implement decrypt_into for chacha20poly1305 (#13783)

* implement decrypt_into for chacha20poly1305

* fix tests maybe

* simplify

Author: reaperhulk

CHANGELOG.rst

@@ -47,9 +47,10 @@ Changelog
   :class:`~cryptography.hazmat.primitives.ciphers.aead.AESSIV`, and
   :class:`~cryptography.hazmat.primitives.ciphers.aead.ChaCha20Poly1305` to
   allow encrypting directly into a pre-allocated buffer.
-* Added a ``decrypt_into`` method to
-  :class:`~cryptography.hazmat.primitives.ciphers.aead.AESSIV` to allow
-  decrypting directly into a pre-allocated buffer.
+* Added ``decrypt_into`` methods to
+  :class:`~cryptography.hazmat.primitives.ciphers.aead.AESSIV` and
+  :class:`~cryptography.hazmat.primitives.ciphers.aead.ChaCha20Poly1305` to
+  allow decrypting directly into a pre-allocated buffer.
 
 .. _v46-0-3:
 

docs/hazmat/primitives/aead.rst

@@ -116,6 +116,34 @@ 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 12 byte value. **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) - 16`` bytes. The plaintext will be written to this
+            buffer.
+        :returns int: The number of bytes written to the buffer (always
+            ``len(data) - 16``).
+        :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.
+
 .. class:: AESGCM(key)
 
     .. versionadded:: 2.0

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

@@ -53,6 +53,13 @@ class ChaCha20Poly1305:
         data: Buffer,
         associated_data: Buffer | None,
     ) -> bytes: ...
+    def decrypt_into(
+        self,
+        nonce: Buffer,
+        data: Buffer,
+        associated_data: Buffer | None,
+        buf: Buffer,
+    ) -> int: ...
 
 class AESCCM:
     def __init__(self, key: Buffer, tag_length: int = 16) -> None: ...

src/rust/src/backend/aead.rs

@@ -358,46 +358,50 @@ impl LazyEvpCipherAead {
         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<'_>,
+        ciphertext: &[u8],
+        aad: Option<Aad<'_>>,
+        nonce: Option<&[u8]>,
+        buf: &mut [u8],
+    ) -> CryptographyResult<()> {
         let key_buf = self.key.bind(py).extract::<CffiBuf<'_>>()?;
 
         let mut decryption_ctx = openssl::cipher_ctx::CipherCtx::new()?;
         if self.is_ccm {
             decryption_ctx.decrypt_init(Some(self.cipher), None, None)?;
             decryption_ctx.set_iv_length(nonce.as_ref().unwrap().len())?;
-
-            if ciphertext.len() < self.tag_len {
-                return Err(CryptographyError::from(exceptions::InvalidTag::new_err(())));
-            }
-
             let (_, tag) = ciphertext.split_at(ciphertext.len() - self.tag_len);
             decryption_ctx.set_tag(tag)?;
-
             decryption_ctx.decrypt_init(None, Some(key_buf.as_bytes()), nonce)?;
         } else {
             decryption_ctx.decrypt_init(Some(self.cipher), Some(key_buf.as_bytes()), None)?;
         }
 
-        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| {
-                EvpCipherAead::decrypt_with_context(
-                    decryption_ctx,
-                    ciphertext,
-                    aad,
-                    nonce,
-                    self.tag_len,
-                    self.tag_first,
-                    self.is_ccm,
-                    b,
-                )?;
-                Ok(())
-            },
-        )?)
+        EvpCipherAead::decrypt_with_context(
+            decryption_ctx,
+            ciphertext,
+            aad,
+            nonce,
+            self.tag_len,
+            self.tag_first,
+            self.is_ccm,
+            buf,
+        )
     }
 }
 
@@ -474,6 +478,29 @@ impl EvpAead {
             },
         )?)
     }
+
+    fn decrypt_into(
+        &self,
+        _py: pyo3::Python<'_>,
+        ciphertext: &[u8],
+        aad: Option<Aad<'_>>,
+        nonce: Option<&[u8]>,
+        buf: &mut [u8],
+    ) -> CryptographyResult<()> {
+        let ad = if let Some(Aad::Single(ad)) = &aad {
+            check_length(ad.as_bytes())?;
+            ad.as_bytes()
+        } else {
+            assert!(aad.is_none());
+            b""
+        };
+
+        self.ctx
+            .decrypt(ciphertext, nonce.unwrap_or(b""), ad, buf)
+            .map_err(|_| exceptions::InvalidTag::new_err(()))?;
+
+        Ok(())
+    }
 }
 
