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seen Sep 12 at 21:35

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, $k_{max} = 26$.
Jul
8
answered What exactly does a key do?
Jul
3
revised SHA256 output to 0-99 number range?
corrected two mistakes
Jul
3
awarded  Commentator
Jul
3
comment SHA256 output to 0-99 number range?
If the values of the bytes of SHA256-hashes are distributed evenly $b'$ should also be distributed evenly in $[0, 100]$. Rounding $b'$ should not change too much about it.
Jul
3
answered SHA256 output to 0-99 number range?
Jun
29
answered How to find out the method used to encrypt
Jun
2
comment Creating a license system based on asymmetric encryption (RSA or ECDSA)
What the data is, is not really important. Still, ID and License key are bound together. Say i am User 10815 and my license key is ABCDF487. I sign a some data (a timestamp or my id or whatever) with that key. Then i send ID and key to the server. The server then checks the signature with the public-key related to the ID. If that signature is valid i have proven to own the license for that ID.
Jun
1
comment Creating a license system based on asymmetric encryption (RSA or ECDSA)
@CodeX: if the software cannot connect to your server you just consider the license valid. RickDemer: The ID is tied to a license. The software sends its ID and signed data, the server checks the signature with the correspponding public-key.
May
31
comment Creating a license system based on asymmetric encryption (RSA or ECDSA)
By not accepting the license code if another program with the same id is already running.
May
31
answered Creating a license system based on asymmetric encryption (RSA or ECDSA)
May
3
answered Uncompress password protected WinRar file without password
May
2
comment How do institutions like banks do RSA with big primes?
I just transcribed the code for the method modPow of Javas BigInteger class as seen here: link
May
2
comment How do institutions like banks do RSA with big primes?
Sorry for this, $t=a$