Merge pull request #2098 from sigmavirus24/cert-builder

Adds certificate builder.

9 files changed, 792 insertions, 1 deletions

diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index 4506a466..f07a9087 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -45,6 +45,13 @@ Changelog
   * :class:`~cryptography.x509.KeyUsage`
   * :class:`~cryptography.x509.SubjectAlternativeName`
 
+* Add support for creating signed certificates with
+  :class:`~cryptography.x509.CertificateBuilder`. This includes support for
+  the following extensions
+
+  * :class:`~cryptography.x509.BasicConstraints`
+  * :class:`~cryptography.x509.SubjectAlternativeName`
+
 0.9.3 - 2015-07-09
 ~~~~~~~~~~~~~~~~~~
 
diff --git a/docs/x509/reference.rst b/docs/x509/reference.rst
index c7d45c77..799126b9 100644
--- a/docs/x509/reference.rst
+++ b/docs/x509/reference.rst
@@ -388,6 +388,140 @@ X.509 CRL (Certificate Revocation List) Object
 
         The extensions encoded in the CRL.
 
+X.509 Certificate Builder
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. class:: CertificateBuilder
+
+    .. versionadded:: 1.0
+
+    .. doctest::
+
+        >>> from cryptography import x509
+        >>> from cryptography.hazmat.backends import default_backend
+        >>> from cryptography.hazmat.primitives import hashes
+        >>> from cryptography.hazmat.primitives.asymmetric import rsa
+        >>> import datetime
+        >>> import uuid
+        >>> one_day = datetime.timedelta(1, 0, 0)
+        >>> private_key = rsa.generate_private_key(
+        ...     public_exponent=65537,
+        ...     key_size=2048,
+        ...     backend=default_backend()
+        ... )
+        >>> public_key = rsa.generate_private_key(
+        ...     public_exponent=65537,
+        ...     key_size=2048,
+        ...     backend=default_backend()
+        ... ).public_key()
+        >>> builder = x509.CertificateBuilder()
+        >>> builder = builder.subject_name(x509.Name([
+        ...     x509.NameAttribute(x509.OID_COMMON_NAME, u'cryptography.io'),
+        ... ]))
+        >>> builder = builder.issuer_name(x509.Name([
+        ...     x509.NameAttribute(x509.OID_COMMON_NAME, u'cryptography.io'),
+        ... ]))
+        >>> builder = builder.not_valid_before(datetime.datetime.today() - one_day)
+        >>> builder = builder.not_valid_after(datetime.datetime(2018, 8, 2))
+        >>> builder = builder.serial_number(int(uuid.uuid4()))
+        >>> builder = builder.public_key(public_key)
+        >>> builder = builder.add_extension(
+        ...     x509.BasicConstraints(ca=False, path_length=None), critical=True,
+        ... )
+        >>> certificate = builder.sign(
+        ...     private_key=private_key, algorithm=hashes.SHA256(),
+        ...     backend=default_backend()
+        ... )
+        >>> isinstance(certificate, x509.Certificate)
+        True
+
+    .. method:: issuer_name(name)
+
+        Sets the issuer's distinguished name.
+
+        :param name: The :class:`~cryptography.x509.Name` that describes the
+            issuer (CA).
+
+    .. method:: subject_name(name)
+
+        Sets the subject's distinguished name.
+
+        :param name: The :class:`~cryptography.x509.Name` that describes the
+            subject.
+
+    .. method:: public_key(public_key)
+
+        Sets the subject's public key.
+
+        :param public_key: The subject's public key. This can be one of
+            :class:`~cryptography.hazmat.primitives.asymmetric.rsa.RSAPublicKey`,
+            :class:`~cryptography.hazmat.primitives.asymmetric.dsa.DSAPublicKey` or
+            :class:`~cryptography.hazmat.primitives.asymmetric.ec.EllipticCurvePublicKey`
+
+    .. method:: serial_number(serial_number)
+
+        Sets the certificate's serial number (an integer).  The CA's policy
+        determines how it attributes serial numbers to certificates.  The only
+        requirement is that this number uniquely identify the certificate given
+        the issuer.
+
+        :param serial_number: Integer number that will be used by the CA to
+            identify this certificate (most notably during certificate
+            revocation checking). Users are encouraged to use a method of
+            generating 20 bytes of entropy, e.g., UUID4. For more information
+            on secure random number generation, see :doc:`/random-numbers`.
+
+    .. method:: not_valid_before(time)
+
+        Sets the certificate's activation time.  This is the time from which
+        clients can start trusting the certificate.  It may be different from
+        the time at which the certificate was created.
+
+        :param time: The :class:`datetime.datetime` object (in UTC) that marks the
+            activation time for the certificate.  The certificate may not be
+            trusted clients if it is used before this time.
+
+    .. method:: not_valid_after(time)
+
+        Sets the certificate's expiration time.  This is the time from which
+        clients should no longer trust the certificate.  The CA's policy will
+        determine how long the certificate should remain in use.
