Add support for decrypting S/MIME messages (#11555)

* first python API proposition

first round-trip tests

feat: made asn1 structures readable

refacto: adapted existing functions accordingly

feat/pkcs12: added symmetric_decrypt

feat: deserialize 3 possible encodings

feat: handling AES-128

feat: raise error when no recipient is found

feat/pkcs7: added decanonicalize function

feat/asn1: added decode_der_data

feat/pkcs7: added smime_enveloped_decode

tests are the round-trip (encrypt & decrypt)

more tests for 100% python coverage

test support pkcs7_encrypt with openssl

added algorithm to pkcs7_encrypt signature

refacto: decrypt function is clearer

flow is more natural

refacto: added all rust error tests

refacto: added another CA chain for checking

fix: const handling

Refactor PKCS7Decryptor to pkcs7_decrypt

refacto: removed SMIME_ENVELOPED_DECODE from rust code

refacto: removed decode_der_data

adapted tests accordingly

removed the PEM tag check

added tests for smime_decnonicalize

one more test case

Update src/rust/src/pkcs7.rs

Co-authored-by: Alex Gaynor <alex.gaynor@gmail.com>

took comments into account

pem to der is now outside of decrypt

fix: removed test_support pkcs7_encrypt

added vector for aes_256_cbc encrypted pkcs7

feat: not using test_support decrypt anymore

added new vectors for PKCS7 tests

feat: using pkcs7 vectors

removed previous ones

fix: changed wrong function

feat: added certificate issuer check

test: generating the RSA chain

removed the vectors accordingly

moved symmetric_decrypt to pkcs7.rs

* Update src/cryptography/hazmat/primitives/serialization/pkcs7.py

Co-authored-by: Alex Gaynor <alex.gaynor@gmail.com>

* fix: removed use of deprecated new_bound for PyBytes

* corrected some error types

* updated tests accordingly

* fix: handling other key encryption algorithms

added vectors & tests accordingly

* first attempts raising error when no header to remove

* one more test to handle text data without header

* fix: went back to the previous implementation

* refacto: removed the return part

* feat: Binary option does not seem useful for decryption

removed decanonicalization function

adapted tests accordingly

* moved logic into rust

only left some checks (for now?)

* removed pyfunction for the inner decrypt one

* added checks in rust now :)

changed name for clarity

* removed unused function

* some checks not needed anymore

* removed a parameter

* took comments into account

* removed unused import

removed excess get_type

* added first unwrap corrections

cleaned tests, added some others

added more vectors

* no more unwrap for parameter checks

* removing headers is Python now

added tests accordingly

will compare with OpenSSL

* final corrections?

* first version of documentation

some minor refactoring

* corrected doctests

* better indentation

* doctest: added RSA private key

* oops

---------

Co-authored-by: Alex Gaynor <alex.gaynor@gmail.com>

Author: nitneuqr

CHANGELOG.rst

@@ -26,6 +26,10 @@ Changelog
 * Added support for :class:`~cryptography.hazmat.primitives.kdf.argon2.Argon2id`
   when using OpenSSL 3.2.0+.
 * Added support for the :class:`~cryptography.x509.Admissions` certificate extension.
+* Added basic support for PKCS7 decryption (including S/MIME 3.2) via
+  :class:`~cryptography.hazmat.primitives.serialization.pkcs7.pkcs7_decrypt_der`, 
+  :class:`~cryptography.hazmat.primitives.serialization.pkcs7.pkcs7_decrypt_pem`, and
+  :class:`~cryptography.hazmat.primitives.serialization.pkcs7.pkcs7_decrypt_smime`.
 
