jarsigner - JAR Signing and Verification Tool

Generates signatures for Java ARchive (JAR) files, and verifies the signatures of signed JAR files.

SYNOPSIS

jarsigner [ options ] jar-file alias

DESCRIPTION

The jarsigner tool is used for two purposes:
  1. to sign Java ARchive (JAR) files, and

  2. to verify the signatures and integrity of signed JAR files.

The JAR feature enables the packaging of class files, images, sounds, and other digital data in a single file for faster and easier distribution. A tool named jar enables developers to produce JAR files. (Technically, any zip file can also be considered a JAR file, although when created by jar or processed by jarsigner, JAR files also contain a META-INF/MANIFEST.MF file.)

A digital signature is a string of bits that is computed from some data (the data being "signed") and the private key of an entity (a person, company, etc.). Like a handwritten signature, a digital signature has many useful characteristics:

In order for an entity's signature to be generated for a file, the entity must first have a public/private key pair associated with it, and also one or more certificates authenticating its public key. A certificate is a digitally signed statement from one entity, saying that the public key of some other entity has a particular value.

jarsigner uses key and certificate information from a keystore to generate digital signatures for JAR files. A keystore is a database of private keys and their associated X.509 certificate chains authenticating the corresponding public keys. The keytool utility is used to create and administer keystores.

jarsigner uses an entity's private key to generate a signature. The signed JAR file contains, among other things, a copy of the certificate from the keystore for the public key corresponding to the private key used to sign the file. jarsigner can verify the digital signature of the signed JAR file using the certificate inside it (in its signature block file).

At this time, jarsigner can only sign JAR files created by the JDK jar tool or zip files. (JAR files are the same as zip files, except they also have a META-INF/MANIFEST.MF file. Such a file will automatically be created when jarsigner signs a zip file.)

The default jarsigner behavior is to sign a JAR (or zip) file. Use the -verify option to instead have it verify a signed JAR file.

Compatibility with JDK 1.1

The keytool and jarsigner tools completely replace the javakey tool provided in JDK 1.1. These new tools provide more features than javakey, including the ability to protect the keystore and private keys with passwords, and the ability to verify signatures in addition to generating them.

The new keystore architecture replaces the identity database that javakey created and managed. There is no backwards compatibility between the keystore format and the database format used by javakey in 1.1. However,

Keystore Aliases

All keystore entities are accessed via unique aliases.

When using jarsigner to sign a JAR file, you must specify the alias for the keystore entry containing the private key needed to generate the signature. For example, the following will sign the JAR file named "MyJarFile.jar", using the private key associated with the alias "duke" in the keystore named "mystore" in the "working" directory. Since no output file is specified, it overwrites MyJarFile.jar with the signed JAR file. 

    jarsigner -keystore /working/mystore -storepass myspass
      -keypass dukekeypasswd MyJarFile.jar duke

Keystores are protected with a password, so the store password (in this case "myspass") must be specified. You will be prompted for it if you don't specify it on the command line. Similarly, private keys are protected in a keystore with a password, so the private key's password (in this case "dukekeypasswd") must be specified, and you will be prompted for it if you don't specify it on the command line and it isn't the same as the store password.

Keystore Location

jarsigner has a -keystore option for specifying the URL of the keystore to be used. The keystore is by default stored in a file named .keystore in the user's home directory, as determined by the "user.home" system property. On Solaris systems "user.home" defaults to the user's home directory.

Keystore Implementation

The KeyStore class provided in the java.security package supplies well-defined interfaces to access and modify the information in a keystore. It is possible for there to be multiple different concrete implementations, where each implementation is that for a particular type of keystore.

Currently, there are two command-line tools that make use of keystore implementations (keytool and jarsigner), and also a GUI-based tool named policytool. Since KeyStore is publicly available, JDK users can write additional security applications that use it.

There is a built-in default implementation, provided by Sun Microsystems. It implements the keystore as a file, utilizing a proprietary keystore type (format) named "JKS". It protects each private key with its individual password, and also protects the integrity of the entire keystore with a (possibly different) password.

