implemented method to verify token

src/modules/security/security_util.py

+# global import, red is normal don't worry
+import network_constants
+
+import requests
+import json
 from typing import Dict
 
+class TokenStash:
+    '''
+    used to keep track of already verified tokens in order to mitigate the traffic
+    to the user-microservice
+    '''
+    trusted_tokens = {}
+
+    @staticmethod
+    def add(token: str, username: str):
+        '''
+        adds a verified token to the stash
+        '''
+        TokenStash.trusted_tokens[token] = username
+
+    @staticmethod
+    def is_token_cached(token: str) -> str:
+        '''
+        returns the associated username to a token, None otherwise
+        '''
+
+        if token in TokenStash.trusted_tokens:
+            return TokenStash.trusted_tokens[token]
+
+        return None
+
+
 def decodeToken(token: str) -> Dict:
-    pass
\ No newline at end of file
+    '''
+    verifies the passed token on the user-microservice and returns a dictionary with the
+    subject entry if the verification was successful, an error is raised otherwise
+
+    @params:
+            token - Required : JWT token from authorization header, must start with "Bearer "
+    '''
+
+    cached_username = TokenStash.is_token_cached(token)
+    if cached_username != None:
+        print("Re-using cached token!")
+        return {"sub": cached_username}
+
+    if not token.startswith("Bearer "):
+        raise ValueError('Invalid JWT token (must be a Bearer string)')
+    token = token[7:]
+
+    response = requests.get(
+        f'https://{REST_GATEWAY_HOSTNAME}/api/tokens/{token}')
+
+    if response.status_code != 200:
+        raise ValueError(
+            f"Validation of token failed ({response.status_code})!")
+
+    data = json.dumps(response.text)
+
+    if not "username" in data:
+        raise ValueError(
+            f"Validation of token failed (missing field in verification response)!")
+
+    TokenStash.add(token, data["username"])
+    return {"sub": data["username"]}