Import the Certificate as a Trusted Certificate

Suppose that you are Ruth and have received from Stan Smith

The signed JAR file sContract.jar containing a contract

The file StanSmith.cer containing the public key certificate for the public key corresponding to the private key used to sign the JAR file

Before you can use the jarsigner tool to check the authenticity of the JAR file's signature, you need to import into your keystore the certificate from Stan.
Even though you (acting as Stan) created these files and they haven't actually been transported anywhere, you can simulate being someone other than the creater and sender, Stan. Acting as Ruth, type the following to create a keystore named ruthstore and import the certificate into an entry with an alias of stan.
keytool -import -alias stan -file StanSmith.cer -keystore ruthstore
Since the keystore doesn't yet exist, it will be created. You will be prompted for a keystore password; type whatever password you want.
The keytool will print out the certificate information and ask you to verify it, for example, by comparing the displayed certificate fingerprints with those obtained from another (trusted) source of information. (Each fingerprint is a relatively short number that uniquely and reliably identifies the certificate.) For example, in the real world you might call up Stan and ask him what the fingerprints should be. He can get the fingerprints of the StanSmith.cer file he created by executing the command
keytool -printcert -file StanSmith.cer
If the fingerprints he sees are the same as the ones reported to you by keytool, the certificate has not been modified in transit. In that case you let keytool proceed with placing a "trusted certificate" entry in the keystore. The entry contains the public key certificate data from the file StanSmith.cer and is assigned the alias stan.

Exchanging Files