aes-gcm-siv | NET Core 3.0 implementation | Cryptography library

Part of your problem is that you're using a password directly as a key. You don't want to do that. AES requires a 128-bit, 192-bit, or 256-bit key, and ideally you want it to be indistinguishable from random, which a password typically is not.

If you have a low-entropy secret, you'll probably want to use something like Argon2id to take that passphrase and a random salt of sufficient length to derive a key. If you have a strong secret with lots of entropy, then you can use something like HKDF to generate a key. If you do either of those things, you can generate a key that's exactly 32 bytes long, and, if you want to, a random nonce for the encryption as well. Since you're using AES-GCM-SIV, you can also use a random nonce and just derive the key this way.