How to determine certificate type from file

There doesn't seem to be any sort of standard naming convention for OpenSSL certificates, so I'd like to know if there's a simple command to get important information about any OpenSSL certificate, regardless of type. I'd like to know at least the certificate type (x509, RSA, DSA) and whether it's a public or private key. Looking at the contents of a certificate I just extracted from a PKCS12 file, neither of these are explicitly shown.


Firstly, you have a few terminology problems:

  • the X509 standard defines certificates, and RSA and DSA are two of the public key algorithms that can be used in those certificates;
  • certificates are used to hold public keys, and never private keys.
  • PKCS#12 is a standard for a container which can hold an X509 client certificates and the corresponding private keys, as well as (optionally) the X509 certificates of the CAs that signed the X509 client certificate(s).

So, if you're examining a PKCS#12 file (typically .p12 extension), then you already know:

  • It contains at least one X509 client certificate, which contains a public key; and
  • It contains the corresponding private keys.

All you don't know is whether those certificate & private key are RSA or DSA. You can check this by extracting the certificate(s), and then examine them:

openssl pkcs12 -in mycert.p12 -clcerts -nokeys -out mycert.crt
openssl x509 -in mycert.crt -text

The text output of the openssl x509 command should include a Subject Public Key section, which will include fields that let you see if it's an RSA or DSA key (along with the key size).

Certificates can also be restricted to certain use.

For example it might be restricted so that it cannot be used as a Certificate Authority. Read also more in this StackOverflow post.

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