What's the basis behind SHA-1 or SHA-2 or other Checksum algorithms? I read about it here http://en.wikipedia.org/wiki/SHA-1#Data_Integrity
But I am still wondering about an answer in a layman's language.
Can I understand it as a very, very compressed code that can be translated back into original data?
Let's say, I have a letter written in notepad. Then the whole of my 1 A4 page size data can be converted into something like this "9b90417b6a186b6f314f0b679f439c89a3b0cdf5". So whenever I want my original data back, I can convert this back into original data?
I am very sure that I am wrong here, because it is weird how data that itself contains combination of letters and numbers can be represented by smaller set of letters and numbers. Illogical!
Then, what's the basic?