+
+        :param time: The :class:`datetime.datetime` object (in UTC) that marks the
+            expiration time for the certificate.  The certificate may not be
+            trusted clients if it is used after this time.
+
+    .. method:: add_extension(extension, critical)
+
+        Adds an X.509 extension to the certificate.
+
+        :param extension: The extension to add to the certificate. Can be one
+            of :class:`~cryptography.x509.BasicConstraints` or
+            :class:`~cryptography.x509.SubjectAlternativeName`.
+
+        :param critical: Set to ``True`` if the extension must be understood and
+             handled by whoever reads the certificate.
+
+    .. method:: sign(backend, private_key, algorithm)
+
+        Sign the certificate using the CA's private key.
+
+        :param backend: Backend that will be used to build the certificate.
+            Must support the
+            :class:`~cryptography.hazmat.backends.interfaces.X509Backend`
+            interface.
+
+        :param private_key: The
+            :class:`~cryptography.hazmat.primitives.asymmetric.rsa.RSAPrivateKey`,
+            :class:`~cryptography.hazmat.primitives.asymmetric.dsa.DSAPrivateKey` or
+            :class:`~cryptography.hazmat.primitives.asymmetric.ec.EllipticCurvePrivateKey`
+            that will be used to sign the certificate.
+
+        :param algorithm: The
+            :class:`~cryptography.hazmat.primitives.hashes.HashAlgorithm` that
+            will be used to generate the signature.
+
+
 X.509 CSR (Certificate Signing Request) Object
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
diff --git a/src/cryptography/hazmat/backends/interfaces.py b/src/cryptography/hazmat/backends/interfaces.py
index 4d378e6b..49ccda18 100644
--- a/src/cryptography/hazmat/backends/interfaces.py
+++ b/src/cryptography/hazmat/backends/interfaces.py
@@ -280,6 +280,12 @@ class X509Backend(object):
         Create and sign an X.509 CSR from a CSR builder object.
         """
 
+    @abc.abstractmethod
+    def sign_x509_certificate(self, builder, private_key, algorithm):
+        """
+        Sign an X.509 Certificate from a CertificateBuilder object.
+        """
+
 
 @six.add_metaclass(abc.ABCMeta)
 class DHBackend(object):
diff --git a/src/cryptography/hazmat/backends/multibackend.py b/src/cryptography/hazmat/backends/multibackend.py
index 6e911fd5..8008989e 100644
--- a/src/cryptography/hazmat/backends/multibackend.py
+++ b/src/cryptography/hazmat/backends/multibackend.py
@@ -351,3 +351,12 @@ class MultiBackend(object):
             "This backend does not support X.509.",
             _Reasons.UNSUPPORTED_X509
         )
+
+    def sign_x509_certificate(self, builder, private_key, algorithm):
+        for b in self._filtered_backends(X509Backend):
+            return b.sign_x509_certificate(builder, private_key, algorithm)
+
+        raise UnsupportedAlgorithm(
+            "This backend does not support X.509.",
+            _Reasons.UNSUPPORTED_X509
+        )
diff --git a/src/cryptography/hazmat/backends/openssl/backend.py b/src/cryptography/hazmat/backends/openssl/backend.py
index 5ab46d44..0176de21 100644
--- a/src/cryptography/hazmat/backends/openssl/backend.py
+++ b/src/cryptography/hazmat/backends/openssl/backend.py
@@ -4,6 +4,7 @@
 
 from __future__ import absolute_import, division, print_function
 
+import calendar
 import collections
 import itertools
 from contextlib import contextmanager
@@ -78,6 +79,12 @@ def _encode_asn1_int(backend, x):
     return i
 
 
+def _encode_asn1_int_gc(backend, x):
+    i = _encode_asn1_int(backend, x)
+    i = backend._ffi.gc(i, backend._lib.ASN1_INTEGER_free)
+    return i
+
+
 def _encode_asn1_str(backend, data, length):
     """
     Create an ASN1_OCTET_STRING from a Python byte string.
@@ -1063,6 +1070,106 @@ class Backend(object):
 
         return _CertificateSigningRequest(self, x509_req)
 
+    def sign_x509_certificate(self, builder, private_key, algorithm):
+        if not isinstance(builder, x509.CertificateBuilder):
+            raise TypeError('Builder type mismatch.')
+        if not isinstance(algorithm, hashes.HashAlgorithm):
+            raise TypeError('Algorithm must be a registered hash algorithm.')
+
+        if self._lib.OPENSSL_VERSION_NUMBER <= 0x10001000:
+            if isinstance(private_key, _DSAPrivateKey):
+                raise NotImplementedError(
+                    "Certificate signatures aren't implemented for DSA"
+                    " keys on OpenSSL versions less than 1.0.1."
+                )
+            if isinstance(private_key, _EllipticCurvePrivateKey):
+                raise NotImplementedError(
+                    "Certificate signatures aren't implemented for EC"
+                    " keys on OpenSSL versions less than 1.0.1."
