Here's a proposal that's been kicking around on my laptop for a while. I cleaned it up to use the revised certificate format from the revised proposal 220.
My math and crypto claims might be incorrect here; I'm sure hoping somebody will tell me if they are.
Filename: 228-cross-certification-onionkeys.txt Title: Cross-certifying identity keys with onion keys Author: Nick Mathewson Created: 25 February 2014 Status: Open
0. Abstract
In current Tor router descriptor designs, routers prove ownership of an identity key (by signing the router descriptors), but not of their onion keys. This document describes a method for them to do so.
1. Introduction.
Signing router descriptors with identity keys prevents attackers from impersonating a server and advertising their own onion keys and IP addresses. That's good.
But there's nothing in Tor right now that effectively stops you (an attacker) from listing somebody else's public onion key in your descriptor. If you do, you can't actually recover any keys negotiated using that key, and you can't MITM circuits made with that key (since you don't have the private key). You _could_ do something weird in the TAP protocol where you .
Nonetheless, it's probably undesirable that this is possible at all. Just because it isn't obvious today how to exploit this doesn't mean it will never be possible.
2. Cross-certifying identities with onion keys
2.1. What to certify
Once proposal 220 is implemented, we'll sign our Ed25519 identity key as described in proposal 220. Since the Ed25519 identity key certifies the RSA key, there's no strict need to certify both separately.
On the other hand, this proposal may be implemented before proposal 220. If it is, we'll need a way for it to certify the RSA1024 key too.
2.2. TAP onion keys
We add to each router descriptor a new element, "onion-key-crosscert", containing a RSA signature of:
A SHA1 hash of the identity key [20 bytes] The Ed25519 identity key, if any [32 bytes]
If there is no ed25519 identity key, or if in some future version there is no RSA identity key, the corresponding field must be zero-filled.
Parties verifying this signature MUST allow additional data beyond the 52 bytes listed above.
2.3. ntor onion keys
Here, we need to convert the ntor key to an ed25519 key for signing. See the appendix A for how to do that. We'll also need to transmit a sign bit.
We can add an element "ntor-onion-key-crosscert", containing an Ed25519 certificate in the format from proposal 220 section 2.1, with a sign indicator to indicate which ed25519 public key to use to check the key:
"ntor-onion-key-crosscert" SP SIGNBIT SP CERT NL
SIGNBIT = "0" / "1"
Note that this cert format has 32 bytes of of redundant data, since it includes the identity key an extra time. That seems okay to me.
The TYPE field in this certificate should be set to [0A] - ntor onion key cross-certifying ntor identity key
3. Authority behavior
Authorities should reject any router descriptor with an invalid onion-key-crosscert element or ntor-onion-key-crosscert element.
Both elements should be required on any cert containing an ed25519 identity key.
See section 3.1 of proposal 220 for rules requiring routers to eventually have ed25519 keys.
4. Performance impact
Routers do not generate new descriptors frequently enough for them to need to
Checking an extra ed25519 signature when parsing a descriptor is very cheap, since we can use batch signature checking.
The point decompression algorithm will require us to calculate 1/(u+1), which costs as much as an exponentiation in GF(2^255-19).
Checking an RSA1024 signature is also cheap, since we use small public exponents.
Adding an extra RSA signature and an extra ed25519 signature to each descriptor will make each descriptor, after compression, about 128+100 bytes longer. (Compressed base64-encoded random bytes are about as long as the original random bytes.) Most clients don't download raw descriptors, though, so it shouldn't matter too much.
A. Converting a curve25519 public key to an ed25519 public key
Given a curve25519 x-coordinate (u), we can get the y coordinate of the ed25519 key using
y = (u-1)/(u+1)
and then we can apply the usual ed25519 point decomporession algorithm to find the x coordinate of the ed25519 point to check signatures with.
Note that we need the sign of the X coordinate to do this operation; otherwise, we'll have two possible X coordinates that might have correspond to the key. Therefore, we need the 'sign' of the X coordinate, as used by the ed25519 key expansion algorithm.
To get the sign, the easiest way is to take the same private key, feed it to the ed25519 public key generation algorithm, and see what the sign is.
B. Security notes
It would be very bad for security if we provided a diffie-hellman oracle for our curve25519 ntor keys. Fortunately, we don't, since nobody else can influence the certificate contents.