Timeline for What exactly does a key do?
Current License: CC BY-SA 3.0
7 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Jul 12, 2014 at 15:17 | vote | accept | Wad | ||
| Jul 12, 2014 at 15:16 | vote | accept | Wad | ||
| Jul 12, 2014 at 15:17 | |||||
| Jul 12, 2014 at 8:35 | comment | added | marstato | 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 11, 2014 at 10:09 | comment | added | Wad | Thanks. I am still not getting though how we can have keys that are 128 bits in length; the only way I can visualize this is to imagine that we have in fact $2^{128}$ letters instead of 26, meaning that the key could be any value between 0 and $2^{128}$-1 (which would make it harder to decipher). Is this about right? | |
| Jul 8, 2014 at 16:51 | comment | added | marstato | 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, 2014 at 16:39 | comment | added | Wad | Thanks; how do we decide what the maximum value of k can be though in this case? | |
| Jul 8, 2014 at 16:19 | history | answered | marstato | CC BY-SA 3.0 |