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Secret Backend: PKI
docs-secrets-pki
The PKI secret backend for Vault generates TLS certificates.

PKI Secret Backend

Name: pki

The PKI secret backend for Vault generates X.509 certificates dynamically based on configured roles. This means services can get certificates needed for both client and server authentication without going through the usual manual process of generating a private key and CSR, submitting to a CA, and waiting for a verification and signing process to complete. Vault's built-in authentication and authorization mechanisms provide the verification functionality.

By keeping TTLs relatively short, revocations are less likely to be needed, keeping CRLs short and helping the backend scale to large workloads. This in turn allows each instance of a running application to have a unique certificate, eliminating sharing and the accompanying pain of revocation and rollover.

In addition, by allowing revocation to mostly be forgone, this backend allows for ephemeral certificates; certificates can be fetched and stored in memory upon application startup and discarded upon shutdown, without ever being written to disk.

This page will show a quick start for this backend. For detailed documentation on every path, use vault path-help after mounting the backend.

Considerations

To successfully deploy this backend, 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 backend or generating the CA to use with this backend.

Never use 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. Your long-lived self-signed root CA's private key should instead be used to issue a shorter-lived intermediate CA certificate, and this is what you should put into Vault. This aligns with industry best practices.

One CA Certificate, One Backend

In order to vastly simplify both the configuration and codebase of the PKI backend, only one CA certificate is allowed per backend. If you want to issue certificates from multiple CAs, mount the PKI backend at multiple mount points with separate CA certificates in each.

This also provides a convenient method of switching to a new CA certificate while keeping CRLs valid from the old CA certificate; simply mount a new backend and issue from there.

Keep certificate lifetimes short, for CRL's sake

This backend 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 backend does not store generated private keys). 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 backend storage).

This backend 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 backend. A good rule of thumb for this backend 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.

You must configure CRL information in advance

This backend serves CRLs from a predictable location. That location must be encoded into your CA certificate if you want to allow applications to use the CRL endpoint encoded in certificates to find the CRL. Instructions for doing so are below. If you need to adjust this later, you will have to generate a new CA certificate using the same private key if you want to keep validity for already-issued certificates.

No OCSP support, yet

Vault's architecture does not currently allow for a binary protocol such as OCSP to be supported by a backend. As such, you should configure your software to use CRLs for revocation information, with a caching lifetime that feels good to you. Since you are following the advice above about keeping lifetimes short (right?), CRLs should not grow too large.

Quick Start

CA certificate

In order for this backend to serve CRL information at the expected location, you will need to generate your CA certificate with this information. For OpenSSL, this means putting a value in the CA section with the appropriate URL; in this example the PKI backend is mounted at pki:

crlDistributionPoints = URI:https://vault.example.com:8200/v1/pki/crl

Adjust the URI as appropriate.

Vault

The first step to using the PKI backend is to mount it. Unlike the generic backend, the pki backend is not mounted by default.

$ vault mount pki
Successfully mounted 'pki' at 'pki'!

Next, Vault must be configured with a root certificate and associated private key. This is done by writing the contents of a file or stdin:

$ vault write pki/config/ca pem_bundle="@ca_bundle.pem"
Success! Data written to: pki/config/ca

or

$ cat bundle.pem | vault write pki/config/ca pem_bundle="-"
Success! Data written to: pki/config/ca

Although in this example the value being piped into stdin could be passed directly into the Vault CLI command, a more complex usage might be to use Ansible to securely store the certificate and private key in an ansible-vault file, then have an ansible-playbook command decrypt this value and pass it in to Vault.

The next step is to configure a role. A role is a logical name that maps to a policy used to generated those credentials. For example, let's create an "example-dot-com" role:

$ vault write pki/roles/example-dot-com \
    allowed_base_domain="example.com" \
    allow_subdomains="true" max_ttl="72h"
Success! Data written to: pki/roles/example-dot-com

By writing to the roles/example-dot-com path we are defining the example-dot-com role. To generate a new set of credentials, we simply write to the issue endpoint with that role name: Vault is now configured to create and manage certificates!

$ vault write pki/issue/example-dot-com common_name=blah.example.com
Key            	Value
lease_id       	pki/issue/example-dot-com/819393b5-e1a1-9efd-b72f-4dc3a1972e31
lease_duration 	259200
lease_renewable	false
certificate    	-----BEGIN CERTIFICATE-----
MIIECDCCAvKgAwIBAgIUXmLrLkTdBIOOIYg2/BXO7docKfUwCwYJKoZIhvcNAQEL
...
az3gfwlOqVTdgi/ZVAtIzhSEJ0OY136bq4NOaw==
-----END CERTIFICATE-----
issuing_ca      -----BEGIN CERTIFICATE-----
MIIDUTCCAjmgAwIBAgIJAKM+z4MSfw2mMA0GCSqGSIb3DQEBCwUAMBsxGTAXBgNV
...
-----END CERTIFICATE-----
private_key    	-----BEGIN RSA PRIVATE KEY-----
MIIEowIBAAKCAQEA0cczc7Y2yIu7aD/IaDi23Io+tvvDS9XaXXDUFW1kqd58P83r
...
3xhCNnZ3CMQaM2I48sloVK/XoikMLb5MZwOUQn/V+TrhWP4Lu7qD
-----END RSA PRIVATE KEY-----
serial         	5e:62:eb:2e:44:dd:04:83:8e:21:88:36:fc:15:ce:ed:da:1c:29:f5

Note that this is a write, not a read, to allow values to be passed in at request time.

