Support en-/decryption using XChaCha20+Poly1305

In order to have a reasonable alternative to AES256-GCM, I want to have the data encrypted using XChaCha20+Poly1305.

There are some use-cases to this:

• providing choices to the user
• prepare for broken or flawed hardware implementations
• avoid the risk of some potential attacks (e.g. timing)
• higher throughput on platforms without hardware support

Although this sounds like advocating for the alternative, I am just paranoid enough to simply want an alternative to be there.

References:

• https://golang.org/pkg/crypto/aes/
• https://tools.ietf.org/html/draft-irtf-cfrg-xchacha-03
• https://github.com/golang/crypto/blob/master/chacha20poly1305/chacha20poly1305.go
• https://doc.libsodium.org/secret-key_cryptography/aead/aes-256-gcm
• https://nacl.cr.yp.to/secretbox.html
• https://soatok.blog/2020/07/12/comparison-of-symmetric-encryption-methods/#aes-gcm-vs-xchacha20poly1305
• https://keymaterial.net/2020/09/07/invisible-salamanders-in-aes-gcm-siv/

The issue names no repository files or tests. Start by comparing the referenced Go chacha20poly1305 implementation with the repository’s current AES-256-GCM path, then determine the needed API, nonce handling, and data-format compatibility. Done means XChaCha20-Poly1305 encryption and decryption are supported with tests covering the new option.