I'm using XTEA to encrypt passwords that I store in a database. (I need to be able to decrypt these passwords later to log in on another system.) But now I have noticed something: if I decrypt a password using a different key I can still see some of the patterns in the plain text. I am not worried about an attack against the XTEA encryption, I am worried about an attack against the other systems that use the encrypted passwords.
Password A = "111222111" Password B = "222111222"
I encrypt these with some key, then decrypt them with another key. The output is something like:
A: qqqwwwqqq B: wwwqqqwww
(Using q and w for readability, it's actually 2 illegible characters.) As you can see, for weak passwords this can reveal a lot of information that can make brute force or dictionary attacks against these passwords a lot easier:
- The length of the password.
- Patterns of the same character in a password.
- Patterns in different passwords.
The assumptions here are:
- The attacker has access to several encrypted passwords and salts.
- The attacker has no access to the encryption key and this key is sufficiently strong.
- The attacker is only interested in the plain-text. Getting the encryption key can be a means but is not an end.
- The attacker has decompiled the code, obfuscation won't do any good.
- The users are idiots and the passwords are as weak and similar as the above examples.
My questions are :
- Can this be considered a weakness in my program? Or can I wash my hands by telling the users to use stronger passwords?
- Is this normal behaviour for XTEA, or is there something wrong with the implementation I am using?
- Are there other encryption methods that are better suited for this?