From: http://docs.oracle.com/javase/7/docs/technotes/tools/windows/jarsigner.html
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, .RSA, or .EC extension.