Should I use an initialization vector (IV) along with my encryption?

Solution 1:

An IV is essential when the same key might ever be used to encrypt more than one message.

The reason is because, under most encryption modes, two messages encrypted with the same key can be analyzed together. In a simple stream cipher, for instance, XORing two ciphertexts encrypted with the same key results in the XOR of the two messages, from which the plaintext can be easily extracted using traditional cryptanalysis techniques.

A weak IV is part of what made WEP breakable.

An IV basically mixes some unique, non-secret data into the key to prevent the same key ever being used twice.

Solution 2:

In most cases you should use IV. Since IV is generated randomly each time, if you encrypt same data twice, encrypted messages are going to be different and it will be impossible for the observer to say if this two messages are the same.

Solution 3:

Take a good look at a picture (see below) of CBC mode. You'll quickly realize that an attacker knowing the IV is like the attacker knowing a previous block of ciphertext (and yes they already know plenty of that).

Here's what I say: most of the "problems" with IV=0 are general problems with block encryption modes when you don't ensure data integrity. You really must ensure integrity.

Here's what I do: use a strong checksum (cryptographic hash or HMAC) and prepend it to your plaintext before encrypting. There's your known first block of ciphertext: it's the IV of the same thing without the checksum, and you need the checksum for a million other reasons.

Finally: any analogy between CBC and stream ciphers is not terribly insightful IMHO.

Just look at the picture of CBC mode, I think you'll be pleasantly surprised.

Here's a picture:

http://en.wikipedia.org/wiki/Block_cipher_modes_of_operation

link text

Solution 4:

If the same key is used multiple times for multiple different secrets patterns could emerge in the encrypted results. The IV, that should be pseudo random and used only once with each key, is there to obfuscate the result. You should never use the same IV with the same key twice, that would defeat the purpose of it.

To not have to bother keeping track of the IV the simplest thing is to prepend, or append it, to the resulting encrypted secret. That way you don't have to think much about it. You will then always know that the first or last N bits is the IV.

When decrypting the secret you just split out the IV, and then use it together with the key to decrypt the secret.