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Create valid self-signed certificates using OpenSSL

I was debugging a WebSocket connection failing with error net::ERR_INSECURE_RESPONSE, in Chrome, when I learnt that the self-signed certificate I was using was missing subject alternative names. This post brings together information I found in several different places, to create valid self-signed server certificates, using OpenSSL, that work with internet browsers such as Chrome.

Valid Certificate on IIS

To create a certificate with subject alternative names

openssl req -x509 -newkey rsa:4096 -nodes -subj '/CN=localhost' -keyout key.pem -out cert.pem -days 365 -config openssl.cnf -extensions req_ext

Additional distinguished name properties may be specified by changing the subj option

-subj "/C=US/ST=private/L=province/O=city/"

A minimalist openssl.cnf file that contains req_ext extension section with subjectAltName

[ req ]
distinguished_name = req_distinguished_name
req_extensions     = req_ext
[ req_distinguished_name ]
[ req_ext ]
subjectAltName = @alt_names
DNS.1   = localhost
DNS.2   =

Print certificate to view subject alternative names and thumbprint/fingerprint

openssl x509 -noout -text -fingerprint -in cert.pem

Create pfx from private key and certificate in pem format

openssl pkcs12 -inkey key.pem -in cert.pem -export -out key.pfx

Create crt file from certificate in pem format

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

Add private key to the appropriate key store and reconfigure server application.

Add certificate file to trusted root authorities key store. Restart the browser. It should be happy with the certificate provided by the server.

On Windows, PowerShell’s New-SelfSignedCertificate command can also be used to automate self-signed certificate creation and installation

New-SelfSignedCertificate -DnsName "localhost", "" -CertStoreLocation "cert:\LocalMachine\My"