Get the Whitepaper called Unprotecting the Crypter, a Generic Approach. It discusses how crypters work and unpacking malware.
You can download it from the following link: https://packetstormsecurity.com/files/download/107814/ucrypter.pdf
Source: https://packetstormsecurity.com/files/107814/Unprotecting-The-Crypter-A-Generic-Approach.html