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How Pebble Supports ACME Client Developers

How Pebble Supports ACME Client Developers Together with the IETF community, we created the ACME standard to support completely automated certificate issuance. This open standard is now supported by dozens of clients. On the server side, did you know that we have not one but two open-source ACME server implementations? The big implementation, which we use ourselves in production, is called Boulder. Boulder handles all of the facets and details needed for a production certificate authority, including policy compliance, database interfaces, challenge verifications, and logging. You can adapt and use Boulder yourself if you need to run a real certificate authority, including an internal, non-publicly-trusted ACME certificate authority within an organization. The small implementation is called Pebble. It’s meant entirely for testing, not for use as a real certificate authority, and we and ACME client developers use it for various automated and manual testing purposes. For example, Certbot has used Pebble in its development process for years in order to perform a series of basic but realistic checks of the ability to request and obtain certificates from an ACME server. Pebble is Easy to Use for ACME Client Testing For any developer or team creating an ACME client application, Pebble solves a range of problems along the lines of “how do I check whether I’ve implemented ACME correctly, so that I could actually get certificates from a CA, without necessarily using a real domain name, and without running into CA rate limits during my routine testing?” Pebble is quick and easy to set up if you need to test an ACME client’s functionality. It runs in RAM without dependencies or persistence; you won’t need to set up a database or a configuration for it. You can get Pebble running with a single golang command in just a few seconds, and immediately start making local ACME requests. That’s suitable for inclusion in a client’s integration test suite, making much more realistic integration tests possible without needing to worry about real domains, CA rate limits, or network outages. We see Pebble getting used in the official test suites for ACME clients including getssl, Lego, Certbot, simp_le, and others. In many cases, every change committed to the ACME client’s code base is automatically tested against Pebble. Pebble is Intentionally Different From Boulder Pebble is also deliberately different from Boulder in some places in order to provide clients with an opportunity to interoperate with slightly different ACME implementations. The Pebble code explains that [I]n places where the ACME specification allows customization/CA choice Pebble aims to make choices different from Boulder. For instance, Pebble changes the path structures for its resources and directory endpoints to differ from Boulder. The goal is to emphasize client specification compatibility and to avoid "over-fitting" on Boulder and the Let's Encrypt production service. For instance, the Let’s Encrypt service currently offers its newAccount resource at the path /acme/new-acct, whereas Pebble uses a different name /sign-me-up, so clients will be reminded to check the directory rather than assuming a specific path. Other substantive differences include: Pebble rejects 5% of all requests as having an invalid nonce, even if the nonce was otherwise valid, so clients can test how they respond to this error condition Pebble only reuses valid authorizations 50% of the time, so clients can check their ability to perform validations when they might not have expected to Pebble truncates timestamps to a different degree of precision than Boulder Unlike Boulder, Pebble respects the notBefore and notAfter fields of new-order requests The ability of ACME clients to work with both versions is a good test of their conformance to the ACME specification, rather than making assumptions about the current behavi

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