implement decrypt_into for aesccm

CHANGELOG.rst

@@ -48,6 +48,7 @@ Changelog
   :class:`~cryptography.hazmat.primitives.ciphers.aead.ChaCha20Poly1305` to
   allow encrypting directly into a pre-allocated buffer.
 * Added ``decrypt_into`` methods to
+  :class:`~cryptography.hazmat.primitives.ciphers.aead.AESCCM`,
   :class:`~cryptography.hazmat.primitives.ciphers.aead.AESGCM`,
   :class:`~cryptography.hazmat.primitives.ciphers.aead.AESSIV`, and
   :class:`~cryptography.hazmat.primitives.ciphers.aead.ChaCha20Poly1305` to

docs/hazmat/primitives/aead.rst

@@ -767,4 +767,33 @@ also support providing integrity for associated data which is not encrypted.
             when the ciphertext has been changed, but will also occur when the
             key, nonce, or associated data are wrong.
 
+    .. method:: decrypt_into(nonce, data, associated_data, buf)
+
+        .. versionadded:: 47.0.0
+
+        Decrypts the ``data`` and authenticates the ``associated_data``. If you
+        called encrypt with ``associated_data`` you must pass the same
+        ``associated_data`` in decrypt or the integrity check will fail. The
+        output is written into the ``buf`` parameter.
+
+        :param nonce: A value of between 7 and 13 bytes. This
+            is the same value used when you originally called encrypt.
+            **NEVER REUSE A NONCE** with a key.
+        :type nonce: :term:`bytes-like`
+        :param data: The data to decrypt (with tag appended).
+        :type data: :term:`bytes-like`
+        :param associated_data: Additional data to authenticate. Can be
+            ``None`` if none was passed during encryption.
+        :type associated_data: :term:`bytes-like`
+        :param buf: A writable :term:`bytes-like` object that must be exactly
+            ``len(data) - tag_length`` bytes. The plaintext will be written to
+            this buffer.
+        :returns int: The number of bytes written to the buffer (always
+            ``len(data) - tag_length``).
+        :raises ValueError: If the buffer is not the correct size.
+        :raises cryptography.exceptions.InvalidTag: If the authentication tag
+            doesn't validate this exception will be raised. This will occur
+            when the ciphertext has been changed, but will also occur when the
+            key, nonce, or associated data are wrong.
+
 .. _`recommends a 96-bit IV length`: https://csrc.nist.gov/pubs/sp/800/38/d/final

src/cryptography/hazmat/bindings/_rust/openssl/aead.pyi

@@ -91,6 +91,13 @@ class AESCCM:
         data: Buffer,
         associated_data: Buffer | None,
     ) -> bytes: ...
+    def decrypt_into(
+        self,
+        nonce: Buffer,
+        data: Buffer,
+        associated_data: Buffer | None,
+        buf: Buffer,
+    ) -> int: ...
 
 class AESSIV:
     def __init__(self, key: Buffer) -> None: ...

src/rust/src/backend/aead.rs

@@ -351,26 +351,6 @@ impl LazyEvpCipherAead {
         )
     }
 
-    fn decrypt<'p>(
-        &self,
-        py: pyo3::Python<'p>,
-        ciphertext: &[u8],
-        aad: Option<Aad<'_>>,
-        nonce: Option<&[u8]>,
-    ) -> CryptographyResult<pyo3::Bound<'p, pyo3::types::PyBytes>> {
-        if ciphertext.len() < self.tag_len {
-            return Err(CryptographyError::from(exceptions::InvalidTag::new_err(())));
-        }
-        Ok(pyo3::types::PyBytes::new_with(
-            py,
-            ciphertext.len() - self.tag_len,
-            |b| {
-                self.decrypt_into(py, ciphertext, aad, nonce, b)?;
-                Ok(())
-            },
-        )?)
-    }
-
     fn decrypt_into(
         &self,
         py: pyo3::Python<'_>,
@@ -1064,27 +1044,76 @@ impl AesCcm {
     ) -> CryptographyResult<pyo3::Bound<'p, pyo3::types::PyBytes>> {
         let nonce_bytes = nonce.as_bytes();
         let data_bytes = data.as_bytes();
+
+        if nonce_bytes.len() < 7 || nonce_bytes.len() > 13 {
+            return Err(CryptographyError::from(
+                pyo3::exceptions::PyValueError::new_err("Nonce must be between 7 and 13 bytes"),
+            ));
+        }
+        if data_bytes.len() < self.tag_length {
+            return Err(CryptographyError::from(exceptions::InvalidTag::new_err(())));
+        }
+
+        Ok(pyo3::types::PyBytes::new_with(
+            py,
+            data_bytes.len() - self.tag_length,
+            |b| {
+                let buf = CffiMutBuf::from_bytes(py, b);
+                self.decrypt_into(py, nonce, data, associated_data, buf)?;
+                Ok(())
+            },
+        )?)
+    }
+
+    #[pyo3(signature = (nonce, data, associated_data, buf))]
+    fn decrypt_into(
+        &self,
+        py: pyo3::Python<'_>,
+        nonce: CffiBuf<'_>,
+        data: CffiBuf<'_>,
+        associated_data: Option<CffiBuf<'_>>,
+        mut buf: CffiMutBuf<'_>,
+    ) -> CryptographyResult<usize> {
+        let nonce_bytes = nonce.as_bytes();
+        let data_bytes = data.as_bytes();
         let aad = associated_data.map(Aad::Single);
 
