to do with OTP at all. In order to get something similar to OTP, then there must be a possible key for every pair of ciphertext/plaintext. With multiplication in mind, you would need a finite field and consider the multiplicative group for plaintext, key and ciphertext.
1d
answered Why don’t all AES encryption tools produce the same key from the same password?
1d
revised Sharing a secret key between many users
added 88 characters in body
1d
comment Sharing a secret key between many users
You're right. I messed up with the very basic byte $\neq$ bit error.
1d
revised Do hand-based hash functions / MACs exist?
added something more about classic hash functions
2d
answered Sharing a secret key between many users
2d
comment Image sharing without data overhead
On the first glance it seems to work, however you are missing a specification of what kind of attacks you want to consider. If you allow the attacker to know B and one of the shares, he can reconstruct A from that. Assuming two parties have the pictures A and B, and hand them over to a trusted third party and receive one share each, then the party knowing B can reconstruct everything and the party with A is still in the dark.
2d
comment How to prove that someone encrypted a specific (large) chunk of data
Alice can cheat in this case, and simple create a double encrypted message of anything of her choice even after the 2. step, under $K_1$ and $K_2$, which she both knows. Bob only signed the key $K_2$ (well, encrypted with Alice public key), not the message itself.
2d
comment Do hand-based hash functions / MACs exist?
If you consider hash functions to be realizations of one-way functions, it raises the question: Did they think of it back then, when classical ciphers were created? And if they had any use? The historic point of view of cryptography was quite different from today, and as far as I know, pretty much limited to encryption. So I doubt there is a classic hash algorithm, because it had no application.
May
22
comment prepaid meters that rely on a disconnected system
The "without knowing the algorithm" is a classical (and today wrong) point of view. One reason for making that assumption might be, that security understanding of the banking sector is most of the time decades behind current standards (which are also a few years behind current research). Knowing the algorithm involved is the very basic ability we grant any serious attacker. Any weaker attacker is irrelevant today.
May
22
comment How to find the encryption method and password?
If you already decrypted it... then it was the algorithm you used to decrypt it?
May
22
comment Alice and Bob's crush
Or you could just use a different algorithm, namely to compute $a \wedge b$ in two-party-computation, instead of making it unnecessarily complex.
May
19
comment Can spatial filters be used to factor composite numbers?
"otherwise they have to behave like functions that depend on brute force searching" That's the part telling you how (in)efficient it is. And brute search is very, very slow, compared to state-of-the-art factorization algorithms.
May
19
answered Do hand-based hash functions / MACs exist?
May
19
comment Is the hash function defined in this exercise collision-resistant?
You're right. I adapted the answer to make up for my earlier error.
May
19
revised Is the hash function defined in this exercise collision-resistant?
Correcting errors
May
19
answered Is the hash function defined in this exercise collision-resistant?
May
8
answered How can I map arbitrary group elements to unique integers without using Hash functions?
May
8
answered Discrete logarithm over prime modulo: small input, large exponent, larger prime
May
8
comment How can I map arbitrary group elements to unique integers without using Hash functions?
$\mathbb{Z}_p$ does not have order $p$, unless you use the addition as group operation. The multiplicative group $\mathbb{Z}_p^*$ has order $p-1$.