smallstep-certificates/docs/questions.md

Frequently Asked Questions

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

We encourage you to read our blog post on everything relating to PKI as we believe it to be a solid resource that answers many of of the questions listed below.

What are 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.

1. 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.
2. 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.
3. The CA performs some form of identity proofing, certifying that the request is coming from the principal named in the CSR.
4. 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?

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.

What are the security risks of exposing the OAuth Client Secret in the output of step ca provisioner list?

It would be nice if we could have the CA operate as an OAuth confidential client, keeping the client secret private and redirecting back to the CA instead of to loopback. But, to be clear, this is not an abuse of the OAuth spec. The way this was implemented in step, as an OAuth native application using a public client, is standard, was intentional, (mostly) conforms to best current practices, and the flow we're using is widely used in practice. A confidential client is (strictly?) more secure. But a public client that redirects to loopback isn’t a significant security risk under a normal threat model.

The current flow

The advantage of the current flow is that it’s more general purpose. For example, step oauth works without any additional infrastructure. An issued access token can be used from the command line, and OIDC identity tokens can be safely used to authenticate to remote services (including remote services that don’t speak OAuth OIDC, or don’t even speak HTTP, but can validate a JWT). step-ca is one example of a remote service that can authenticate step users via OIDC identity token. You can also use step crypto jwt verify to authenticate using OIDC at the command line.

The particular details of the OAuth flow we selected has pros & cons, as does any flow. The relevant security risks are:

1. Since the OAuth access token isn’t issued directly to a remote server (e.g., step-ca), remote servers can’t safely use the issued access tokens without significant care. If they did, an attacker might be able to maliciously trick the remote server into using an access token that was issued to a different client.

2. The redirect back from the OAuth authorization server to the client can be intercepted by another process running on the local machine. This isn’t really necessary though, because...

3. The client_secret is public, so anyone can initiate (and complete) an OAuth flow using our client (but it will always redirect back to 127.0.0.1).

The first threat is moot since we don't actually use the access token for anything when we're connecting to step-ca. Unfortunately there's no way to not get an access token. So we just ignore it.

Note that it is safe to use the access token from the command line to access resources at a remote API. For example, it’s safe to user step oauth to obtain an OAuth access token from Google and use it to access Google’s APIs in a bash script.

More generally, access tokens are for accessing resources (authorization) and are not useful for authenticating a user since they're not audience-addressed. If you and I both have a Google OAuth client, I could get Alice to OAuth into my app and use the issued access token to masquerade as Alice to you. But OIDC identity tokens are audience-addressed. An identity token is a JWT with the client_id baked in as the aud (audience) parameter. As long as clients check this parameter (which step-ca does) they're not susceptible to this attack. In fact, OIDC identity tokens were designed and developed precisely to solve this problem.

So it's completely safe for one entity to obtain an identity token from an IdP on behalf of a user and use it to authenticate to another entity (like step does). That's exactly the use case OIDC was designed to support.

The second and third threats are related. They involve a malicious attempt to initiate an OAuth OIDC flow using our client credentials. There's a lot of analysis we could do here comparing this situation to a non-native (e.g., web) client and to other flows (e.g., the implicit flow, which also makes the client secret public). Skipping that detail, we know two things for sure:

1. OAuth flows generally require user consent to complete (e.g., a user has to "approve" an application's authentication / authorization request)

2. An OAuth flow initiated using our client will always redirect back to 127.0.0.1

So a malicious attacker trying to obtain an identity token needs two things:

1. They need to get user consent to complete an OAuth flow
2. They need to have local access to the user's machine

This is already a pretty high bar. It’s worth noting, however, that the first part is much easier if the user is already logged in and the identity provider is configured to not require consent (i.e., the OAuth flow is automatically completed without the user having to click any buttons). Okta seems to do this for some applications by default.

It's also worth noting that a process with local access could probably obtain an access/identity token for a confidential client without knowing the client secret. That's the main reason I don't think the flow we're using has a meaningful security impact under most threat models. The biggest difference is that attacking a confidential client would probably require privileged (root) access, whereas our flow could be attacked by an unprivileged process. But the fruit of our OAuth flow — the SSH certificate — is also available for use by an unprivileged process running locally via the ssh-agent. So the only thing possibly gained is the ability to exfiltrate.

Stuff we should consider doing

There are at least three OAuth features that are relevant to this discussion. Two have already been mentioned:

1. OAuth public clients for native applications can be (er, are supposed to be) created without a client secret

2. Proof Key for Code Exchange (PKCE) helps ensure that the process requesting the access token / identity token is the same process that initiated the flow

The first feature, clients without secrets, is mostly cosmetic. There's no real difference between a public secret and no secret, except that it's confusing to have something called a "secret" that's not actually secret. (Caveat: IdPs that support "native applications" without secrets typically enforce other rules for these clients — they often require PKCE and might not issue a renew token, for example. But these features can often be turned on/of for other client types, too.)

The reason we don't assume a public client without a secret is that, unfortunately, not all IdPs support them. Significantly, Google does not. In fact, gcloud (Google Cloud's CLI tool) uses OAuth OIDC and uses the exact same technique we're using. If you look at the source you'll find their "NOTSOSECRET" All of that said, we should support "native clients" without secrets at some point.

We should also implement Proof Key for Code Exchange (PKCE). This has been on our backlog for a while, and it's actually really simple and useful. It's definitely low-hanging fruit. Before initiating the OAuth flow your client generates a random number. It hashes that number and passes the hash to the IdP as part of the authorization request (the URL that users are sent to for login). After authenticating and consenting, when the user is redirected back to the client, the client makes a request to the IdP to get an access token & identity token. In that request the client must include the unhashed random number. The IdP re-hashes it and compares it to the value it received in the authorization request. If they match, the IdP can be certain that the entity making the access token request is the same entity that initiated the flow. In other words, the request has not been intercepted by some malicious intermediary.

