python3Packages.pyaes: raise on default IV (#491877)

From 034c7eea63a155582109233d2fc1de8e14121908 Mon Sep 17 00:00:00 2001

From: Martin Weinelt <hexa@darmstadt.ccc.de>

Date: Mon, 2 Mar 2026 12:55:44 +0100

Subject: [PATCH] Raise on default IV

This disables the static default IV for CBC, CFB and OFB by raising when

not IV gets passed. We make sure not to break the API contract this way,

so that existing consumers who rely on the default IV get a useful

exception message instead of an API break, which could be done in a

future version.

In CBC mode an IV cannot be predictable or it breaks IND-CPA, this is

also described as CWE-329.

In CFB and OFB mode an IV still requires to be unique, which does not

really hold when initializing it statically.

---

 pyaes/aes.py | 6 +++---

 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/pyaes/aes.py b/pyaes/aes.py

index c6e8bc0..fd25547 100644

--- a/pyaes/aes.py

+++ b/pyaes/aes.py

@@ -376,7 +376,7 @@ class AESModeOfOperationCBC(AESBlockModeOfOperation):

     def __init__(self, key, iv = None):

         if iv is None:

-            self._last_cipherblock = [ 0 ] * 16

+            raise ValueError("Missing IV parameter. This is a security problem, see https://github.com/ricmoo/pyaes/issues/56.")

         elif len(iv) != 16:

             raise ValueError('initialization vector must be 16 bytes')

         else:

@@ -423,7 +423,7 @@ def __init__(self, key, iv, segment_size = 1):

         if segment_size == 0: segment_size = 1

         if iv is None:

-            self._shift_register = [ 0 ] * 16

+            raise ValueError("Missing IV parameter. This is a security problem, see https://github.com/ricmoo/pyaes/issues/56.")

         elif len(iv) != 16:

             raise ValueError('initialization vector must be 16 bytes')

         else:

@@ -495,7 +495,7 @@ class AESModeOfOperationOFB(AESStreamModeOfOperation):

     def __init__(self, key, iv = None):

         if iv is None:

-            self._last_precipherblock = [ 0 ] * 16

+            raise ValueError("Missing IV parameter. This is a security problem, see https://github.com/ricmoo/pyaes/issues/56.")

         elif len(iv) != 16:

             raise ValueError('initialization vector must be 16 bytes')

         else:

    sha256 = "02c1b1405c38d3c370b085fb952dd8bea3fadcee6411ad99f312cc129c536d8f";

  };

  patches = [

    # https://github.com/ricmoo/pyaes/issues/56

    # https://blog.trailofbits.com/2026/02/18/carelessness-versus-craftsmanship-in-cryptography/

    ./default-iv.patch

  ];

  meta = {

    description = "Pure-Python AES";

    license = lib.licenses.mit;