feat: add crypto explanation for the stream gateway

explanation/cryptography/crypto_stream_gateway.md

@@ -0,0 +1,25 @@
+(exp-security-crypto-stream-gateway)=
+# Anbox Stream Gateway
+
+Anbox Streaming Gateway is using cryptographic technology for:
+
+* TLS transport encryption
+* Mutual TLS based authentication
+* Token based authentication
+
+## TLS transport encryption
+
+All network endpoints exposed by the Anbox Stream Gateway are secured with TLS using an 4096 bit RSA key. The Anbox Stream Gateway strictly enforces TLS 1.3 or better and does not provide backward compatibility with older TLS versions.
+
+## Mutual TLS based authentication
+
+In order to exchange messages with the Anbox Stream Agent through the [NATS](https://nats.io/) message queue, the Anbox Stream Gateway uses a CA certificated signed by a 4096 bit RSA key to ensure trust with the NATS server.
+
+## Token based authentication
+
+Users can generate API tokens to authenticate with the HTTP API provided by the Anbox Stream Gateway. For the API tokens a scope-limited [Macaroon](http://theory.stanford.edu/~ataly/Papers/macaroons.pdf) is used. The token is signed with a [HMAC](https://www.okta.com/identity-101/hmac/) using SHA-256 (HS256) and a 64 byte secret key. The [`macaroon.New`](https://pkg.go.dev/gopkg.in/[email protected]#New) method is used internally to generate the JWT token.
+
+## Packages used
+
+* [Go standard library](https://pkg.go.dev/std)
+* [`gopkg.in/macaroon.v2`](gopkg.in/macaroon.v2)