Kindle for PC DRM

[LaBBa]

Hi all,
my friend just showed me this application "Kindle for PC" - http://www.amazon.com/gp/feature.html/ref=kcp_pc_mkt_lnd?docId=1000426311
that it able to read files (.prc) that are DRM and encrypted.

now i have seen in this url:
http://www.makeuseof.com/tag/how-to-strip-mobi-and-prc-ebooks-of-encryption/

that someone already found how to decrypt the DRM on a given PID of the device but for the "Kindle for PC" they didn't found yet what is the PID or where it is stored.

I'm starting to reverse the application of Amazon to see if it is possible for a user to extract his PID so he can later decrypt the book files and use it on other softwer to read other then "Kindle for PC" Amazon application.
i'm buying a sample book so i can start tracing/RE it :-)

anyone wanna join me ? (

LaBBa.

[LaBBa]

i have compared so far 2 type of running one without DRM and the other with DRM i have found that the behavior that decides if to continue or to show an error message is in this sub:

.text:00414270 sub_414270      proc near               ; CODE XREF: sub_414240:loc_41425Bp
.text:00414270                                         ; sub_4197F0:loc_4199C1p ...
.text:00414270
.text:00414270 var_30          = dword ptr -30h
.text:00414270 var_2C          = dword ptr -2Ch
.text:00414270 var_28          = dword ptr -28h
.text:00414270 var_24          = byte ptr -24h
.text:00414270 var_20          = byte ptr -20h
.text:00414270 var_C           = dword ptr -0Ch
.text:00414270 var_4           = dword ptr -4
.text:00414270
.text:00414270                 push    ebp
.text:00414271                 mov     ebp, esp
.text:00414273                 and     esp, 0FFFFFFF8h
.text:00414276                 mov     eax, large fs:0
.text:0041427C                 push    0FFFFFFFFh
.text:0041427E                 push    offset sub_A01580
.text:00414283                 push    eax
.text:00414284                 mov     large fs:0, esp
.text:0041428B                 sub     esp, 28h
.text:0041428E                 push    ebx
.text:0041428F                 push    esi
.text:00414290                 push    edi
.text:00414291                 mov     edi, ecx
.text:00414293                 mov     ecx, [edi+3Ch]
.text:00414296                 xor     ebx, ebx
.text:00414298                 cmp     ecx, ebx
.text:0041429A                 jz      short loc_4142A4
.text:0041429C                 mov     eax, [ecx]
.text:0041429E                 mov     edx, [eax+2Ch]
.text:004142A1                 push    ebx
.text:004142A2                 call    edx
.text:004142A4
.text:004142A4 loc_4142A4:                             ; CODE XREF: sub_414270+2Aj
.text:004142A4                 mov     eax, dword_D1CB60
.text:004142A9                 mov     [esp+40h+var_30], eax
.text:004142AD                 mov     ecx, 1
.text:004142B2                 lock xadd [eax], ecx
.text:004142B6                 mov     [esp+40h+var_4], ebx
.text:004142BA                 mov     eax, [edi+20h]
.text:004142BD                 cmp     eax, ebx
.text:004142BF                 jz      loc_41443B
.text:004142C5                 cmp     [eax+1Dh], bl
.text:004142C8                 jnz     loc_41443B
.text:004142CE                 cmp     [eax+14h], ebx
.text:004142D1                 jz      loc_4143C4
.text:004142D7                 lea     esi, [esp+40h+var_20]
.text:004142DB                 call    sub_46BFF0
.text:004142E0                 mov     byte ptr [esp+40h+var_4], 1
.text:004142E5                 mov     edx, [edi+20h]
.text:004142E8                 mov     ecx, [edx+14h]
.text:004142EB                 mov     eax, [ecx]
.text:004142ED                 mov     eax, [eax+8]
.text:004142F0                 mov     edx, esi
.text:004142F2                 push    edx
.text:004142F3                 lea     edx, [esp+44h+var_2C]
.text:004142F7                 push    edx
.text:004142F8                 call    eax
.text:004142FA                 lea     ecx, [esp+40h+var_28]
.text:004142FE                 push    ecx
.text:004142FF                 mov     byte ptr [esp+44h+var_4], 2
.text:00414304                 call    sub_43EC40
.text:00414309                 add     esp, 4
.text:0041430C                 push    eax
.text:0041430D                 lea     ecx, [esp+44h+var_30]
.text:00414311                 mov     byte ptr [esp+44h+var_4], 3
.text:00414316                 call    sub_904EE0
.text:0041431B                 mov     byte ptr [esp+40h+var_4], 2
.text:00414320                 mov     edx, [esp+40h+var_28]
.text:00414324                 or      eax, 0FFFFFFFFh
.text:00414327                 lock xadd [edx], eax
.text:0041432B                 jnz     short loc_41433A
.text:0041432D                 mov     ecx, [esp+40h+var_28]
.text:00414331                 push    ecx             ; void *
.text:00414332                 call    j_free
.text:00414337                 add     esp, 4
.text:0041433A
.text:0041433A loc_41433A:                             ; CODE XREF: sub_414270+BBj
.text:0041433A                 mov     ecx, [esp+40h+var_2C]
.text:0041433E                 cmp     ecx, ebx
.text:00414340                 jz      short loc_4143B7
.text:00414342                 push    1
.text:00414344                 push    ecx
.text:00414345                 mov     eax, esp
.text:00414347                 mov     [eax], ecx
.text:00414349                 mov     ecx, [esp+48h+var_2C]
.text:0041434D                 mov     [esp+48h+var_28], esp
.text:00414351                 cmp     ecx, ebx
.text:00414353                 jz      short loc_41435B
.text:00414355                 mov     edx, [ecx]
.text:00414357                 mov     eax, [edx]
.text:00414359                 call    eax
.text:0041435B
.text:0041435B loc_41435B:                             ; CODE XREF: sub_414270+E3j
.text:0041435B                 mov     byte ptr [esp+48h+var_4], 4
.text:00414360                 mov     ecx, [edi+20h]
.text:00414363                 mov     eax, [ecx+14h]
.text:00414366                 push    eax
.text:00414367                 call    sub_402AD0
.text:0041436C                 push    eax
.text:0041436D                 mov     byte ptr [esp+50h+var_4], 2
.text:00414372                 call    sub_403FA0
.text:00414377                 mov     byte ptr [esp+40h+var_4], 1
.text:0041437C                 mov     ecx, [esp+40h+var_2C]
.text:00414380                 cmp     ecx, ebx
.text:00414382                 jz      short loc_41438B
.text:00414384                 mov     edx, [ecx]
.text:00414386                 mov     eax, [edx+4]
.text:00414389                 call    eax
.text:0041438B
.text:0041438B loc_41438B:                             ; CODE XREF: sub_414270+112j
.text:0041438B                 lea     edi, [esp+40h+var_20]