+                )
+
+        # Resolve the signature algorithm.
+        evp_md = self._lib.EVP_get_digestbyname(
+            algorithm.name.encode('ascii')
+        )
+        assert evp_md != self._ffi.NULL
+
+        # Create an empty certificate.
+        x509_cert = self._lib.X509_new()
+        x509_cert = self._ffi.gc(x509_cert, backend._lib.X509_free)
+
+        # Set the x509 version.
+        res = self._lib.X509_set_version(x509_cert, builder._version.value)
+        assert res == 1
+
+        # Set the subject's name.
+        res = self._lib.X509_set_subject_name(
+            x509_cert, _encode_name(self, list(builder._subject_name))
+        )
+        assert res == 1
+
+        # Set the subject's public key.
+        res = self._lib.X509_set_pubkey(
+            x509_cert, builder._public_key._evp_pkey
+        )
+        assert res == 1
+
+        # Set the certificate serial number.
+        serial_number = _encode_asn1_int_gc(self, builder._serial_number)
+        res = self._lib.X509_set_serialNumber(x509_cert, serial_number)
+        assert res == 1
+
+        # Set the "not before" time.
+        res = self._lib.ASN1_TIME_set(
+            self._lib.X509_get_notBefore(x509_cert),
+            calendar.timegm(builder._not_valid_before.timetuple())
+        )
+        assert res != self._ffi.NULL
+
+        # Set the "not after" time.
+        res = self._lib.ASN1_TIME_set(
+            self._lib.X509_get_notAfter(x509_cert),
+            calendar.timegm(builder._not_valid_after.timetuple())
+        )
+        assert res != self._ffi.NULL
+
+        # Add extensions.
+        for i, extension in enumerate(builder._extensions):
+            if isinstance(extension.value, x509.BasicConstraints):
+                pp, r = _encode_basic_constraints(self, extension.value)
+            elif isinstance(extension.value, x509.SubjectAlternativeName):
+                pp, r = _encode_subject_alt_name(self, extension.value)
+            else:
+                raise NotImplementedError('Extension not yet supported.')
+
+            obj = _txt2obj(self, extension.oid.dotted_string)
+            extension = self._lib.X509_EXTENSION_create_by_OBJ(
+                self._ffi.NULL,
+                obj,
+                1 if extension.critical else 0,
+                _encode_asn1_str_gc(self, pp[0], r)
+            )
+            res = self._lib.X509_add_ext(x509_cert, extension, i)
+            assert res == 1
+
+        # Set the issuer name.
+        res = self._lib.X509_set_issuer_name(
+            x509_cert, _encode_name(self, list(builder._issuer_name))
+        )
+        assert res == 1
+
+        # Sign the certificate with the issuer's private key.
+        res = self._lib.X509_sign(
+            x509_cert, private_key._evp_pkey, evp_md
+        )
+        if res == 0:
+            errors = self._consume_errors()
+            assert errors[0][1] == self._lib.ERR_LIB_RSA
+            assert errors[0][3] == self._lib.RSA_R_DIGEST_TOO_BIG_FOR_RSA_KEY
+            raise ValueError("Digest too big for RSA key")
+
+        return _Certificate(self, x509_cert)
+
     def load_pem_private_key(self, data, password):
         return self._load_key(
             self._lib.PEM_read_bio_PrivateKey,
diff --git a/src/cryptography/x509.py b/src/cryptography/x509.py
index 75552fc1..a831506e 100644
--- a/src/cryptography/x509.py
+++ b/src/cryptography/x509.py
@@ -5,6 +5,7 @@
 from __future__ import absolute_import, division, print_function
 
 import abc
+import datetime
 import ipaddress
 from email.utils import parseaddr
 from enum import Enum
@@ -17,6 +18,7 @@ from six.moves import urllib_parse
 
 from cryptography import utils
 from cryptography.hazmat.primitives import hashes
+from cryptography.hazmat.primitives.asymmetric import dsa, ec, rsa
 
 
 _OID_NAMES = {
@@ -95,6 +97,8 @@ _GENERAL_NAMES = {
     8: "registeredID",
 }
 
+_UNIX_EPOCH = datetime.datetime(1970, 1, 1)
+
 
 class Version(Enum):
     v1 = 0
@@ -1598,3 +1602,146 @@ class CertificateSigningRequestBuilder(object):
         if self._subject_name is None:
             raise ValueError("A CertificateSigningRequest must have a subject")
         return backend.create_x509_csr(self, private_key, algorithm)
+
+
+class CertificateBuilder(object):
+    def __init__(self, issuer_name=None, subject_name=None,
+                 public_key=None, serial_number=None, not_valid_before=None,
+                 not_valid_after=None, extensions=[]):
+        self._version = Version.v3
+        self._issuer_name = issuer_name
+        self._subject_name = subject_name
+        self._public_key = public_key
+        self._serial_number = serial_number
+        self._not_valid_before = not_valid_before
+        self._not_valid_after = not_valid_after
+        self._extensions = extensions
+
+    def issuer_name(self, name):
+        """
+        Sets the CA's distinguished name.
