Terence Eden’s Blog

2024-02-26 12:34:28

HTTP Signature Infinite Loop?
https://shkspr.mobi/blog/2024/02/http-signature-infinite-loop/

I'm trying to get my head round HTTP Signatures as they're used extensively in the Fediverse.

Conceptually, they're relatively straightforward.

You send me a normal HTTP request. For example, you want to POST something to https://example.com/data

You send me these headers:

POST /dataHost: example.comDate: Sat, 24 Feb 2024 14:43:48 GMTAccept-Encoding: gzipDigest: SHA-256=aaC57TDzM0Wq+50We2TkCsdMDvdqON92edg7KI+Hk8M=Content-Type: application/activity+jsonSignature: keyId="https://your_website.biz/publicKey",algorithm="rsa-sha256",headers="(request-target) host date digest content-type",signature="JGQ53kEoIiMWRp9By9jajVGCOCu4n7XBeiA1uY5xLcnAxL2Y1GIgU/...=="Connection: Keep-AliveContent-Length: 751

In order to verify the contents of the message, I need to do three things:

1. Check the SHA-256 hash of the message matches the content of the "Digest" header.
2. Check the timestamp is somewhat fresh.
3. Check the signature matches.

The first is simple: base64_encode( hash( "sha256", $request_body, true ) ).
The second is a matter of opinion. I might be happy to receive messages from the distant past or far in the future. For the sake of a little clock drift, let's allow 60 seconds either way.
The third gets complicated.

First, I need to get the public key published at keyId="https://your_website.biz/publicKey".

Next, I need to know which algorithm is being used to sign the headers: algorithm="rsa-sha256"

Then, I need to know which headers - and in what order - are being signed: headers="(request-target) host date digest content-type"

So I create a string using the received details which matches those headers in that specific order:

(request-target) POST /dataHost: example.comDate: Sat, 24 Feb 2024 14:43:48 GMTDigest: SHA-256=aaC57TDzM0Wq+50We2TkCsdMDvdqON92edg7KI+Hk8M=Content-Type: application/activity+json

I can verify if the signature - signature="JGQ53kEoIiMWRp9By9jajVGCOCu4n7XBeiA1uY5xLcnAxL2Y1GIgU/...==" matches by:

openssl_verify(    $headersString,     $signature,     $publicKey,     $algorithm);

If that's TRUE then all is well.

But can you spot the implicit problem?

How do I get your server's public key?

I just GET https://your_website.biz/publicKey - but if your server uses something like Authorised Fetch then I have to sign my request to you.

Which means your server will need to validate my signature by obtaining my public key. Which it will get by signing a request and sending it to me. Which, before I return my public key, I will need to validate your signature by obtaining your public key. Which I will get by signing a request... and so on.

This deadlock loop is documented. The usual way around it is either for the sending server to use an instance-specific signature which can be retrieved by an unsigned request, or to allow any unsigned request to access a user's public key.

I get why things happen this way - I just wish it were easier to implement!

https://shkspr.mobi/blog/2024/02/http-signature-infinite-loop/

#ActivityPub #CyberSecurity #encryption #fediverse #http

Authorized Fetch and the Instance Actor

Using an instance actor for mitigating the authorized fetch key retrieval bug seems strange to me so I dug into the historical record to try to understand how this all evolved.

^SocialHub