inkylatenoth inkylatenoth@protonmail.com writes:

Whilst implementing v3 hidden services myself I found some inconsistencies between the specs and the current implementation. I wanted to share these in case someone from the Tor organization wants to update the specs and/or the implementation.

Hello inkylatenoth!

That's a great post and thanks for catching all these issues and innacuracies! We are definitely interested in consistency and fixing the spec (and implementation if needed).

# rend-spec-v3.txt

## 2.4

• after decrypting the `superencrypted' object from a descriptor, the resulting document does not end with the NL character. This means that it does not strictly conform to the document meta-format described in section 1.2 of dir-spec.txt.

Hmm... This might be worth fixing on the implementation if possible (and if it won't break things). Otherwise, let's patch the spec.

## A.2

• the blinded key param is defined as H(BLIND_STRING | A | s | B | N). In practice I found that I had to add a null byte after BLIND_STRING in order to reach the same value as the C implementation:

  param = H(BLIND_STRING | INT_1(\x00) | A | s | B | N)

  In all other cases where a string constant is used like this (e.g. computing the nonce N above), I found that the trailing null byte is not required.

Ouch. This might be an artifact of the way strings are implemented in C.

I guess a spec patch might be the right thing to do here, otherwise too much stuff will break.

• when clamping the blinding factor, the second bitwise operation is `param[31] &= 127' in the spec but `param[31] &= 63' in the C implementation. These are equivalent in practice when followed by the third operation (`param[31] |= 64'), but it might be nice to use a consistent representation for the benefit of human readers.

Hmm... Yeah there are various ways to do the clamping for ed25519 keys.

I think we should edit the spec to reflect the clamping we do on the code.

# 220-ecc-ids-keys.txt

# 2.1

• 'The signature is formed by signing the first N-64 bytes of the certificate prefixed with the string "Tor node signing key certificate v1".' I found this to be false; the signatures only validate without the string prefix.

Ouch... I think we should edit the spec and consider if there are any security risks here.

## A.1

• I realized that the certificate types here are outdated. The signing-key extension is listed as type [04], when in rend-spec-v3.txt and the C implementation it is type [08].

Let's fix the spec here too...

---

Inkylatenoth, let me know if you are interested in drafting a spec/code patch for the issues you found!!! If you are not interested, I can try to do them myself at some point in the next weeks (been pretty busy with stuff lately).

Also, let us know if your independent implementation is a public thing we should know about. Seems interesting :)

Thanks again!