My original post, which is preserved below for historical purposes but has its text struck through, was wrong in certain important details and quickly outdated in others, for which I humbly apologize. It was an attempt to help you cope with the release of the 1.7.2 update to Adobe Digital Editions. ADE 1.7.2 changed the way a crucial decryption key is stored when the ADE activation takes place — when, that is, you "authorize" your computer to use your Adobe ID and password.
Here is the latest (hopefully correct) information, which is fleshed out in iPhone: Reading Adobe/EPUB eBooks:
There is now (see Step 4 in iPhone: Reading Adobe/EPUB eBooks) a new version (v.4.3) of the ineptkey Python script for Windows users, the purpose of which is to derive a key to decrypt Adobe eBooks, and also a brand new ineptkeymac Python script (v.1) for Mac users, the purpose of which is the same.
Either the ineptkey v.4.3 Python script for Windows users or the new ineptkeymac v.1 script for Mac users will produce an adeptkey.der output file that can be input to version 2 of the ineptepub script. v.2 has been the "standard" ineptepub version for a long time now, and it is again. The purpose of ineptepub is to use the key generated by either ineptkey or ineptkeymac to decrypt Adobe eBooks.
Windows or Mac users can alternatively use ineptepub_auto_version3.pyw (v.3 of ineptepub) or aineptepub.pyw (v.4.1 of ineptepub, which is different from v.4), both of which decrypt whole folders/directories of Adobe eBooks rather than just a single Adobe/EPUB file.
Version 4 of ineptepub does not work with ineptkey v.4.3, the latest ineptkey version, but only with ineptkey v.4.2, which is already obsolete ... as ineptepub v.4 is likewise already obsolete.
While ineptkey v.4.2 was current, it generated an adeptkey4.der (not adeptkey.der) file in a different internal format than adeptkey.der. That file was intended for ineptepub v.4. When ineptkey v.4.3 came out, I mistakenly told readers in my original post below to input its output file to ineptepub v.4, which led to "index out of range" errors.
Any adeptkey.der file that was generated prior to ADE version 1.7.2 is no longer usable with eBooks downloaded with ADE 1.7.2. Even though it's the right format for ineptepub v.2, it will produce a "problem decrypting session key" error when used with ADE 1.7.2-downloaded books. It will work only with books downloaded with ADE 1.7.1 or earlier.
Users that have an old adeptkey.der file that was generated while ADE 1.7.1 was active can, if they wish, hold onto it and use it to decrypt books that they downloaded before the ADE 1.7.2 update. Or they can re-download those books in ADE 1.7.2 and use a newly generated adeptkey.der file, from ineptkey v.4.3 or ineptkeymac v.1, to decrypt the newly downloaded books in any version of ineptepub other than v.4. I recommend the latter course, as it avoids having to keep track of which books were downloaded with which ADE versions.
Again, sorry for all the mix-ups,
The original post:
Error: problem decrypting session key