Terence Eden
🆕 blog! “A simple(ish) guide to verifying HTTP Message Signatures in PHP”
Mastodon makes heavy use of HTTP Message Signatures. They're a newish almost-standard which allows a server to verify that a request made to it came from the person who sent it. This is a quick example to show how to verify these signatures using P…
👀 Read more: https://shkspr.mobi/blog/2024/02/a-simpleish-guide-to-verifying-http-message-signatures-in-php/
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#ActivityPub #cryptography #http #mastodon #security
Eugen Rochko
@Edent @evan The key points to the actor, and the actor points to the key.
Terence Eden
@evan but how do I test that?
If the keyID is "example.com/keys/123456" that might lead to a fake page which claims to be from the user in the body.
(Feel free to tell me I've overthinking this.)
Evan Prodromou
@Edent and no, absolutely only use the key in the header!
Evan Prodromou
@Edent The "owner" of the key should be identical to the actor.
Terence Eden
Another #ActivityPub question about verifying signatures.
A header contains:
`keyID="example.com/user/1#main-key`
But the body of the message might have:
"actor": "example.com/user/2"
How do I check that that message has been signed by the actor in the body?
The URls might not be in the same format. So I guess back to webfinger to request the key from the actor - ignoring the one provided in the header?
Terence Eden
@Gargron @evan
I see. So both need to be verified?
Or, to put it another way, the location of the key should be taken from the actor, not the signature header?
Eugen Rochko
@Edent @erincandescent @evan Presumably at some point you'll want to resolve the actor that owns the key anyway (to attach whatever is in the payload to a proper profile). When you resolve the actor, verify that it points to the key that points to the actor.
Terence Eden
@erincandescent @evan @Gargron
I'm sorry if I'm being thick (and feel free to tell me) - but...
I get a signature with a keyID of `evil.com/evil#sig`
I get the public key from evil.com
I verify the time, digest, and signature all match.
The body of the message says actor example.com/edent Creates...
When / how do I then check that edent's public key is the same as the one in the keyID?
erincandescent
Evan Prodromou
@Edent @Gargron I'm not sure how to convey to you how important it is to ONLY use the keyId field in the signature header for validating the signature. Do not go spelunking around in the body of the message. If you can't figure it out from the Signature header, the signature is invalid.
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