[tor-commits] [stem/master] Expose classes for HSv3 blinded keys.

[atagar at torproject.org]

commit 7358ca1263cdbc88161c67103fec4201242036cc
Author: George Kadianakis <desnacked at riseup.net>
Date:   Thu Oct 10 20:50:56 2019 +0300

    Expose classes for HSv3 blinded keys.
    
    HSv3 blinded keys are not compatible with hazmat keys because their private key
    format is different. This means that we cannot store them in a hazmat or pynacl
    data structure.
    
    This commit adds an abstraction layer on top of the ed25519 blinded keys that
    implement the fundamental methods of the hazmat ed25519 module (duck typing),
    so that they can be used interchangeably to sign and verify documents.
---
 stem/descriptor/hsv3_crypto.py | 59 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 59 insertions(+)

diff --git a/stem/descriptor/hsv3_crypto.py b/stem/descriptor/hsv3_crypto.py
new file mode 100644
index 00000000..bd090265
--- /dev/null
+++ b/stem/descriptor/hsv3_crypto.py
@@ -0,0 +1,59 @@
+import base64
+import hashlib
+import struct
+import os
+
+from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PublicKey
+from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
+from cryptography.hazmat.backends import default_backend
+from cryptography.hazmat.primitives import serialization
+
+import stem.descriptor.ed25519_exts_ref as ed25519_exts_ref
+import stem.descriptor.slow_ed25519 as slow_ed25519
+
+"""
+HSv3 Key blinding
+
+Expose classes for HSv3 blinded keys which can mimic the hazmat ed25519
+public/private key classes, so that we can use them interchangeably in the
+certificate module.
+
+- HSv3PublicBlindedKey: represents the public blinded ed25519 key of an onion
+  service and should expose a public_bytes() method and a verify() method.
+
+- HSv3PrivateBlindedKey: represents the private part of a blinded ed25519 key
+  of an onion service and should expose a public_key() method and a sign() method.
+"""
+class HSv3PrivateBlindedKey(object):
+    def __init__(self, hazmat_private_key, blinding_param):
+        secret_seed = hazmat_private_key.private_bytes(encoding=serialization.Encoding.Raw,
+                                                       format=serialization.PrivateFormat.Raw,
+                                                       encryption_algorithm=serialization.NoEncryption())
+        assert(len(secret_seed) == 32)
+
+        expanded_identity_priv_key = ed25519_exts_ref.expandSK(secret_seed)
+        identity_public_key = slow_ed25519.publickey(secret_seed)
+
+        self.blinded_secret_key = ed25519_exts_ref.blindESK(expanded_identity_priv_key, blinding_param)
+        blinded_public_key = ed25519_exts_ref.blindPK(identity_public_key, blinding_param)
+        self.blinded_public_key = HSv3PublicBlindedKey(blinded_public_key)
+
+    def public_key(self):
+        return self.blinded_public_key
+
+    def sign(self, msg):
+        return ed25519_exts_ref.signatureWithESK(msg, self.blinded_secret_key, self.blinded_public_key.public_bytes())
+
+class HSv3PublicBlindedKey(object):
+    def __init__(self, public_key):
+        self.public_key = public_key
+
+    def public_bytes(self, encoding=None, format=None):
+        return self.public_key
+
+    def verify(self, signature, message):
+        """
+        raises exception if sig not valid
+        """
+        ext.slow_ed25519.checkvalid(signature, message, self.public_key)
+