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From 89af85f9d4fc2ef3e89ad1b2a58c751f00f54a4f Mon Sep 17 00:00:00 2001
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From: Alex Gaynor <alex.gaynor@gmail.com>
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Date: Thu, 3 Mar 2022 16:24:21 -0500
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Subject: [PATCH 5/5] Fixed serialization of keyusage ext with no bits (#6930)
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fixes #6926
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---
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 src/rust/src/x509/extensions.rs | 17 +++++++++++------
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 tests/x509/test_x509_ext.py     | 14 ++++++++++++++
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 2 files changed, 25 insertions(+), 6 deletions(-)
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diff --git a/src/rust/src/x509/extensions.rs b/src/rust/src/x509/extensions.rs
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index 606566dd9..68b9839a0 100644
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--- a/src/rust/src/x509/extensions.rs
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+++ b/src/rust/src/x509/extensions.rs
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@@ -135,12 +135,17 @@ pub(crate) fn encode_extension(
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             certificate::set_bit(&mut bs, 7, ext.getattr("encipher_only")?.is_true()?);
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             certificate::set_bit(&mut bs, 8, ext.getattr("decipher_only")?.is_true()?);
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         }
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-        let bits = if bs[1] == 0 { &bs[..1] } else { &bs[..] };
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-        let unused_bits = bits.last().unwrap().trailing_zeros() as u8;
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-        Ok(Some(asn1::write_single(&asn1::BitString::new(
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-            bits,
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-            unused_bits,
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-        ))))
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+        let (bits, unused_bits) = if bs[1] == 0 {
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+            if bs[0] == 0 {
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+                (&[][..], 0)
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+            } else {
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+                (&bs[..1], bs[0].trailing_zeros() as u8)
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+            }
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+        } else {
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+            (&bs[..], bs[1].trailing_zeros() as u8)
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+        };
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+        let v = asn1::BitString::new(bits, unused_bits).unwrap();
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+        Ok(Some(asn1::write_single(&v)))
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     } else if oid == &*oid::AUTHORITY_INFORMATION_ACCESS_OID
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         || oid == &*oid::SUBJECT_INFORMATION_ACCESS_OID
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     {
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diff --git a/tests/x509/test_x509_ext.py b/tests/x509/test_x509_ext.py
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index 66ac43d95..2bbba8ec6 100644
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--- a/tests/x509/test_x509_ext.py
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+++ b/tests/x509/test_x509_ext.py
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@@ -1137,6 +1137,20 @@ class TestKeyUsage(object):
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                 ),
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                 b"\x03\x02\x02\x94",
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             ),
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+            (
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+                x509.KeyUsage(
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+                    digital_signature=False,
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+                    content_commitment=False,
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+                    key_encipherment=False,
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+                    data_encipherment=False,
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+                    key_agreement=False,
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+                    key_cert_sign=False,
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+                    crl_sign=False,
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+                    encipher_only=False,
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+                    decipher_only=False,
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+                ),
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+                b"\x03\x01\x00",
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+            ),
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         ],
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     )
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     def test_public_bytes(self, ext, serialized):
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-- 
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2.35.1
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