Keystore implementations are provider-based. More specifically, the application interfaces supplied by KeyStore are implemented in terms of a "Service Provider Interface" (SPI). That is, there is a corresponding abstract KeystoreSpi class, also in the java.security package, which defines the Service Provider Interface methods that "providers" must implement. (The term "provider" refers to a package or a set of packages that supply a concrete implementation of a subset of services that can be accessed by the Java Security API.) Thus, to provide a keystore implementation, clients must implement a "provider" and supply a KeystoreSpi subclass implementation, as described in How to Implement a Provider for the Java Cryptography Architecture.

Applications can choose different types of keystore implementations from different providers, using the "getInstance" factory method supplied in the KeyStore class. A keystore type defines the storage and data format of the keystore information, and the algorithms used to protect private keys in the keystore and the integrity of the keystore itself. Keystore implementations of different types are not compatible. The keytool, jarsigner, and policytool tools can be used with any type of keystore, because they only use the common application interfaces supplied by KeyStore.

The tools currently choose a keystore implementation based simply on the value of the keystore.type property specified in the security properties file. The security properties file is called java.security, and it resides in the JDK security properties directory, java.home/lib/security, where java.home is the JDK installation directory.

Each tool gets the keystore.type value and then examines all the currently-installed providers until it finds one that implements keystores of that type. It then uses the keystore implementation from that provider.

The KeyStore class defines a static method named getDefaultType that lets applications and applets retrieve the value of the keystore.type property. The following line of code creates an instance of the default keystore type (as specified in the keystore.type property): 

    KeyStore keyStore = KeyStore.getInstance(KeyStore.getDefaultType());

The default keystore type is "jks" (the proprietary type of the keystore implementation provided by Sun). This is specified by the following line in the security properties file: 

    keystore.type=jks

To have the tools utilize a keystore implementation other than the default, change that line to specify a different keystore type.

For example, if you have a provider package that supplies a keystore implementation for a keystore type called "pkcs12", change the line to 

    keystore.type=pkcs12
Note: case doesn't matter in keystore type designations. For example, "JKS" would be considered the same as "jks".

Alternatively, for keytool, you can specify a keystore type at the command line, via the -storetype option; and for policytool, you can specify a keystore type via the "Change Keystore" command in the Edit menu.

Supported Algorithms and Key Sizes

At this time, jarsigner can sign a JAR file using either

That is, if the signer's public and private keys are DSA keys, jarsigner will sign the JAR file using the "SHA1withDSA" algorithm. If the signer's keys are RSA keys, jarsigner will sign the JAR file using the "MD5withRSA" algorithm. This is only possible if there is a statically installed provider supplying an implementation for the "MD5withRSA" algorithm. (There is always a "SHA1withDSA" algorithm available, from the default "SUN" provider.)

The Signed JAR File

When jarsigner is used to sign a JAR file, the output signed JAR file is exactly the same as the input JAR file, except that it has two additional files placed in the META-INF directory:

The base file names for these two files come from the value of the -sigFile option. For example, if the option appears as 
  -sigFile MKSIGN
the files are named "MKSIGN.SF" and "MKSIGN.DSA".

If no -sigfile option appears on the command line, the base file name for the .SF and .DSA files will be the first 8 characters of the alias name specified on the command line, all converted to upper case. If the alias name has fewer than 8 characters, the full alias name is used. If the alias name contains any characters that are not legal in a signature file name, each such character is converted to an underscore ("_") character in forming the file name.

The Signature (.SF) File

A signature file (the .SF file) looks similar to the manifest file that is always included in a JAR file when jarsigner is used to sign the file. That is, for each source file included in the JAR file, the .SF file has three lines, just as in the manifest file, listing the following:
  • the file name,

  • the name of the digest algorithm used (SHA), and

  • a SHA digest value.
In the manifest file, the SHA digest value for each source file is the digest (hash) of the binary data in the source file. In the .SF file, on the other hand, the digest value for a given source file is the hash of the three lines in the manifest file for the source file.

The signature file also, by default, includes a header containing a hash of the whole manifest file. The presence of the header enables verification optimization, as described in JAR File Verification.

The Signature Block (.DSA) File

The .SF file is signed and the signature is placed in the .DSA file. The .DSA file also contains, encoded inside it, the certificate or certificate chain from the keystore which authenticates the public key corresponding to the private key used for signing.

