PKI realms structure

The concept of PKI realms is designed to provide a standardized way for various applications to access X.509 certificates and private keys. The management of the keys and certificates is moved outside of the application into a fixed directory and file structure, which other applications can access.

The application view

Different applications have different requirements for X.509 certificates and private keys. Some of them support keys and certificates in separate files, others require them combined in a single file. The PKI realm is designed to support both schemes at once.

The realms are located in the /etc/pki/realms/ directory. Each realm is contained in it's own subdirectory. By default a domain realm is configured by the role, and it's simplified directory structure looks like this:

/etc/pki/realms/
└── domain/
    ├── CA.crt
    ├── default.crt
    ├── default.key
    ├── default.pem
    └── trusted.crt

Each of these symlinks point to a file contained in another subdirectory (see the next section). The contents of these files are:

CA.crt

This is the "trust anchor" or Root Certificate Authority certificate used by the application to check the validity of client certificates. This file is publicly readable.

CA.crt may contain a CA certificate different than the one that issued the server certificate. In this case, this can be used to have separate client and server Certificate Authorities.

default.crt
This is the server certificate with optionally bundled Intermediate Certificate Authorities. It is sent to the clients during connection establishment by the application. This file is publicly readable.
default.key
This is the server private key. It's readable only by the root account and by selected UNIX group – this can be used to limit access to different private keys by different UNIX accounts. FIXME: selected UNIX group
default.pem
This file contains the private key, server certificate and Intermediate CA certificate(s). It has the same restrictions as the private key – can be read only by the root account and selected UNIX group.
trusted.crt
This is the complete trust chain of intermediate and root CA certificates, without the server certificate, similar to CA.crt. It is used for automatic OCSP stapling verification by the server, and works with the primary CA in case the alternative Certificate Authority is used for client certificates.

All of the above filenames are static, which means that the only thing you need to change to select a different PKI realm is the realm directory name.

Example nginx configuration

To use the domain realm in your nginx configuration, you can add something similar to this example in your configuration file:

server {
    listen [::]:443 ssl;

    # HTTPS support
    ssl_certificate         /etc/pki/realms/domain/default.crt;
    ssl_certificate_key     /etc/pki/realms/domain/default.key;

    # OCSP Stapling support
    ssl_stapling            on;
    ssl_stapling_verify     on;
    ssl_trusted_certificate /etc/pki/realms/domain/trusted.crt;

    # X.509 Client certificate support
    ssl_verify_client       optional;
    ssl_verify_depth        2;
    ssl_trusted_certificate /etc/pki/realms/domain/CA.crt;
}

This configuration explains where each certificate is used, but this is not sufficient to enable HTTPS for the webserver. Refer to the nginx documentation for the rest of the required configuration options.

If you use the debops.nginx Ansible role provided with the project, it has extensive integration with the debops.pki role and can configure the webserver automatically. Usually all you need to do is to make sure the default realm matches the one you would like to use for each server configuration.

The PKI realm directory structure

This is an example domain realm directory, created on each remote host managed by debops.pki. The current set of certificates active in this realm is provided by the internal debops.pki Certificate Authority:

/etc/pki/realms/
└── domain/
    ├── acme/
    ├── config/
    │   └── realm.conf
    ├── external/
    ├── internal/
    │   ├── alt_intermediate.pem
    │   ├── alt_root.pem
    │   ├── cert.pem
    │   ├── gnutls.conf
    │   ├── intermediate.pem
    │   ├── request.pem
    │   └── root.pem
    ├── private/
    │   ├── key_chain_dhparam.pem
    │   ├── key_chain.pem
    │   ├── key.pem
    │   └── realm_key.pem
    ├── public/
    │   ├── alt_intermediate.pem -> ../internal/alt_intermediate.pem
    │   ├── alt_intermediate_root.pem
    │   ├── alt_root.pem -> ../internal/alt_root.pem
    │   ├── alt_trusted.pem -> alt_intermediate_root.pem
    │   ├── cert_intermediate_dhparam.pem
    │   ├── cert_intermediate.pem
    │   ├── cert.pem -> ../internal/cert.pem
    │   ├── cert.pem.sig
    │   ├── chain.pem -> cert_intermediate_dhparam.pem
    │   ├── intermediate_root.pem
    │   ├── root.pem -> ../internal/root.pem
    │   └── trusted.pem -> intermediate_root.pem
    ├── selfsigned/ (optional)
    │   ├── cert.pem
    │   ├── gnutls.conf
    │   ├── request.pem
    │   └── root.pem -> cert.pem
    ├── CA.crt -> public/alt_trusted.pem
    ├── default.crt -> public/chain.pem
    ├── default.key -> private/key.pem
    ├── default.pem -> private/key_chain_dhparam.pem
    └── trusted.crt -> public/trusted.pem

On the Ansible Controller, there's a corresponding directory structure located in the secret/ directory maintained by the debops.secret Ansible role:

secret/pki/
├── realms/
│   ├── by-group/
│   │   └── all/
│   │       └── domain/
│   │           ├── external/
│   │           └── private/
│   └── by-host/
│       └── hostname.example.com/
│           └── domain/
│               ├── external/
│               ├── internal/
│               │   ├── alt_intermediate.pem
│               │   ├── alt_root.pem
│               │   ├── cert.pem
│               │   ├── intermediate.pem
│               │   └── root.pem
│               └── private/
└── requests/
    └── domain/
        └── hostname.example.com/
            └── domain/
                └── request.pem

Your version might not contain all of the shown files and symlinks, for example the alt_*.pem versions of intermediate and root CA certificates are only present if an alternative CA is configured.

