Write a function that takes a plaintext, determines if it has valid PKCS#7 padding, and strips the padding off.
"ICE ICE BABY\x04\x04\x04\x04"
... has valid padding, and produces the result "ICE ICE BABY".
"ICE ICE BABY\x05\x05\x05\x05"
... does not have valid padding, nor does:
"ICE ICE BABY\x01\x02\x03\x04"
If you are writing in a language with exceptions, like Python or Ruby, make your function throw an exception on bad padding.
Crypto nerds know where we're going with this. Bear with us.