open-vault/website/content/docs/secrets/pki/considerations.mdx

---
layout: docs
page_title: PKI - Secrets Engines
description: The PKI secrets engine for Vault generates TLS certificates.
---

# PKI Secrets Engine - Considerations

To successfully deploy this secrets engine, there are a number of important
considerations to be aware of, as well as some preparatory steps that should be
undertaken. You should read all of these _before_ using this secrets engine or
generating the CA to use with this secrets engine.

## Be Careful with Root CAs

Vault storage is secure, but not as secure as a piece of paper in a bank vault.
It is, after all, networked software. If your root CA is hosted outside of
Vault, don't put it in Vault as well; instead, issue a shorter-lived
intermediate CA certificate and put this into Vault. This aligns with industry
best practices.

Since 0.4, the secrets engine supports generating self-signed root CAs and
creating and signing CSRs for intermediate CAs. In each instance, for security
reasons, the private key can _only_ be exported at generation time, and the
ability to do so is part of the command path (so it can be put into ACL
policies).

If you plan on using intermediate CAs with Vault, it is suggested that you let
Vault create CSRs and do not export the private key, then sign those with your
root CA (which may be a second mount of the `pki` secrets engine).

### Managed Keys

Since 1.10, Vault Enterprise can access private key material in a
[_managed key_](../enterprise/managed-keys).  In this case, Vault never sees the
private key, and the external KMS or HSM performs certificate signing operations.
Managed keys are configured by selecting the `kms` type when generating a root
or intermediate.

## One CA Certificate, One Secrets Engine

Since Vault 1.11.0, the PKI Secrets Engine supports multiple issuers in a single
mount. However, in order to simplify the configuration, it is _strongly_
recommended that operators limit a mount to a single issuer. If you want to issue
certificates from multiple disparate CAs, mount the PKI secrets engine at multiple
mount points with separate CA certificates in each.

A common pattern is to have one mount act as your root CA and to use this CA
only to sign intermediate CA CSRs from other PKI secrets engines.

To keep old CAs active, there's two approaches to achieving rotation:

 1. Use multiple secrets engines. This allows a fresh start, preserving the
    old issuer and CRL. Vault ACL policy can be updated to deny new issuance
    under the old mount point and roles can be re-evaluated before being
    imported into the new mount point.
 2. Use multiple issuers in the same mount point. The usage of the old issuer
    can be restricted to CRL signing, and existing roles and ACL policy can be
    kept as-is. This allows cross-signing within the same mount, and consumers
    of the mount won't have to update their configuration. Once the transitional
    period for this rotation has completed and all past issued certificate have
    expired, it is encouraged to fully remove the old issuer and any unnecessary
    cross-signed issuers from the mount point.

Another suggested use case for multiple issuers in the same mount is splitting
issuance by TTL lifetime. For short-lived certificates, an intermediate
stored in Vault will often out-perform a HSM-backed intermediate. For
longer-lived certificates, however, it is often important to have the
intermediate key material secured throughout the lifetime of the end-entity
certificate. This means that two intermediates in the same mount -- one backed
by the HSM and one backed by Vault -- can satisfy both use cases. Operators
can make roles setting maximum TTLs for each issuer and consumers of the
mount can decide which to use.

## Keep certificate lifetimes short, for CRL's sake

This secrets engine aligns with Vault's philosophy of short-lived secrets. As
such it is not expected that CRLs will grow large; the only place a private key
is ever returned is to the requesting client (this secrets engine does _not_
store generated private keys, except for CA certificates). In most cases, if the
key is lost, the certificate can simply be ignored, as it will expire shortly.

If a certificate must truly be revoked, the normal Vault revocation function can
be used; alternately a root token can be used to revoke the certificate using
the certificate's serial number. Any revocation action will cause the CRL to be
regenerated. When the CRL is regenerated, any expired certificates are removed
from the CRL (and any revoked, expired certificate are removed from secrets
engine storage). This is an expensive operation! Due to the structure of the
CRL standard, Vault must read **all** revoked certificates into memory in order
to rebuild the CRL and clients must fetch the regenerated CRL.

This secrets engine does not support multiple CRL endpoints with sliding date
windows; often such mechanisms will have the transition point a few days apart,
but this gets into the expected realm of the actual certificate validity periods
issued from this secrets engine. A good rule of thumb for this secrets engine
would be to simply not issue certificates with a validity period greater than
your maximum comfortable CRL lifetime. Alternately, you can control CRL caching
behavior on the client to ensure that checks happen more often.

Often multiple endpoints are used in case a single CRL endpoint is down so that
clients don't have to figure out what to do with a lack of response. Run Vault
in HA mode, and the CRL endpoint should be available even if a particular node
is down.

~> Note: Since Vault 1.11.0, with multiple issuers in the same mount point,
   different issuers may have different CRLs (depending on subject and key
   material). This means that Vault may need to regenerate multiple CRLs.
   This is again a rationale for keeping TTLs short and avoiding revocation
   if possible.

