I understand the the statement that "AES is not currently vulnerable to known-plaintext attack" but I assume that there is an implicit rider in that statement that should be read "when used in an appropriate mode AES is not currently vulnerable to known-plaintext attack".
Note: we do not currently do this and I am just trying to build an argument (if valid) to show why we will continue to not do this....
If we have a database that stores short strings (less than 256 bits). Each plain-text input needs to have the same cipher - this rules out use of nonces or IVs and any chaining that uses multiple blocks other than the two required to store the string.
We assume that an attacker can determine the original plaintext for any given cipher text (for reasons I wont go in to, but it seems like good practice).
In this case, where we have removed some of the additional inputs, and we are left with just the key, the known-plaintext and the known-cipher text: why is AES still safe? What am I missing in my understanding that means the algorithm cannot be reversed?
EDIT: The response from John Deters make it very clear that if the system is available to the attacker then they are able to "fish" for the plain-text:cipher pairs. But I am going to assume that we would notice such activity (i.e. the system logs usage and this cannot be subverted, may be an unsafe assumption but I am comfortable with it for the moment as I want to focus on areas outside of my control).
So I would rephrase the question: Assume we have given away / lost some of the cipher values to an external party who has no access to the system itself. Assume the third party can guess one or more of the plain-text values and associate those with their ciphers.
Are (and if so why) the rest of the ciphers still secure? Why cant the key be calculated and the rest of the ciphers decrypted? Does the whole key need to be determined?