This page documents the x509 extensions (e.g. for CRLs, etc.) set by other CAs. The information here is used by django-ca to initialize and sign certificate authorities and certificates.
Helpful descriptions of the meaning of various extensions can also be found in x509v3_config(5SSL) (online).
The issuer is an X509 Name naming who signed the certificate. For root CAs, the issuer has the same value as the subject.
The "CA Issuers" is a URI pointing to the signing certificate. The certificate is in DER/ASN1 format and has a Content-Type: application/x-x509-ca-cert header (except where noted).
Let's Encrypt is notable here because its CA Issuers field points to a PKCS#7 file and the HTTP response returns a Content-Type: application/x-pkcs7-mime header.
The certificate pointed to by the CA Issuers field is the root certificate (so the Comodo DV CA points to the AddTrust CA that signed the Comodo Root CA).
Let's Encrypt is again special in that the response has a Content-Type: application/pkix-cert header (but at least it's in DER format like every other certificate). RapidSSL uses Content-Type: text/plain.
The CA Issuers field sometimes points to the signing certificate (e.g. StartSSL) or to the root CA (e.g. Comodo DV, which points to the AddTrust Root CA)
A hash identifying the CA used to sign the certificate. In theory the identifier may also be based on the issuer name and serial number, but in the wild, all certificates reference the subjectKeyIdentifier. Self-signed certificates (e.g. Root CAs, like StartSSL and Comodo below) will reference themself, while signed certificates reference the signed CA, e.g.:
| Name | SubjectKeyIdentifier | AuthorityKeyIdentifier |
|---|---|---|
| Root CA | foo | foo |
| Intermediate CA | bar | foo |
| Client Cert | foobar | bar |
Root CAs usually have a value identical to the subjectKeyIdentifier, but some root CAs do not include this extension at all.
The BasicConstraints extension specifies if the certificate can be used as a certificate authority. It is always marked as critical. The pathlen attribute specifies the levels of possible intermediate CAs. If not present, the level of intermediate CAs is unlimited, a pathlen:0 means that the CA itself can not issue certificates with CA:TRUE itself.
Most root CAs do not set a Path Length, while most (but not all) intermediate CAs set a Path Length of 0.
Notable here that some end-user certificates do not mark this extension as critical.
In theory a complex multi-valued extension, this extension usually just holds a URI pointing to a Certificate Revocation List (CRL).
Root certificate authorities (StartSSL, GeoTrust Global, GlobalSign) do not set this field. This usually isn't a problem since clients have a list of trusted root certificates anyway, and browsers and distributions should get regular updates on the list of trusted certificates.
All CRLs linked here are all in DER/ASN1 format, and the Content-Type header in the response is set to application/pkix-crl. Only Comodo uses application/x-pkcs7-crl, but it is also in DER/ASN1 format.
Let's Encrypt is so far the only CA that does not maintain a CRL for signed certificates. Major CAs usually don't fancy CRLs much because they are a large file (e.g. the CRL from Comodo is 1.5MB) containing all certificates and cause major traffic for CAs. OCSP is just better in every way.
A list of purposes for which the certificate can be used for. CA certificates usually do not set this field.
Only StartSSL sets this field in its signed certificates. It's a URI pointing to their homepage.
List of permitted key usages. Usually marked as critical, except for certificates signed by StartSSL.
This extension is only valid in CAs and must be marked as critical, according to RFC 5280.
Only the expired Let's Encrypt X1 sets this extension to exclude .mil, and does not set this extension as critical.
This extension is used for Certificate Transparency and only makes sense in client certificates. It is usually not marked as critical (since many clients do not support Certificate Transparency).
The SubjectAlternativeName extension is not present in any CA certificate, and of course whatever the customer requests in signed certificates.
| CA | Value |
|---|---|
| Let's Encrypt | - |
| StartSSL | - |
| StartSSL Class 2 | - |
| StartSSL Class 3 | - |
| GeoTrust Global | - |
| RapidSSL G3 | - |
| Comodo | - |
| Comodo DV | - |
| GlobalSign | - |
| GlobalSign DV | - |
The SubjectKeyIdentifier extension provides a means of identifying certificates. It is a mandatory extension for CA certificates. Currently only RapidSSL does not set this for signed certificates.
The value of the SubjectKeyIdentifier extension reappears in the authorityKeyIdentifier extension.
Extensions used by certificates encountered in the wild that django-ca does not (yet) support in any way.
Currently only the old StartSSL root CA has any unknown extension.
Currently no tested cert has any unknown extensions.
The values of extensions and values of CRLs found in the wild.
The value of this extension matches the SubjectKeyIdentifier of the CA that signed the CRL.