What are the general ideas of decrypting a cipher?

Question

I am trying to improve my skills in encryption and decryption, put there are too many ways to encryption, what ideas are common in this subject, are there any books that talk about that?

thanks alot.

Answer 1

Are you asking how to crack a cipher? When you're dealing with a modern, strong cipher, (AES, Blowfish, Twofish, etc.) there is no known way to decrypt the ciphertext unless you have the key, or can brute-force guess it.

If you want to learn more about using cryptography in general, Schneier's book is probably the best there is. For a more popular description, take a look at The Code Book, which is a fun read.

Answer 2

Your question title and question details don't match very well. I think you should consider clarifying what you are looking for.

A cipher is put quite simply, a mapping. To encrypt you map from one value to another, to decrypt you map in the opposite direction.

An example known as Caeser's cipher:

To encrypt, simply replace each letter with the letter underneath it.
To decrypt, replace each letter in the ciphertext with the letter above.

• ABCDEFGHIJKLMNOPQRSTUVWXYZ
• DEFGHIJKLMNOPQRSTUVWXYZABC
• hello -> khoor
• khoor -> hello

For a general introduction to modern encryption, this link from a quick Google search looks pretty good: http://computer.howstuffworks.com/encryption7.htm