Extract the public key from a certificate

publickey

Generate public key + sha256 rely on X.509
1. openssl s_client -connect api.github.com:443 | openssl x509 -pubkey -noout | openssl rsa -pubin -outform der | openssl dgst -sha256 -binary | openssl enc -base64
2. openssl x509 -noout -in certificate.pem -pubkey | openssl asn1parse -noout -inform pem -out public.key openssl dgst -sha256 -binary public.key | openssl enc -base64
3. openssl rsa -in my-rsa-key-file.key -outform der -pubout | openssl dgst -sha256 -binary | openssl enc -base64
4. openssl req -in my-signing-request.csr -pubkey -noout | openssl pkey -pubin -outform der | openssl dgst -sha256 -binary | openssl enc -base64
5. openssl x509 -in my-certificate.crt -pubkey -noout | openssl pkey -pubin -outform der | openssl dgst -sha256 -binary | openssl enc -base64

Covert PEM to DER
openssl x509 -outform der -in certificate.pem -out certificate.der

Covert DER to PEM
openssl x509 -inform der -in certificate.cer -out certificate.pem

Check or View Certificate (PEM)
openssl x509 -in cert.pem -text -noout
openssl x509 -in cert.cer -text -noout
openssl x509 -in cert.crt -text -noout

Check or View DER encoded Certificate
openssl x509 -in certificate.der -inform der -text -noout
openssl x509 -inform der -in cerfile.cer -noout -text
openssl x509 -inform pem -in cerfile.cer -noout -text

Source

1. https://developer.mozilla.org/en-US/docs/Web/HTTP/Public_Key_Pinning#Extracting_the_Base64_encoded_public_key_information
2. https://tools.ietf.org/html/rfc7469#appendix-A
3. https://gist.github.com/webtobesocial/5313b0d7abc25e06c2d78f8b767d4bc3
4. https://www.sslshopper.com/ssl-converter.html
5. https://support.ssl.com/Knowledgebase/Article/View/19/0/der-vs-crt-vs-cer-vs-pem-certificates-and-how-to-convert-them
6. https://github.com/scottyab/ssl-pin-generator