wip: per-device keys proposal with ECDH and ECDSA

Author: Soxasora

docs/dev/proposals/secure-custom-domain-sessions.md

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+###### Problem
+Synchronizing a user session between stacker.news and a user-owned custom domain, means giving up the whole JWT, which can be stolen and replayed.
+
+For example, in a scenario like DNS hijacking, the malicious user that owns the custom domain can point their DNS record to a server of their own, **log** the incoming requests, **collect** the JWTs and **replay** them.
+
+Usually, mitigations are used for this kind of problem:
+- fingerprinting
+- dns polling
+- short-lived sessions, with refresh tokens
+- aggressive checks on custom domains
+
+I think that some mitigations are correct, such as **dns polling** to take measures against the malicious owner; others might be too much.
+
+##### Per-device keys
+**The case for per-device ECDH Key Pairs**
+As a form of authentication, we can use ECDH shared secrets to either
+- sign requests for the GraphQL endpoint
+- encrypt JWT payloads
+To accomplish this, each device will have its own ECDH key pair, generated and stored in IndexedDB on first visit.
+It can also be used in the future for encrypted messaging
+
+An example flow can be:
+```
+
+1. Device generates key pair
+2. On login, server creates an ECDH key pair
+3. Client and server exchanges public keys
+4. Server and device derives the same shared secret with each other
+-- Stores:
+    user, server_priv_key, client_pub_key, shared_secret_hash
+5. Server issues an encrypted JWT using the shared secret
+6. Device decrypts JWT with the same shared secret
+```
+
+Shared secret rotation is part of best practices in this context.
+
+**The case for ECDSA**
+ECDSA, much like the ECDH route above, can be used to generate a key pair on the first visit and share its public key with the server, to bind it to the user we're importing from `stacker.news`.
+
+ECDSA can be used to sign every request or JWTs, the server will then verify the signature and accept the request if the signature is valid.
+
+An example flow can be:
+```
+1. Device generates key pair
+2. Public key is sent to server and bound to a user
+3. Server issues a JWT that includes the public key fingerprint
+4. On each requests, client signs a message with private key
+5. Server verifies the JWT and the signature, proving possession
+```
+
+If the JWT gets stolen, it's useless without the device private key.
+
+###### Bonus
+A bonus point of using key pairs, is the capability of tracking devices connected to an account, enabling session revocation.