Vault has now generated a new set of credentials using the example-dot-com role configuration. Here we see the dynamically generated private key and certificate. The issuing CA certificate is returned as well.

Using ACLs, it is possible to restrict using the pki backend such that trusted operators can manage the role definitions, and both users and applications are restricted in the credentials they are allowed to read.

If you get stuck at any time, simply run vault path-help pki or with a subpath for interactive help output.

API

/pki/ca(/pem)

GET

Description
Retrieves the CA certificate *in raw DER-encoded form*. This is a bare endpoint that does not return a standard Vault data structure. If `/pem` is added to the endpoint, the CA certificate is returned in PEM format.

This is an unauthenticated endpoint.
Method
GET
URL
`/pki/ca(/pem)`
Parameters
None
Returns
```
<binary DER-encoded certficiate>
```

/pki/cert/

GET

Description
Retrieves one of a selection of certificates. Valid values: `ca` for the CA certificate, `crl` for the current CRL, or a serial number in either hyphen-separated or colon-separated octal format. This endpoint returns the certificate in PEM formatting in the `certificate` key of the JSON object.

This is an unauthenticated endpoint.
Method
GET
URL
`/pki/cert/`
Parameters
None
Returns
```javascript
{
    "data": {
        "certificate": "-----BEGIN CERTIFICATE-----\nMIIGmDCCBYCgAwIBAgIHBzEB3fTzhTANBgkqhkiG9w0BAQsFADCBjDELMAkGA1UE\n..."
    }
}
...
```

/pki/config/ca

POST

Description
A PEM file containing the issuing CA certificate and its private key, concatenated.

This is a root-protected endpoint.

The information can be provided from a file via a `curl` command similar to the following:
```text
curl -X POST --data "@cabundle.json" http://127.0.0.1:8200/v1/pki/config/ca -H X-Vault-Token:06b9d...
```

Note that if you provide the data through the HTTP API it must be
JSON-formatted, with newlines replaced with `\n`, like so:

```text
{ "pem_bundle": "-----BEGIN RSA PRIVATE KEY-----\n...\n-----END CERTIFICATE-----" }
```
Method
POST
URL
`/pki/config/ca`
Parameters
  • pem_bundle required The key and certificate concatenated in PEM format.
Returns
A `204` response code.

/pki/crl(/pem)

GET

Description
Retrieves the current CRL *in raw DER-encoded form*. This endpoint is suitable for usage in the CRL Distribution Points extension in a CA certificate. This is a bare endpoint that does not return a standard Vault data structure. If `/pem` is added to the endpoint, the CRL is returned in PEM format.

This is an unauthenticated endpoint.
Method
GET
URL
`/pki/crl(/pem)`
Parameters
None
Returns
```
<binary DER-encoded CRL>
```

/pki/crl/rotate

GET

Description
This endpoint forces a rotation of the CRL. This can be used by administrators to cut the size of the CRL if it contains a number of certificates that have now expired, but has not been rotated due to no further certificates being revoked.

This is a root-protected endpoint.
Method
GET
URL
`/pki/crl/rotate`
Parameters
None
Returns
{
    "data": {
        "success": true
    }
}

/pki/issue/

POST

Description
Generates a new set of credentials (private key and certificate) based on the named role. The issuing CA certificate is returned as well, so that only the root CA need be in a client's trust store.

*The private key is _not_ stored. If you do not save the private key, you will need to request a new certificate.*
Method
POST
URL
`/pki/issue/`
Parameters
  • common_name required The requested CN for the certificate. If the CN is allowed by role policy, it will be issued.
  • alt_names optional Requested Subject Alternative Names, in a comma-delimited list. If any requested names do not match role policy, the entire request will be denied.
  • ip_sans optional Requested IP Subject Alternative Names, in a comma-delimited list. Only valid if the role allows IP SANs (which is the default).
  • ttl optional Requested Time To Live. Cannot be greater than the role's `max_ttl` value. If not provided, the role's `ttl` value will be used. Note that the role values default to system values if not explicitly set.
Returns
{
    "lease_id": "pki/issue/test/7ad6cfa5-f04f-c62a-d477-f33210475d05",
    "renewable": false,
    "lease_duration": 21600,
    "data": {
        "certificate": "-----BEGIN CERTIFICATE-----\nMIIDzDCCAragAwIBAgIUOd0ukLcjH43TfTHFG9qE0FtlMVgwCwYJKoZIhvcNAQEL\n...\numkqeYeO30g1uYvDuWLXVA==\n-----END CERTIFICATE-----\n",
        "issuing_ca": "-----BEGIN CERTIFICATE-----\nMIIDUTCCAjmgAwIBAgIJAKM+z4MSfw2mMA0GCSqGSIb3DQEBCwUAMBsxGTAXBgNV\n...\nG/7g4koczXLoUM3OQXd5Aq2cs4SS1vODrYmgbioFsQ3eDHd1fg==\n-----END CERTIFICATE-----\n",
        "private_key": "-----BEGIN RSA PRIVATE KEY-----\nMIIEowIBAAKCAQEAnVHfwoKsUG1GDVyWB1AFroaKl2ImMBO8EnvGLRrmobIkQvh+\n...\nQN351pgTphi6nlCkGPzkDuwvtxSxiCWXQcaxrHAL7MiJpPzkIBq1\n-----END RSA PRIVATE KEY-----\n",
        "serial": "39:dd:2e:90:b7:23:1f:8d:d3:7d:31:c5:1b:da:84:d0:5b:65:31:58"
    },
    "auth": null
}

