javascript - Why CryptoJS produced different value every time the browser loads

Question

Following code is run on a web page via script tag. Every time I load the page or run the code in the browser console - I am getting different value...

var key = 'key-123:456';
var uid = 1234567890;
var encrypted = CryptoJS.AES.encrypt(id, key);
encrypted.toString();

How can I have single "encrypted value"for "single id" regardless of how many times I load the page or run the code in console?

Answer 1

AES is a "block" cipher, which means it operates deterministically on fixed-length blocks from plaintext to ciphertext (and vice versa). However, it's typical (and generally preferred) to use a "mode of operation" that adds non-determinism to the encryption process. For example, CBC mode (which CryptoJS uses by default) XORs a random initialization vector with the plaintext before encrypting it (and, correspondingly, after decrypting it):

This is vastly preferred because otherwise an eavesdropper can detect duplicate blocks, which might allow an attacker to eventually understand what is being communicated -- undoing the entire point of your encryption.

However, it sounds like you want your encryption to have this specific weakness, which suggests to me that maybe you don't really want encryption at all. Instead, you might want a hash, which is a deterministic one-way transformation. (CryptoJS supports several hashes.) With a hash, a given input A will always hash to the same hash value H, so you can compare Hash(A) == Hash(B) to see if A == B. (This isn't a perfect comparison, since hashes have an infinite input space and finite output space, but hashes are deliberately designed so that it's very, very difficult to find two inputs that produce the same output.) This is how websites securely store your password: the service stores Hash(password) instead of password itself, then when a user submits a password entry, the sites compares Hash(entry) and Hash(password) to see if the entry is correct.

var hash = CryptoJS.SHA3(message);

However, if you really do need to reverse the transformed value back into plaintext and not just compare it to another hashed value, then you do need encryption. In that case, you can use the cryptographically inferior ECB mode, which has the weaknesses described above. In CryptoJS, you can do this by supplying an options object with a mode property:

CryptoJS.AES.encrypt(msg, key, { mode: CryptoJS.mode.ECB });