+        """
+        if not isinstance(name, Name):
+            raise TypeError('Expecting x509.Name object.')
+        if self._issuer_name is not None:
+            raise ValueError('The issuer name may only be set once.')
+        return CertificateBuilder(
+            name, self._subject_name, self._public_key,
+            self._serial_number, self._not_valid_before,
+            self._not_valid_after, self._extensions
+        )
+
+    def subject_name(self, name):
+        """
+        Sets the requestor's distinguished name.
+        """
+        if not isinstance(name, Name):
+            raise TypeError('Expecting x509.Name object.')
+        if self._subject_name is not None:
+            raise ValueError('The subject name may only be set once.')
+        return CertificateBuilder(
+            self._issuer_name, name, self._public_key,
+            self._serial_number, self._not_valid_before,
+            self._not_valid_after, self._extensions
+        )
+
+    def public_key(self, key):
+        """
+        Sets the requestor's public key (as found in the signing request).
+        """
+        if not isinstance(key, (dsa.DSAPublicKey, rsa.RSAPublicKey,
+                                ec.EllipticCurvePublicKey)):
+            raise TypeError('Expecting one of DSAPublicKey, RSAPublicKey,'
+                            ' or EllipticCurvePublicKey.')
+        if self._public_key is not None:
+            raise ValueError('The public key may only be set once.')
+        return CertificateBuilder(
+            self._issuer_name, self._subject_name, key,
+            self._serial_number, self._not_valid_before,
+            self._not_valid_after, self._extensions
+        )
+
+    def serial_number(self, number):
+        """
+        Sets the certificate serial number.
+        """
+        if not isinstance(number, six.integer_types):
+            raise TypeError('Serial number must be of integral type.')
+        if self._serial_number is not None:
+            raise ValueError('The serial number may only be set once.')
+        if number < 0:
+            raise ValueError('The serial number should be non-negative.')
+        if utils.bit_length(number) > 160:  # As defined in RFC 5280
+            raise ValueError('The serial number should not be more than 160 '
+                             'bits.')
+        return CertificateBuilder(
+            self._issuer_name, self._subject_name,
+            self._public_key, number, self._not_valid_before,
+            self._not_valid_after, self._extensions
+        )
+
+    def not_valid_before(self, time):
+        """
+        Sets the certificate activation time.
+        """
+        if not isinstance(time, datetime.datetime):
+            raise TypeError('Expecting datetime object.')
+        if self._not_valid_before is not None:
+            raise ValueError('The not valid before may only be set once.')
+        if time <= _UNIX_EPOCH:
+            raise ValueError('The not valid before date must be after the unix'
+                             ' epoch (1970 January 1).')
+        return CertificateBuilder(
+            self._issuer_name, self._subject_name,
+            self._public_key, self._serial_number, time,
+            self._not_valid_after, self._extensions
+        )
+
+    def not_valid_after(self, time):
+        """
+        Sets the certificate expiration time.
+        """
+        if not isinstance(time, datetime.datetime):
+            raise TypeError('Expecting datetime object.')
+        if self._not_valid_after is not None:
+            raise ValueError('The not valid after may only be set once.')
+        if time <= _UNIX_EPOCH:
+            raise ValueError('The not valid after date must be after the unix'
+                             ' epoch (1970 January 1).')
+        return CertificateBuilder(
+            self._issuer_name, self._subject_name,
+            self._public_key, self._serial_number, self._not_valid_before,
+            time, self._extensions
+        )
+
+    def add_extension(self, extension, critical):
+        """
+        Adds an X.509 extension to the certificate.
+        """
+        if isinstance(extension, BasicConstraints):
+            extension = Extension(OID_BASIC_CONSTRAINTS, critical, extension)
+        elif isinstance(extension, SubjectAlternativeName):
+            extension = Extension(
+                OID_SUBJECT_ALTERNATIVE_NAME, critical, extension
+            )
+        elif isinstance(extension, InhibitAnyPolicy):
+            extension = Extension(OID_INHIBIT_ANY_POLICY, critical, extension)
+        else:
+            raise NotImplementedError('Unsupported X.509 extension.')
+
+        # TODO: This is quadratic in the number of extensions
+        for e in self._extensions:
+            if e.oid == extension.oid:
+                raise ValueError('This extension has already been set.')
+
+        return CertificateBuilder(
+            self._issuer_name, self._subject_name,
+            self._public_key, self._serial_number, self._not_valid_before,
+            self._not_valid_after, self._extensions + [extension]
+        )
+
+    def sign(self, backend, private_key, algorithm):
+        """
+        Signs the certificate using the CA's private key.
