mirror of
https://gitlab.torproject.org/tpo/core/tor.git
synced 2024-11-20 10:12:15 +01:00
d230827912
Tor doesn't use SVN anymore, making $Revision$, $Id$ and $Date$ meaningless. Remove them without replacement.
103 lines
3.6 KiB
Plaintext
103 lines
3.6 KiB
Plaintext
Filename: 157-specific-cert-download.txt
|
|
Title: Make certificate downloads specific
|
|
Author: Nick Mathewson
|
|
Created: 2-Dec-2008
|
|
Status: Accepted
|
|
Target: 0.2.1.x
|
|
|
|
History:
|
|
|
|
2008 Dec 2, 22:34
|
|
Changed name of cross certification field to match the other authority
|
|
certificate fields.
|
|
|
|
Status:
|
|
|
|
As of 0.2.1.9-alpha:
|
|
Cross-certification is implemented for new certificates, but not yet
|
|
required. Directories support the tor/keys/fp-sk urls.
|
|
|
|
Overview:
|
|
|
|
Tor's directory specification gives two ways to download a certificate:
|
|
by its identity fingerprint, or by the digest of its signing key. Both
|
|
are error-prone. We propose a new download mechanism to make sure that
|
|
clients get the certificates they want.
|
|
|
|
Motivation:
|
|
|
|
When a client wants a certificate to verify a consensus, it has two choices
|
|
currently:
|
|
- Download by identity key fingerprint. In this case, the client risks
|
|
getting a certificate for the same authority, but with a different
|
|
signing key than the one used to sign the consensus.
|
|
|
|
- Download by signing key fingerprint. In this case, the client risks
|
|
getting a forged certificate that contains the right signing key
|
|
signed with the wrong identity key. (Since caches are willing to
|
|
cache certs from authorities they do not themselves recognize, the
|
|
attacker wouldn't need to compromise an authority's key to do this.)
|
|
|
|
Current solution:
|
|
|
|
Clients fetch by identity keys, and re-fetch with backoff if they don't get
|
|
certs with the signing key they want.
|
|
|
|
Proposed solution:
|
|
|
|
Phase 1: Add a URL type for clients to download certs by identity _and_
|
|
signing key fingerprint. Unless both fields match, the client doesn't
|
|
accept the certificate(s). Clients begin using this method when their
|
|
randomly chosen directory cache supports it.
|
|
|
|
Phase 1A: Simultaneously, add a cross-certification element to
|
|
certificates.
|
|
|
|
Phase 2: Once many directory caches support phase 1, clients should prefer
|
|
to fetch certificates using that protocol when available.
|
|
|
|
Phase 2A: Once all authorities are generating cross-certified certificates
|
|
as in phase 1A, require cross-certification.
|
|
|
|
Specification additions:
|
|
|
|
The key certificate whose identity key fingerprint is <F> and whose signing
|
|
key fingerprint is <S> should be available at:
|
|
|
|
http://<hostname>/tor/keys/fp-sk/<F>-<S>.z
|
|
|
|
As usual, clients may request multiple certificates using:
|
|
|
|
http://<hostname>/tor/keys/fp-sk/<F1>-<S1>+<F2>-<S2>.z
|
|
|
|
Clients SHOULD use this format whenever they know both key fingerprints for
|
|
a desired certificate.
|
|
|
|
|
|
Certificates SHOULD contain the following field (at most once):
|
|
|
|
"dir-key-crosscert" NL CrossSignature NL
|
|
|
|
where CrossSignature is a signature, made using the certificate's signing
|
|
key, of the digest of the PKCS1-padded hash of the certificate's identity
|
|
key. For backward compatibility with broken versions of the parser, we
|
|
wrap the base64-encoded signature in -----BEGIN ID SIGNATURE---- and
|
|
-----END ID SIGNATURE----- tags. (See bug 880.) Implementations MUST allow
|
|
the "ID " portion to be omitted, however.
|
|
|
|
When encountering a certificate with a dir-key-crosscert entry,
|
|
implementations MUST verify that the signature is a correct signature of
|
|
the hash of the identity key using the signing key.
|
|
|
|
(In a future version of this specification, dir-key-crosscert entries will
|
|
be required.)
|
|
|
|
Why cross-certify too?
|
|
|
|
Cross-certification protects clients who haven't updated yet, by reducing
|
|
the number of caches that are willing to hold and serve bogus certificates.
|
|
|
|
References:
|
|
|
|
This is related to part 2 of bug 854.
|