# AES-ECB known ciphertext/plaintext attack

i'm new to crypto and am working on a little exercise to understand the weakness of ECB mode encryption. Here is the scenario.

I have the plain text

I am not tall

and the associated cipher-text generated using an unknown key with AES-ECB.

Is there any way, apart from brute force, I can change generate the ciphertext for the string

I am yes tall

using the information that's available to me?

Knowing the AES-ECB ciphertext for I am not tall does not help you find you the ciphertext for I am yes tall. (Well, it does, but it only rules out one possibility among $2^{128}$ — it can't be the same ciphertext.) ECB has several weaknesses, but not this one. In fact, if you only want to encrypt a single 16-byte message, ECB is fine. The reason ECB is almost never used except by people who don't know better is that encrypting a single message with a forced size is not common.