These functions accept an array buffer and extract an AES and MAC key
from it without verifying it has the appropriate length. Ciphertext
messages are similarly dissected. The slice function does not raise an
error on out of bounds accesses but instead returns an empty or
partially-filled array. Empty or short arrays will be passed through to
the window.crypto.subtle API, where they will raise an error. We should
not rely on the Web Crypto API to validate key lengths or for MAC checks
to fail. Instead, validate the lengths of given parameters before
extracting their components.
// FREEBIE
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