         if nonce_bytes.len() < 7 || nonce_bytes.len() > 13 {
             return Err(CryptographyError::from(
                 pyo3::exceptions::PyValueError::new_err("Nonce must be between 7 and 13 bytes"),
             ));
         }
+
+        if data_bytes.len() < self.tag_length {
+            return Err(CryptographyError::from(exceptions::InvalidTag::new_err(())));
+        }
+
         // For information about computing this, see
         // https://tools.ietf.org/html/rfc3610#section-2.1
         let l_val = 15 - nonce_bytes.len();
         let max_length = 1usize.checked_shl(8 * l_val as u32);
         // If `max_length` overflowed, then it's not possible for data to be
         // longer than it.
-        let pt_length = data_bytes.len().saturating_sub(self.tag_length);
-        if max_length.map(|v| v < pt_length).unwrap_or(false) {
+        let expected_len = data_bytes.len() - self.tag_length;
+        if max_length.map(|v| v < expected_len).unwrap_or(false) {
             return Err(CryptographyError::from(
                 pyo3::exceptions::PyValueError::new_err("Data too long for nonce"),
             ));
         }
 
-        self.ctx.decrypt(py, data_bytes, aad, Some(nonce_bytes))
+        if buf.as_mut_bytes().len() != expected_len {
+            return Err(CryptographyError::from(
+                pyo3::exceptions::PyValueError::new_err(format!(
+                    "buffer must be {} bytes",
+                    expected_len
+                )),
+            ));
+        }
+
+        self.ctx
+            .decrypt_into(py, data_bytes, aad, Some(nonce_bytes), buf.as_mut_bytes())?;
+
+        Ok(expected_len)
     }
 }
 

tests/hazmat/primitives/test_aead.py

@@ -321,6 +321,14 @@ def test_invalid_nonce_length(self, backend):
         with pytest.raises(ValueError):
             aesccm.encrypt(nonce[:6], pt, None)
 
+        with pytest.raises(ValueError):
+            buf = bytearray(16)
+            aesccm.decrypt_into(nonce, b"x" * 20, None, buf)
+
+        with pytest.raises(ValueError):
+            buf = bytearray(16)
+            aesccm.decrypt_into(nonce[:6], b"x" * 20, None, buf)
+
     def test_vectors(self, subtests, backend):
         vectors = _load_all_params(
             os.path.join("ciphers", "AES", "CCM"),
@@ -423,6 +431,10 @@ def test_decrypt_data_too_short(self, backend):
         with pytest.raises(InvalidTag):
             aesccm.decrypt(b"0" * 12, b"0", None)
 
+        with pytest.raises(InvalidTag):
+            buf = bytearray(16)
+            aesccm.decrypt_into(b"0" * 12, b"0", None, buf)
+
     def test_buffer_protocol(self, backend):
         key = AESCCM.generate_key(128)
         aesccm = AESCCM(key)
@@ -472,6 +484,53 @@ def test_encrypt_into_buffer_incorrect_size(self, ptlen, buflen, backend):
         with pytest.raises(ValueError, match="buffer must be"):
             aesccm.encrypt_into(nonce, pt, None, buf)
 
+    def test_decrypt_into(self, backend):
+        key = AESCCM.generate_key(128)
+        aesccm = AESCCM(key)
+        nonce = os.urandom(12)
+        pt = b"decrypt me"
+        ad = b"additional"
+        ct = aesccm.encrypt(nonce, pt, ad)
+        buf = bytearray(len(pt))
+        n = aesccm.decrypt_into(nonce, ct, ad, buf)
+        assert n == len(pt)
+        assert buf == pt
+
+    @pytest.mark.parametrize(
+        ("ctlen", "buflen"), [(26, 9), (26, 11), (31, 14), (36, 21)]
+    )
+    def test_decrypt_into_buffer_incorrect_size(self, ctlen, buflen, backend):
+        key = AESCCM.generate_key(128)
+        aesccm = AESCCM(key)
+        nonce = os.urandom(12)
+        ct = b"x" * ctlen
+        buf = bytearray(buflen)
+        with pytest.raises(ValueError, match="buffer must be"):
+            aesccm.decrypt_into(nonce, ct, None, buf)
+
+    def test_decrypt_into_invalid_tag(self, backend):
+        key = AESCCM.generate_key(128)
+        aesccm = AESCCM(key)
+        nonce = os.urandom(12)
+        pt = b"some data"
+        ad = b"additional"
+        ct = aesccm.encrypt(nonce, pt, ad)
+        # Corrupt the ciphertext
+        corrupted_ct = bytearray(ct)
+        corrupted_ct[0] ^= 1
+        buf = bytearray(len(pt))
+        with pytest.raises(InvalidTag):
+            aesccm.decrypt_into(nonce, bytes(corrupted_ct), ad, buf)
+
+    def test_decrypt_into_nonce_too_long(self, backend):
+        key = AESCCM.generate_key(128)
+        aesccm = AESCCM(key)
+        pt = b"encrypt me" * 6600
+        nonce = os.urandom(13)
+        buf = bytearray(len(pt))
+        with pytest.raises(ValueError, match="Data too long for nonce"):
+            aesccm.decrypt_into(nonce, pt, None, buf)
+
 
 def _load_gcm_vectors():
     vectors = _load_all_params(