## You must configure issuing/CRL/OCSP information _in advance_

This secrets engine serves CRLs from a predictable location, but it is not
possible for the secrets engine to know where it is running. Therefore, you must
configure desired URLs for the issuing certificate, CRL distribution points, and
OCSP servers manually using the `config/urls` endpoint. It is supported to have
more than one of each of these by passing in the multiple URLs as a
comma-separated string parameter.

## Safe Minimums

Since its inception, this secrets engine has enforced SHA256 for signature
hashes rather than SHA1. As of 0.5.1, a minimum of 2048 bits for RSA keys is
also enforced. Software that can handle SHA256 signatures should also be able to
handle 2048-bit keys, and 1024-bit keys are considered unsafe and are disallowed
in the Internet PKI.

## Token Lifetimes and Revocation

When a token expires, it revokes all leases associated with it. This means that
long-lived CA certs need correspondingly long-lived tokens, something that is
easy to forget. Starting with 0.6, root and intermediate CA certs no longer have
associated leases, to prevent unintended revocation when not using a token with
a long enough lifetime. To revoke these certificates, use the `pki/revoke`
endpoint.

## Certificate Storage

Unlike many secrets engines which replicate their state to all clusters
in multi-cluster architectures, the PKI secrets engine stores leaf certificates
issued with `no_store` set to `false` local to the cluster that issued them.
This allows for both primary and secondary clusters' nodes to issue
certificates for greater scalability.  As a result, these certificates
and any revocations are visible only on the issuing cluster. This additionally
means each cluster has its own set of CRLs, distinct from other clusters. These
CRLs should either be unified into a single CRL for distribution from a single
URI, or server operators should know to fetch all CRLs from all clusters.

## Telemetry

Beyond Vault's default telemetry around request processing, PKI exposes count and
duration metrics for the issue, sign, sign-verbatim, and revoke calls. The
metrics keys take the form `mount-path,operation,[failure]` with labels for
namespace and role name.

Note that these metrics are per-node and thus would need to be aggregated across
nodes and clusters.

## Auditing

Because Vault HMACs audit string keys by default, it is necessary to tune
PKI secrets mounts to get an accurate view of issuance that is occurring under
this mount.

Some suggested keys to un-HMAC for requests are as follows:

 - `csr` - the requested CSR to sign,
 - `certificate` - the requested self-signed certificate to re-sign or
   when importing issuers,
 - Various issuance-related overriding parameters, such as:
   - `issuer_ref` - the issuer requested to sign this certificate,
   - `common_name` - the requested common name,
   - `alt_names` - alternative requested DNS-type SANs for this certificate,
   - `other_sans` - other (non-DNS, non-Email, non-IP, non-URI) requested SANs for this certificate,
   - `ip_sans` - requested IP-type SANs for this certificate,
   - `uri_sans` - requested URI-type SANs for this certificate,
   - `ttl` - requested expiration date of this certificate,
   - `not_after` - requested expiration date of this certificate,
   - `serial_number` - the subject's requested serial number,
   - `key_type` - the requested key type,
   - `private_key_format` - the requested key format which is also
     used for the public certificate format as well,
 - Various role- or issuer-related generation parameters, such as:
   - `managed_key_name` - when creating an issuer, the requested managed
     key name,
   - `managed_key_id` -  when creating an issuer, the requested managed
     key identifier,
   - `ou` - the subject's organizational unit,
   - `organization` - the subject's organization,
   - `country` - the subject's country code,
   - `locality` - the subject's locality,
   - `province` - the subject's province,
   - `street_address` - the subject's street address,
   - `postal_code` - the subject's postal code,
   - `permitted_dns_domains` - permitted DNS domains,
   - `policy_identifiers` - the requested policy identifiers when creating a role, and
   - `ext_key_usage_oids` - the extended key usage OIDs for the requested certificate.

Some suggested keys to un-HMAC for responses are as follows:

 - `certificate` - the certificate that was issued,
 - `issuing_ca` - the certificate of the CA which issued the requested
   certificate,
 - `serial_number` - the serial number of the certificate that was issued,
 - `error` - to show errors associated with the request, and
 - `ca_chain` - optional due to noise; the full CA chain of the issuer of
   the requested certificate.

~> Note: These list of parameters to un-HMAC are provided as a suggestion and
   may not be exhaustive.

The following keys are suggested **NOT** to un-HMAC, due to their sensitive
nature:

 - `private_key` - this response parameter contains the private keys
   generated by Vault during issuance, and
 - `pem_bundle` this request parameter is only used on the issuer-import
   paths and may contain sensitive private key material.

## Learn

Refer to the [Build Your Own Certificate Authority (CA)](https://learn.hashicorp.com/vault/secrets-management/sm-pki-engine)
guide for a step-by-step tutorial.

Have a look at the [PKI Secrets Engine with Managed Keys](https://learn.hashicorp.com/tutorials/vault/managed-key-pki?in=vault/enterprise)
for more about how to use externally managed keys with PKI.

## API

The PKI secrets engine has a full HTTP API. Please see the
[PKI secrets engine API](/api-docs/secret/pki) for more
details.