JAR File Verification

A successful JAR file verification occurs if the signature(s) are valid, and none of the files that were in the JAR file when the signatures were generated have been changed since then. JAR file verification involves the following steps:

  1. Verify the signature of the .SF file itself.

    That is, the verification ensures that the signature stored in each signature block (.DSA) file was in fact generated using the private key corresponding to the public key whose certificate (or certificate chain) also appears in the .DSA file. It also ensures that the signature is a valid signature of the corresponding signature (.SF) file, and thus the .SF file has not been tampered with.

  2. Verify the digest listed in each entry in the .SF file with each corresponding section in the manifest.

    The .SF file by default includes a header containing a hash of the entire manifest file. When the header is present, then the verification can check to see whether or not the hash in the header indeed matches the hash of the manifest file. If that is the case, verification proceeds to the next step.

    If that is not the case, a less optimized verification is required to ensure that the hash in each source file information section in the .SF file equals the hash of its corresponding section in the manifest file (see The Signature (.SF) File).

    One reason the hash of the manifest file that is stored in the .SF file header may not equal the hash of the current manifest file would be because one or more files were added to the JAR file (using the jar tool) after the signature (and thus the .SF file) was generated. When the jar tool is used to add files, the manifest file is changed (sections are added to it for the new files), but the .SF file is not. A verification is still considered successful if none of the files that were in the JAR file when the signature was generated have been changed since then, which is the case if the hashes in the non-header sections of the .SF file equal the hashes of the corresponding sections in the manifest file.

  3. Read each file in the JAR file that has an entry in the .SF file. While reading, compute the file's digest, and then compare the result with the digest for this file in the manifest section. The digests should be the same, or verification fails.

If any serious verification failures occur during the verification process, the process is stopped and a security exception is thrown. It is caught and displayed by jarsigner.

Multiple Signatures for a JAR File

A JAR file can be signed by multiple people simply by running the jarsigner tool on the file twice, specifying the alias for a different person each time, as in: 
  jarsigner myBundle.jar susan
  jarsigner myBundle.jar kevin
When a JAR file is signed multiple times, there are multiple .SF and .DSA files in the resulting JAR file, one pair for each signature. Thus, in the example above, the output JAR file includes files with the following names: 
  SUSAN.SF
  SUSAN.DSA
  KEVIN.SF
  KEVIN.DSA
Note: It is also possible for a JAR file to have mixed signatures, some generated by the JDK 1.1 javakey tool and others by jarsigner. That is, jarsigner can be used to sign JAR files already previously signed using javakey.

OPTIONS

The various jarsigner options are listed and described below. Note:

-keystore url
Specifies the URL that tells the keystore location. This defaults to the file .keystore in the user's home directory, as determined by the "user.home" system property.

A keystore is required when signing, so you must explicitly specify one if the default keystore does not exist (or you want to use one other than the default).

A keystore is not required when verifying, but if one is specified, or the default exists, and the -verbose option was also specified, additional information is output regarding whether or not any of the certificates used to verify the JAR file are contained in that keystore.

Note: the -keystore argument can actually be a file name (and path) specification rather than a URL, in which case it will be treated the same as a "file:" URL. That is, 

  -keystore filePathAndName
is treated as equivalent to 
  -keystore file:filePathAndName

-storepass password
Specifies the password which is required to access the keystore. This is only needed when signing (not verifying) a JAR file. In that case, if a -storepass option is not provided at the command line, the user is prompted for the password.

Note: The password shouldn't be specified on the command line or in a script unless it is for testing purposes, or you are on a secure system. Also, when typing in a password at the password prompt, the password is echoed (displayed exactly as typed), so be careful not to type it in front of anyone.

-keypass password
Specifies the password used to protect the private key of the keystore entry addressed by the alias specified on the command line. The password is required when using jarsigner to sign a JAR file. If no password is provided on the command line, and the required password is different from the store password, the user is prompted for it.

Note: The password shouldn't be specified on the command line or in a script unless it is for testing purposes, or you are on a secure system. Also, when typing in a password at the password prompt, the password is echoed (displayed exactly as typed), so be careful not to type it in front of anyone.

-sigfile file
Specifies the base file name to be used for the generated .SF and .DSA files. For example, if file is "DUKESIGN", the generated .SF and .DSA files will be named "DUKESIGN.SF" and "DUKESIGN.DSA", and will be placed in the "META-INF" directory of the signed JAR file.