 .. _v43-0-3:
 

docs/development/test-vectors.rst

@@ -882,6 +882,9 @@ Custom PKCS7 Test Vectors
 * ``pkcs7/enveloped-rsa-oaep.pem``- A PEM encoded PKCS7 file with 
   enveloped data, with key encrypted using RSA-OAEP, under the public key of 
   ``x509/custom/ca/rsa_ca.pem``.
+* ``pkcs7/enveloped-no-content.der``- A DER encoded PKCS7 file with 
+  enveloped data, without encrypted content, with key encrypted under the 
+  public key of ``x509/custom/ca/rsa_ca.pem``.
 
 Custom OpenSSH Test Vectors
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~

docs/hazmat/primitives/asymmetric/serialization.rst

@@ -1001,11 +1001,6 @@ PKCS7 is a format described in :rfc:`2315`, among other specifications. It can
 contain certificates, CRLs, and much more. PKCS7 files commonly have a ``p7b``,
 ``p7m``, or ``p7s`` file suffix but other suffixes are also seen in the wild.
 
-.. note::
-
-    ``cryptography`` only supports parsing certificates from PKCS7 files at
-    this time.
-
 .. data:: PKCS7HashTypes
 
     .. versionadded:: 40.0.0
@@ -1126,6 +1121,60 @@ contain certificates, CRLs, and much more. PKCS7 files commonly have a ``p7b``,
     -----END CERTIFICATE-----
     """.strip()
 
