Certificate authority management¶

django-ca supports managing multiple certificate authorities as well as child certificate authorities.

The command-line interface is the only way to create certificate authorities. It is obviously most important that the private keys are never exposed to any attacker, and any web interface would pose an unnecessary risk. Some details, like the x509 extensions used for signing certificates, can be configured using the web interface.

For the same reason, the private key of a certificate authority is stored on the file system and not in the database. The initial location of the private key is configured by the CA_DIR setting. This also means that you can run your django-ca on two hosts, where one host has the private key and only uses the command line, and one with the web interface that can still be used to revoke certificates.

Index of commands¶

To manage certificate authorities, use the following manage.py commands:

Command	Description
`dump_ca`	Write the CA certificate to a file.
`edit_ca`	Edit a certificate authority.
`import_ca`	Import an existing certificate authority.
`init_ca`	Create a new certificate authority.
`list_cas`	List all currently configured certificate authorities.
`view_ca`	View details of a certificate authority.

Like all manage.py subcommands, you can run manage.py <subcomand> -h to get a list of available parameters.

Create a new CA¶

You should be very careful when creating a new certificate authority, especially if it is used by a large number of clients. If you make a mistake here, it could make your CA unusable and you have to redistribute new public keys to all clients, which is usually a lot of work.

Please think carefully about how you want to run your CA: Do you want intermediate CAs? Do you want to use CRLs and/or run an OCSP responder?

Hostname¶

Running a CA with an OCSP responder or CRLs for certificate validation requires a web server providing HTTP. Please configure CA_DEFAULT_HOSTNAME accordingly. You can always override that setting by passing manual URLs when creating a new CA.

`pathlen` attribute¶

The pathlen attribute says how many levels of intermediate CAs can be used below a given CA. If present, it is an integer attribute (>= 0) meaning how many intermediate CAs can be below this CA. If not present, the number is unlimited. For a valid setup, all pathlen attributes of all intermediate CAs must be correct. Here is a typical (correct) example:

root   # pathlen: 2
|- child_A  # pathlen 1
   |- child_A.1  # pathlen 0
|- child_B  # pathlen 0

In this example, root and child_A can have intermediate CAs, while child_B and child_A.1 can not.

The default value for the pathlen attribute is 0, meaning that any CA cannot have any intermediate CAs. You can use the --pathlen parameter to set a different value or the --no-pathlen parameter if you don’t want to set the attribute:

# Two sublevels of intermediate CAs:
$ python manage.py init_ca --pathlen=2 ...

# unlimited number of intermediate CAs:
$ python manage.py init_ca --no-pathlen ...

CRL URLs¶

Certificate Revocation Lists (CRLs) are signed files that contain a list of all revoked certificates. Certificates (including those for CAs) can contain pointers to CRLs, usually a single URL, in the CRLDistributionPoints extension. Clients that support this extension can query the URL and refuse to establish a connection if the certificate is revoked.

Since a CRL has to be signed by the issuing CA, root CAs cannot sensibly contain a CRL: You could only revoke the root CA with it, and it would have to be signed by the (compromised) root CA.

If you have correctly configured CA_DEFAULT_HOSTNAME, you can use CRL URLs out of the box. You can also embed custom URLs in certificates, please see Host a Certificate Revocation List (CRL) for more information.

OCSP responder¶

The Online Certificate Status Protocol or OCSP is a service (called “OCSP responder”) run by a certificate authority that allows clients to query for revoked certificates. It is an improvement over CRLs particularly for larger CAs because a full CRL can grow quite big.

The same restrictions as for CRLs apply: You cannot add a OCSP URL to a root CA, it runs via HTTP (not HTTPS) and if you decide to add such URLs, you also have to actually run that service, or clients will refuse to connect.

If you have correctly configured CA_DEFAULT_HOSTNAME, you can use an OCSP responder almost out of the box, the only thing you have to do is regularly create OCSP responder keys:

$ python manage.py regenerate_ocsp_keys

Name constraints¶

NameConstraints are a little-used extension (see RFC 5280, section 4.2.1.10 that allows you to create CAs that are limited to issuing certificates for a particular set of addresses. The parsing of this syntax is quite complex, see e.g. this blog post for a good explanation.

Warning

This extension is marked as “critical”. Any client that does not understand this extension will refuse a connection.

To add name constraints to a CA, use the --name-constraint option, which can be given multiple times. Values are any valid name, see Names on the command-line for detailed documentation. Prefix the value with either permitted, or excluded, to add them to the Permitted or Excluded subtree:

$ python manage.py init_ca \
   --name-constraint permitted,DNS:com
   --name-constraint permitted,DNS:net
   --name-constraint excluded,DNS:evil.com
   ...

This will restrict the CA to issuing certificates for .com and .net subdomains, except for evil.com, which obviously should never have a certificate (evil.net is good, though).

Examples¶

Here is a shell session that illustrates the respective manage.py commands:

$ python manage.py init_ca --pathlen=2
>     --crl-url=http://ca.example.com/crl \
>     --ocsp-url=http://ocsp.ca.example.com \
>     --issuer-url=http://ca.example.com/ca.crt \
>     TestCA /C=AT/L=Vienna/L=Vienna/O=Example/OU=ExampleUnit/CN=ca.example.com
$ python manage.py list_cas
BD:5B:AB:5B:A2:1C:49:0D:9A:B2:AA:BC:68:ED:ED:7D - TestCA

$ python manage.py view_ca BD:5B:AB:5B:A2
...
* OCSP URL: http://ocsp.ca.example.com
$ python manage.py edit_ca --ocsp-url=http://new-ocsp.ca.example.com \
>     BD:5B:AB:5B:A2
$ python manage.py view_ca BD:5B:AB:5B:A2
...
* OCSP URL: http://new-ocsp.ca.example.com

Note that you can just use the start of a serial to identify the CA, as long as that still uniquely identifies the CA.

Create intermediate CAs¶

Intermediate CAs are created, just like normal CAs, using manage.py init_ca. For intermediate CAs to be valid, CAs however must have a correct pathlen x509 extension. Its value is an integer describing how many levels of intermediate CAs a CA may have. A pathlen of “0” means that a CA cannot have any intermediate CAs, if it is not present, a CA may have an infinite number of intermediate CAs.

Note

django-ca by default sets a pathlen of “0”, as it aims to be secure by default. The pathlen attribute cannot be changed in hindsight (not without resigning the CA). If you plan to create intermediate CAs, you have to consider this when creating the root CA.

So for example, if you want two levels of intermediate CAs, , you’d need the following pathlen values (the pathlen value is the minimum value, it could always be a larger number):

index	CA	`pathlen`	description
1	example.com	2	Your root CA.
2	sub1.example.com	1	Your first intermediate CA, a sub-CA from (1).
3	sub2.example.com	0	A second intermediate CA, also a sub-CA from (1).
4	sub.sub1.example.com	0	An intermediate CA of (2).

If in the above example, CA (1) had pathlen of “1” or CA (2) had a pathlen of “0”, CA (4) would no longer be a valid CA.

By default, django-ca sets a pathlen of 0, so CAs will not be able to have any intermediate CAs. You can configure the value by passing --pathlen to init_ca:

$ python manage.py init_ca --pathlen=2 ...

When creating a sub-ca, you must name its parent using the --parent parameter:

$ python manage.py list_cas
BD:5B:AB:5B:A2:1C:49:0D:9A:B2:AA:BC:68:ED:ED:7D - Root CA
$ python manage.py init_ca --parent=BD:5B:AB:5B ...