crypto: Add two comments to _chacha20poly1305_final()

It's actually not hard to understand, but add comments to make it quite
clear.

crypto/chachapoly: Allow output be NULL in decrypting empty plaintext

Don't distinguish the input cipher data from AD by checking whether the
output buffer is NULL, because it's actually valid to pa

crypto/chachapoly: Allow output be NULL in decrypting empty plaintext

Don't distinguish the input cipher data from AD by checking whether the
output buffer is NULL, because it's actually valid to pass it as NULL
when to decrypt a message of empty plaintext. And it's really used by
WireGuard.

So separate the AD process code into a separate helper function named
_chacha20poly1305_update_ad(). Update the assertions to not blindly
assert 'out != NULL'. Also add a note about this special case to the
header file.

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crypto: Implement chacha20poly1305 in-place process for mbuf chains

Implement the chacha20poly1305_{encrypt,decrypt}_mbuf() functions that
performs in-place encryption and decryption for data in an

crypto: Implement chacha20poly1305 in-place process for mbuf chains

Implement the chacha20poly1305_{encrypt,decrypt}_mbuf() functions that
performs in-place encryption and decryption for data in an mbuf chain.

The in-kernel WireGuard will use these two functions to encrypt/decrypt
packets.

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crypto: Refactor the chacha20poly1305 code to be more flexible

Introduce the 'chacha20poly1305_ctx' struct to hold the context, and
implement the _init()/_update()/_final() functions as the building

crypto: Refactor the chacha20poly1305 code to be more flexible

Introduce the 'chacha20poly1305_ctx' struct to hold the context, and
implement the _init()/_update()/_final() functions as the building
blocks to perform encryption/decryption in a more generic way.

The main intention is to help implement the in-place encryption and
decryption of data in an mbuf chain. That would reduce the unnecessary
memory allocations and data copies in packet manipulations, as needed by
the in-kernel WireGuard. This API will be done in a later commit.

Rewrite the original chacha20poly1305_{encrypt,decrypt}() functions
using the new blocks.

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crypto: Add ChaCha20-Poly1305 and XChaCha20-Poly1305 AEAD

Derived from OpenBSD with significant modifications by me:

- Removed unused code to hook into the cryptosoft framework.
- Adjusted the inte

crypto: Add ChaCha20-Poly1305 and XChaCha20-Poly1305 AEAD

Derived from OpenBSD with significant modifications by me:

- Removed unused code to hook into the cryptosoft framework.
- Adjusted the interface to align with the IETF RFC document
(e.g., make the nonce a byte string other than a uint64_t),
so that the code becomes more generic.

References:
- RFC 8439: ChaCha20 and Poly1305 for IETF Protocols
- RFC draft: XChaCha: eXtended-nonce ChaCha and AEAD_XChaCha20_Poly1305

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