I would like to export my private key from a Java Keytool keystore, so I can use it with openssl. How can I do that?
Use Java keytool to convert from JKS to P12...

Export from keytool's proprietary format (called "JKS") to standardized format PKCS #12:

keytool -importkeystore \
    -srckeystore keystore.jks \
    -destkeystore keystore.p12 \
    -deststoretype PKCS12 \
    -srcalias <jkskeyalias> \
    -deststorepass <password> \
    -destkeypass <password>


...then use openssl to export from P12 to PEM

Export certificate using openssl:

openssl pkcs12 -in keystore.p12  -nokeys -out cert.pem


Export unencrypted private key:

openssl pkcs12 -in keystore.p12  -nodes -nocerts -out key.pem

