Make HostKeyTest class reusable (#278)

Because the `HostKeyTest` class was mutating its static/global
`HOST_KEY_TYPES` dict, this class could not actually be used more than once
in a single thread!

Rather than mutate this dict after parsing each key type
(`HOST_KEY_TYPES[host_key_type]['parsed'] = True`), the `perform_test`
method should simple add the parsed key types to a local `set()`.

src/ssh_audit/hostkeytest.py

@@ -40,7 +40,7 @@ class HostKeyTest:
     # Tracks the RSA host key types.  As of this writing, testing one in this family yields valid results for the rest.
     RSA_FAMILY = ['ssh-rsa', 'rsa-sha2-256', 'rsa-sha2-512']
 
-    # Dict holding the host key types we should extract & parse.  'cert' is True to denote that a host key type handles certificates (thus requires additional parsing).  'variable_key_len' is True for host key types that can have variable sizes (True only for RSA types, as the rest are of fixed-size).  After the host key type is fully parsed, the key 'parsed' is added with a value of True.
+    # Dict holding the host key types we should extract & parse.  'cert' is True to denote that a host key type handles certificates (thus requires additional parsing).  'variable_key_len' is True for host key types that can have variable sizes (True only for RSA types, as the rest are of fixed-size).
     HOST_KEY_TYPES = {
         'ssh-rsa':      {'cert': False, 'variable_key_len': True},
         'rsa-sha2-256': {'cert': False, 'variable_key_len': True},
@@ -93,6 +93,7 @@ def run(out: 'OutputBuffer', s: 'SSH_Socket', server_kex: 'SSH2_Kex') -> None:
     def perform_test(out: 'OutputBuffer', s: 'SSH_Socket', server_kex: 'SSH2_Kex', kex_str: str, kex_group: 'KexDH', host_key_types: Dict[str, Dict[str, bool]]) -> None:
         hostkey_modulus_size = 0
         ca_modulus_size = 0
+        parsed_host_key_types = set()
 
         # If the connection still exists, close it so we can test
         # using a clean slate (otherwise it may exist in a non-testable
@@ -106,7 +107,7 @@ def perform_test(out: 'OutputBuffer', s: 'SSH_Socket', server_kex: 'SSH2_Kex', k
             key_warn_comments = []
 
             # Skip those already handled (i.e.: those in the RSA family, as testing one tests them all).
-            if 'parsed' in host_key_types[host_key_type] and host_key_types[host_key_type]['parsed']:
+            if host_key_type in parsed_host_key_types:
                 continue
 
             # If this host key type is supported by the server, we test it.
@@ -216,7 +217,7 @@ def perform_test(out: 'OutputBuffer', s: 'SSH_Socket', server_kex: 'SSH2_Kex', k
                 # If this host key type is in the RSA family, then mark them all as parsed (since results in one are valid for them all).
                 if host_key_type in HostKeyTest.RSA_FAMILY:
                     for rsa_type in HostKeyTest.RSA_FAMILY:
-                        host_key_types[rsa_type]['parsed'] = True
+                        parsed_host_key_types.add(rsa_type)
 
                         # If the current key is a member of the RSA family, then populate all RSA family members with the same
                         # failure and/or warning comments.
@@ -228,7 +229,7 @@ def perform_test(out: 'OutputBuffer', s: 'SSH_Socket', server_kex: 'SSH2_Kex', k
                         db['key'][rsa_type][2].extend(key_warn_comments)
 
                 else:
-                    host_key_types[host_key_type]['parsed'] = True
+                    parsed_host_key_types.add(host_key_type)
                     db = SSH2_KexDB.get_db()
                     while len(db['key'][host_key_type]) < 3:
                         db['key'][host_key_type].append([])