//components/encrypted_messages

Do not add new uses of this component without consulting the security team.

This component implements an extremely simple cryptographic scheme, which allows encrypting anonymous messages to a remote end with a known, static public key. The scheme is as follows, with a server public key spub:

1. Generate a random X25519 keypair (cpub, cpriv)
2. Compute a shared secret using spub and cpriv - note that the server will be able to compute the same shared secret using cpub and spriv later
3. Compute a subkey from that shared secret and a client-supplied fixed label string (to provide domain separation) using HKDF
4. Use that subkey with an AEAD (currently always AES-128-CTR with HMAC-SHA256) to encrypt and authenticate the payload
5. Pack the result into a protobuf containing the message itself, a server public key identifier, the random cpub from step 1, and an algorithm identifier for the inner AEAD

This is used in situations where Chromium is forced to communicate maybe-sensitive data over plain HTTP - namely metrics and variation requests, which are scenarios in which HTTPS might be unavailable.

Do not add new uses of this component without consulting the security team.