How to import a .cer certificate into a java keystore?

Importing .cer certificate file downloaded from browser (open the url and dig for details) into cacerts keystore in java_home\jre\lib\security worked for me, as opposed to attemps to generate and use my own keystore.

1. Go to your java_home\jre\lib\security
2. (Windows) Open admin command line there using cmd and CTRL+SHIFT+ENTER
3. Run keytool to import certificate:
   • (Replace yourAliasName and path\to\certificate.cer respectively)

 ..\..\bin\keytool -import -trustcacerts -keystore cacerts -storepass changeit -noprompt -alias yourAliasName -file path\to\certificate.cer

This way you don't have to specify any additional JVM options and the certificate should be recognized by the JRE.

You shouldn't have to make any changes to the certificate. Are you sure you are running the right import command?

The following works for me:

keytool -import -alias joe -file mycert.cer -keystore mycerts -storepass changeit

where mycert.cer contains:

-----BEGIN CERTIFICATE-----
MIIFUTCCBDmgAwIBAgIHK4FgDiVqczANBgkqhkiG9w0BAQUFADCByjELMAkGA1UE
BhMCVVMxEDAOBgNVBAgTB0FyaXpvbmExEzARBgNVBAcTClNjb3R0c2RhbGUxGjAY
...
RLJKd+SjxhLMD2pznKxC/Ztkkcoxaw9u0zVPOPrUtsE/X68Vmv6AEHJ+lWnUaWlf
zLpfMEvelFPYH4NT9mV5wuQ1Pgurf/ydBhPizc0uOCvd6UddJS5rPfVWnuFkgQOk
WmD+yvuojwsL38LPbtrC8SZgPKT3grnLwKu18nm3UN2isuciKPF2spNEFnmCUWDc
MMicbud3twMSO6Zbm3lx6CToNFzP
-----END CERTIFICATE-----

• If you want to authenticate you need the private key - there is no other option.
• A certificate is a public key with extra properties (like company name, country,...) that is signed by some Certificate authority that guarantees that the attached properties are true.
• .CER files are certificates and don't have the private key. The private key is provided with a .PFX keystore file normally. If you really authenticate is because you already had imported the private key.

• You normally can import .CER certificates without any problems with

  keytool -importcert -file certificate.cer -keystore keystore.jks -alias "Alias" 
  

Here is the code I've been using for programatically importing .cer files into a new KeyStore.

import java.io.BufferedInputStream;
import java.io.IOException;
import java.io.InputStream;
//VERY IMPORTANT.  SOME OF THESE EXIST IN MORE THAN ONE PACKAGE!
import java.security.GeneralSecurityException;
import java.security.KeyStore;
import java.security.cert.Certificate;
import java.security.cert.CertificateFactory;

//Put everything after here in your function.
KeyStore trustStore  = KeyStore.getInstance(KeyStore.getDefaultType());
trustStore.load(null);//Make an empty store
InputStream fis = /* insert your file path here */;
BufferedInputStream bis = new BufferedInputStream(fis);

CertificateFactory cf = CertificateFactory.getInstance("X.509");

while (bis.available() > 0) {
    Certificate cert = cf.generateCertificate(bis);
    trustStore.setCertificateEntry("fiddler"+bis.available(), cert);
}