JAR signature block file format

Summary: this post explains the content of the JAR signature block file - that is, the file META-INF/*.RSA, META-INF/*.DSA, META-INF/*.EC or SIG-* inside the JAR.

Oracle does not document it

Signed JAR file contains the following additions over a non-signed JAR:

  1. Checksums over the JAR content, stored in text files META-INF/MANIFEST.MF and META-INF/*.SF
  2. 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
  • 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 META-INF/SEC_PAD.RSA.

What does this block contain

The file appears to be a DER-encoded ASN.1 PKCS#7 data structure. DER-encoded ASN.1 file can be examined with asn1parse subcommand of the OpenSSL:

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 openssl rsautl. 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 META-INF/SEC_PAD.RSA 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 structures: 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 with jarsigner:

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 jarsigner. 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:

  1. 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.

  2. 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).

Summary (again)

  • 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 asn1parse.
  • OpenSSL can verify signatures in signature block files and create almost identical structures, which have been reported to be accepted by Java tools.

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