I am revising for my exams and I have come across this question in a past paper and I am not confident about it at all.
Assuming that only the smallest permissible key and block sizes can be used I have derived the following:
Possible plaintext blocks: $2^{128}$
Possible ciphertext blocks: $2^{128}$
Possible mappings: $(2^{128})!$
Possible keys: $2^{128}$
My assumption is that it is true that you can find a key given a plaintext and ciphertext since there are significantly way more mappings than there are keys. But then again, are there even enough mappings for that to be possible?
I really feel like I am missing something important here. Are there any hints or suggestions you can give?