+        """
+        return backend.sign_x509_certificate(self, private_key, algorithm)
diff --git a/tests/hazmat/backends/test_multibackend.py b/tests/hazmat/backends/test_multibackend.py
index 3c05cdfa..d516af16 100644
--- a/tests/hazmat/backends/test_multibackend.py
+++ b/tests/hazmat/backends/test_multibackend.py
@@ -206,6 +206,9 @@ class DummyX509Backend(object):
     def create_x509_csr(self, builder, private_key, algorithm):
         pass
 
+    def sign_x509_certificate(self, builder, private_key, algorithm):
+        pass
+
 
 class TestMultiBackend(object):
     def test_ciphers(self):
@@ -484,6 +487,7 @@ class TestMultiBackend(object):
         backend.load_pem_x509_csr(b"reqdata")
         backend.load_der_x509_csr(b"reqdata")
         backend.create_x509_csr(object(), b"privatekey", hashes.SHA1())
+        backend.sign_x509_certificate(object(), b"privatekey", hashes.SHA1())
 
         backend = MultiBackend([])
         with raises_unsupported_algorithm(_Reasons.UNSUPPORTED_X509):
@@ -496,3 +500,7 @@ class TestMultiBackend(object):
             backend.load_der_x509_csr(b"reqdata")
         with raises_unsupported_algorithm(_Reasons.UNSUPPORTED_X509):
             backend.create_x509_csr(object(), b"privatekey", hashes.SHA1())
+        with raises_unsupported_algorithm(_Reasons.UNSUPPORTED_X509):
+            backend.sign_x509_certificate(
+                object(), b"privatekey", hashes.SHA1()
+            )
diff --git a/tests/hazmat/backends/test_openssl.py b/tests/hazmat/backends/test_openssl.py
index 040e3677..c2a4f544 100644
--- a/tests/hazmat/backends/test_openssl.py
+++ b/tests/hazmat/backends/test_openssl.py
@@ -4,6 +4,7 @@
 
 from __future__ import absolute_import, division, print_function
 
+import datetime
 import os
 import subprocess
 import sys
@@ -14,6 +15,7 @@ import pretend
 import pytest
 
 from cryptography import utils
+from cryptography import x509
 from cryptography.exceptions import InternalError, _Reasons
 from cryptography.hazmat.backends.interfaces import RSABackend
 from cryptography.hazmat.backends.openssl.backend import (
@@ -505,6 +507,33 @@ class TestOpenSSLCreateX509CSR(object):
             backend.create_x509_csr(object(), private_key, hashes.SHA1())
 
 
+class TestOpenSSLSignX509Certificate(object):
+    def test_requires_certificate_builder(self):
+        private_key = RSA_KEY_2048.private_key(backend)
+
+        with pytest.raises(TypeError):
+            backend.sign_x509_certificate(object(), private_key, DummyHash())
+
+    def test_checks_for_unsupported_extensions(self):
+        private_key = RSA_KEY_2048.private_key(backend)
+        builder = x509.CertificateBuilder().subject_name(x509.Name([
+            x509.NameAttribute(x509.OID_COUNTRY_NAME, u'US'),
+        ])).public_key(
+            private_key.public_key()
+        ).serial_number(
+            777
+        ).not_valid_before(
+            datetime.datetime(1999, 1, 1)
+        ).not_valid_after(
+            datetime.datetime(2020, 1, 1)
+        ).add_extension(
+            x509.InhibitAnyPolicy(0), False
+        )
+
+        with pytest.raises(NotImplementedError):
+            builder.sign(backend, private_key, hashes.SHA1())
+
+
 class TestOpenSSLSerialisationWithOpenSSL(object):
     def test_pem_password_cb_buffer_too_small(self):
         ffi_cb, cb = backend._pem_password_cb(b"aa")
diff --git a/tests/test_x509.py b/tests/test_x509.py
index 98cf49be..ac20f649 100644
--- a/tests/test_x509.py
+++ b/tests/test_x509.py
@@ -22,7 +22,7 @@ from cryptography.hazmat.primitives import hashes, serialization
 from cryptography.hazmat.primitives.asymmetric import dsa, ec, rsa
 
 from .hazmat.primitives.fixtures_dsa import DSA_KEY_2048
-from .hazmat.primitives.fixtures_rsa import RSA_KEY_2048
+from .hazmat.primitives.fixtures_rsa import RSA_KEY_2048, RSA_KEY_512
 from .hazmat.primitives.test_ec import _skip_curve_unsupported
 from .utils import load_vectors_from_file
 
@@ -775,6 +775,350 @@ class TestRSACertificateRequest(object):
         assert hash(request1) == hash(request2)
         assert hash(request1) != hash(request3)
 
+    def test_build_cert(self, backend):
+        issuer_private_key = RSA_KEY_2048.private_key(backend)
+        subject_private_key = RSA_KEY_2048.private_key(backend)
+
+        not_valid_before = datetime.datetime(2002, 1, 1, 12, 1)
+        not_valid_after = datetime.datetime(2030, 12, 31, 8, 30)
+
+        builder = x509.CertificateBuilder().serial_number(
+            777
+        ).issuer_name(x509.Name([
