From 991b7e310497bea7583eb62d08a83c274e2b2d09 Mon Sep 17 00:00:00 2001
From: Jean-Paul Calderone <exarkun@twistedmatrix.com>
Date: Tue, 28 Apr 2020 10:30:53 -0400
Subject: [PATCH] improve the veracity of the DummyRedeemer's passes
Previously the implementation allowed colliding passes if unblinded tokens
shared a prefix or suffix.
---
src/_zkapauthorizer/controller.py | 19 ++++++++++++++-----
1 file changed, 14 insertions(+), 5 deletions(-)
diff --git a/src/_zkapauthorizer/controller.py b/src/_zkapauthorizer/controller.py
index b438680..687316e 100644
--- a/src/_zkapauthorizer/controller.py
+++ b/src/_zkapauthorizer/controller.py
@@ -41,6 +41,10 @@ from base64 import (
b64encode,
b64decode,
)
+from hashlib import (
+ sha256,
+)
+
import attr
from zope.interface import (
@@ -346,11 +350,16 @@ class DummyRedeemer(object):
def tokens_to_passes(self, message, unblinded_tokens):
def token_to_pass(token):
- # Smear the unblinded token value across the two new values we
- # need.
- bs = b64decode(token.unblinded_token.encode("ascii"))
- preimage = bs[:48] + b"x" * 16
- signature = bs[48:] + b"y" * 16
+ # Generate distinct strings based on the unblinded token which we
+ # can include in the resulting Pass. This ensures the pass values
+ # will be unique if and only if the unblinded tokens were unique
+ # (barring improbable hash collisions).
+ token_digest = sha256(
+ token.unblinded_token.encode("ascii")
+ ).hexdigest().encode("ascii")
+
+ preimage = b"preimage-" + token_digest[len(b"preimage-"):]
+ signature = b"signature-" + token_digest[len(b"signature-"):]
return Pass(
b64encode(preimage).decode("ascii"),
b64encode(signature).decode("ascii"),
--
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