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:
Its authenticity can be verified, via a computation that
uses the public key corresponding to the private key used to
generate the signature.
It cannot be forged, assuming the private key is kept secret.
It is a function of the data signed and thus can't be
claimed to be the signature for other data as well.
The signed data cannot be changed; if it is, the signature
will no longer verify as being authentic.
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,
It is possible to import the information from
an identity database into a keystore, via the keytool-identitydb command.
jarsigner can sign JAR files also previously signed using
javakey.
jarsigner can verify JAR files signed using javakey.
Thus, it recognizes and can work with signer aliases that are from a
JDK 1.1 identity database rather than a JDK 1.2 keystore.
The following table explains how JAR files that were signed
in JDK 1.1.x are treated in JDK 1.2.
JAR File Type
Identity in 1.1 database
Trusted Identity imported into 1.2 keystore from 1.1
database (4)
Policy File grants privileges to Identity/Alias
Privileges Granted
Signed JAR
NO
NO
NO
Default privileges granted to all code.
Unsigned JAR
NO
NO
NO
Default privileges granted to all code.
Signed JAR
NO
YES
NO
Default privileges granted to all code.
Signed JAR
YES/Untrusted
NO
NO
Default privileges granted to all code. (3)
Signed JAR
YES/Untrusted
NO
YES
Default privileges granted to all code. (1,3)
Signed JAR
NO
YES
YES
Default privileges granted to all code plus privileges
granted in policy file.
Signed JAR
YES/Trusted
YES
YES
Default privileges granted to all code plus privileges
granted in policy file. (2)
Signed JAR
YES/Trusted
NO
NO
All privileges
Signed JAR
YES/Trusted
YES
NO
All privileges (1)
Signed JAR
YES/Trusted
NO
YES
All privileges (1)
Notes:
If an identity/alias is mentioned in the policy file, it must be
imported into the keystore for the policy file to have any effect on
privileges granted.
The policy file/keystore combination has precedence over a
trusted identity in the identity database.
Untrusted identities are ignored in JDK 1.2.
Only trusted identities can be imported into JDK 1.2
keystores.
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.
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 Policy Tool.
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.
keytool works on any file-based keystore
implementation. (It treats the keytore location that is passed to
it at the command line as a filename and converts it to a
FileInputStream, from which it loads the keystore information.)
The jarsigner and policytool tools, on the other hand,
can read a keystore from any location that can be specified
using a URL.
For jarsigner and keytool, you can specify a keystore
type at the command line, via the -storetype option. For
Policy Tool, you can specify a keystore type via the "Change Keystore"
command in the Edit menu.
If you don't explicitly specify a keystore type,
the tools 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 runtime environment's directory (the jre directory in
the SDK or the top-level directory of the Java 2 Runtime Environment).
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):
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".
At this time, jarsigner can sign a JAR file using either
DSA (Digital Signature Algorithm) with the SHA-1 digest algorithm, or
the RSA algorithm with the MD5 digest algorithm.
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 attempt to 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.)
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:
a signature file, with a .SF extension, and
a signature block file, with a .DSA extension.
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 allowed in a signature file name,
each such character is converted to an underscore ("_")
character in forming the file name. Legal characters
include letters, digits, underscores, and hyphens.
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.
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:
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.
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.
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 multiple times, 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.
The various jarsigner options are listed and described
below. Note:
All option names are preceded by a minus sign (-).
The options may be provided in any order.
Items in italics (option values) represent the actual values
that must be supplied.
The -keystore, -storepass, -keypass,
-sigfile , and -signedjar options are only relevant when
signing a JAR file, not when verifying a signed
JAR file. Similarly, an alias is only specified on the command line
when signing a JAR file.
-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
-storetype storetype
Specifies the type of keystore to be instantiated. The
default keystore type is the one that is specified as the value of the
"keystore.type" property in the security properties file, which is
returned by the static getDefaultType method in
java.security.KeyStore.
-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-zA-Z0-9_-". That is, only letters, numbers, underscore, and
hyphen characters are allowed. Note: All lowercase
characters will be converted to uppercase for the
.SF and .DSA file names.
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.
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
The keystore is also examined.
If no keystore value is specified on the command line,
the default keystore file (if any) will be checked.
If the public key certificate for a signer matches an
entry in the keystore, then the following information will
also be displayed:
in parentheses, the alias name for the
keystore entry 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.
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":
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
if the public key certificate in the JAR file matches that in a
keystore entry. For example,
jarsigner -keystore /working/mystore -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 -keystore /working/mystore -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.