can a key be applied to half the cipher text
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.