snmp: T2687: precalculate snmpv3 encrypted keys

As of now when adding new credentials for any SNMPv3 user we submit the
credential either plaintext or encrypted. A plaintext credential will be hashed
by SNMPd in the background and then passed back into the CLI so it's not stored
in cleartext. This feels like the wrong way in changing the CLI content with
data produced by a 3rd party daemon which implements the service.

It feels like the tail wiggles the entire dog.

This should be changed in the following way:
- After retrieving the plaintext password from CLI, use Python to hash the key
  in advance
- Re-populate the encrypted key into the CLI and drop the plaintext one
- Generate service configuration and continue startup of SNMPd

This also fixes a race condition when SNMPd started up but not properly
provided the hasehd keys in the configuration resulting in a ConfigurationError.
Now as we also support binding SNMPd to a VRF this fixes a deadlock situation
on bootup as we can only bind late to the VRF and require up to 5 restarts of
the service - but the service will never start.

1 files changed, 51 insertions, 0 deletions

diff --git a/python/vyos/snmpv3_hashgen.py b/python/vyos/snmpv3_hashgen.py
new file mode 100644
index 000000000..38eca5314
--- /dev/null
+++ b/python/vyos/snmpv3_hashgen.py
@@ -0,0 +1,51 @@
+# Imported from https://github.com/TheMysteriousX/SNMPv3-Hash-Generator
+
+import hashlib
+import string
+import secrets
+
+from itertools import repeat
+
+P_LEN = 32
+E_LEN = 16
+
+class Hashgen(object):
+    @staticmethod
+    def md5(bytes):
+        return hashlib.md5(bytes).digest().hex()
+
+    @staticmethod
+    def sha1(bytes):
+        return hashlib.sha1(bytes).digest().hex()
+
+    @staticmethod
+    def expand(s, l):
+        reps = l // len(s) + 1 # approximation; worst case: overrun = l + len(s)
+        return ''.join(list(repeat(s, reps)))[:l]
+
+    @classmethod
+    def kdf(cls, password):
+        data = cls.expand(password, 1048576).encode('utf-8')
+        return hashlib.sha1(data).digest()
+
+    @staticmethod
+    def random_string(len=P_LEN, alphabet=(string.ascii_letters + string.digits)):
+        return ''.join(secrets.choice(alphabet) for _ in range(len))
+
+    @staticmethod
+    def random_engine(len=E_LEN):
+        return secrets.token_hex(len)
+
+    @classmethod
+    def derive_msg(cls, passphrase, engine):
+        # Parameter derivation á la rfc3414
+        Ku = cls.kdf(passphrase)
+        E = bytearray.fromhex(engine)
+
+        return b''.join([Ku, E, Ku])
+
+    # Define available hash algorithms
+    algs = {
+        'sha1': sha1,
+        'md5': md5,
+    }