Extract passphrase from Jenkins' credentials.xml

First, you need to get the encrypted value which is conveniently placed in the value attribute of the password field of that credentials item you are interested in. Navigate to the credentials item in Jenkins UI you, click Inspect Element on the password field, and copy its value attribute (something like {AQAABAAAa6VBbyzg5AWMW2RnfaBaj46}

Then, go to JENKINS_URL/script and execute println( hudson.util.Secret.decrypt("{AQAABAAAa6VBbyzg5AWMW2RnfaBaj46}") ); decrypted password appears under the input field

Open your Jenkins' installation's script console by visiting http(s)://${JENKINS_ADDRESS}/script.

There, execute the following Groovy script:

println( hudson.util.Secret.decrypt("${ENCRYPTED_PASSPHRASE_OR_PASSWORD}") )

where ${ENCRYPTED_PASSPHRASE_OR_PASSWORD} is the encrypted content of the <password> or <passphrase> XML element that you are looking for.