In this post, I am going to explain how to sign a jar file using jarsigner tool. ‘jarsigner’ tool is bundled as part of jdk distribution. To proceed with this tutorial, install jdk in your system.

Signing a jar file is a three step process.

a.   Create a public and private key pair

b.   Export the Certificate to file system

c.    Sign the jar using jarsigner tool

d.   Verify the signed jar

Generate key pair

Syntax

keytool -genkey -alias {ALIAS_NAME} -keyalg {ALGORITHM_TO_GENERATE_KEY_PAIR} -keypass {KEY_PASSWORD} -storepass {STORE_PASSWORD} -keystore {KEY_STORE_FILE}

Example

keytool -genkey -alias keyPairToSignJar -keyalg RSA -keypass password123 -storepass password123 -keystore demo.jks

$ keytool -genkey -Alias Keypairtosignjar -keyalg RSA -keypass password123 -storepass password123 -Keystore demo.jks
What is your first and last name?
  [Unknown]:  java blogspot
What is the name of your organizational unit?
  [Unknown]:  bloggers
What is the name of your organization?
  [Unknown]:  https://self-learning-java-tutorial.blogspot.com/
What is the name of your City or Locality?
  [Unknown]:  Bangalore
What is the name of your State or Province?
  [Unknown]:  Karnataka
What is the two-letter country code for this unit?
  [Unknown]:  in
Is CN=java blogspot, OU=bloggers, O=https://self-learning-java-tutorial.blogspot.com/, L=Bangalore, ST=Karnataka, C=in correct?
  [no]:  y


Warning:
The JKS keystore uses a proprietary format. It is recommended to migrate to PKCS12 which is an industry standard format using "keytool -importkeystore -srckeystore demo.jks -destkeystore demo.jks -deststoretype pkcs12".

Migrate the demo.jks file to pkcs12 by executing below command.

keytool -importkeystore -srckeystore demo.jks -destkeystore demo.jks -deststoretype pkcs12

bash-3.2$ keytool -importkeystore -srckeystore demo.jks -destkeystore demo.jks -deststoretype pkcs12
Enter source keystore password:  
Entry for alias keypairtosignjar successfully imported.
Import command completed:  1 entries successfully imported, 0 entries failed or cancelled

Warning:
Migrated "demo.jks" to Non JKS/JCEKS. The JKS keystore is backed up as "demo.jks.old".

When you list the content of this directory, you can see two files.

bash-3.2$ ls
demo.jks	demo.jks.old

Export the certificate to file system

Syntax

keytool -export -alias {ALIAS_NAME} -storepass {STORE_PASSWORD} -file {FILE_TO_STORE_THE_CERTIFICATE} -keystore {KEY_STORE_FILE}

Example

keytool -export -alias keyPairToSignJar -storepass password123 -file myCert.cer -keystore demo.jks

bash-3.2$ keytool -export -alias keyPairToSignJar -storepass password123 -file myCert.cer -keystore demo.jks
Certificate stored in file 
bash-3.2$
bash-3.2$
bash-3.2$
bash-3.2$ ls
demo.jks	demo.jks.old	myCert.cer

Sign the jar using jarsigner tool

Syntax

jarsigner -keystore {KEY_STORE_FILE} -signedjar {SIGNED_JAR_NAME} {JAR_TO_SIGN} {ALIAS_NAME}

Example

jarsigner -keystore demo.jks -signedjar signed_hello.jar hello.jar keyPairToSignJar

Above command sign hello.jar file and generate signed_hello.jar file.

bash-3.2$ jarsigner -keystore demo.jks -signedjar signed_hello.jar hello.jar keyPairToSignJar
Enter Passphrase for keystore: 
jar signed.

Warning: 
The signer's certificate is self-signed.
bash-3.2$ 
bash-3.2$ 
bash-3.2$ 
bash-3.2$ ls
demo.jks		demo.jks.old		hello.jar		myCert.cer		signed_hello.jar

Verify the signed jar file

Syntax

jarsigner -verify {SIGNED_JAR_NAME}

Example

jarsigner -verify signed_hello.jar

bash-3.2$ jarsigner -verify signed_hello.jar

jar verified.

Warning: 
This jar contains entries whose certificate chain is invalid. Reason: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
This jar contains entries whose signer certificate is self-signed.
This jar contains entries whose signer certificate will expire within six months. 
This jar contains signatures that do not include a timestamp. Without a timestamp, users may not be able to validate this jar after any of the signer certificates expire (as early as 2022-09-15).

Re-run with the -verbose and -certs options for more details.

We can remove PKIX and other warning, by adding this self-signed certificate to the java cacerts file.

Add the self signed certificate to Java cacerts file

Syntax

keytool -import -v -trustcacerts -alias {ALIAS_NAME}  -file {CERTIFICATE_FILE} -keystore {CACERTS_FILE_PATH}

Example

keytool -import -v -trustcacerts -alias keyPairToSignJar  -file myCert.cer -keystore /Library/Java/JavaVirtualMachines/jdk1.8.0_311.jdk/Contents/Home/jre/lib/security/cacerts

Since this command require admin privileges to update cacerts file, I ran this command in sudo mode.

bash-3.2$ sudo keytool -import -v -trustcacerts -alias keyPairToSignJar  -file myCert.cer -keystore /Library/Java/JavaVirtualMachines/jdk1.8.0_311.jdk/Contents/Home/jre/lib/security/cacerts
Password:
Enter keystore password:  
Owner: CN=java blogspot, OU=bloggers, O=https://self-learning-java-tutorial.blogspot.com/, L=Bangalore, ST=Karnataka, C=in
Issuer: CN=java blogspot, OU=bloggers, O=https://self-learning-java-tutorial.blogspot.com/, L=Bangalore, ST=Karnataka, C=in
Serial number: 396d9471
Valid from: Fri Jun 17 14:59:01 IST 2022 until: Thu Sep 15 14:59:01 IST 2022
Certificate fingerprints:
	 SHA1: 0C:0E:F4:4A:14:85:92:4B:78:F2:6A:74:EB:FF:E5:E0:93:2D:A1:93
	 SHA256: 23:7E:DD:B1:BC:5F:92:24:4F:C1:55:9B:03:54:0E:7C:1C:18:F7:59:04:3C:18:08:EF:09:C4:A0:F8:00:3E:CE
Signature algorithm name: SHA256withRSA
Subject Public Key Algorithm: 2048-bit RSA key
Version: 3

Extensions: 

#1: ObjectId: 2.5.29.14 Criticality=false
SubjectKeyIdentifier [
KeyIdentifier [
0000: F3 81 B5 68 6D F8 4D D9   69 DB 59 B9 77 04 23 58  ...hm.M.i.Y.w.#X
0010: 9F 83 9B FA                                        ....
]
]

Trust this certificate? [no]:  yes
Certificate was added to keystore
[Storing /Library/Java/JavaVirtualMachines/jdk1.8.0_311.jdk/Contents/Home/jre/lib/security/cacerts]

‘changeit’ is the default password for cacerts file.

Let’s re verify the signed jar file.

bash-3.2$ jarsigner -verify signed_hello.jar

jar verified.

Warning: 
This jar contains entries whose signer certificate is self-signed.

Re-run with the -verbose and -certs options for more details.

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