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  • 18 votes cast
Feb
5
comment Would the rotors in Enigma machines always advance by one position? Or was there a way to set this?
Advantage? It would not add to security if it was done always; actually, always advancing two steps in the second rotor would half the keyspace.
Jan
10
comment Need for salt with IV
You are getting something wrong here: IVs are used in encryption, e.g. AES. Salts are used in hash functions.
Jan
10
comment Need for salt with IV
I dont understand your question. A salt randomizes a hash to protect against rainbow tables. IVs make cryptoanalysis very hard. Thats two very different things.
Jan
3
comment end-to-end encrypted chat app with symmetric + public key encryption, TLS and cert pinning
Guessing the password gives access to the account, the private key is effectively useless; the private key needs to be derived from the password. Also no forward secrecy. In general, this is possible. Its just that nobody does it because you would need to charge people for the service (as, obviously, you cannot sell other peoples data) and other, free alternatives are "secure" enough for most people.
Dec
29
awarded  Critic
Dec
28
comment Modular exponentiation on calculator for textbook RSA
As said: there are algorithms. Google for mod pow operation. But you dont want to type these out on your small calculator. Use sth. like this: javascripter.net/math/calculators/100digitbigintcalculator.htm
Dec
28
comment Modular exponentiation on calculator for textbook RSA
Standard calculators cannot do that because $15^{23}$ is too big for a calculator to store. Computers use special algorithms to combine the exponentiation and the modulus in one calcuation step thus avoiding having to calculate $m^e$. If you get hands on a scientific calcuator that allows custom programs to be written on it, you can implement such an algorithm.
Dec
27
revised How should I generate secure random numbers deterministically?
added 6 characters in body; added 4 characters in body
Dec
26
comment How should I generate secure random numbers deterministically?
The hash is not required. I dont know about Any padding techniques for the counter in CTR; i added the hash to be on the safe side. I frickled the 128bit cipher in there because i believe AES is the most battle-tested cipher available - and it has 128bit (yes, Rihjandel 256 would work, too, but again i'd rather be on the safe side)
Dec
26
answered How should I generate secure random numbers deterministically?
Dec
24
comment What is the best current way to cryptographically secure databases?
What about running your favourite DB on a hardware-encrypted hdd?
Jun
23
awarded  Caucus
Mar
5
awarded  Yearling
Aug
10
comment Help deciphering caesar cipher
Does that underscore represent a sapce or is it part of the alphabet/character set of this cipher?
Aug
5
comment How to only encrypt a subset of the plaintext
What do you mean with "subset of P"? You can use any cipher. What data you put through and what not is completely your choice and of no relevance for the cipher.
Jul
26
comment Probability that two different files can have same MD5 hash for both an unencrypted and encrypted version (AES256, same key between files)
Oh sorry, excuse the typo.
Jul
26
comment Probability that two different files can have same MD5 hash for both an unencrypted and encrypted version (AES256, same key between files)
AES25 is not very precise. If a truly randomized Padding is used this is so extraordinary unlikely that you could consider it impossible. However if a deterministic padding is used it would still depend on the mode being used. The probability of this effect occuring randomly stays the same for each mode. But it is easier to construct such a case with ECB or CTR mode than with, say, OFB or CBC mode.
Jul
19
comment KDF based on HMAC-SHA-256
See RFC 2898 for KDFs based on cryptographically secure hash algorithms and MACs.
Jul
12
comment What exactly does a key do?
No. In real use there are bytes ($[0 ... 2^8-1]$). A 128-bit key has $128 : 8 = 16$ bytes. In most symmteric ciphers the bytes are operated on isolated; the 128bit portion of key is never used as a whole for one operation. Take a look at this explanation of AES, it will clear things up: moserware.com/2009/09/stick-figure-guide-to-advanced.html In asymmetric cryptography (RSA, DSA, ElGamal ...) the keys (in RSA up to 4096 bits in everyday-use) the plaintext and key are both used as one number. And yes, in these cases the sheer length makes deciphering without key impossible.
Jul
8
comment What exactly does a key do?
The alphabet has 26 letters so the index ranges from 0 to 25 (lets call that interval $[0, 25]$ $I$). We have to choose $k_{max}$ such that $(x \mod k_{max}) \in I$. That yields $k_{max} = 26$. To put it more simple: $x \mod y \gt 0$ for all $x$ and $y$ greater than $0$. Concerning that condition $x \mod y \leq y - 1$. So if $m + k \mod k_{max}$ has to be less than or equal to 25, $k_{max} = 26$.