ed25519 support in x509 certificate builder (#4937)

* ed25519 support in x509 certificate builder

This adds minimal ed25519 support. More to come.

* Apply suggestions from code review

Co-Authored-By: Alex Gaynor <alex.gaynor@gmail.com>

3 files changed, 23 insertions, 7 deletions

diff --git a/src/cryptography/hazmat/backends/openssl/backend.py b/src/cryptography/hazmat/backends/openssl/backend.py
index ca0a11bd..c24d334a 100644
--- a/src/cryptography/hazmat/backends/openssl/backend.py
+++ b/src/cryptography/hazmat/backends/openssl/backend.py
@@ -798,7 +798,12 @@ class Backend(object):
     def create_x509_certificate(self, builder, private_key, algorithm):
         if not isinstance(builder, x509.CertificateBuilder):
             raise TypeError('Builder type mismatch.')
-        if not isinstance(algorithm, hashes.HashAlgorithm):
+        if isinstance(private_key, ed25519.Ed25519PrivateKey):
+            if algorithm is not None:
+                raise ValueError(
+                    "algorithm must be None when signing via ed25519"
+                )
+        elif not isinstance(algorithm, hashes.HashAlgorithm):
             raise TypeError('Algorithm must be a registered hash algorithm.')
 
         if (
@@ -806,11 +811,11 @@ class Backend(object):
             isinstance(private_key, rsa.RSAPrivateKey)
         ):
             raise ValueError(
-                "MD5 is not a supported hash algorithm for EC/DSA certificates"
+                "MD5 is only (reluctantly) supported for RSA certificates"
             )
 
         # Resolve the signature algorithm.
-        evp_md = self._evp_md_non_null_from_algorithm(algorithm)
+        evp_md = self._evp_md_x509_null_if_ed25519(private_key, algorithm)
 
         # Create an empty certificate.
         x509_cert = self._lib.X509_new()
@@ -878,6 +883,13 @@ class Backend(object):
 
         return _Certificate(self, x509_cert)
 
+    def _evp_md_x509_null_if_ed25519(self, private_key, algorithm):
+        if isinstance(private_key, ed25519.Ed25519PrivateKey):
+            # OpenSSL requires us to pass NULL for EVP_MD for ed25519 signing
+            return self._ffi.NULL
+        else:
+            return self._evp_md_non_null_from_algorithm(algorithm)
+
     def _set_asn1_time(self, asn1_time, time):
         if time.year >= 2050:
             asn1_str = time.strftime('%Y%m%d%H%M%SZ').encode('ascii')
diff --git a/src/cryptography/x509/base.py b/src/cryptography/x509/base.py
index 63c2e3c6..dc7eee94 100644
--- a/src/cryptography/x509/base.py
+++ b/src/cryptography/x509/base.py
@@ -12,7 +12,7 @@ from enum import Enum
 import six
 
 from cryptography import utils
-from cryptography.hazmat.primitives.asymmetric import dsa, ec, rsa
+from cryptography.hazmat.primitives.asymmetric import dsa, ec, ed25519, rsa
 from cryptography.x509.extensions import Extension, ExtensionType
 from cryptography.x509.name import Name
 
@@ -474,9 +474,10 @@ class CertificateBuilder(object):
         Sets the requestor's public key (as found in the signing request).
         """
         if not isinstance(key, (dsa.DSAPublicKey, rsa.RSAPublicKey,
-                                ec.EllipticCurvePublicKey)):
+                                ec.EllipticCurvePublicKey,
+                                ed25519.Ed25519PublicKey)):
             raise TypeError('Expecting one of DSAPublicKey, RSAPublicKey,'
-                            ' or EllipticCurvePublicKey.')
+                            ' EllipticCurvePublicKey, or Ed25519PublicKey.')
         if self._public_key is not None:
             raise ValueError('The public key may only be set once.')
         return CertificateBuilder(
diff --git a/src/cryptography/x509/oid.py b/src/cryptography/x509/oid.py
index 1bfe58ca..ab01d67b 100644
--- a/src/cryptography/x509/oid.py
+++ b/src/cryptography/x509/oid.py
@@ -96,6 +96,7 @@ class SignatureAlgorithmOID(object):
     DSA_WITH_SHA1 = ObjectIdentifier("1.2.840.10040.4.3")
     DSA_WITH_SHA224 = ObjectIdentifier("2.16.840.1.101.3.4.3.1")
     DSA_WITH_SHA256 = ObjectIdentifier("2.16.840.1.101.3.4.3.2")
+    ED25519 = ObjectIdentifier("1.3.101.112")
 
 
 _SIG_OIDS_TO_HASH = {
@@ -113,7 +114,8 @@ _SIG_OIDS_TO_HASH = {
     SignatureAlgorithmOID.ECDSA_WITH_SHA512: hashes.SHA512(),
     SignatureAlgorithmOID.DSA_WITH_SHA1: hashes.SHA1(),
     SignatureAlgorithmOID.DSA_WITH_SHA224: hashes.SHA224(),
-    SignatureAlgorithmOID.DSA_WITH_SHA256: hashes.SHA256()
+    SignatureAlgorithmOID.DSA_WITH_SHA256: hashes.SHA256(),
+    SignatureAlgorithmOID.ED25519: None,
 }
 
 
@@ -181,6 +183,7 @@ _OID_NAMES = {
     SignatureAlgorithmOID.DSA_WITH_SHA1: "dsa-with-sha1",
     SignatureAlgorithmOID.DSA_WITH_SHA224: "dsa-with-sha224",
     SignatureAlgorithmOID.DSA_WITH_SHA256: "dsa-with-sha256",
+    SignatureAlgorithmOID.ED25519: "ed25519",
     ExtendedKeyUsageOID.SERVER_AUTH: "serverAuth",
     ExtendedKeyUsageOID.CLIENT_AUTH: "clientAuth",
     ExtendedKeyUsageOID.CODE_SIGNING: "codeSigning",