+    ca_key_rsa = b"""
+    -----BEGIN PRIVATE KEY-----
+    MIIJQgIBADANBgkqhkiG9w0BAQEFAASCCSwwggkoAgEAAoICAQDQSIXkXNR0+DM1
+    eRr1Gw5PQhVOg06JkQKTakZos64kapujmOB7d3e9QV6IOvyAZKgJ2eP1yUONBuLF
+    Q2+dpNdaD73yfxeaXPulKjwS/kBs2BpCaLmwKlxaSOqMNKmshTUC79E/aOModEED
+    qBr4Apr/daporS62TV7uFPUu+hvg4hkk/kMjJDMY/lbBkbEUQbn1dbq3J7xVo1Ok
+    NvnK9nKdJjABvejU8iLJGIifLy9N1s+A1+JJTuF+O3z5g51PzjJ+Em7zGfPeo9S9
+    CdOEvrlU4U5MUFnBXKl4V+ajPJM3IyVJsmxZW39edI91ornFuPCv4+3ydMfat4lK
+    OBr2tHKEnIJSVnIKPwQQsBQ8PDVW2u56cUkTImkt6k79HRBXEZ7wcnPu4chscZVn
+    UxPbR4rFCNXmVZPT/c4qjTmSrHGPGV9fvwuDPV+vWOwPCO+BeXTtuyEcnBIDq0qN
+    s9TYX0sG6ia/WtkwbUbBYp5/K4ygSMzZ9BOafYztVo8bZHIx3116SzfBRTL6GCPZ
+    fyvmVg5vbG6GhfI64KM0nNNOABXpgB+/ZpghlUSl59bwwKOAywuqdzYgRWEHGG1v
+    Vfm3hg+rK7BesSbbmP1MLT0Ti1ks7ggq2f+AZZqTbEdHoSBRb8xCo1+q0dsqd2Cp
+    YLg2zATCjKX0hsQBcHGezomsUdtFBwIDAQABAoICAQDH6YQRvwPwzTWhkn7MWU6v
+    xjbbJ+7e3T9CrNOttSBlNanzKU31U6KrFS4dxbgLqBEde3Rwud/LYZuRSPu9rLVC
+    bS+crF3EPJEQY2xLspu1nOn/abMoolAIHEp7jiR5QVWzXulRWmQFtSed0eEowJ9y
+    qMaKOAdI1RRToev/TfIqM/l8Z0ubVChzSdONcUAsuDU7ouc22r3K2Lv0Nwwkwc0a
+    hse3NEdg9JNsvs6LM2fM52w9N3ircjm+xmxatPft3HTcSucREIzg2hDb7K2HkOQj
+    0ykq2Eh97ml+56eocADBAEvO46FZVxf2WhxEBY8Xdz4VJMmDWJFmnZj5ksZWmrX6
+    U5BfFY7DZvE2EpoZ5ph1Fm6dcXrJFkaZEyJLlzFKehXMipVenjCanIPpEEUvIz+p
+    m0QVoNJRj/GcNyIEZ0BCXedBOUWU4XE1pG4r6oZqwUvcjsVrqXP5kbJMVybiS6Kd
+    6T8ve+4qsn3ZvGRVKjInqf2WI0Wvum2sTF+4OAkYvFel9dKNjpYnnj4tLFc/EKWz
+    9+pE/Zz5fMOyMD9qXM6bdVkPjWjy1vXmNW4qFCZljrb395hTvsAPMsO6bbAM+lu6
+    YcdOAf8k7awTb79kPMrPcbCygyKSGN9C9T3a/Nhrbr3TPi9SD9hC5Q8bL9uSHcR2
+    hgRQcApxsfDRrGwy2lheEQKCAQEA/Hrynao+k6sYtlDc/ueCjb323EzsuhOxPqUZ
+    fKtGeFkJzKuaKTtymasvVpAAqJBEhTALrptGWlJQ0Y/EVaPpZ9pmk791EWNXdXsX
+    wwufbHxm6K9aOeogev8cd+B/9wUAQPQVotyRzCcOfbVe7t81cBNktqam5Zb9Y4Zr
