Function encode_aead 

pub fn encode_aead(
    shared_secret: &[u8],
    plaintext: &[u8],
    aad: &[u8],
) -> Result<KkAeadPacket>

Encode plaintext with authenticated associated data (AEAD).

The AAD is included in the commitment MAC but is NOT encrypted. This is useful for metadata (headers, routing info, version tags) that must be readable in the clear but tamper-proof.

Arguments

• shared_secret - the pre-shared key
• plaintext - data to encrypt
• aad - associated data to authenticate (not encrypted)