# PKCS#7 padding validation

Write a function that takes a plaintext, determines if it has valid
PKCS#7 padding, and strips the padding off.

The string:

    "ICE ICE BABY\x04\x04\x04\x04"

... has valid padding, and produces the result "ICE ICE BABY".

The string:

    "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.