feat: verify allocated join authorisations with hmac

server/domain/join_auth.go

@@ -1,6 +1,8 @@
 package domain
 
 import (
+	"crypto/hmac"
+	"crypto/sha256"
 	"fmt"
 	"time"
 )
@@ -29,6 +31,18 @@ func SignJoinAuthorisation(auth JoinAuthorisation, sign func([]byte) ([]byte, er
 	return SignedJoinAuthorisation{Authorisation: auth, Signature: append([]byte(nil), signature...)}, nil
 }
 
+// SignJoinAuthorisationHMAC is the interoperable production profile used by
+// the Godot allocated server. The key is mounted out-of-band; the signed
+// bytes remain the same canonical claim bytes used by the generic signer.
+func SignJoinAuthorisationHMAC(auth JoinAuthorisation, key []byte) (SignedJoinAuthorisation, error) {
+	if len(key) == 0 {
+		return SignedJoinAuthorisation{}, ErrJoinAuthorisation
+	}
+	mac := hmac.New(sha256.New, key)
+	_, _ = mac.Write(JoinAuthorisationBytes(auth))
+	return SignedJoinAuthorisation{Authorisation: auth, Signature: mac.Sum(nil)}, nil
+}
+
 func (r *RankedConnections) AdmitSigned(signed SignedJoinAuthorisation, verify func([]byte, []byte) bool, now time.Time) (uint64, error) {
 	if len(signed.Signature) == 0 || verify == nil || !verify(JoinAuthorisationBytes(signed.Authorisation), signed.Signature) {
 		return 0, ErrJoinAuthorisation