Block cipher swap key and message Say I use AES. If I insert the message the way the key normally is used and vice-versa, would this be equivalent secure to the intended encryption?
From a theoretical perspective - speaking of general pseudo-random permutations (PRP, the theoretical model used for block ciphers) - no.
An examples: Let $F(k,m)$ be any secure PRP.
Then $F'(k,m)=\begin{cases}m&amp;\text{if }k=0^n\\F(k,m)&amp;\text{else}\end{cases}$ is a secure PRP as well as the chance of actually randomly picking $0^n$ is negligible. Of course once you use more chosen strings for the key - as you'd when swapping the inputs - this guarantee breaks down and you need further guarantees from your cipher for security to hold.
This is not a property known to hold for AES, but the AES key schedule is known to not be the most robust when not choosing random keys, so caution is strongly advised.
Also on a performance note: AES key generation is slow and would be required for each message block in this scenario, increasing the computational cost by 3-10x depending on the implementation.