The characters in file must come from the set "A-Z0-9_-". That is, only letters, numbers, underscore, and hyphen characters are allowed.

If no -sigfile option appears on the command line, the base file name for the .SF and .DSA files will be the first 8 characters of the alias name specified on the command line, all converted to upper case. If the alias name has fewer than 8 characters, the full alias name is used. If the alias name contains any characters that are not legal in a signature file name, each such character is converted to an underscore ("_") character in forming the file name.

-signedjar file
Specifies the name to be used for the signed JAR file.

If no name is specified on the command line, the name used is the same as the input JAR file name (the name of the JAR file to be signed); in other words, that file is overwritten with the signed JAR file.

-verify
If this appears on the command line, the specified JAR file will be verified, not signed. If the verification is successful, "jar verified" will be displayed. If you try to verify an unsigned JAR file, or a JAR file signed with an unsupported algorithm (e.g., RSA when you don't have an RSA provider installed), the following is displayed: "jar is unsigned. (signatures missing or not parsable)"

It is possible to verify JAR files signed using either jarsigner or the JDK 1.1 javakey tool, or both.

For further information on verification, see JAR File Verification.

-certs
If this appears on the command line, along with the -verify and -verbose options, the output includes certificate information for each signer of the JAR file. This information includes

  • the name of the type of certificate (stored in the .DSA file) that certifies the signer's public key,

  • if the certificate is an X.509 certificate (more specifically, an instance of java.security.cert.X509Certificate): the distinguished name of the signer, and

  • in parentheses, the alias name for the keystore entry (if any) for that signer. If the signer actually comes from a JDK 1.1 identity database instead of from a keystore, the alias name will appear in brackets instead of parentheses.

-verbose
If this appears on the command line, it indicates "verbose" mode, which causes jarsigner to output extra information as to the progress of the JAR signing or verification.

-internalsf
In the past, the .DSA (signature block) file generated when a JAR file was signed used to include a complete encoded copy of the .SF file (signature file) also generated. This behavior has been changed. To reduce the overall size of the output JAR file, the .DSA file by default doesn't contain a copy of the .SF file anymore. But if -internalsf appears on the command line, the old behavior is utilized. This option is mainly useful for testing; in practice, it should not be used, since doing so eliminates a useful optimization.

-sectionsonly
If this appears on the command line, the .SF file (signature file) generated when a JAR file is signed does not include a header containing a hash of the whole manifest file. It just contains information and hashes related to each individual source file included in the JAR file, as described in The Signature (.SF) File .

By default, this header is added, as an optimization. When the header is present, then whenever the JAR file is verified, the verification can first check to see whether or not the hash in the header indeed matches the hash of the whole manifest file. If so, verification proceeds to the next step. If not, it is necessary to do a less optimized verification that the hash in each source file information section in the .SF file equals the hash of its corresponding section in the manifest file.

For further information, see JAR File Verification.

This option is mainly useful for testing; in practice, it should not be used, since doing so eliminates a useful optimization.

-Jjavaoption
Passes through the specified javaoption string directly to the Java interpreter. (jarsigner is actually a "wrapper" around the interpreter.) This option should not contain any spaces. It is useful for adjusting the execution environment or memory usage. For a list of possible interpreter options, type java -h or java -X at the command line.

EXAMPLES

Signing a JAR File

Suppose you have a JAR file named "bundle.jar" and you'd like to sign it using the private key of the user whose keystore alias is "jane" in the keystore named "mystore" in the "working" directory. Suppose the keystore password is "myspass" and the password for jane's private key is "j638klm". You can use the following to sign the JAR file and name the signed JAR file "sbundle.jar": 

    jarsigner -keystore /working/mystore -storepass myspass
      -keypass j638klm -signedjar sbundle.jar bundle.jar jane

Note that there is no -sigfile specified in the command above, so the generated .SF and .DSA files to be placed in the signed JAR file will have default names based on the alias name. That is, they will be named JANE.SF and JANE.DSA.

