Added allow_larger_keys flag to custom policies to control whether targets can have larger keys, and added Docker tests to complete work started in PR #242.

README.md

@@ -191,6 +191,7 @@ For convenience, a web front-end on top of the command-line tool is available at
  - Snap builds are now architecture-independent.
  - Changed Docker base image from `python:3-slim` to `python:3-alpine`, resulting in a 59% reduction in image size; credit [Daniel Thamdrup](https://github.com/dallemon).
  - Custom policies now support the `allow_algorithm_subset_and_reordering` directive to allow targets to pass with a subset and/or re-ordered list of host keys, kex, ciphers, and MACs.  This allows for the creation of a baseline policy where targets can optionally implement stricter controls; partial credit [yannik1015](https://github.com/yannik1015).
+ - Custom policies now support the `allow_larger_keys` directive to allow targets to pass with larger host keys, CA keys, and Diffie-Hellman keys.  This allows for the creation of a baseline policy where targets can optionally implement stricter controls; partial credit [Damian Szuberski](https://github.com/szubersk).
  - Added 1 new key exchange algorithm: `gss-nistp384-sha384-*`.
 
 ### v3.1.0 (2023-12-20)

docker_test.sh

@@ -790,6 +790,9 @@ run_custom_policy_test "config2" "test15" "${PROGRAM_RETVAL_GOOD}"
 # Failing test with algorithm subset matching.
 run_custom_policy_test "config2" "test16" "${PROGRAM_RETVAL_FAILURE}"
 
+# Passing test with larger key matching.
+run_custom_policy_test "config2" "test17" "${PROGRAM_RETVAL_GOOD}"
+
 # Failing test for built-in OpenSSH 8.0p1 server policy (RSA host key size is 3072 instead of 4096).
 run_builtin_policy_test "Hardened OpenSSH Server v8.0 (version 4)" "8.0p1" "test1" "-o HostKeyAlgorithms=rsa-sha2-512,rsa-sha2-256,ssh-ed25519 -o KexAlgorithms=curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256 -o Ciphers=chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr -o MACs=hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com" "${PROGRAM_RETVAL_FAILURE}"
 

src/ssh_audit/policy.py

@@ -55,6 +55,7 @@ class Policy:
         self._dh_modulus_sizes: Optional[Dict[str, int]] = None
         self._server_policy = True
         self._allow_algorithm_subset_and_reordering = False
+        self._allow_larger_keys = False
         self._errors: List[Any] = []
 
         self._name_and_version: str = ''
@@ -114,7 +115,7 @@ class Policy:
             key = key.strip()
             val = val.strip()
 
-            if key not in ['name', 'version', 'banner', 'compressions', 'host keys', 'optional host keys', 'key exchanges', 'ciphers', 'macs', 'client policy', 'host_key_sizes', 'dh_modulus_sizes', 'allow_algorithm_subset_and_reordering'] and not key.startswith('hostkey_size_') and not key.startswith('cakey_size_') and not key.startswith('dh_modulus_size_'):
+            if key not in ['name', 'version', 'banner', 'compressions', 'host keys', 'optional host keys', 'key exchanges', 'ciphers', 'macs', 'client policy', 'host_key_sizes', 'dh_modulus_sizes', 'allow_algorithm_subset_and_reordering', 'allow_larger_keys'] and not key.startswith('hostkey_size_') and not key.startswith('cakey_size_') and not key.startswith('dh_modulus_size_'):
                 raise ValueError("invalid field found in policy: %s" % line)
 
             if key in ['name', 'banner']:
@@ -209,6 +210,8 @@ class Policy:
                 self._server_policy = False
             elif key == 'allow_algorithm_subset_and_reordering' and val.lower() == 'true':
                 self._allow_algorithm_subset_and_reordering = True
+            elif key == 'allow_larger_keys' and val.lower() == 'true':
+                self._allow_larger_keys = True
 
         if self._name is None:
             raise ValueError('The policy does not have a name field.')
@@ -296,9 +299,12 @@ name = "Custom Policy (based on %s on %s)"
 # The version of this policy (displayed in the output during scans).  Not parsed, and may be any value, including strings.
 version = 1
 
-# When false, host keys, kex, ciphers, and MAC lists must match exactly.  When true, the target host may support a subset of the specified algorithms and/or algorithms may appear in a different order; this is useful for specifying a baseline and allowing some hosts the option to implement stricter controls.
+# When false, host keys, kex, ciphers, and MAC lists must match exactly.  When true, the target host may support a subset of the specified algorithms and/or algorithms may appear in a different order; this feature is useful for specifying a baseline and allowing some hosts the option to implement stricter controls.
 allow_algorithm_subset_and_reordering = false
 
+# When false, host keys, CA keys, and Diffie-Hellman key sizes must exactly match what's specified in this policy.  When true, target systems are allowed to have larger keys; this feature is useful for specifying a baseline and allowing some hosts the option to implement stricter controls.
+allow_larger_keys = false
+
 # The banner that must match exactly.  Commented out to ignore banners, since minor variability in the banner is sometimes normal.
 # banner = "%s"
 
