Firmware signing doesn't work like what you describe. The image is signed by Juniper using their code signing private key and the installer use their code signing public key. The calculation is done and the signature is used as a reference. If someone were to modify the contents the public key will calculate a different value which won't match the reference signature.
The file checksum is not a code signing signature but an integrity signature. It provides proof the file has not been damaged during transport. You can verify the integrity signature by going to Juniper's website to obtain them without any entitlement account.
support.juniper.net
Then select the appropriate values from the OS and Version pull down menus and click on the checksum links. It's advisable you verify a file using multiple hash algorithms as it is less likely a hash collision would occur using different hashing algorithms.
The file checksum is not a code signing signature but an integrity signature. It provides proof the file has not been damaged during transport. You can verify the integrity signature by going to Juniper's website to obtain them without any entitlement account.
Downloads
Then select the appropriate values from the OS and Version pull down menus and click on the checksum links. It's advisable you verify a file using multiple hash algorithms as it is less likely a hash collision would occur using different hashing algorithms.


