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SSL Certificates

To create the Self sighned SSL Certificate using configuration

distinguished_name = req_distinguished_name
x509_extensions = v3_req
prompt = no

C = US
ST = California
L = San Francisco
CN =

subjectKeyIdentifier = hash
authorityKeyIdentifier = keyid,issuer
basicConstraints = CA:TRUE
subjectAltName = @alt_names

DNS.1 = *
#DNS.2 = <2nd Domain>
#DNS.3 = <3rd Domain>
#DNS.4 = <4th Domain>
## on so on, and so forth.
openssl genrsa -out mysite.key 2048 -nodes
openssl req -new -x509 -key mysite.key -sha256 -config mysite.cnf -out mysite.crt -days 730

Another way to create the certificate

Create a root certificate and private key to sign the certificates for your services:

$ openssl req -x509 -sha256 -nodes -days 365 -newkey rsa:2048 -subj '/O=example Inc./' -keyout -out

Create a certificate and a private key for

$ openssl req -out -newkey rsa:2048 -nodes -keyout -subj "/ organization"
$ openssl x509 -req -days 365 -CA -CAkey -set_serial 0 -in -out

Ceritficates and encoding

X.509 certificate is a digital document that has been encoded and/or digitally signed according to RFC 5280. The term X.509 certificate usually refers to PKIX for Public Key Infrastructure (X.509).

There is a lot of confusion about what DER, PEM, CRT, and CER are. While in certain cases some can be interchanged the best practice is to identify how your certificate is encoded and then label it correctly. Correctly labeled certificates will be much easier to manipulat

Encodings (also used as extensions)

  • .DER = The DER extension is used for binary DER encoded certificates. These files may also bear the CER or the CRT extension. Proper English usage would be “I have a DER encoded certificate” not “I have a DER certificate”.
  • .PEM = The PEM extension is used for different types of X.509v3 files which contain ASCII (Base64) armored data prefixed with a “—– BEGIN …” line.

Common Extensions

  • .CRT = The CRT extension is used for certificates. The certificates may be encoded as binary DER or as ASCII PEM. The CER and CRT extensions are nearly synonymous. Most common among *nix systems
  • CER = alternate form of .crt (Microsoft Convention) You can use MS to convert .crt to .cer (.both DER encoded .cer, or base64[PEM] encoded .cer) The .cer file extension is also recognized by IE as a command to run a MS cryptoAPI command (specifically rundll32.exe cryptext.dll,CryptExtOpenCER) which displays a dialogue for importing and/or viewing certificate contents.
  • .KEY = The KEY extension is used both for public and private PKCS#8 keys. The keys may be encoded as binary DER or as ASCII PEM.

The only time CRT and CER can safely be interchanged is when the encoding type can be identical. (ie PEM encoded CRT = PEM encoded CER)

Common OpenSSL Certificate Manipulations

Even though PEM encoded certificates are ASCII they are not human readable. Here are some commands that will let you output the contents of a certificate in human readable form;

View PEM encoded certificate

Use the command that has the extension of your certificate replacing with the name of your certificate

openssl x509 -in cert.pem -text -noout
openssl x509 -in cert.cer -text -noout
openssl x509 -in cert.crt -text -noout

View DER encoded Certificate

openssl x509 -in certificate.der -inform der -text -noout



openssl x509 -in cert.crt -outform der -out cert.der


openssl x509 -in cert.crt -inform der -outform pem -out cert.pem


In some cases it is advantageous to combine multiple pieces of the X.509 infrastructure into a single file. One common example would be to combine both the private key and public key into the same certificate.

The easiest way to combine certs keys and chains is to convert each to a PEM encoded certificate then simple copy the contents of each file into a new file. This is suitable for combining files to use in applications like Apache.


Some certs will come in a combined form. Where one file can contain any one of: Certificate, Private Key, Public Key, Signed Certificate, Certificate Authority (CA), and/or Authority Chain.


Convert a PKCS#12 file (.pfx .p12) containing a private key and certificates to PEM

openssl pkcs12 -in keyStore.pfx -out keyStore.pem -nodes

You can add -nocerts to only output the private key or add -nokeys

Convert a PEM certificate file and a private key to PKCS#12 (.pfx .p12)

openssl pkcs12 -export -out certificate.pfx -inkey privateKey.key -in certificate.crt -certfile CACert.crt