+            x509.NameAttribute(x509.OID_COUNTRY_NAME, u'US'),
+            x509.NameAttribute(x509.OID_STATE_OR_PROVINCE_NAME, u'Texas'),
+            x509.NameAttribute(x509.OID_LOCALITY_NAME, u'Austin'),
+            x509.NameAttribute(x509.OID_ORGANIZATION_NAME, u'PyCA'),
+            x509.NameAttribute(x509.OID_COMMON_NAME, u'cryptography.io'),
+        ])).subject_name(x509.Name([
+            x509.NameAttribute(x509.OID_COUNTRY_NAME, u'US'),
+            x509.NameAttribute(x509.OID_STATE_OR_PROVINCE_NAME, u'Texas'),
+            x509.NameAttribute(x509.OID_LOCALITY_NAME, u'Austin'),
+            x509.NameAttribute(x509.OID_ORGANIZATION_NAME, u'PyCA'),
+            x509.NameAttribute(x509.OID_COMMON_NAME, u'cryptography.io'),
+        ])).public_key(
+            subject_private_key.public_key()
+        ).add_extension(
+            x509.BasicConstraints(ca=False, path_length=None), True,
+        ).add_extension(
+            x509.SubjectAlternativeName([x509.DNSName(u"cryptography.io")]),
+            critical=False,
+        ).not_valid_before(
+            not_valid_before
+        ).not_valid_after(
+            not_valid_after
+        )
+
+        cert = builder.sign(backend, issuer_private_key, hashes.SHA1())
+
+        assert cert.version is x509.Version.v3
+        assert cert.not_valid_before == not_valid_before
+        assert cert.not_valid_after == not_valid_after
+        basic_constraints = cert.extensions.get_extension_for_oid(
+            x509.OID_BASIC_CONSTRAINTS
+        )
+        assert basic_constraints.value.ca is False
+        assert basic_constraints.value.path_length is None
+        subject_alternative_name = cert.extensions.get_extension_for_oid(
+            x509.OID_SUBJECT_ALTERNATIVE_NAME
+        )
+        assert list(subject_alternative_name.value) == [
+            x509.DNSName(u"cryptography.io"),
+        ]
+
+
+class TestCertificateBuilder(object):
+    def test_issuer_name_must_be_a_name_type(self):
+        builder = x509.CertificateBuilder()
+
+        with pytest.raises(TypeError):
+            builder.issuer_name("subject")
+
+        with pytest.raises(TypeError):
+            builder.issuer_name(object)
+
+    def test_issuer_name_may_only_be_set_once(self):
+        name = x509.Name([
+            x509.NameAttribute(x509.OID_COUNTRY_NAME, u'US'),
+        ])
+        builder = x509.CertificateBuilder().issuer_name(name)
+
+        with pytest.raises(ValueError):
+            builder.issuer_name(name)
+
+    def test_subject_name_must_be_a_name_type(self):
+        builder = x509.CertificateBuilder()
+
+        with pytest.raises(TypeError):
+            builder.subject_name("subject")
+
+        with pytest.raises(TypeError):
+            builder.subject_name(object)
+
+    def test_subject_name_may_only_be_set_once(self):
+        name = x509.Name([
+            x509.NameAttribute(x509.OID_COUNTRY_NAME, u'US'),
+        ])
+        builder = x509.CertificateBuilder().subject_name(name)
+
+        with pytest.raises(ValueError):
+            builder.subject_name(name)
+
+    @pytest.mark.requires_backend_interface(interface=RSABackend)
+    @pytest.mark.requires_backend_interface(interface=X509Backend)
+    def test_public_key_must_be_public_key(self, backend):
+        private_key = RSA_KEY_2048.private_key(backend)
+        builder = x509.CertificateBuilder()
+
+        with pytest.raises(TypeError):
+            builder.public_key(private_key)
+
+    @pytest.mark.requires_backend_interface(interface=RSABackend)
+    @pytest.mark.requires_backend_interface(interface=X509Backend)
+    def test_public_key_may_only_be_set_once(self, backend):
+        private_key = RSA_KEY_2048.private_key(backend)
+        public_key = private_key.public_key()
+        builder = x509.CertificateBuilder().public_key(public_key)
+
+        with pytest.raises(ValueError):
+            builder.public_key(public_key)
+
+    def test_serial_number_must_be_an_integer_type(self):
+        with pytest.raises(TypeError):
+            x509.CertificateBuilder().serial_number(10.0)
+
+    def test_serial_number_must_be_non_negative(self):
+        with pytest.raises(ValueError):
+            x509.CertificateBuilder().serial_number(-10)
+
+    def test_serial_number_must_be_less_than_160_bits_long(self):
+        with pytest.raises(ValueError):
+            # 2 raised to the 160th power is actually 161 bits
+            x509.CertificateBuilder().serial_number(2 ** 160)
+
+    def test_serial_number_may_only_be_set_once(self):
