Getting the encryption method and key from the encrypted data and the raw data

Question

I have some pairs of plaintext and ciphertext data, from which I need to be able to decrypt the other passwords stored in the database.

The password field typically contains something like

• ENC:LCznyFywN+3J+AD330rqHg==
• ENC:D3H2ODVCAzkzQzRr8ZbJdw==

The raw data for the first is 1234, and the second 62369. ENC: denotes that it is an encrypted field, and the == at the end is always the same (no matter what the password).

If this can be done, then any prods in the right direction would be greatly received.

Answer 1

There is no general method for that. Cryptography is about making it impossible in practice.

In the particular examples, it looks like "encrypted data" is encoded in Base64, as 22 characters, representing 128 arbitrary bits. From that it can't be deduced much about the scheme used.

Answer 2

What you are trying to do is called a known-plaintext attack.

A proper cipher will not be susceptible to a known-plaintext attack, so if the encryption method used on the strings in your database is any good, it can't be done.

Imagine that you were in fact using a (128 bit block) cipher that permits deducting the password given the plaintext and the ciphertext and you decide to use it to send encrypted emails to somebody.

If another person knows that your email ends with

Sincerely yours,

CYMR0

since this string is over 128 bits long, he can recover the password and, therefore, decrypt the entire email.