+    qu63gBB1UttdmIF5qitl3JcFztlBjiza2UrqgVdKE+d9vLR84IBRy3dyQIOi6C1c
+    y37GNgObjx8ZcUVV54/KgvoVvDkvN6TEbUdC9eQz7FW7DA7MMVqyDvWZrSjBzVhK
+    2bTrd+Pi6S4n/ETvA6XRufHC8af4bdE2hzuq5VZO1kkgH37djwKCAQEA0y/YU0b4
+    vCYpZ1MNhBFI6J9346DHD55Zu5dWFRqNkC0PiO6xEMUaUMbG4gxkiQPNT5WvddQs
+    EbRQTnd4FFdqB7XWoH+wERN7zjbT+BZVrHVC4gxEEy33s5oXGn7/ATxaowo7I4oq
+    15MwgZu3hBNxVUtuePZ6D9/ePNGOGOUtdMRrusmVX7gZEXxwvlLJXyVepl2V4JV1
+    otI8EZCcoRhSfeYNEs4VhN0WmfMSV7ge0eFfVb6Lb+6PCcasYED8S0tBN2vjzvol
+    zCMv8skPATm7SopqBDoBPcXCHwN/gUFXHf/lrvE6bbeX1ZMxnRYKdQLLNYyQK9cr
+    nCUJXuNM21tVCQKCAQBapCkFwWDF0t8EVPOB78tG57QAUv2JsBgpzUvhHfwmqJCE
+    Efc+ZkE2Oea8xOX3nhN7XUxUWxpewr6Q/XQW6smYpye8UzfMDkYPvylAtKN/Zwnq
+    70kNEainf37Q6qAGJp14tCgwV89f44WoS7zRNQESQ2QczqeMNTCy0kdFDn6CU2ZL
+    YMWxQopTNVFUaEOFhympySCoceTOmm/VxX22iXVrg6XZzgAOeTO69s4hoFm4eoMW
+    Vqvjpmi4wT6K1w2GjWEOMPDz6ml3rX2WkxCbu5RDA7R4+mM5bzBkcBYvImyGliGY
+    ZSGlx3mnbZhlkQ3Tg+IESt+wnRM1Uk7rT0VhCUKxAoIBABWYuPibM2iaRnWoiqNM
+    2TXgyPPgRzsTqH2ElmsGEiACW6pXLohWf8Bu83u+ZLGWT/Kpjg3wqqkM1YGQuhjq
+    b49mSxKSvECiy3BlLvwZ3J0MSNCxDG0hsEkPovk0r4NC1soBi9awlH0DMlyuve+l
+    xVtBoYSBQC5LaICztWJaXXGpfJLXdo0ZWIbvQOBVuv4d5jYBMAiNgEAsW7Q4I6xd
+    vmHdmsyngo/ZxCvuLZwG2jAAai1slPnXXY1UYeBeBO72PS8bu2o5LpBXsNmVMhGg
+    A8U1rm3MOMBGbvmY8/sV4YDR4H0pch4yPja7HMHBtUQOCxXoz/2LvYv0RacMe5mb
+    F3ECggEAWxQZnT8pObxKrISZpHSKi54VxuLYbemS63Tdr4HE/KuiFAvbM6AeZOki
+    jbiMnqrCTOhJRS/i9HV78zSxRZZyVm961tnsjqMyaamX/S4yD7v3Vzu1mfsdVCa2
+    Sl+JUUxsEgs/G3Fu6I/0TsCSn/HgNLM8b3f8TDkbpnOqKX165ddojXqSCfxjuYau
+    Szih/+jF1dz2/zBye1ARkLRdY/SzlzGl0cVn8bfkE0YEde7wvQ624Biy7r9i1o40
+    7cy/8EQBR2FcXpOAZ7UgOqgGLNhXnd4FPsX4ldKOf5De8FErQOFirJ8pCUxFGr0U
+    fDWXtBuybAb5u+ZaVwHgqaaPCkKkVQ==
+    -----END PRIVATE KEY-----
+    """.strip()
 
 .. class:: PKCS7SignatureBuilder
 
@@ -1261,28 +1310,204 @@ contain certificates, CRLs, and much more. PKCS7 files commonly have a ``p7b``,
             this operation only
             :attr:`~cryptography.hazmat.primitives.serialization.pkcs7.PKCS7Options.Text` and
             :attr:`~cryptography.hazmat.primitives.serialization.pkcs7.PKCS7Options.Binary`
-            are supported.
+            are supported, and cannot be used at the same time.
 
         :returns bytes: The enveloped PKCS7 message.
 
+.. function:: pkcs7_decrypt_der(data, certificate, private_key, options)
+
+    .. versionadded:: 44.0.0
+
+    .. doctest::
+
+        >>> from cryptography import x509
+        >>> from cryptography.hazmat.primitives import serialization
+        >>> from cryptography.hazmat.primitives.serialization import pkcs7
+        >>> cert = x509.load_pem_x509_certificate(ca_cert_rsa)
+        >>> key = serialization.load_pem_private_key(ca_key_rsa, None)
+        >>> options = [pkcs7.PKCS7Options.Text]
+        >>> enveloped = pkcs7.PKCS7EnvelopeBuilder().set_data(
+        ...     b"data to encrypt"
+        ... ).add_recipient(
+        ...     cert
+        ... ).encrypt(
+        ...     serialization.Encoding.DER, options
+        ... )
+        >>> pkcs7.pkcs7_decrypt_der(enveloped, cert, key, options)
+        b'data to encrypt'
+
+    Deserialize and decrypt a DER-encoded PKCS7 message. PKCS7 (or S/MIME) has multiple versions,
+    but this supports a subset of :rfc:`5751`, also known as S/MIME Version 3.2.
+
+    :param data: The data, encoded in DER format.
+    :type data: bytes
+
+    :param certificate: A :class:`~cryptography.x509.Certificate` for an intended
+        recipient of the encrypted message. Only certificates with public RSA keys
+        are currently supported.
+
+    :param private_key: The :class:`~cryptography.hazmat.primitives.asymmetric.rsa.RSAPrivateKey`
+        associated with the certificate provided. Only private RSA keys are supported.
+
+    :param options: A list of
+        :class:`~cryptography.hazmat.primitives.serialization.pkcs7.PKCS7Options`. For 
+        this operation only
+        :attr:`~cryptography.hazmat.primitives.serialization.pkcs7.PKCS7Options.Text` is supported.
+    
+    :returns bytes: The decrypted message.
+
+    :raises ValueError: If the recipient certificate does not match any of the encrypted keys in the
+        PKCS7 data.
+
+    :raises cryptography.exceptions.UnsupportedAlgorithm: If any of the PKCS7 keys are encrypted
+        with another algorithm than RSA with PKCS1 v1.5 padding.
+
+    :raises cryptography.exceptions.UnsupportedAlgorithm: If the content is encrypted with
+        another algorithm than AES-128-CBC.
+
+    :raises ValueError: If the PKCS7 data does not contain encrypted content.
+    
+    :raises ValueError: If the PKCS7 data is not of the enveloped data type.
+
+.. function:: pkcs7_decrypt_pem(data, certificate, private_key, options)
+
+    .. versionadded:: 44.0.0
+
+    .. doctest::
+
+        >>> from cryptography import x509
+        >>> from cryptography.hazmat.primitives import serialization
+        >>> from cryptography.hazmat.primitives.serialization import pkcs7
+        >>> cert = x509.load_pem_x509_certificate(ca_cert_rsa)
+        >>> key = serialization.load_pem_private_key(ca_key_rsa, None)
+        >>> options = [pkcs7.PKCS7Options.Text]
+        >>> enveloped = pkcs7.PKCS7EnvelopeBuilder().set_data(
+        ...     b"data to encrypt"
+        ... ).add_recipient(
+        ...     cert
+        ... ).encrypt(
+        ...     serialization.Encoding.PEM, options
+        ... )
+        >>> pkcs7.pkcs7_decrypt_pem(enveloped, cert, key, options)
+        b'data to encrypt'
+
+    Deserialize and decrypt a PEM-encoded PKCS7E message. PKCS7 (or S/MIME) has multiple versions,
+    but this supports a subset of :rfc:`5751`, also known as S/MIME Version 3.2.
+
+    :param data: The data, encoded in PEM format.
+    :type data: bytes
+
+    :param certificate: A :class:`~cryptography.x509.Certificate` for an intended
+        recipient of the encrypted message. Only certificates with public RSA keys
+        are currently supported.
+
+    :param private_key: The :class:`~cryptography.hazmat.primitives.asymmetric.rsa.RSAPrivateKey`
+        associated with the certificate provided. Only private RSA keys are supported.
+
+    :param options: A list of
+        :class:`~cryptography.hazmat.primitives.serialization.pkcs7.PKCS7Options`. For 
+        this operation only
+        :attr:`~cryptography.hazmat.primitives.serialization.pkcs7.PKCS7Options.Text` is supported.
+    
+    :returns bytes: The decrypted message.
+
+    :raises ValueError: If the PEM data does not have the PKCS7 tag.
+
+    :raises ValueError: If the recipient certificate does not match any of the encrypted keys in the
+        PKCS7 data.
+
+    :raises cryptography.exceptions.UnsupportedAlgorithm: If any of the PKCS7 keys are encrypted
+        with another algorithm than RSA with PKCS1 v1.5 padding.
+
+    :raises cryptography.exceptions.UnsupportedAlgorithm: If the content is encrypted with
+        another algorithm than AES-128-CBC.
+
+    :raises ValueError: If the PKCS7 data does not contain encrypted content.
+    
+    :raises ValueError: If the PKCS7 data is not of the enveloped data type.
+
+.. function:: pkcs7_decrypt_smime(data, certificate, private_key, options)
+
+    .. versionadded:: 44.0.0
+
+    .. doctest::
+
+        >>> from cryptography import x509
+        >>> from cryptography.hazmat.primitives import serialization
+        >>> from cryptography.hazmat.primitives.serialization import pkcs7
+        >>> cert = x509.load_pem_x509_certificate(ca_cert_rsa)
+        >>> key = serialization.load_pem_private_key(ca_key_rsa, None)
+        >>> options = [pkcs7.PKCS7Options.Text]
+        >>> enveloped = pkcs7.PKCS7EnvelopeBuilder().set_data(
+        ...     b"data to encrypt"
+        ... ).add_recipient(
+        ...     cert
+        ... ).encrypt(
+        ...     serialization.Encoding.SMIME, options
+        ... )
+        >>> pkcs7.pkcs7_decrypt_smime(enveloped, cert, key, options)
+        b'data to encrypt'
+
+    Deserialize and decrypt a S/MIME-encoded PKCS7 message. PKCS7 (or S/MIME) has multiple versions,
+    but this supports a subset of :rfc:`5751`, also known as S/MIME Version 3.2.
+
+    :param data: The data. It should be in S/MIME format, meaning MIME with content type
+        ``application/pkcs7-mime`` or ``application/x-pkcs7-mime``.
+    :type data: bytes
+
+    :param certificate: A :class:`~cryptography.x509.Certificate` for an intended
+        recipient of the encrypted message. Only certificates with public RSA keys
+        are currently supported.
+
+    :param private_key: The :class:`~cryptography.hazmat.primitives.asymmetric.rsa.RSAPrivateKey`
+        associated with the certificate provided. Only private RSA keys are supported.
+
+    :param options: A list of
+        :class:`~cryptography.hazmat.primitives.serialization.pkcs7.PKCS7Options`. For 
+        this operation only
+        :attr:`~cryptography.hazmat.primitives.serialization.pkcs7.PKCS7Options.Text` is supported.
+    
+    :returns bytes: The decrypted message.
+
+    :raises ValueError: If the S/MIME data is not one of the correct content types.
+
+    :raises ValueError: If the recipient certificate does not match any of the encrypted keys in the
+        PKCS7 data.
+
+    :raises cryptography.exceptions.UnsupportedAlgorithm: If any of the PKCS7 keys are encrypted
+        with another algorithm than RSA with PKCS1 v1.5 padding.
+
+    :raises cryptography.exceptions.UnsupportedAlgorithm: If the content is encrypted with
+        another algorithm than AES-128-CBC.
+
+    :raises ValueError: If the PKCS7 data does not contain encrypted content.
+    
+    :raises ValueError: If the PKCS7 data is not of the enveloped data type.
+
 