+        builder = x509.CertificateBuilder().serial_number(10)
+
+        with pytest.raises(ValueError):
+            builder.serial_number(20)
+
+    def test_invalid_not_valid_after(self):
+        with pytest.raises(TypeError):
+            x509.CertificateBuilder().not_valid_after(104204304504)
+
+        with pytest.raises(TypeError):
+            x509.CertificateBuilder().not_valid_after(datetime.time())
+
+        with pytest.raises(ValueError):
+            x509.CertificateBuilder().not_valid_after(
+                datetime.datetime(1960, 8, 10)
+            )
+
+    def test_not_valid_after_may_only_be_set_once(self):
+        builder = x509.CertificateBuilder().not_valid_after(
+            datetime.datetime.now()
+        )
+
+        with pytest.raises(ValueError):
+            builder.not_valid_after(
+                datetime.datetime.now()
+            )
+
+    def test_invalid_not_valid_before(self):
+        with pytest.raises(TypeError):
+            x509.CertificateBuilder().not_valid_before(104204304504)
+
+        with pytest.raises(TypeError):
+            x509.CertificateBuilder().not_valid_before(datetime.time())
+
+        with pytest.raises(ValueError):
+            x509.CertificateBuilder().not_valid_before(
+                datetime.datetime(1960, 8, 10)
+            )
+
+    def test_not_valid_before_may_only_be_set_once(self):
+        builder = x509.CertificateBuilder().not_valid_before(
+            datetime.datetime.now()
+        )
+
+        with pytest.raises(ValueError):
+            builder.not_valid_before(
+                datetime.datetime.now()
+            )
+
+    def test_add_extension_checks_for_duplicates(self):
+        builder = x509.CertificateBuilder().add_extension(
+            x509.BasicConstraints(ca=False, path_length=None), True,
+        )
+
+        with pytest.raises(ValueError):
+            builder.add_extension(
+                x509.BasicConstraints(ca=False, path_length=None), True,
+            )
+
+    def test_add_unsupported_extension(self):
+        builder = x509.CertificateBuilder()
+
+        with pytest.raises(NotImplementedError):
+            builder.add_extension(object(), False)
+
+    @pytest.mark.requires_backend_interface(interface=RSABackend)
+    @pytest.mark.requires_backend_interface(interface=X509Backend)
+    def test_sign_with_unsupported_hash(self, backend):
+        private_key = RSA_KEY_2048.private_key(backend)
+        builder = x509.CertificateBuilder()
+
+        with pytest.raises(TypeError):
+            builder.sign(backend, private_key, object())
+
+    @pytest.mark.requires_backend_interface(interface=DSABackend)
+    @pytest.mark.requires_backend_interface(interface=X509Backend)
+    def test_sign_with_dsa_private_key_is_unsupported(self, backend):
+        if backend._lib.OPENSSL_VERSION_NUMBER >= 0x10001000:
+            pytest.skip("Requires an older OpenSSL. Must be < 1.0.1")
+
+        private_key = DSA_KEY_2048.private_key(backend)
+        builder = x509.CertificateBuilder()
+
+        with pytest.raises(NotImplementedError):
+            builder.sign(backend, private_key, hashes.SHA512())
+
+    @pytest.mark.requires_backend_interface(interface=EllipticCurveBackend)
+    @pytest.mark.requires_backend_interface(interface=X509Backend)
+    def test_sign_with_ec_private_key_is_unsupported(self, backend):
+        if backend._lib.OPENSSL_VERSION_NUMBER >= 0x10001000:
+            pytest.skip("Requires an older OpenSSL. Must be < 1.0.1")
+
+        _skip_curve_unsupported(backend, ec.SECP256R1())
+        private_key = ec.generate_private_key(ec.SECP256R1(), backend)
+        builder = x509.CertificateBuilder()
+
+        with pytest.raises(NotImplementedError):
+            builder.sign(backend, private_key, hashes.SHA512())
+
+    @pytest.mark.requires_backend_interface(interface=DSABackend)
+    @pytest.mark.requires_backend_interface(interface=X509Backend)
+    def test_build_cert_with_dsa_private_key(self, backend):
+        if backend._lib.OPENSSL_VERSION_NUMBER < 0x10001000:
+            pytest.skip("Requires a newer OpenSSL. Must be >= 1.0.1")
+
+        issuer_private_key = DSA_KEY_2048.private_key(backend)
+        subject_private_key = DSA_KEY_2048.private_key(backend)
+
+        not_valid_before = datetime.datetime(2002, 1, 1, 12, 1)
+        not_valid_after = datetime.datetime(2030, 12, 31, 8, 30)
+
+        builder = x509.CertificateBuilder().serial_number(
+            777
+        ).issuer_name(x509.Name([
+            x509.NameAttribute(x509.OID_COUNTRY_NAME, u'US'),
