Signing jar files with jarsigner
Most of the information in this note can be found in the `help' section of the jarsigner and keytool utilities:
- 1 Criteria for Signature Validity
- 2 Use of the JarSigner Tool
- 3 Sign a given jar archive
- 4 Check that a signed jar is valid
- 5 Verify a signed jar with unknown signer
- 6 Some test with an invalid archive signature
- 7 Use of the Keytool Utility
- 8 Create a new DSA Key Pair for Bob
- 9 Visualize the content of the keystore
- 10 Extract of the public key certificate of Bob for dissemination
- 11 Visualize a certificate
- 12 Import a certificate
- 13 Visualize the content of the keystore
Criteria for Signature Validity
The criteria of validity of a digital signature are the following:
- No modification of the archive resources after the signature,
- certificate not outdated (or not yet valid).
Moreover, the signer of the archive must be known, i.e. its public key certificate must be identified as trusted before the validation. Otherwise, any malicious third party can forge a similar certificate, potentially with the same signer name, and present a coherent signed archive.
Additional criteria of archive signature validity are defined in the context of the OSGi framework, that are specific to the deployment of components from third party repositories:
- No resource removed from the archive after the signature,
- No resource added from the archive after the signature,
- The digital signature must immediately follow the Manifest file of the archive, to prevent caching malicious files.
This means that according to the security level you need, the Sun criteria of signature validity may not be sufficient.
Use of the JarSigner Tool
The Sun `Jarsigner' is a utility delivered along with Sun JDK. It has the ability to sign Java Archives (Jars), and to verify the validity their signature.
The use of the Sun Jarsigner tool is highlighted with an example of an OSGi test bundle called fridgebundle-1.1.jar. OSGi archives are a specific type of jar files.
So as to test the jarsigner tool, you need to have a public/private key pair. The example are given with Bob's key pair.
- If you want to make the tests with Bob's key pair, download the keystore file named testkeystore. Store it in the
- If you want to create your own public/private key pair, see the paragraph relative to the keytool utility to learn about it.
Following operations can be performed with the jarsigner tool. Create a
refArchive directory, and store each example bundle in it:
Sign a given jar archive
Sign the archive File:Fridgebundle-1.1.jar fridgebundle-1.1.jar with bob's private key:
jarsigner -keystore refArchive/testkeystore -signedjar refArchive/fridgebundle-1.1.signed.jar refArchive/fridgebundle-1.1.jar bob Enter Passphrase for keystore: password Enter key password for bob: bobspwd
Check that a signed jar is valid
jarsigner -verify -keystore refArchive/testkeystore refArchive/fridgebundle-1.1.signed.jar
Verify a signed jar with unknown signer
jarsigner -verify -keystore refArchive/testkeystore refArchive/fridgebundle-1.1.unknownsigner.jar
Some test with an invalid archive signature
jarsigner -verify refArchive/bindex-manifestMainAttrsModified-1.0.jar jarsigner: java.lang.SecurityException: Invalid signature file digest for Manifest main attributes
Remark: No warning is issued by the Sun Jarsigner if the signer of the archive is unknow to you. No matter who has signed the archive, this latter will be considered as valid !
Use of the Keytool Utility
The Sun keytool utility supports the management of DSA and RSA asymetric key pairs, as well as the management of public key certificates of third party actors.
- have a keystore file
Example: the keystore file is named <a href="docs_techNotes/refArchive/testkeystore">testkeystore</a>, and is accessible with the password
If you specify a keystore that does not exist in the keytool options, it is automatically created and initialized with the given parameters (e.g. the password).
The default keystore in *nix systems is /home/user/.keystore. It is overridden by the
You can perform following tests so as to learn how to use the keytool:
Create a new DSA Key Pair for Bob
Visualize the content of the keystore
Extract of the public key certificate of Bob for dissemination
The certificate is stored in the
Visualize a certificate
Import a certificate
Import 'e.g. the one of alice
Visualize the content of the keystore
You can find further informations here: