PAST: Platform-Agnostic Security Tokens

PAST is everything you love about JOSE (JWT, JWE, JWS) without any of the many design deficits that plague the JOSE standards.

What follows is a reference implementation. Requires PHP 7 or newer.

What is PAST?

PAST (Platform-Agnostic Security Tokens) is a specification and reference implementation for secure stateless tokens.

Unlike JSON Web Tokens (JWT), which gives developers more than enough rope with which to hang themselves, PAST only allows secure operations. JWT gives you "algorithm agility", PAST gives you "versioned protocols". It's incredibly unlikely that you'll be able to use PAST in an insecure way.

Key Differences between PAST and JWT

PAST

v2.auth.eyJkYXRhIjoidGhpcyBpcyBhIHNpZ25lZCBtZXNzYWdlIiwiZXhwIjoiMjAzOS0wMS0wMVQwMDowMDowMCJ9VpWy4KU60YnKUzTkixFi9foXhXKTHbcDBtpg7oWllm8=

This decodes to:

Version: v2
Purpose: auth (shared-key authentication)

Payload:

{
  "data": "this is a signed message",
  "exp": "2039-01-01T00:00:00"
}

Authentication tag:

VpWy4KU60YnKUzTkixFi9foXhXKTHbcDBtpg7oWllm8=

To learn what each version means, please see this page in the documentation.

JWT

An example JWT (taken from JWT.io) might look like this:

eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiYWRtaW4iOnRydWV9.TJVA95OrM7E2cBab30RMHrHDcEfxjoYZgeFONFh7HgQ

This decodes to:

Header:

{
  "alg": "HS256",
  "typ": "JWT"
}

Body:

{
  "sub": "1234567890",
  "name": "John Doe",
  "admin": true
}

Signature:

TJVA95OrM7E2cBab30RMHrHDcEfxjoYZgeFONFh7HgQ

Motivation

As you can see, with JWT, you get to specify an alg header. There are a lot of options to choose from (including none).

There have been ways to exploit JWT libraries by replacing RS256 with HS256 and using the known public key as the HMAC-SHA256 key, thereby allowing arbitrary token forgery.

With PAST, your options are version and a purpose. There are four possible values for purpose:

auth -- shared-key authentication
enc -- shared-key encryption
seal -- public-key encryption
sign -- public-key authentication (a.k.a. digital signatures)

All encryption modes use authenticated modes.

Regardless of the purpose selected, the header (and an optional footer, which is always cleartext but base64url-encoded) is included in the signature or authentication tag.

How to Use this Library

See the documentation.

The section dedicated to this PHP implementation may be more relevant.

b / past