+        ])).subject_name(x509.Name([
+            x509.NameAttribute(x509.OID_COUNTRY_NAME, u'US'),
+        ])).public_key(
+            subject_private_key.public_key()
+        ).add_extension(
+            x509.BasicConstraints(ca=False, path_length=None), True,
+        ).add_extension(
+            x509.SubjectAlternativeName([x509.DNSName(u"cryptography.io")]),
+            critical=False,
+        ).not_valid_before(
+            not_valid_before
+        ).not_valid_after(
+            not_valid_after
+        )
+
+        cert = builder.sign(backend, issuer_private_key, hashes.SHA1())
+
+        assert cert.version is x509.Version.v3
+        assert cert.not_valid_before == not_valid_before
+        assert cert.not_valid_after == not_valid_after
+        basic_constraints = cert.extensions.get_extension_for_oid(
+            x509.OID_BASIC_CONSTRAINTS
+        )
+        assert basic_constraints.value.ca is False
+        assert basic_constraints.value.path_length is None
+        subject_alternative_name = cert.extensions.get_extension_for_oid(
+            x509.OID_SUBJECT_ALTERNATIVE_NAME
+        )
+        assert list(subject_alternative_name.value) == [
+            x509.DNSName(u"cryptography.io"),
+        ]
+
+    @pytest.mark.requires_backend_interface(interface=EllipticCurveBackend)
+    @pytest.mark.requires_backend_interface(interface=X509Backend)
+    def test_build_cert_with_ec_private_key(self, backend):
+        if backend._lib.OPENSSL_VERSION_NUMBER < 0x10001000:
+            pytest.skip("Requires a newer OpenSSL. Must be >= 1.0.1")
+
+        _skip_curve_unsupported(backend, ec.SECP256R1())
+        issuer_private_key = ec.generate_private_key(ec.SECP256R1(), backend)
+        subject_private_key = ec.generate_private_key(ec.SECP256R1(), backend)
+
+        not_valid_before = datetime.datetime(2002, 1, 1, 12, 1)
+        not_valid_after = datetime.datetime(2030, 12, 31, 8, 30)
+
+        builder = x509.CertificateBuilder().serial_number(
+            777
+        ).issuer_name(x509.Name([
+            x509.NameAttribute(x509.OID_COUNTRY_NAME, u'US'),
+        ])).subject_name(x509.Name([
+            x509.NameAttribute(x509.OID_COUNTRY_NAME, u'US'),
+        ])).public_key(
+            subject_private_key.public_key()
+        ).add_extension(
+            x509.BasicConstraints(ca=False, path_length=None), True,
+        ).add_extension(
+            x509.SubjectAlternativeName([x509.DNSName(u"cryptography.io")]),
+            critical=False,
+        ).not_valid_before(
+            not_valid_before
+        ).not_valid_after(
+            not_valid_after
+        )
+
+        cert = builder.sign(backend, issuer_private_key, hashes.SHA1())
+
+        assert cert.version is x509.Version.v3
+        assert cert.not_valid_before == not_valid_before
+        assert cert.not_valid_after == not_valid_after
+        basic_constraints = cert.extensions.get_extension_for_oid(
+            x509.OID_BASIC_CONSTRAINTS
+        )
+        assert basic_constraints.value.ca is False
+        assert basic_constraints.value.path_length is None
+        subject_alternative_name = cert.extensions.get_extension_for_oid(
+            x509.OID_SUBJECT_ALTERNATIVE_NAME
+        )
+        assert list(subject_alternative_name.value) == [
+            x509.DNSName(u"cryptography.io"),
+        ]
+
+    @pytest.mark.requires_backend_interface(interface=RSABackend)
+    @pytest.mark.requires_backend_interface(interface=X509Backend)
+    def test_build_cert_with_rsa_key_too_small(self, backend):
+        issuer_private_key = RSA_KEY_512.private_key(backend)
+        subject_private_key = RSA_KEY_512.private_key(backend)
+
+        not_valid_before = datetime.datetime(2002, 1, 1, 12, 1)
+        not_valid_after = datetime.datetime(2030, 12, 31, 8, 30)
+
+        builder = x509.CertificateBuilder().serial_number(
+            777
+        ).issuer_name(x509.Name([
+            x509.NameAttribute(x509.OID_COUNTRY_NAME, u'US'),
+        ])).subject_name(x509.Name([
+            x509.NameAttribute(x509.OID_COUNTRY_NAME, u'US'),
+        ])).public_key(
+            subject_private_key.public_key()
+        ).not_valid_before(
+            not_valid_before
+        ).not_valid_after(
+            not_valid_after
+        )
+
+        with pytest.raises(ValueError):
+            builder.sign(backend, issuer_private_key, hashes.SHA512())
+
 
 @pytest.mark.requires_backend_interface(interface=X509Backend)
 class TestCertificateSigningRequestBuilder(object):