kh2reg.py: handle OpenSSH hashed hostnames.

Obviously we can't do that by inverting the hash function itself, but
if the user provides one or more host names on the command line that
they're expecting to appear in the file, we can at least compare the
stored hashes against those.

contrib/kh2reg.py

@@ -19,6 +19,7 @@ import sys
 import argparse
 import itertools
 import collections
+import hashlib
 from functools import reduce
 
 def winmungestr(s):
@@ -52,6 +53,34 @@ def warn(s):
     sys.stderr.write("%s:%d: %s\n"
                      % (fileinput.filename(), fileinput.filelineno(), s))
 
+class HMAC(object):
+    def __init__(self, hashclass, blocksize):
+        self.hashclass = hashclass
+        self.blocksize = blocksize
+        self.struct = struct.Struct(">{:d}B".format(self.blocksize))
+    def pad_key(self, key):
+        return key + b'\0' * (self.blocksize - len(key))
+    def xor_key(self, key, xor):
+        return self.struct.pack(*[b ^ xor for b in self.struct.unpack(key)])
+    def keyed_hash(self, key, padbyte, string):
+        return self.hashclass(self.xor_key(key, padbyte) + string).digest()
+    def compute(self, key, string):
+        if len(key) > self.blocksize:
+            key = self.hashclass(key).digest()
+        key = self.pad_key(key)
+        return self.keyed_hash(key, 0x5C, self.keyed_hash(key, 0x36, string))
+
+def openssh_hashed_host_match(hashed_host, try_host):
+    if hashed_host.startswith(b'|1|'):
+        salt, expected = hashed_host[3:].split(b'|')
+        salt = base64.decodestring(salt)
+        expected = base64.decodestring(expected)
+        mac = HMAC(hashlib.sha1, 64)
+    else:
+        return False # unrecognised magic number prefix
+
+    return mac.compute(salt, try_host) == expected
+
 def invert(n, p):
     """Compute inverse mod p."""
     if n % p == 0:
@@ -172,7 +201,7 @@ class KeyFormatError(Exception):
     def __init__(self, msg):
         self.msg = msg
 
-def handle_line(line, output_formatter):
+def handle_line(line, output_formatter, try_hosts):
     try:
         # Remove leading/trailing whitespace (should zap CR and LF)
         line = line.strip()
@@ -308,10 +337,17 @@ def handle_line(line, output_formatter):
             if re.search (r"[*?!]", host):
                 warn("skipping wildcard host pattern '%s'" % host)
                 continue
-            elif re.match (r"\|", host):
+
-                warn("skipping hashed hostname '%s'" % host)
+            if re.match (r"\|", host):
-                continue
+                for try_host in try_hosts:
+                    if openssh_hashed_host_match(host.encode('ASCII'),
+                                                 try_host.encode('UTF-8')):
+                        host = try_host
+                        break
                 else:
+                    warn("unable to match hashed hostname '%s'" % host)
+                    continue
+
             m = re.match (r"\[([^]]*)\]:(\d*)$", host)
             if m:
                 (host, port) = m.group(1,2)
@@ -385,15 +421,19 @@ def main():
     parser.add_argument("-o", "--output", type=argparse.FileType("w"),
                         default=argparse.FileType("w")("-"),
                         help="Output file to write to (default stdout).")
+    parser.add_argument("--hostname", action="append",
+                        help="Host name(s) to try matching against hashed "
+                        "host entries in input.")
     parser.add_argument("infile", nargs="*",
                         help="Input file(s) to read from (default stdin).")
-    parser.set_defaults(output_formatter_class=WindowsOutputFormatter)
+    parser.set_defaults(output_formatter_class=WindowsOutputFormatter,
+                        hostname=[])
     args = parser.parse_args()
 
     output_formatter = args.output_formatter_class(args.output)
     output_formatter.header()
     for line in fileinput.input(args.infile):
-        handle_line(line, output_formatter)
+        handle_line(line, output_formatter, args.hostname)
     output_formatter.trailer()
 
 if __name__ == "__main__":