Encryption – Can Encryption Be Reversed?

encryption

I am under the impression that an encrypted string cannot be decrypted so the original value is lost forever.

However, if the following string always equals "dominic" (my name), then can't there be some logical way to reverse it; being as it's not random nor is it based on the date/time, but there is a logical method to it?

0WrtCkg6IdaV/l4hDaYq3seMIWMbW+X/g36fvt8uYkE=

No matter what or how many times I encrypt "dominic" (string), it always equals as above. So, shouldn't there be some way to decrypt a string like that?

Example of what I'm talking about:

public string EncryptPassword(string password)
{
    return Convert.ToBase64String(
        System.Security.Cryptography.SHA256.Create()
        .ComputeHash(Encoding.UTF8.GetBytes(password)));
}

Best Answer

Encryption can always be reversed. The point of encryption is to take a message and encode it with a secret key so that only another person who has the key can reverse the encryption and read the message.

What you're looking at here is hashing, which is not the same as encryption, though cryptographic techniques are often used in implementing hashes. The idea of a hash is that it uses complicated mathematical techniques to build a new value that maps to an old value, which is repeatable. There's no key, and it's not meant to be reversed. A cryptographically strong hash is created with the mathematical property that, if you have value A whose hash is value B, it's very, very difficult to intentionally create another value C that also hashes to B.

Hashes don't need to be reversible, because they're used for authentication. If you give me a username and a password, you really don't want me storing that password in my database, because if someone hacks in and gains access to my database, they could get ahold of your password! So instead, I'd store the hash of your password in the database. Then when you log in, I check to see if there's a username that matches yours, with a password entry that matches the hash of the password you sent, and if so you're authenticated, because it's very difficult to create a hash collision (two values that hash to the same value) with a good hash, so I'm almost perfectly certain that the password you used is the right one.

The other property of a strong cryptographic hash is that it's very difficult to reverse. You know that the value 0WrtCkg6IdaV/l4hDaYq3seMIWMbW+X/g36fvt8uYkE= is the hash for "dominic" because you just worked it out, but if you didn't know that, and didn't know where to start looking, and all you had was 0WrtCkg6IdaV/l4hDaYq3seMIWMbW+X/g36fvt8uYkE=, it could literally take you billions of years to figure out that the original was "dominic", if the hash is a good one. Again, this is useful to prevent collateral damage in case a password list gets stolen.

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