Summary: this post explains the content of the JAR signature block file -
that is, the file
SIG-* inside the JAR.
Oracle does not document it
Signed JAR file contains the following additions over a non-signed JAR:
- Checksums over the JAR content, stored in text files
- The actual cryptographic signature (created with the private key of the signer) over the checksums in a binary signature block file.
Surprisingly, format of the latter does not seem to be documented by Oracle. JAR file specification provides only a useful knowledge that "These are binary files not intended to be interpreted by humans".
Here, the content of this "signature block file" is explained. We show how it can be created and verified with non-Java tool: OpenSSL.
Create a sample signature block file
For our investigation, generate such file by signing some data with jarsigner:
- Make an RSA private key (and store it unencrypted), corresponding self-signed certificate, pack them in a format jarsigner understands:
openssl genrsa -out key.pem openssl req -x509 -new -key key.pem -out cert.pem -subj '/CN=foo' openssl pkcs12 -export -in cert.pem -inkey key.pem -out keystore.pfx -passout pass:123456 -name SEC_PAD
openssl req command reads the default OpenSSL configuration file.
If you care about exact content of your certificate, you should
not rely on the system default but write your own (you can start with a
- Create the data, jar it, sign the JAR, and unpack the "META-INF" directory:
echo 'Hello, world!' > data jar cf data.jar data jarsigner -keystore keystore.pfx -storetype PKCS12 -storepass 123456 data.jar SEC_PAD unzip data.jar META-INF/*
The "signature block file" is
What does this block contain
openssl asn1parse -in META-INF/SEC_PAD.RSA -inform der -i > jarsigner.txt
For more verbosity, you may use some ASN.1 decoder such as one at lapo.it.
You'll see that the two top-level components are:
- The certificate.
- 256-byte RSA signature.
You can extract the signature bytes from the binary data and
verify (=decrypt with the public key) them with
That includes some "low-level" operations and brings you one more step down
to understanding the file's content.
A simple "high-level" verification command, not involving manual byte
manipulation, would be:
openssl cms -verify -noverify -content META-INF/SEC_PAD.SF -in META-INF/SEC_PAD.RSA -inform der
This command tells: "Check that the CMS structure in
is really a signature of
META-INF/SEC_PAD.SF; do not attempt to validate
the certificate". Congratulations, we have verified the JAR signature
without Java tools.
Creating the signature block file with OpenSSL
For this example, we created the signature block file with jarsigner.
There are at least two OpenSSL commands which can produce similar
openssl cms and
openssl smime, with the options given below:
openssl cms -sign -binary -noattr -in META-INF/SEC_PAD.SF -outform der -out openssl-cms.der -signer cert.pem -inkey key.pem -md sha256 openssl smime -sign -noattr -in META-INF/SEC_PAD.SF -outform der -out openssl-smime.der -signer cert.pem -inkey key.pem -md sha256
Let's decode the created files and compare them to what has been produced
openssl asn1parse -inform der -in openssl-cms.der -i > openssl-cms.txt openssl asn1parse -inform der -in openssl-smime.der -i > openssl-smime.txt
Testing the "DIY signature"
Underlying ASN.1 structures are, in both cms and smime cases,
very close but not identical to those made by
As the format of the signature block file is not specified,
we can only do tests to have some ground to say that "it works".
Just replace the original signature block file with our signature
created by OpenSSL:
cp openssl-cms.der META-INF/SEC_PAD.RSA zip -u data.jar META-INF/SEC_PAD.RSA jarsigner -verify -keystore keystore.pfx -storetype PKCS12 -storepass 123456 data.jar SEC_PAD
Lucky strike: a signature produced by
openssl cms is recognized by
jarsigner (that is, at least "it worked for me").
Note that the data which is signed is
SEC_PAD.SF, and it was
itself created by jarsigner. If not using the latter, you'll need to
produce that file in some way.
What's the use for this knowledge?
Besides better understanding your data, one can think of at least two reasons to sign JARs with non-native tools. Both are somewhat untypical, but not completely irrelevant:
The signature must be produced in a system, where native Java tools are not available. Such system must have access to private key, and security administrators may like the idea of not having such overbloated software as JRE in a tightly controlled environment.
The signature must be produced or verified in a system, where available tools do not support the required signature algorithm. Examples "why" include compliance with regulations or compatibility with legacy systems. There are systems where testing which elliptic curves are supported by jarsigner reveals just three curves (which is not much).
- JAR signature block file is a DER-encoded PKCS#7 structure.
- Its exact content can be viewed with any ASN.1 decoder, e.g. with
- OpenSSL can verify signatures in signature block files and create almost identical structures, which have been reported to be accepted by Java tools.