If you want to be prompted for the store password and the private key password, you could shorten the above command to 

    jarsigner -keystore /working/mystore
      -signedjar sbundle.jar bundle.jar jane
If the keystore to be used is the default keystore (the one named ".keystore" in your home directory), you don't need to specify a keystore, as in: 
    jarsigner -signedjar sbundle.jar bundle.jar jane
Finally, if you want the signed JAR file to simply overwrite the input JAR file (bundle.jar), you don't need to specify a -signedjar option: 
    jarsigner bundle.jar jane

Verifying a Signed JAR File

To verify a signed JAR file, that is, to verify that the signature is valid and the JAR file has not been tampered with, use a command such as the following: 
    jarsigner -verify sbundle.jar

If the verification is successful, 

    jar verified.
is displayed. Otherwise, an error message appears.

You can get more information if you use the -verbose option. A sample use of jarsigner with the -verbose option is shown below, along with sample output: 

    jarsigner -verify -verbose sbundle.jar

           198 Fri Sep 26 16:14:06 PDT 1997 META-INF/MANIFEST.MF
           199 Fri Sep 26 16:22:10 PDT 1997 META-INF/JANE.SF
          1013 Fri Sep 26 16:22:10 PDT 1997 META-INF/JANE.DSA
    smk   2752 Fri Sep 26 16:12:30 PDT 1997 AclEx.class
    smk    849 Fri Sep 26 16:12:46 PDT 1997 test.class

      s = signature was verified
      m = entry is listed in manifest
      k = at least one certificate was found in keystore

    jar verified.

Verification with Certificate Information

If you specify the -certs option when verifying, along with the -verify and -verbose options, the output includes certificate information for each signer of the JAR file, including the certificate type, the signer distinguished name information (iff it's an X.509 certificate), and, in parentheses, the keystore alias for the signer. For example, 
    jarsigner -verify -verbose -certs myTest.jar

           198 Fri Sep 26 16:14:06 PDT 1997 META-INF/MANIFEST.MF
           199 Fri Sep 26 16:22:10 PDT 1997 META-INF/JANE.SF
          1013 Fri Sep 26 16:22:10 PDT 1997 META-INF/JANE.DSA
           208 Fri Sep 26 16:23:30 PDT 1997 META-INF/JAVATEST.SF
          1087 Fri Sep 26 16:23:30 PDT 1997 META-INF/JAVATEST.DSA
    smk   2752 Fri Sep 26 16:12:30 PDT 1997 Tst.class

      X.509, CN=Test Group, OU=Java Software, O=Sun Microsystems, L=CUP, S=CA, C=US (javatest)
      X.509, CN=Jane Smith, OU=Java Software, O=Sun, L=cup, S=ca, C=us (jane)

      s = signature was verified
      m = entry is listed in manifest
      k = at least one certificate was found in keystore

    jar verified.
If the certificate for a signer is not an X.509 certificate, there is no distinguished name information. In that case, just the certificate type and the alias are shown. For example, if the certificate is a PGP certificate, and the alias is "bob", you'd get 
      PGP, (bob)

Verification of a JAR File that Includes Identity Database Signers

If a JAR file has been signed using the JDK 1.1 javakey tool, and thus the signer is an alias in an identity database, the verification output includes an "i" symbol. If the JAR file has been signed by both an alias in an identity database and an alias in a keystore, both "k" and "i" appear.

When the -certs option is used, any identity database aliases are shown in square brackets rather than the parentheses used for keystore aliases. For example: 

    jarsigner -verify -verbose -certs writeFile.jar

           198 Fri Sep 26 16:14:06 PDT 1997 META-INF/MANIFEST.MF
           199 Fri Sep 26 16:22:10 PDT 1997 META-INF/JANE.SF
          1013 Fri Sep 26 16:22:10 PDT 1997 META-INF/JANE.DSA
           199 Fri Sep 27 12:22:30 PDT 1997 META-INF/DUKE.SF
          1013 Fri Sep 27 12:22:30 PDT 1997 META-INF/DUKE.DSA
   smki   2752 Fri Sep 26 16:12:30 PDT 1997 writeFile.html

      X.509, CN=Jane Smith, OU=Java Software, O=Sun, L=cup, S=ca, C=us (jane)
      X.509, CN=Duke, OU=Java Software, O=Sun, L=cup, S=ca, C=us [duke]

      s = signature was verified
      m = entry is listed in manifest
      k = at least one certificate was found in keystore
      i = at least one certificate was found in identity scope

    jar verified.
Note that the alias "duke" is in brackets to denote that it is an identity database alias, not a keystore alias.

SEE ALSO