Stop validating keys in ECDH exchange (#8490)

The theory here is that we're already doing sufficient validation key loading, and this is purely duplicative.

Note that there's at least _some_ validationg that was previously occurring only ECDH, the LowOrderPublic check that can be seen in wycheproof.

src/_cffi_src/openssl/evp.py

@@ -38,6 +38,7 @@ static const long Cryptography_HAS_EVP_DIGESTFINAL_XOF;
 static const long Cryptography_HAS_300_FIPS;
 static const long Cryptography_HAS_300_EVP_CIPHER;
 static const long Cryptography_HAS_EVP_PKEY_DH;
+static const long Cryptography_HAS_EVP_PKEY_SET_PEER_EX;
 """
 
 FUNCTIONS = """
@@ -119,6 +120,7 @@ int EVP_PKEY_keygen_init(EVP_PKEY_CTX *);
 int EVP_PKEY_keygen(EVP_PKEY_CTX *, EVP_PKEY **);
 int EVP_PKEY_derive_init(EVP_PKEY_CTX *);
 int EVP_PKEY_derive_set_peer(EVP_PKEY_CTX *, EVP_PKEY *);
+int EVP_PKEY_derive_set_peer_ex(EVP_PKEY_CTX *, EVP_PKEY *, int);
 int EVP_PKEY_derive(EVP_PKEY_CTX *, unsigned char *, size_t *);
 
 int EVP_PKEY_id(const EVP_PKEY *);
@@ -198,6 +200,13 @@ static const long Cryptography_HAS_RAW_KEY = 1;
 static const long Cryptography_HAS_EVP_DIGESTFINAL_XOF = 1;
 #endif
 
+#if CRYPTOGRAPHY_OPENSSL_300_OR_GREATER
+static const long Cryptography_HAS_EVP_PKEY_SET_PEER_EX = 1;
+#else
+static const long Cryptography_HAS_EVP_PKEY_SET_PEER_EX = 0;
+int (*EVP_PKEY_derive_set_peer_ex)(EVP_PKEY_CTX *, EVP_PKEY *, int) = NULL;
+#endif
+
 /* This is tied to X25519 support so we reuse the Cryptography_HAS_X25519
    conditional to remove it. OpenSSL 1.1.0 didn't have this define, but
    1.1.1 will when it is released. We can remove this in the distant

src/cryptography/hazmat/backends/openssl/utils.py

@@ -17,10 +17,16 @@ def _evp_pkey_derive(backend: "Backend", evp_pkey, peer_public_key) -> bytes:
     ctx = backend._ffi.gc(ctx, backend._lib.EVP_PKEY_CTX_free)
     res = backend._lib.EVP_PKEY_derive_init(ctx)
     backend.openssl_assert(res == 1)
-    res = backend._lib.EVP_PKEY_derive_set_peer(ctx, peer_public_key._evp_pkey)
-    if res != 1:
-        errors_with_text = backend._consume_errors_with_text()
-        raise ValueError("Error computing shared key.", errors_with_text)
+
+    if backend._lib.Cryptography_HAS_EVP_PKEY_SET_PEER_EX:
+        res = backend._lib.EVP_PKEY_derive_set_peer_ex(
+            ctx, peer_public_key._evp_pkey, 0
+        )
+    else:
+        res = backend._lib.EVP_PKEY_derive_set_peer(
+            ctx, peer_public_key._evp_pkey
+        )
+    backend.openssl_assert(res == 1)
 
     keylen = backend._ffi.new("size_t *")
     res = backend._lib.EVP_PKEY_derive(ctx, backend._ffi.NULL, keylen)

src/cryptography/hazmat/bindings/openssl/_conditional.py

@@ -271,6 +271,10 @@ def cryptography_has_get_extms_support() -> typing.List[str]:
     return ["SSL_get_extms_support"]
 
 
+def cryptography_has_evp_pkey_set_peer_ex() -> typing.List[str]:
+    return ["EVP_PKEY_derive_set_peer_ex"]
+
+
 # This is a mapping of
 # {condition: function-returning-names-dependent-on-that-condition} so we can
 # loop over them and delete unsupported names at runtime. It will be removed
@@ -322,4 +326,7 @@ CONDITIONAL_NAMES = {
         cryptography_has_ssl_op_ignore_unexpected_eof
     ),
     "Cryptography_HAS_GET_EXTMS_SUPPORT": cryptography_has_get_extms_support,
+    "Cryptography_HAS_EVP_PKEY_SET_PEER_EX": (
+        cryptography_has_evp_pkey_set_peer_ex
+    ),
 }

tests/wycheproof/test_ecdh.py

@@ -86,13 +86,7 @@ def test_ecdh(backend, wycheproof):
     except UnsupportedAlgorithm:
         return
 
-    if wycheproof.valid or (
-        wycheproof.acceptable
-        and not (
-            wycheproof.has_flag("LowOrderPublic")
-            and backend._lib.CRYPTOGRAPHY_OPENSSL_300_OR_GREATER
-        )
-    ):
+    if wycheproof.valid or wycheproof.acceptable:
         computed_shared = private_key.exchange(ec.ECDH(), public_key)
         expected_shared = binascii.unhexlify(wycheproof.testcase["shared"])
         assert computed_shared == expected_shared