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.
This must include the curly braces surrounding the text.