 .. class:: PKCS7Options
 
     .. versionadded:: 3.2
 
-    An enumeration of options for PKCS7 signature and envelope creation.
+    An enumeration of options for PKCS7 signature, envelope creation, and decryption.
 
     .. attribute:: Text
 
-        The text option adds ``text/plain`` headers to an S/MIME message when
-        serializing to
+        For signing, the text option adds ``text/plain`` headers to an S/MIME message when
+        serializing to 
         :attr:`~cryptography.hazmat.primitives.serialization.Encoding.SMIME`.
         This option is disallowed with ``DER`` serialization.
+        For envelope creation, it adds ``text/plain`` headers to the encrypted content, regardless
+        of the specified encoding.
+        For envelope decryption, it parses the decrypted content headers (if any), checks if the
+        content type is 'text/plain', then removes all headers (keeping only the payload) of this
+        decrypted content. If there is no header, or the content type is not "text/plain", it
+        raises an error.
 
     .. attribute:: Binary
 
-        Signing normally converts line endings (LF to CRLF). When
-        passing this option the data will not be converted.
+        Signature and envelope creation normally converts line endings (LF to CRLF). When
+        passing this option, the data will not be converted.
 
     .. attribute:: DetachedSignature
 

src/cryptography/hazmat/bindings/_rust/pkcs7.pyi

@@ -6,6 +6,7 @@ import typing
 
 from cryptography import x509
 from cryptography.hazmat.primitives import serialization
+from cryptography.hazmat.primitives.asymmetric import rsa
 from cryptography.hazmat.primitives.serialization import pkcs7
 
 def serialize_certificates(
@@ -22,6 +23,24 @@ def sign_and_serialize(
     encoding: serialization.Encoding,
     options: typing.Iterable[pkcs7.PKCS7Options],
 ) -> bytes: ...
+def decrypt_der(
+    data: bytes,
+    certificate: x509.Certificate,
+    private_key: rsa.RSAPrivateKey,
+    options: typing.Iterable[pkcs7.PKCS7Options],
+) -> bytes: ...
+def decrypt_pem(
+    data: bytes,
+    certificate: x509.Certificate,
+    private_key: rsa.RSAPrivateKey,
+    options: typing.Iterable[pkcs7.PKCS7Options],
+) -> bytes: ...
+def decrypt_smime(
+    data: bytes,
+    certificate: x509.Certificate,
+    private_key: rsa.RSAPrivateKey,
+    options: typing.Iterable[pkcs7.PKCS7Options],
+) -> bytes: ...
 def load_pem_pkcs7_certificates(
     data: bytes,
 ) -> list[x509.Certificate]: ...

src/cryptography/hazmat/bindings/_rust/test_support.pyi

@@ -13,13 +13,6 @@ class TestCertificate:
     subject_value_tags: list[int]
 
 def test_parse_certificate(data: bytes) -> TestCertificate: ...
-def pkcs7_decrypt(
-    encoding: serialization.Encoding,
-    msg: bytes,
-    pkey: serialization.pkcs7.PKCS7PrivateKeyTypes,
-    cert_recipient: x509.Certificate,
-    options: list[pkcs7.PKCS7Options],
-) -> bytes: ...
 def pkcs7_verify(
     encoding: serialization.Encoding,
     sig: bytes,

src/cryptography/hazmat/primitives/serialization/pkcs7.py

@@ -263,6 +263,11 @@ def encrypt(
         return rust_pkcs7.encrypt_and_serialize(self, encoding, options)
 
 
+pkcs7_decrypt_der = rust_pkcs7.decrypt_der
+pkcs7_decrypt_pem = rust_pkcs7.decrypt_pem
+pkcs7_decrypt_smime = rust_pkcs7.decrypt_smime
+
+
 def _smime_signed_encode(
     data: bytes, signature: bytes, micalg: str, text_mode: bool
 ) -> bytes:
@@ -328,6 +333,34 @@ def _smime_enveloped_encode(data: bytes) -> bytes:
     return m.as_bytes(policy=m.policy.clone(linesep="\n", max_line_length=0))
 
 
+def _smime_enveloped_decode(data: bytes) -> bytes:
+    m = email.message_from_bytes(data)
+    if m.get_content_type() not in {
+        "application/x-pkcs7-mime",
+        "application/pkcs7-mime",
+    }:
+        raise ValueError("Not an S/MIME enveloped message")
+    return bytes(m.get_payload(decode=True))
+
+
+def _smime_remove_text_headers(data: bytes) -> bytes:
+    m = email.message_from_bytes(data)
+    # Using get() instead of get_content_type() since it has None as default,
+    # where the latter has "text/plain". Both methods are case-insensitive.
+    content_type = m.get("content-type")
+    if content_type is None:
+        raise ValueError(
+            "Decrypted MIME data has no 'Content-Type' header. "
+            "Please remove the 'Text' option to parse it manually."
+        )
+    if "text/plain" not in content_type:
+        raise ValueError(
+            f"Decrypted MIME data content type is '{content_type}', not "
+            "'text/plain'. Remove the 'Text' option to parse it manually."
+        )
+    return bytes(m.get_payload(decode=True))
+
+
 class OpenSSLMimePart(email.message.MIMEPart):
     # A MIMEPart subclass that replicates OpenSSL's behavior of not including
     # a newline if there are no headers.