/pki/revoke

POST

Description
Revokes a certificate using its serial number. This is an alternative option to the standard method of revoking using Vault lease IDs. A successful revocation will rotate the CRL.

This is a root-protected endpoint.
Method
POST
URL
`/pki/revoke`
Parameters
  • serial_number required The serial number of the certificate to revoke, in hyphen-separated or colon-separated octal.
Returns
```javascript { "data": { "revocation_time": 1433269787 } } ```

/pki/roles/

POST

Description
Creates or updates the role definition. Note that the `allowed_base_domain`, `allow_token_displayname`, `allow_subdomains`, and `allow_any_name` attributes are additive; between them nearly and across multiple roles nearly any issuing policy can be accommodated. `server_flag`, `client_flag`, and `code_signing_flag` are additive as well. If a client requests a certificate that is not allowed by the CN policy in the role, the request is denied.
Method
POST
URL
`/pki/roles/`
Parameters
  • ttl optional The Time To Live value provided as a string duration with time suffix. Hour is the largest suffix. If not set, uses the system default value or the value of `max_ttl`, whichever is shorter.
  • max_ttl optional The maximum Time To Live provided as a string duration with time suffix. Hour is the largest suffix. If not set, defaults to the system maximum lease TTL.
  • allow_localhost optional If set, clients can request certificates for `localhost` as one of the requested common names. This is useful for testing and to allow clients on a single host to talk securely. Defaults to true.
  • allowed_base_domain optional If set, clients can request certificates for subdomains directly off of this base domain. _This includes the wildcard subdomain._ For instance, a base_domain of `example.com` allows clients to request certificates for `foo.example.com` and `*.example.com`. To allow further levels of subdomains, enable the `allow_subdomains` option. There is no default.
  • allow_token_displayname optional If set, clients can request certificates matching the value of Display Name from the requesting token. Remember, this stacks with the other CN options, including `allowed_base_domain`. Defaults to `false`.
  • allow_subdomains optional If set, clients can request certificates with CNs that are subdomains of the CNs allowed by the other role options. _This includes wildcard subdomains._ This is redundant when using the `allow_any_name` option. Defaults to `false`.
  • allow_any_name optional If set, clients can request any CN. Useful in some circumstances, but make sure you understand whether it is appropriate for your installation before enabling it. Defaults to `false`.
  • allow_ip_sans optional If set, clients can request IP Subject Alternative Names. Unlike CNs, no authorization checking is performed except to verify that the given values are valid IP addresses. Defaults to `true`.
  • server_flag optional If set, certificates are flagged for server use. Defaults to `true`.
  • client_flag optional If set, certificates are flagged for client use. Defaults to `true`.
  • code_signing_flag optional If set, certificates are flagged for code signing use. Defaults to `false`.
  • key_type optional The type of key to generate for generated private keys. Currently, `rsa` and `ec` are supported. Defaults to `rsa`.
  • key_bits optional The number of bits to use for the generated keys. Defaults to `2048`; this will need to be changed for `ec` keys. See https://golang.org/pkg/crypto/elliptic/#Curve for an overview of allowed bit lengths for `ec`.
Returns
A `204` response code.

GET

Description
Queries the role definition.
Method
GET
URL
`/pki/roles/`
Parameters
None
Returns
```javascript
{
    "data": {
        "allow_any_name": false,
        "allow_ip_sans": true,
        "allow_localhost": true,
        "allow_subdomains": false,
        "allow_token_displayname": false,
        "allowed_base_domain": "example.com",
        "client_flag": true,
        "code_signing_flag": false,
        "key_bits": 2048,
        "key_type": "rsa",
        "ttl": "6h",
        "max_ttl": "12h",
        "server_flag": true
    }
}
```

DELETE

Description
Deletes the role definition. Deleting a role does not revoke certificates previously issued under this role.
Method
DELETE
URL
`/pki/roles/`
Parameters
None
Returns
A `204` response code.