@@ -371,7 +377,8 @@ macs = %s
                 server_host_keys = kex.host_keys()
                 if hostkey_type in server_host_keys:
                     actual_hostkey_size = cast(int, server_host_keys[hostkey_type]['hostkey_size'])
-                    if actual_hostkey_size < expected_hostkey_size:
+                    if (self._allow_larger_keys and actual_hostkey_size < expected_hostkey_size) or \
+                       (not self._allow_larger_keys and actual_hostkey_size != expected_hostkey_size):
                         ret = False
                         self._append_error('Host key (%s) sizes' % hostkey_type, [str(expected_hostkey_size)], None, [str(actual_hostkey_size)])
 
@@ -387,7 +394,8 @@ macs = %s
                             ret = False
                             self._append_error('CA signature type', [expected_ca_key_type], None, [actual_ca_key_type])
                         # Ensure that the actual and expected signature sizes match.
-                        elif actual_ca_key_size < expected_ca_key_size:
+                        elif (self._allow_larger_keys and actual_ca_key_size < expected_ca_key_size) or \
+                             (not self._allow_larger_keys and actual_ca_key_size != expected_ca_key_size):
                             ret = False
                             self._append_error('CA signature size (%s)' % actual_ca_key_type, [str(expected_ca_key_size)], None, [str(actual_ca_key_size)])
 
@@ -446,7 +454,8 @@ macs = %s
                 expected_dh_modulus_size = self._dh_modulus_sizes[dh_modulus_type]
                 if dh_modulus_type in kex.dh_modulus_sizes():
                     actual_dh_modulus_size = kex.dh_modulus_sizes()[dh_modulus_type]
-                    if expected_dh_modulus_size > actual_dh_modulus_size:
+                    if (self._allow_larger_keys and actual_dh_modulus_size < expected_dh_modulus_size) or \
+                       (not self._allow_larger_keys and actual_dh_modulus_size != expected_dh_modulus_size):
                         ret = False
                         self._append_error('Group exchange (%s) modulus sizes' % dh_modulus_type, [str(expected_dh_modulus_size)], None, [str(actual_dh_modulus_size)])
 

test/docker/expected_results/openssh_8.0p1_custom_policy_test17.json

@@ -0,0 +1,6 @@
+{
+    "errors": [],
+    "host": "localhost",
+    "passed": true,
+    "policy": "Docker policy: test17 (version 1)"
+}

test/docker/expected_results/openssh_8.0p1_custom_policy_test17.txt

@@ -0,0 +1,3 @@
+Host:   localhost:2222
+Policy: Docker policy: test17 (version 1)
+Result: ✔ Passed

test/docker/policies/policy_test17.txt

@@ -0,0 +1,15 @@
+#
+# Docker policy: test17
+#
+
+name = "Docker policy: test17"
+version = 1
+allow_larger_keys = true
+banner = "SSH-2.0-OpenSSH_8.0"
+compressions = none, zlib@openssh.com
+host keys = rsa-sha2-512, rsa-sha2-256, ssh-rsa, ecdsa-sha2-nistp256, ssh-ed25519
+key exchanges = curve25519-sha256, curve25519-sha256@libssh.org, ecdh-sha2-nistp256, ecdh-sha2-nistp384, ecdh-sha2-nistp521, diffie-hellman-group-exchange-sha256, diffie-hellman-group16-sha512, diffie-hellman-group18-sha512, diffie-hellman-group14-sha256, diffie-hellman-group14-sha1
+ciphers = chacha20-poly1305@openssh.com, aes128-ctr, aes192-ctr, aes256-ctr, aes128-gcm@openssh.com, aes256-gcm@openssh.com
+macs = umac-64-etm@openssh.com, umac-128-etm@openssh.com, hmac-sha2-256-etm@openssh.com, hmac-sha2-512-etm@openssh.com, hmac-sha1-etm@openssh.com, umac-64@openssh.com, umac-128@openssh.com, hmac-sha2-256, hmac-sha2-512, hmac-sha1
+host_key_sizes = {"ssh-rsa": {"hostkey_size": 2048}, "rsa-sha2-256": {"hostkey_size": 2048}, "rsa-sha2-512": {"hostkey_size": 2048}, "ssh-ed25519": {"hostkey_size": 256}}
+dh_modulus_sizes = {"diffie-hellman-group-exchange-sha256": 2048}

test/test_policy.py

@@ -297,7 +297,7 @@ macs = mac_alg1, mac_alg2, mac_alg3'''
         pol_data = pol_data.replace(date.today().strftime('%Y/%m/%d'), '[todays date]')
 
         # Instead of writing out the entire expected policy--line by line--just check that it has the expected hash.
-        assert hashlib.sha256(pol_data.encode('ascii')).hexdigest() == '4b504b799f6b964a20ccbe8af7edd26c7b5f0e0b98070e754ea41dccdace33b4'
+        assert hashlib.sha256(pol_data.encode('ascii')).hexdigest() == 'fb84bce442cff2bce9bf653d6373a8a938e3bfcfbd1e876f51a08c1842df3cff'
 
 
     def test_policy_evaluate_passing_1(self):