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wallcraft · 11-26-2009, 11:32 PM

Perhaps it is worth summarizing what happens in a normal MOBI ebook. Here is what I think happens, but I am nor certain this is right and I am skipping over some technical details.

The actual encryption is done before the ebook leaves the publisher, with a (book specific) key. Only a small part of this file is then customized for the 1-4 PIDs allowed in a delivered MOBI ebook. The customization encodes an encrypted marker of the PID and the book key in the file, and this can then be extracted by entering the PID (e.g. by a MobiPocket Reader or by mobidedrm). So the process is PID -> book key -> plain text.

The K4PC AZW files are largely the same as AZWs for other devices, in other words the book key and the basic encryption is the same. What clarknova is suggesting is that Amazon has changed the PID -> book key process. This is plausible, because Amazon only needs 1 PID per file (not 4) and in principle they could do anything that used the same basic structure as the standard MOBI file. If this is the case, then this would be disappointing because MobiPocket never designed the PID to be a secret. Since it is only 8 characters long, with 36 values per character, it is in principle discoverable using a Brute force attack. In effect the PID is a password, subject to Password cracking. I don't know how long it would take to crack the PID from a MOBI file, but this seems practical even using brute force and there might be faster ways. This is a complete waste of effort for standard PIDs, because we already know them. Even the "secret" PIDs of Kindles and the Kindle iPhone app were discovered by other techniques, not relying of cracking the PID from a MOBI file. Since we don't know the PID of K4PC, cracking might be appropriate - but only if we know that K4PC is actually using a conventional MOBI PID.

Note that most DRM schemes are not broken by brute force or other attacks on the cipher, but rather on some flaw in the system that allows discovering the PID (or whatever). This "flaw" has to be there because DRM is being asked to do the impossible. The reader app is in the possession of the "attacker" and it has to know how to decrypt the ebook.

11-26-2009, 11:32 PM	#4
wallcraft reader Posts: 6,977 Karma: 5183568 Join Date: Mar 2006 Location: Mississippi, USA Device: Kindle 3, Kobo Glo HD	Perhaps it is worth summarizing what happens in a normal MOBI ebook. Here is what I think happens, but I am nor certain this is right and I am skipping over some technical details. The actual encryption is done before the ebook leaves the publisher, with a (book specific) key. Only a small part of this file is then customized for the 1-4 PIDs allowed in a delivered MOBI ebook. The customization encodes an encrypted marker of the PID and the book key in the file, and this can then be extracted by entering the PID (e.g. by a MobiPocket Reader or by mobidedrm). So the process is PID -> book key -> plain text. The K4PC AZW files are largely the same as AZWs for other devices, in other words the book key and the basic encryption is the same. What clarknova is suggesting is that Amazon has changed the PID -> book key process. This is plausible, because Amazon only needs 1 PID per file (not 4) and in principle they could do anything that used the same basic structure as the standard MOBI file. If this is the case, then this would be disappointing because MobiPocket never designed the PID to be a secret. Since it is only 8 characters long, with 36 values per character, it is in principle discoverable using a Brute force attack. In effect the PID is a password, subject to Password cracking. I don't know how long it would take to crack the PID from a MOBI file, but this seems practical even using brute force and there might be faster ways. This is a complete waste of effort for standard PIDs, because we already know them. Even the "secret" PIDs of Kindles and the Kindle iPhone app were discovered by other techniques, not relying of cracking the PID from a MOBI file. Since we don't know the PID of K4PC, cracking might be appropriate - but only if we know that K4PC is actually using a conventional MOBI PID. Note that most DRM schemes are not broken by brute force or other attacks on the cipher, but rather on some flaw in the system that allows discovering the PID (or whatever). This "flaw" has to be there because DRM is being asked to do the impossible. The reader app is in the possession of the "attacker" and it has to know how to decrypt the ebook. Last edited by wallcraft; 11-26-2009 at 11:38 PM.