How to Compute an HTTP Signature for Mastodon (and an example in NodeJS)

I am reading this documentation from Mastodon.

And from it, I understand that Mastodon requires an HTTP Signature, signing at least these headers:
(request-target) host date digest

If your client is written in JavaScript and runs on Nodejs, an example for how to build a signature is given on the npmjs site.

But I believe this example is out of date. It does not use the (request-target) pseudo header. So that’s not gonna work.

So what must you do? Go back to the Mastodon documentation. Unfortunately, that too, is either out of date or confusing. For a post request, the mastodon documentation states that you must first compute the “RSA-SHA256 digest hash of your request’s body”. This is not correct. There is no such thing as “RSA-SHA256 digest”! RSA-SHA256 is not the name of a digest. Message Digests include: SHA1, SHA256, MD5 (old and insecure at this point) and others. According to my reading of the code, Mastodon supports only SHA-256 digests. The documentation should state that you must compute the “SHA256 digest”. (There is no RSA key involved in computing a digest).

Regardless of the digest algorithm you use, the computed digest is a byte array. That brings us to the next question: how to encode that byte array as a string, in order to pass it to Mastodon. Some typical options for encoding are: hex encoding (aka base16 encoding), base64 encoding, or base64-url encoding. The documentation does not state which of those encodings is accepted. Helpfully, the example provided in the documentation shows a digest string that appears to be hex-encoded. Unhelpfully, again according to my reading of the code, Mastodon requires a base64-encoded digest!

With these gaps and misleading things in the documentation, I think it would be impossible for a neophyte to navigate the documentation and successfully implement a client that passes a validatable signature.

  1. produce the POST body
  2. compute the SHA-256 digest of the POST body, including all whitepsace and leading or trailing newlines. Try this online tool to help you verify your work.
  3. Encode that computed digest (which is a byte array) with base64. This should produce a string of about 44 characters.
  4. Set the Digest header to be SHA-256=xxxyyyyy , where xxxyyyy is the base64 encoding of the SHA-256 digest.
  5. Set the http headers for the pending outbound request to include at least host, date, and digest.
  6. compute the signature following the example from the site, with headers of “(request-target) host date digest”, and using the appropriate RSA key pair.

If it were me, I would also include a :created: and an :expires: field in the http signature.

You can play around with HTTP Signatures using this online tool. That tool does not yet support computing a Digest of a POST body, but I’ll look into extending it to do that too.

Let me know in the comments if any of this is not clear.

I posted a working example for Nodejs as a gist on Github.

It depends only on nodejs and the builtin libraries for crypto and URL to compute the hash/digest and signature. It does not actually send a request to Mastodon; that is left for you to do.

Pre-request script for Postman, to calculate HttpSignature

If you do REST, you probably have a favorite REST client testing tool.
Mine is Postman, a Google Chrome app.

Postman has a nifty feature called Pre-request scripts, which allows you to write some Javascript code that performs a calculation and then reads and writes the “environment” object for the request. This means you can calculate … hashes or digests or signatures or anything you like.

Here’s an example of a script that calculates an HMAC-SHA256 HttpSignature, using the keyId and secret-key embedded in the environment. It also computes a digest on the message payload. Postman helpfully includes CrytoJS in the JS sandbox, so it’s easy to do HMAC and SHA.

In this particular case, the HttpSignature verification on the server requires 2 headers (date and digest) plus the ‘(request-target)’ value. The digest is a SHA-256 of the payload, which is then base64 encoded.

Anyone can start with this and modify it to do other variations.
Good luck!


I should have mentioned this: Postman, even the latest Chrome app version, uses XmlHTTPRequest to send out requests. XHR is purposefully limited, in the specification, to restrict some headers from being set explicitly on outbound requests. The list of restricted headers includes: Origin, Date, Cookie, Via, and others. The reason for this restriction: it is desired that the user-agent be fully in control of such request headers.

My example above uses an HttpSignature that signs the Date header. This means the code must also SET the Date header to a known value; in my case I am using a value generated by the pre-request script.


The value corresponds to “now”, the current moment. But the point is the value has to be absolutely known.

This is not possible in standard Postman, because of the XHR restriction. The effect you will see is that the intended Date header silently does not get sent with the request.

This may confound you if you don’t have an ability to trace the request on the receiving (server) side. In the context of a request that uses HttpSignature, the server will throw an error saying “Missing Date header”.

But! in Postman v0.9.6 and above, it is possible to configure Postman with something the Postman people call “the Intercptor plugin”, which then allows the lifting of this restriction. The Date header gets sent, and everything works.

If you don’t want to rely on the Interceptor plugin, and you want the HttpSignature to include the date value, then you’ll have to use a differently named header to hold the date. Use X-Date or anything other than “Date”. You need to change the client as well as the server of course, to make everything hold together.

Online calculator for SHA and HMAC-SHA

Here’s a thing I built. It’s just a webpage that calculates SHA-(1,224,256,384,512) and HMAC with the same algorithms.

I was using this to help with building a system that relies on HttpSignature. Developers need some help in constructing and validating their HMACs and SHAs.