Commonly Asked Questions

These are some commonly asked questions on the topics of PKI, TLS, X509, cryptography, threshold-cryptography, etc. We hope to reduce the amount of hand-waving in these responses as we add more features to the Step toolkit over time.

What's TLS & PKI?

TLS stands for transport layer security. It used to be called secure sockets layer (or SSL), but technically SSL refers to an older version of the protocol. Normal TCP connections communicate in plain text, allowing attackers to eavesdrop and spoof messages. If used properly, TLS provides confidentiality and integrity for TCP traffic, ensuring that messages can only be seen by their intended recipient, and cannot be modified in transit.

TLS is a complicated protocol with lots of options, but the most common mode of operation establishes a secure channel using asymmetric cryptography with digital certificates (or just certificates for short). First, some quick definitions:

Asymmetric cryptography (a.k.a., public key cryptography) is an underappreciated gift from mathematics to computer science. It uses a key pair: a private key known only to the recipient of the message, and a public key that can be broadly distributed, even to adversaries, without compromising security.
Digital certificates are data structures that map a public key to the well-known name of the owner of the corresponding private key (e.g., a DNS host name). They bind a name to the public key so you can address recipients by name instead of using public keys directly (which are big random numbers).

Briefly, there are two functions that can be achieved using asymmetric cryptography:

Messages can be encrypted using the public key to ensure that only the private key holder can decrypt them, and
Messages can be signed using the private key so that anyone with the public key knows the message came from the private key holder. With digital certificates, you can replace "private key holder" with "named entity," which makes things a whole lot more useful. It lets you use names, instead of public keys, to address messages.

PKI stands for public key infrastructure. Abstractly, it's a set of policies and procedures for managing digital certificates (i.e., managing the bindings between names and public keys). Without proper secure PKI, an attacker can fake a binding and undermine security.

What's a certificate authority?

A certificate authority (CA) stores, issues, and signs digital certificates. CAs have their own key pair, with the private key carefully secured (often offline). The CA binds its name to its public key by signing a digital certificate using its own private key (called self signing). The CA's self-signed certificate, or root certificate, is distributed to all principals in the system (e.g., all of the clients and servers in your infrastructure).

So, the CA is tasked with securely binding names to public keys. Here's how that process works.

When a named principal wants a certificate, it generates its own key pair. Nobody else ever needs to know the private key, not even the CA.
The principal creates a certificate signing request (CSR), containing its name and public key (and some other stuff), and submits it to the CA. The CSR is self-signed, like the root certificate, so the CA knows that the requestor has the corresponding private key.
The CA performs some form of identity proofing, certifying that the request is coming from the principal named in the CSR.
Once satisfied, the CA issues a certificate by using its own private key to sign a certificate binding the name and public key from the CSR.

Certificates signed by the CA are used to securely introduce principals that don't already know one anothers' public keys. Assuming both principals agree on a trusted CA, they can exchange digital certificates and authenticate the signatures to gain some assurance that they are communicating with the named entity.

Technically, smallstep's certificate authority is more than just a certificate authority. It combines several PKI roles into one simple, flexible package. It acts as a registration authority, accepting requests for digital certificates and verifying the identity of the requesting entities before establishing bindings. It also acts as a central directory and more generally as a certificate management system, a secure location for storing and distributing key material.

Why not just use Verisign, Entrust, Let's Encrypt, etc?

The web's open public key infrastructure (web PKI), while far from perfect, is an important foundation for securing the web. So why not use it for securing communication for your own internal infrastructure? There are several reasons:

It's expensive to provision certificates from a public CA for all of your services
Public CAs can't handle client certificates (mutual TLS)
It's much harder (and more expensive) to revoke or roll certificates from public CAs
It relies on a third party that can subvert your security

More broadly, the answer is that web PKI was designed for the web. A lot of the web PKI design decisions aren't appropriate for internal systems.

How does identity proofing work?

Good question. However you want it to. Details here. Give some options: simple meat-space token-based method should probably be default. But we should also support the use of a CI/CD pipeline or orchestrator as a registration authority so you can automate certificate provisioning.

Also talk about ACME, if we decide to support it.

In general, trust will always flow back out to you, the operator of your system. With that in mind, the simplest form of identity proofing is manual: [describe token-based manual mechanism here]. As your system grows, this process can become onerous. Automated identity proofing requires careful coordination between different parts of your system. Smallstep provides additional tooling, and vetted designs, to help with this. If you integrate with our other tools its easy to start with a manual identity proofing mechanism and move to a more sophisticated automated method as your system grows.

I already have PKI in place. Can I use this with my own root certificate?

Absolutely. [Details here].

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