Add support for checking presence of a fingerprint (no UI yet)

src/check_fingerprint.py

@@ -0,0 +1,51 @@
+import enum
+
+import entry
+import hashing
+
+class result(enum.Enum):
+	notfound, ok, fail = range(3)
+
+def check_fingerprint(entries, domain, port, fingerprint):
+	"""check_fingerprint([Entry], str, u16, bytes[32]) → (enum result: result, str / None: comment)
+	Checks if the given host is found with the given fingerprint.
+	Will return the comment on the host if the host is found, regardless
+	of whether the fingerprint checks out."""
+	assert type(entries) == list and all(type(i) == entry.Entry for i in entries)
+	assert type(domain) == str
+	assert type(port) == int and 0 <= port <= (1<<16) - 1
+	assert type(fingerprint) == bytes and len(fingerprint) == 32
+
+	# Normalize the host here, so we don't have to do it every time we
+	# check for a possible match
+	normalized_hosts = [entry.normalize_host(domain, port)]
+
+	# If we are looking at non-22 port, also check the general form of
+	# the host without a port specifier. This seems to be how OpenSSH
+	# does it too
+	if port != 22:
+		normalized_hosts.append(entry.normalize_host(domain, 22))
+
+	for possible_match in entries:
+		for normalized_host in normalized_hosts:
+			hashed_host = hashing.hash_with_salt(normalized_host, possible_match.salt)
+			if hashed_host == possible_match.hashed_host:
+				# Convert the comment to a string
+				# We put replacement characters where
+				# decoding fails instead of throwing an
+				# error, because even whilethe comment
+				# field must be valid utf-8, failing in this
+				# situation is bad UX
+				comment = possible_match.comment.decode('utf-8', errors = 'replace')
+				# TODO: Justify this
+				# We only care about the first match, so we
+				# return here
+				if fingerprint == possible_match.fingerprint:
+					# Fingerprint matches, it passes
+					return (result.ok, comment)
+				else:
+					# Fingerprint different, it fails
+					return (result.fail, comment)
+
+	# We did not match, tell the caller so
+	return (result.notfound, None)

src/process_known_hosts.py

@@ -13,6 +13,8 @@ def process_line(line):
 	a list of Entries based on it."""
 	assert type(line) == str
 
+	# TODO: Skip over IPs somehow?
+
 	# Remove trailing newlines
 	if line[-1] == '\n': line = line[:-1]