From 93af6f2f89a9bf28361e67716c4240e691520f30 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Sybren=20A=2E=20St=C3=BCvel?= <sybren@stuvel.eu>
Date: Wed, 3 Jun 2020 14:39:23 +0200
Subject: Fix CVE-2020-13757: detect cyphertext modifications by prepending
 zero bytes

Reject cyphertexts that have been modified by prepending zero bytes, by
checking the cyphertext length against the expected size (given the
decryption key). This resolves CVE-2020-13757.

The same approach is used when verifying a signature.

Thanks Carnil for pointing this out on https://github.com/sybrenstuvel/python-rsa/issues/146
---
 tests/test_pkcs1.py | 44 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 44 insertions(+)

(limited to 'tests')

diff --git a/tests/test_pkcs1.py b/tests/test_pkcs1.py
index 9fba2d8..702ce2d 100644
--- a/tests/test_pkcs1.py
+++ b/tests/test_pkcs1.py
@@ -64,6 +64,32 @@ class BinaryTest(unittest.TestCase):
         self.assertNotEqual(encrypted1, encrypted2)
 
 
+class ExtraZeroesTest(unittest.TestCase):
+    def setUp(self):
+        # Key, cyphertext, and plaintext taken from https://github.com/sybrenstuvel/python-rsa/issues/146
+        self.private_key = rsa.PrivateKey.load_pkcs1(
+            "-----BEGIN RSA PRIVATE KEY-----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\nXVJl32HuKYPJOMIWfc7hIfxLRHnCN+coz2M6tgqMDs0E/OfjuqVZ\n-----END RSA PRIVATE KEY-----",
+            format='PEM')
+        self.cyphertext = bytes.fromhex(
+            "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"
+        )
+        self.plaintext = bytes.fromhex("54657374")
+
+    def test_unmodified(self):
+        message = rsa.decrypt(self.cyphertext, self.private_key)
+        self.assertEqual(message, self.plaintext)
+
+    def test_prepend_zeroes(self):
+        cyphertext = bytes.fromhex("0000") + self.cyphertext
+        with self.assertRaises(rsa.DecryptionError):
+            rsa.decrypt(cyphertext, self.private_key)
+
+    def test_append_zeroes(self):
+        cyphertext = self.cyphertext + bytes.fromhex("0000")
+        with self.assertRaises(rsa.DecryptionError):
+            rsa.decrypt(cyphertext, self.private_key)
+
+
 class SignatureTest(unittest.TestCase):
     def setUp(self):
         (self.pub, self.priv) = rsa.newkeys(512)
@@ -132,3 +158,21 @@ class SignatureTest(unittest.TestCase):
         signature = pkcs1.sign_hash(msg_hash, self.priv, 'SHA-224')
 
         self.assertTrue(pkcs1.verify(message, signature, self.pub))
+
+    def test_prepend_zeroes(self):
+        """Prepending the signature with zeroes should be detected."""
+
+        message = b'je moeder'
+        signature = pkcs1.sign(message, self.priv, 'SHA-256')
+        signature = bytes.fromhex('0000') + signature
+        with self.assertRaises(rsa.VerificationError):
+            pkcs1.verify(message, signature, self.pub)
+
+    def test_apppend_zeroes(self):
+        """Apppending the signature with zeroes should be detected."""
+
+        message = b'je moeder'
+        signature = pkcs1.sign(message, self.priv, 'SHA-256')
+        signature = signature + bytes.fromhex('0000')
+        with self.assertRaises(rsa.VerificationError):
+            pkcs1.verify(message, signature, self.pub)
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