Both directories are maintained and kept in sync using two Bash scripts provided by the role, pki-realm and pki-authority. Ansible tasks are used to copy files to and from Ansible Controller to remote hosts.

How a PKI realm is created

Each PKI realm starts with a simple directory structure created on the Ansible Controller in the secret/ directory:

secret/pki/
└── realms/
    ├── by-group/
    │   └── all/
    │       └── domain/
    │           ├── external/
    │           └── private/
    └── by-host/
        └── hostname.example.com/
            └── domain/
                ├── external/
                ├── internal/
                └── private/

These directories are created at the beginning, so that Ansible can copy private files before the actual PKI realm creation on remote hosts. This can be used to provide a set of identical private RSA keys to multiple hosts at once (using the directories in private/ subdirectories) or custom scripts that access external Certificate Authorities (using external/ subdirectories).

Next, PKI realm directories are created on the remote host:

/etc/pki/realms/
└── domain/
    ├── acme/
    ├── config/
    │   └── realm.conf
    ├── external/
    ├── internal/
    ├── private/
    └── public/

The config/realm.conf file contains a set of Bash variables that define different parameters of the PKI realm, for example the default DNS domain used to generate the certificates, owner and group of various directories and files, permissions applied to various directory and file types, and so on.

The acme/, external/ and internal/ subdirectories hold data files for different Certificate Authorities. Each CA is described in more detail in a separate document, here is a brief overview:

acme/
This directory is for certificates issued using ACME (for example Let's Encrypt). It will be activated and used automatically when a host has a public IP address and the nginx webserver is installed and configured to support ACME Challenges (see the debops.nginx role for more details).
external/

This directory is used to manage certificates signed by an external Certificate Authority. To do this, you need to provide a special script file, which will be executed with a set of environment variables. This can be used to request a certificate from an external CA, like Active Directory or FreeIPA, or download a certificate from an external location.

An alternative is to provide an already signed cert.pem file and optionally the intermediate.pem and root.pem files.

internal/
This directory is used by the internal debops.pki Certificate Authority to transfer certificate requests as well as certificates.

If the internal CA is disabled either globally for a host, or for a particular PKI realm, an alternative directory, selfsigned/ will be created. It will hold a self-signed certificate, not trusted by anything else (not even the host that has created it). This is done, so that services depending on the existence of the private keys and certificates can function correctly at all times.

The pki-realm script checks which of these directories have valid certificates in order of pki_authority_preference, and the first valid one is used as the "active" directory. Files from the active directory are symlinked to the public/ directory.

The public/ directory holds the currently active certificates which are symlinks to the real certificate files in one of the active directories mentioned above. Some additional files are also created here by the pki-realm script, namely the certificate chain (server certificate + intermediate certificates) and the trusted chain (intermediate certificates + root certificate).

The private/ directory holds the private key of a given realm. Access to this directory and files inside is restricted by UNIX permissions and only a specific system group (usually ssl-cert, but it can be configured) is allowed to access the files inside.

The next step is the creation of all necessary files, like private/public keys, certificate requests, etc. At this point, if Ansible was provided with a private RSA key to use, it will copy it to the private/ directory. After that, all necessary files are created by the pki-realm script on the remote host. The directory structure changes a bit:

/etc/pki/realms/
└── domain/
    ├── acme/
    ├── config/
    │   └── realm.conf
    ├── external/
    ├── internal/
    │   ├── gnutls.conf
    │   └── request.pem
    ├── private/
    │   ├── key.pem
    │   └── realm_key.pem
    ├── public/
    ├── CA.crt -> /etc/ssl/certs/ca-certificates.crt
    └── default.key -> private/key.pem

As you can see, the configuration of a Certificate Request for an internal CA has been created, and the internal/request.pem file has been generated, using the private/key.pem RSA key. By default, if no root.pem certificate is provided, the system CA certificate store is symlinked as CA.crt.

Afterwards, Ansible uploads the generated Certificate Signing Request (CSR) to the Ansible Controller for the internal CA to sign (if it's enabled). The CSR is uploaded to the secret/ directory:

secret/pki/
├── realms/
│   ├── by-group/
│   │   └── all/
│   │       └── domain/
│   │           ├── external/
│   │           └── private/
│   └── by-host/
│       └── hostname.example.com/
│           └── domain/
│               ├── external/
│               ├── internal/
│               └── private/
└── requests/
    └── domain/
        └── hostname.example.com/
            └── domain/
                └── request.pem

To avoid possible confusion, the secret/pki/requests/domain/ directory points to the "domain" internal CA which is an intermediate CA located under the "root" CA. The hostname.example.com/domain/ directory inside the domain/ directory points to the "domain" realm on the hostname.example.com host.

