Fixed parsing of ecdsa-sha2-nistp* CA signatures on host keys. Additi…

…onally, they are now flagged as potentially back-doored, just as standard host keys are. (#239)

README.md

@@ -181,6 +181,7 @@ For convenience, a web front-end on top of the command-line tool is available at
 ### v3.2.0-dev (???)
  - Expanded filter of CBC ciphers to flag for the Terrapin vulnerability.  It now includes more rarely found ciphers.
  - Color output is disabled if the `NO_COLOR` environment variable is set (see https://no-color.org/).
+ - Fixed parsing of ecdsa-sha2-nistp* CA signatures on host keys.  Additionally, they are now flagged as potentially back-doored, just as standard host keys are.
 
 ### v3.1.0 (2023-12-20)
  - Added test for the Terrapin message prefix truncation vulnerability ([CVE-2023-48795](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2023-48795)).

src/ssh_audit/hostkeytest.py

@@ -180,7 +180,7 @@ def perform_test(out: 'OutputBuffer', s: 'SSH_Socket', server_kex: 'SSH2_Kex', k
                         hostkey_min_good = 256
                         hostkey_min_warn = 224
                         hostkey_warn_str = HostKeyTest.SMALL_ECC_MODULUS_WARNING
-                    if ca_key_type.startswith('ssh-ed25519') or host_key_type.startswith('ecdsa-sha2-nistp'):
+                    if ca_key_type.startswith('ssh-ed25519') or ca_key_type.startswith('ecdsa-sha2-nistp'):
                         cakey_min_good = 256
                         cakey_min_warn = 224
                         cakey_warn_str = HostKeyTest.SMALL_ECC_MODULUS_WARNING
@@ -209,6 +209,10 @@ def perform_test(out: 'OutputBuffer', s: 'SSH_Socket', server_kex: 'SSH2_Kex', k
                         elif (0 < ca_modulus_size < cakey_min_good) and (cakey_warn_str not in key_warn_comments):
                             key_warn_comments.append(cakey_warn_str)
 
+                    # If the CA key type uses ECDSA with a NIST P-curve, fail it for possibly being back-doored.
+                    if ca_key_type.startswith('ecdsa-sha2-nistp'):
+                        key_fail_comments.append('CA key uses elliptic curves that are suspected as being backdoored by the U.S. National Security Agency')
+
                 # 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:

src/ssh_audit/kexdh.py

@@ -212,6 +212,15 @@ def __parse_ca_key(self, hostkey: bytes, hostkey_type: str, ptr: int) -> Tuple[s
                 # CA's modulus.  Bingo.
                 ca_key_n, ca_key_n_len, ptr = KexDH.__get_bytes(ca_key, ptr)  # pylint: disable=unused-variable
 
+                if ca_key_type.startswith("ecdsa-sha2-nistp") and ca_key_n_len > 0:
+                    self.out.d("Found ecdsa-sha2-nistp* CA key type.")
+
+                    # 0x04 signifies that this is an uncompressed public key (meaning that full X and Y values are provided in ca_key_n.
+                    if ca_key_n[0] == 4:
+                        ca_key_n_len = ca_key_n_len - 1  # Subtract the 0x04 byte.
+                        ca_key_n_len = int(ca_key_n_len / 2)  # Divide by 2 since the modulus is the size of either the X or Y value.
+
+
         else:
             self.out.d("Certificate type %u found; this is not usually valid in the context of a host key!  Skipping it..." % cert_type)