[update] fix (unexploitable) BB'06 vulnerability in rsa_verify

The rsa_verify code was vulnerable to a BB'06 attack, allowing to forge
signatures for arbitrary messages if and only if the public key exponent is
3.  Since the updates key is hardcoded to 65537, there is no risk for
youtube-dl, but I don't want vulnerable code in the wild.

The new function adopts a way safer approach of encoding-and-comparing to
replace the dangerous parsing code.

test/test_update.py

@@ -0,0 +1,30 @@
+#!/usr/bin/env python
+
+from __future__ import unicode_literals
+
+# Allow direct execution
+import os
+import sys
+import unittest
+sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+
+
+import json
+from youtube_dl.update import rsa_verify
+
+
+class TestUpdate(unittest.TestCase):
+    def test_rsa_verify(self):
+        UPDATES_RSA_KEY = (0x9d60ee4d8f805312fdb15a62f87b95bd66177b91df176765d13514a0f1754bcd2057295c5b6f1d35daa6742c3ffc9a82d3e118861c207995a8031e151d863c9927e304576bc80692bc8e094896fcf11b66f3e29e04e3a71e9a11558558acea1840aec37fc396fb6b65dc81a1c4144e03bd1c011de62e3f1357b327d08426fe93, 65537)
+        with open(os.path.join(os.path.dirname(os.path.abspath(__file__)), 'versions.json'), 'rb') as f:
+            versions_info = f.read().decode()
+        versions_info = json.loads(versions_info)
+        signature = versions_info['signature']
+        del versions_info['signature']
+        self.assertTrue(rsa_verify(
+            json.dumps(versions_info, sort_keys=True).encode('utf-8'),
+            signature, UPDATES_RSA_KEY))
+
+
+if __name__ == '__main__':
+    unittest.main()

test/versions.json

@@ -0,0 +1,34 @@
+{
+    "latest": "2013.01.06", 
+    "signature": "72158cdba391628569ffdbea259afbcf279bbe3d8aeb7492690735dc1cfa6afa754f55c61196f3871d429599ab22f2667f1fec98865527b32632e7f4b3675a7ef0f0fbe084d359256ae4bba68f0d33854e531a70754712f244be71d4b92e664302aa99653ee4df19800d955b6c4149cd2b3f24288d6e4b40b16126e01f4c8ce6", 
+    "versions": {
+        "2013.01.02": {
+            "bin": [
+                "http://youtube-dl.org/downloads/2013.01.02/youtube-dl", 
+                "f5b502f8aaa77675c4884938b1e4871ebca2611813a0c0e74f60c0fbd6dcca6b"
+            ], 
+            "exe": [
+                "http://youtube-dl.org/downloads/2013.01.02/youtube-dl.exe", 
+                "75fa89d2ce297d102ff27675aa9d92545bbc91013f52ec52868c069f4f9f0422"
+            ], 
+            "tar": [
+                "http://youtube-dl.org/downloads/2013.01.02/youtube-dl-2013.01.02.tar.gz", 
+                "6a66d022ac8e1c13da284036288a133ec8dba003b7bd3a5179d0c0daca8c8196"
+            ]
+        }, 
+        "2013.01.06": {
+            "bin": [
+                "http://youtube-dl.org/downloads/2013.01.06/youtube-dl", 
+                "64b6ed8865735c6302e836d4d832577321b4519aa02640dc508580c1ee824049"
+            ], 
+            "exe": [
+                "http://youtube-dl.org/downloads/2013.01.06/youtube-dl.exe", 
+                "58609baf91e4389d36e3ba586e21dab882daaaee537e4448b1265392ae86ff84"
+            ], 
+            "tar": [
+                "http://youtube-dl.org/downloads/2013.01.06/youtube-dl-2013.01.06.tar.gz", 
+                "fe77ab20a95d980ed17a659aa67e371fdd4d656d19c4c7950e7b720b0c2f1a86"
+            ]
+        }
+    }
+}

youtube_dl/update.py

@@ -15,33 +15,17 @@ from .version import __version__
 
 
 def rsa_verify(message, signature, key):
-    from struct import pack
     from hashlib import sha256
-
     assert isinstance(message, bytes)
-    block_size = 0
-    n = key[0]
-    while n:
-        block_size += 1
-        n >>= 8
-    signature = pow(int(signature, 16), key[1], key[0])
-    raw_bytes = []
-    while signature:
-        raw_bytes.insert(0, pack("B", signature & 0xFF))
-        signature >>= 8
-    signature = (block_size - len(raw_bytes)) * b'\x00' + b''.join(raw_bytes)
-    if signature[0:2] != b'\x00\x01':
+    byte_size = (len(bin(key[0])) - 2 + 8 - 1) // 8
+    signature = ('%x' % pow(int(signature, 16), key[1], key[0])).encode()
+    signature = (byte_size * 2 - len(signature)) * b'0' + signature
+    asn1 = b'3031300d060960864801650304020105000420'
+    asn1 += sha256(message).hexdigest().encode()
+    if byte_size < len(asn1) // 2 + 11:
         return False
-    signature = signature[2:]
-    if b'\x00' not in signature:
-        return False
-    signature = signature[signature.index(b'\x00') + 1:]
-    if not signature.startswith(b'\x30\x31\x30\x0D\x06\x09\x60\x86\x48\x01\x65\x03\x04\x02\x01\x05\x00\x04\x20'):
-        return False
-    signature = signature[19:]
-    if signature != sha256(message).digest():
-        return False
-    return True
+    expected = b'0001' + (byte_size - len(asn1) // 2 - 3) * b'ff' + b'00' + asn1
+    return expected == signature
 
 
 def update_self(to_screen, verbose, opener):