When all of the requests from the remote hosts are uploaded to the Ansible Controller, the pki-authority script inside the secret/ directory takes over and performs certificate signing for all of the currently managed hosts. The certificate named cert.pem is placed in the internal/ directory of each host according to the realm the request came from.

In addition to the certificates, the CA intermediate and root certificates are also symlinked to the internal/ directory, so that Ansible can automatically copy their contents to the remote hosts. If a particular Certificate Authority indicates that an alternative CA should be present, the alt_*.pem versions of intermediate and root certificates are also symlinked there:

secret/pki/
├── realms/
│   ├── by-group/
│   │   └── all/
│   │       └── domain/
│   │           ├── external/
│   │           └── private/
│   └── by-host/
│       └── hostname.example.com/
│           └── domain/
│               ├── external/
│               ├── internal/
│               │   ├── alt_intermediate.pem
│               │   ├── alt_root.pem
│               │   ├── cert.pem
│               │   ├── intermediate.pem
│               │   └── root.pem
│               └── private/
└── requests/
    └── domain/
        └── hostname.example.com/
            └── domain/
                └── request.pem

When all of the requests have been processed, Ansible copies the content of the directories to remote hosts. The content of the by-host/ directory is copied first and overwrites all files that are present on remote hosts, the by-group/ directory content is copied only when the corresponding files are not present. This allows the administrator to provide the shared scripts or private keys/certificates as needed, per host, per group or for all managed hosts.

After certificates signed by the internal CA are downloaded to remote hosts, the directory structure might look similar to:

/etc/pki/realms/
└── domain/
    ├── acme/
    ├── config/
    │   └── realm.conf
    ├── external/
    ├── internal/
    │   ├── alt_intermediate.pem
    │   ├── alt_root.pem
    │   ├── cert.pem
    │   ├── gnutls.conf
    │   ├── intermediate.pem
    │   ├── request.pem
    │   └── root.pem
    ├── private/
    │   ├── key.pem
    │   └── realm_key.pem
    ├── public/
    ├── CA.crt -> /etc/ssl/certs/ca-certificates.crt
    └── default.key -> private/key.pem

Other authority directories (acme/ and external/) might also contain various files.

After certificates are copied from the Ansible Controller, the pki-realm script is executed again for each PKI realm configured on a given host. It checks which authority directories have valid certificates, picks the first viable one according to pki_authority_preference and activates them.

Certificate activation entails symlinking the certificate, intermediate and root files to the public/ directory and generation of various chain files: certificate + intermediate, intermediate + root and key + certificate + intermediate (which is stored securely in the private/ directory).

Some applications do not support a separate dhparam file, and instead expect that the DHE parameters are present after the X.509 certificate chain. If the debops.dhparam role has been configured on a host and Diffie-Hellman parameter support is enabled in a given PKI realm, DHE parameters will be appended to the final certificate chains (both public and private). When debops.dhparam regenerates the parameters, the pki-realm script will automatically detect the new ones and update the certificate chains.

The end result is a fully configured PKI realm with a set of valid certificates available for other applications and services:

/etc/pki/realms/
└── domain/
    ├── acme/
    ├── config/
    │   └── realm.conf
    ├── external/
    ├── internal/
    │   ├── alt_intermediate.pem
    │   ├── alt_root.pem
    │   ├── cert.pem
    │   ├── gnutls.conf
    │   ├── intermediate.pem
    │   ├── request.pem
    │   └── root.pem
    ├── private/
    │   ├── key_chain_dhparam.pem
    │   ├── key_chain.pem
    │   ├── key.pem
    │   └── realm_key.pem
    ├── public/
    │   ├── alt_intermediate.pem -> ../internal/alt_intermediate.pem
    │   ├── alt_intermediate_root.pem
    │   ├── alt_root.pem -> ../internal/alt_root.pem
    │   ├── alt_trusted.pem -> alt_intermediate_root.pem
    │   ├── cert_intermediate_dhparam.pem
    │   ├── cert_intermediate.pem
    │   ├── cert.pem -> ../internal/cert.pem
    │   ├── cert.pem.sig
    │   ├── chain.pem -> cert_intermediate_dhparam.pem
    │   ├── intermediate_root.pem
    │   ├── root.pem -> ../internal/root.pem
    │   └── trusted.pem -> intermediate_root.pem
    ├── CA.crt -> public/alt_trusted.pem
    ├── default.crt -> public/chain.pem
    ├── default.key -> private/key.pem
    ├── default.pem -> private/key_chain_dhparam.pem
    └── trusted.crt -> public/trusted.pem

During this process, at various stages special "hook" scripts might be run, which can react to events like realm creation, activation of new certificates and so on.