 #[pyo3::pyclass(frozen, module = "cryptography.hazmat.bindings._rust.openssl.aead")]
@@ -618,7 +645,36 @@ impl ChaCha20Poly1305 {
         data: CffiBuf<'_>,
         associated_data: Option<CffiBuf<'_>>,
     ) -> CryptographyResult<pyo3::Bound<'p, pyo3::types::PyBytes>> {
+        if nonce.as_bytes().len() != 12 {
+            return Err(CryptographyError::from(
+                pyo3::exceptions::PyValueError::new_err("Nonce must be 12 bytes"),
+            ));
+        }
+        if data.as_bytes().len() < self.ctx.tag_len {
+            return Err(CryptographyError::from(exceptions::InvalidTag::new_err(())));
+        }
+        Ok(pyo3::types::PyBytes::new_with(
+            py,
+            data.as_bytes().len() - self.ctx.tag_len,
+            |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() != 12 {
@@ -627,8 +683,24 @@ impl ChaCha20Poly1305 {
             ));
         }
 
+        if data.as_bytes().len() < self.ctx.tag_len {
+            return Err(CryptographyError::from(exceptions::InvalidTag::new_err(())));
+        }
+
+        let expected_len = data_bytes.len() - self.ctx.tag_len;
+        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(py, data.as_bytes(), aad, Some(nonce_bytes))
+            .decrypt_into(py, data_bytes, aad, Some(nonce_bytes), buf.as_mut_bytes())?;
+
+        Ok(expected_len)
     }
 }
 

tests/hazmat/primitives/test_aead.py

@@ -118,12 +118,20 @@ def test_nonce_not_12_bytes(self, backend):
         with pytest.raises(ValueError):
             chacha.decrypt(b"00", b"hello", b"")
 
+        with pytest.raises(ValueError):
+            buf = bytearray(1)
+            chacha.decrypt_into(b"00", b"hello", b"", buf)
+
     def test_decrypt_data_too_short(self, backend):
         key = ChaCha20Poly1305.generate_key()
         chacha = ChaCha20Poly1305(key)
         with pytest.raises(InvalidTag):
             chacha.decrypt(b"0" * 12, b"0", None)
 
+        with pytest.raises(InvalidTag):
+            buf = bytearray(16)
+            chacha.decrypt_into(b"0" * 12, b"0", None, buf)
+
     def test_associated_data_none_equal_to_empty_bytestring(self, backend):
         key = ChaCha20Poly1305.generate_key()
         chacha = ChaCha20Poly1305(key)
@@ -222,6 +230,44 @@ def test_encrypt_into_buffer_incorrect_size(self, ptlen, buflen, backend):
         with pytest.raises(ValueError, match="buffer must be"):
             chacha.encrypt_into(nonce, pt, None, buf)
 
+    def test_decrypt_into(self, backend):
+        key = ChaCha20Poly1305.generate_key()
+        chacha = ChaCha20Poly1305(key)
+        nonce = os.urandom(12)
+        pt = b"decrypt me"
+        ad = b"additional"
+        ct = chacha.encrypt(nonce, pt, ad)
+        buf = bytearray(len(pt))
+        n = chacha.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 = ChaCha20Poly1305.generate_key()
+        chacha = ChaCha20Poly1305(key)
+        nonce = os.urandom(12)
+        ct = b"x" * ctlen
+        buf = bytearray(buflen)
+        with pytest.raises(ValueError, match="buffer must be"):
+            chacha.decrypt_into(nonce, ct, None, buf)
+
+    def test_decrypt_into_invalid_tag(self, backend):
+        key = ChaCha20Poly1305.generate_key()
+        chacha = ChaCha20Poly1305(key)
+        nonce = os.urandom(12)
+        pt = b"some data"
+        ad = b"additional"
+        ct = chacha.encrypt(nonce, pt, ad)
+        # Corrupt the ciphertext
+        corrupted_ct = bytearray(ct)
+        corrupted_ct[0] ^= 1
+        buf = bytearray(len(pt))
+        with pytest.raises(InvalidTag):
+            chacha.decrypt_into(nonce, bytes(corrupted_ct), ad, buf)
+
 
 @pytest.mark.skipif(
     not _aead_supported(AESCCM),