Verify that the strings match

kishib.py

@@ -1,5 +1,7 @@
 import base64
+import binascii
 import hashlib
+import secrets
 import socket
 import sys
 
@@ -118,16 +120,79 @@ def chunk(sliceable, length):
 	for i in range(0, len(sliceable), length):
 		yield sliceable[i:i + length]
 
+def b32_encode(binary):
+	# Kishib uses a modified base32 compared to RFC 4648
+	# * All letters are lowercase
+	# * 'i' is replaced with '8' and 'o' is replaced with '9'
+	encoded = base64.b32encode(binary).decode()
+	encoded = encoded.lower()
+	encoded = encoded.replace('i', '8').replace('o', '9')
+	return encoded
+
 def format_hash(hash_bytes):
-	hash_base32 = base64.b32encode(hash_bytes).decode().lower().replace('=', '')
+	hash_base32 = b32_encode(hash_bytes)
 	chunked_base32 = chunk(hash_base32, 4)
 	return '-'.join(chunked_base32)
 
+class HashFormatError(Exception): pass
+class HashChecksumError(Exception): pass
+
+def parse_hash(auth_hash):
+	# Hash consists of 8 segments of four characters
+	segments = auth_hash.split('-')
+	if len(segments) != 8:
+		raise HashFormatError
+	if not all(len(i) == 4 for i in segments):
+		raise HashFormatError
+
+	combined = ''.join(segments)
+	if 'i' in combined or 'o' in combined:
+		raise HashFormatError
+
+	standard_base32 = combined.replace('8', 'i').replace('9', 'o').upper()
+
+	try:
+		binary = base64.b32decode(standard_base32)
+	except binascii.Error as err:
+		raise HashFormatError from err
+
+	truncated_hash = binary[:16]
+	hash_check = binary[16:]
+
+	# Using secrets.compare_digest is not necessary, but I feel it is a
+	# good habit to avoid comparing hashes with variable-timed comparisons
+	if not secrets.compare_digest(sha512(truncated_hash)[:4], hash_check):
+		raise HashChecksumError
+
+	return binary
+
 def verify(client_pubkey, server_pubkey):
 	own_hash = auth_hash(client_pubkey, server_pubkey)
 	print('Authentication hash: %s' % format_hash(own_hash))
 
-	# TODO: Actually verify
+	while True:
+		user_input = input('Do the hashes match (yes/no/[paste])? ')
+
+		if user_input == 'no':
+			error('Could not transfer the keys')
+
+		elif user_input == 'yes':
+			return
+
+		else:
+			try:
+				other_hash = parse_hash(user_input)
+			except HashFormatError:
+				print('Expected \'yes\', \'no\' or a base32-encoded hash')
+				continue
+			except HashChecksumError:
+				print('Hash checksum check failed')
+				continue
+
+			if secrets.compare_digest(own_hash, other_hash):
+				return
+			else:
+				error('Could not transfer the keys')
 
 def main():
 	# TODO: Actual agument parsing