Java WebStart Cache
As of Java version 6 the Java WebStart Cache can be found in the following locations.
On Windows XP
C:\Documents and Settings\%USERNAME%\Application Data\Sun\Java\Deployment\cache\
On Windows Vista and later
Caveat: The following information is based on analysis of several dozen *.idx files from different Windows 7 systems. As such, the following information should not be considered to have been exhaustively researched.
Analysis of *.idx files have revealed a number of interesting findings. One is that specific offsets appear to be based on the version or "magic number" of the *.idx file. For example, for files with the second DWORD of the binary contents of the file (thought to be the "magic number") of 0x5a02, the URL from which the data was retrieved starts at offset 0x20, and is an ASCII string terminated by "\x00\x00".
For a magic number of 0x5d02, the size of the URL string can be found at offset 0x80. The first two string values to extract from this data are prefaced with their lengths in 4-byte DWORDs, stored in big endian order. To get the first string, read the DWORD at offset 0x80, and translate it as a big endian value (in Perl, use unpack("N",$data)). Beginning at offset 0x84, the string is length characters long. At the end of that string, the next DWORD is the length of the second string, also in big endian format.
Once you've completed reading the initial strings, there is a DWORD value which can be interpreted as a type value, of sorts, and the remaining data (i.e., strings following the apparent type value) appears to follow a fairly regular pattern. The type value appears to represent the number of string pairs in the remaining data. Each string is prefaced by a WORD (2-byte) value, in big endian format, which tells us how long each string is...using this information, it is a fairly straightforward process to parse through the information and collect the strings, and pair them up.
In many cases, the type values of 2 include an HTTP Response code of 302; the values of 6, 7, and 8 (values that have been observed so far) include a response of 200, as well as additional data (including time stamps), and the *.idx files themselves appear to contain certificate (and perhaps other) information.