implemented getting user from signature

internal/game/session.go

@@ -1,6 +1,9 @@
 package game
 
 import (
+	"crypto/ed25519"
+	"encoding/base64"
+
 	petname "github.com/dustinkirkland/golang-petname"
 )
 
@@ -37,3 +40,15 @@ func GetSession(name string) *Session {
 func (s *Session) AddUser(user User) {
 	s.Users = append(s.Users, user)
 }
+
+func (s Session) VerifySignature(signature string, message []byte) *User {
+	for _, u := range s.Users {
+		sig, _ := base64.StdEncoding.DecodeString(signature)
+
+		if ed25519.Verify(u.PublicKey, message, sig) {
+			return &u
+		}
+	}
+
+	return nil
+}