can a key be applied to half the cipher text id like to ask if we encrypt the word "hello" as "olleh" but then we apply the key to only "oll" will it be converted into its orginal form i.e "llo". this was just for example. another thing id like to as is that how much time would it take to encrypt a 1.6 mb file with an average encryption algorthm. in seconds or miliseconds. like half a second?
can a key be applied to half the cipher text


Stream ciphers or Block Cipher with CTR mode can do so.


  how much time would it take to encrypt a 1.6 mb file with an average encryption algorthm. in seconds or miliseconds. like half a second?


It depends on hardware (CPU, speed of the driver, etc), software (OS, the programming language, etc) and the algorithm; but 1.6M Bytes should not take 0.5 seconds.

For example:


With the AES-NI, AES(ECB) can encrypt 2G+ Bytes/s with G4500 using Java, Windows 7;  
without it, AES(ECB) encrypts around 120M Bytes/s with G2020 using Java, Windows 7.  
Twofish encrypts around 110M Bytes/s with G2020 using Java, Windows 7.  
Idea encrypts around 65M Bytes/s with G2020 using Java, Windows 7.