Encrytion of files
Decrypting of encrypted data on a digital storage device relies on being able to read it back un-encrypted.
This places some limitations on how you can scramble the file. eg it will be a standard byte with a start and stop bit (or two) even if these are also encrypted.
The simple method of encryption is to generate a key using one of the key generating algorithms and adding it algebraicly to the unencrypted data stream.
These keys will normally have a finite length so that when the sequence gets to a given point it reverts to the start.
The secret is to know the point on the key that is the start point for the encryption.
This can be found from a plain copy of a file that has been encrypted
If you know the algorithm, decryption without the key involves feeding the key stream to the encrypted data in a set of registers and add them together again.
Step the key forward one bit at a time to the file and when the key is in sync the output will be your decrypted file.
One way we used to sync the two streams was to look for a sequence of 15 start stop bits in the correct place. This also works if the file is double encrypted.
Big Brother has all the gear to decrypt encrypted files so don't think encrypting data on your HDD will save you if you get caught with something you should not have!
Barry