I am trying to generate a private-public key pair and convert the public key into a certificate which can be added into my truststore.

To generate private & public key: openssl rsa -in private.pem -outform PEM -pubout -out public_key.pem

Now I am trying to convert this to a certificate:

openssl x509 -outform der -in  public_key.pem -out  public.cer 

But I get an error:

7962:error:0906D06C:PEM routines:PEM_read_bio:no start line:/BuildRoot/Library/Caches/com.apple.xbs/Sources/OpenSSL098/OpenSSL098-64.30.2/src/crypto/pem/pem_lib.c:648:Expecting: TRUSTED CERTIFICATE

All tutorials show that I have to convert pem to crt before adding to a truststore.


You cannot "convert" a public key to a certificate. A certificate includes the public key but it includes also more information like the subject, the issuer, when the certificate is valid etc. And a certificate is signed by the issuer. Thus what you would need instead is to create a certificate signing request (CSR) which includes the public key but also includes all the additional information. This CSR then needs to be signed by a certificate authority (CA) which then results in the certificate.

For creating a simple self-signed certificate which is not trusted by any browser see How to create a self-signed certificate with openssl?.

  • So in this example: openssl req -x509 -newkey rsa:4096 -keyout key.pem -out cert.pem -days 365 key.pem will contain both private and public key? – user1692342 Feb 8 '17 at 17:41
  • @user1692342: I'm not sure how the question in the comment relates to the original question. But: key.pem is the private key which includes the public key and the public key is also included in cert.pem. – Steffen Ullrich Feb 8 '17 at 17:47

You cannot convert a public key into a certificate.

The original commands will not work since the PEM encoding / file format is expecting to contain the encrypted certificate text like below:


Certificate data here


Therefore if you view the original .PEM file and see something else (like BEGIN RSA ... ) then that is incorrect.

Now according to the thread title you are seeking to convert a PEM into a CRT file format. Note that x509 certificates can be in two encodings - DER and PEM. Also, PEM can be within a .CRT, .CER and also .PEM format.

Therefore if you see that error there is also a chance that you are treating a DER encoded certificate as a PEM encoded certificate. You can try to see if it's actually DER encoded by following the instructions in this page.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.