Uses AEGIS to [AEAD][ae] decrypt the _public_
[`ciphertext`][crate::easy::Ciphertext] to a byte vector containing the _secret_
plaintext.
(The module name specifies the used AEGIS algorithm variant.)
[`auth_tag`][crate::easy::AuthTag] must contain the _public_ authentication
tag produced during encryption. It is used to authenticate the ciphertext and
the associated data.
[`associated_data`][crate::easy::AssociatedData] is _public_ "additional
associated data" that the sender used during encryption and provided along with
the ciphertext to the receiver. This data is authenticated along with
`ciphertext`.
[`key`][crate::easy::Key] is the _secret_ encryption key that was used during
encryption.
[`nonce`][crate::careful::Nonce] is the _public_ "number used only once" that
was used during encryption.
<div class="warning">
To avoid catastrophic system compromise, the provided `nonce` must <span
style="color: red; font-weight: bold">NEVER</span> be re-used to encrypt a
different `plaintext` with the same `key`.
</div>
# Errors
- Returns [`Error::AuthTagInvalid`][crate::easy::Error] if the
provided `auth_tag` does not match the computed authentication tag.
[ae]: https://en.wikipedia.org/wiki/Authenticated_encryption