The last hardening mechanism to be aware of are the acr and amr parameters. Basically, when the OAuth flow is initiated the client can request that the IdP require consent, do 2FA, and a bunch of other stuff. The issued identity token includes parameters to indicate that these processes did, indeed, occur. Leveraging this mechanism one could configure step-ca to check these parameters and be sure that users have consented and undergone a 2FA presence check (e.g., tapped a security token). Unfortunately, like a bunch of other optional OAuth features, many IdPs (cough Google cough) don't support this stuff.

Summary

Implementing PKCE should be our highest priority item. Support for "native" clients without secrets would also be nice. Forcing 2FA & consent via acr & amr is also a good idea. Support for non-native clients that redirect back to the CA, and where the secret is actually secret, would also be nice. But it's a bigger architectural change and the security implications aren't actually that severe.

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

Yes. There's a easy way, and a longer but more secure way to do this.

Option 1: The easy way

If you have your root CA signing key available, you can run:

step ca init --root=[ROOT_CERT_FILE] --key=[ROOT_PRIVATE_KEY_FILE]

The root certificate can be in PEM or DER format, and the signing key can be a PEM file containing a PKCS#1, PKCS#8, or RFC5915 (for EC) key.

Option 2: More secure

That said, CAs are usually pretty locked down and it's bad practice to move the private key around. So I'm gonna assume that's not an option and give you the more complex instructions to do this "the right way", by generating a CSR for step-ca, getting it signed by your existing root, and configuring step-ca to use it.

When you run step ca init we create a couple artifacts under ~/.step/. The important ones for us are:

• ~/.step/certs/root_ca.crt is your root CA certificate
• ~/.step/secrets/root_ca_key is your root CA signing key
• ~/.step/certs/intermediate_ca.crt is your intermediate CA cert
• ~/.step/secrets/intermediate_ca_key is the intermediate signing key used by step-ca

The easiest thing to do is to run step ca init to get this scaffolding configuration in place, then remove/replace these artifacts with new ones that are tied to your existing root CA.

First, step-ca does not actually need the root CA signing key. So you can simply remove that file:

rm ~/.step/secrets/root_ca_key

Next, replace step-ca's root CA cert with your existing root certificate:

mv /path/to/your/existing/root.crt ~/.step/certs/root_ca.crt

Now you need to generate a new signing key and intermediate certificate, signed by your existing root CA. To do that we can use the step certificate create subcommand to generate a certificate signing request (CSR) that we'll have your existing root CA sign, producing an intermediate certificate.

To generate those artifacts run:

step certificate create "Intermediate CA Name" intermediate.csr intermediate_ca_key --csr

Next, you'll need to transfer the CSR file (intermediate.csr) to your existing root CA and get it signed.

Now you need to get the CSR executed by your existing root CA.

Active Directory Certificate Services

certreq -submit -attrib "CertificateTemplate:SubCA" intermediate.csr intermediate.crt

AWS Certificate Manager Private CA

Here's a Python script that uses issue-certificate to process the CSR:

import boto3
import sys

AWS_CA_ARN = '[YOUR_PRIVATE_CA_ARN]'

csr = ''.join(sys.stdin.readlines())

client = boto3.client('acm-pca')
response = client.issue_certificate(
    CertificateAuthorityArn=AWS_CA_ARN,
    Csr=csr,
    SigningAlgorithm='SHA256WITHRSA',
    TemplateArn='arn:aws:acm-pca:::template/SubordinateCACertificate_PathLen1/V1',
    Validity={
        'Value': 5,
        'Type': 'YEARS'
    }
)
print(f"Creating certificate with ARN {response['CertificateArn']}...", file=sys.stderr, end='')
waiter = client.get_waiter('certificate_issued')
waiter.wait(
    CertificateAuthorityArn=AWS_CA_ARN,
    CertificateArn=response['CertificateArn']
)
print('done.', file=sys.stderr)
response = client.get_certificate(
   CertificateArn=response['CertificateArn'],
   CertificateAuthorityArn=AWS_CA_ARN
)
print(response['Certificate'])

To run it, fill in the ARN of your CA and run:

python issue_certificate.py < intermediate.csr > intermediate.crt

OpenSSL

openssl ca -config [ROOT_CA_CONFIG_FILE] \
  -extensions v3_intermediate_ca \
  -days 3650 -notext -md sha512 \
  -in intermediate.csr \
  -out intermediate.crt

CFSSL

For CFSSL you'll need a signing profile that specifies a 10-year expiry:

cat > ca-smallstep-config.json <<EOF
{
  "signing": {
    "profiles": {
      "smallstep": {
        "expiry": "87660h",
        "usages": ["signing"]
      }
    }
  }
}
EOF

Now use that config to sign the intermediate certificate:

cfssl sign -ca ca.pem \
    -ca-key ca-key.pem \
    -config ca-smallstep-config.json \
    -profile smallstep
    -csr intermediate.csr | cfssljson -bare

This process will yield a signed intermediate.crt certificate (or cert.pem for CFSSL). Transfer this file back to the machine running step-ca.

Finally, replace the intermediate .crt and signing key produced by step ca init with the new ones we just created:

mv intermediate.crt ~/.step/certs/intermediate_ca.crt
mv intermediate_ca_key ~/.step/secrets/intermediate_ca_key

That should be it! You should be able to start step-ca and the certificates should be trusted by anything that trusts your existing root CA.

Further Reading

• Use TLS Everywhere
• Everything you should know about certificates and PKI but are too afraid to ask