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 later and does not provide backward compatibility with older TLS versions.
+
+## Mutual TLS based authentication
+
+To exchange messages with the Anbox Stream Agent through the [NATS](https://nats.io/) message queue, the Anbox Stream Gateway uses a CA certificate 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](https://jwt.io/) token.
+
+## Packages used
+
+* [Go standard library](https://pkg.go.dev/std)
+* [`gopkg.in/macaroon.v2`](gopkg.in/macaroon.v2)

explanation/cryptography/landing.md

@@ -11,5 +11,6 @@ This information is currently available for the following components:
 
 crypto_ams
 crypto_stream_agent
+crypto_stream_gateway
 crypto_dashboard
 ```