.text:004143B1                 push    eax
.text:004143B2                 jmp     loc_414455
.text:004143B7 ; ---------------------------------------------------------------------------
.text:004143B7
.text:004143B7 loc_4143B7:                             ; CODE XREF: sub_414270+D0j
.text:004143B7                 lea     edi, [esp+40h+var_20]
.text:004143BB                 mov     byte ptr [esp+40h+var_4], bl
.text:004143BF                 call    sub_403F60
.text:004143C4
.text:004143C4 loc_4143C4:                             ; CODE XREF: sub_414270+61j
.text:004143C4                 lea     ecx, [esp+40h+var_28]
.text:004143C8                 push    ecx
.text:004143C9                 call    sub_401500
.text:004143CE                 add     esp, 4
.text:004143D1                 push    offset aCouldNotOpenBo ; "Could not open book, shoot!"
.text:004143D6                 mov     ecx, eax
.text:004143D8                 mov     byte ptr [esp+44h+var_4], 5

.text:00414436                 call    BadBoy
.text:0041443B
.text:0041443B loc_41443B:                             ; CODE XREF: sub_414270+4Fj
.text:0041443B                                         ; sub_414270+58j
.text:0041443B                 mov     [esp+40h+var_4], 0FFFFFFFFh
.text:00414443                 mov     edx, [esp+40h+var_30]
.text:00414447                 or      eax, 0FFFFFFFFh
.text:0041444A                 lock xadd [edx], eax
.text:0041444E                 jnz     short loc_41445D
.text:00414450                 mov     ecx, [esp+40h+var_30]
.text:00414454                 push    ecx             ; void *
.text:00414455
.text:00414455 loc_414455:                             ; CODE XREF: sub_414270+142j
.text:00414455                 call    j_free
.text:0041445A                 add     esp, 4
.text:0041445D
.text:0041445D loc_41445D:                             ; CODE XREF: sub_414270+137j
.text:0041445D                                         ; sub_414270+1DEj
.text:0041445D                 mov     ecx, [esp+40h+var_C]
.text:00414461                 pop     edi
.text:00414462                 pop     esi
.text:00414463                 mov     large fs:0, ecx
.text:0041446A                 pop     ebx
.text:0041446B                 mov     esp, ebp
.text:0041446D                 pop     ebp
.text:0041446E                 retn

the critical point of good and bad jump is in this place
00414340 JE SHORT

if all is good the the jump shouldn't be taken if its bad then it is taken
and then we will get to the private error string:
"Could not open book, shoot!"

thats all for now..

Regards,
LaBBa.

[Shub-Nigurrath]

as long as it is possible to help without owning a kindle, I am onboard. Send me details.

BR

[LaBBa]

well
I have purchased a book from amazon that is protected with DRM and compared the execution with my computer and illegal VM and go those results.

(only 5$ so i don't care because it is a challange for me to do it)

[Nukacola]

So i'm on Board as well. I guess u (LABBA) also didn't have a Kindle and u use the PC reader published a short time ago. One time on the PC u bought the Book and 2 time in a VM without licence. Or was i wrong and this is not the case....

Regards
Nukacola

[LaBBa]

this is the case indeed.
the current vesion that i use to do my reversing is:
Kindle For PC v1.0 Beta 1 (25338)

you can set a BP your self on the code i have pasted above and start to to the reversing..
i'm currently moving realy slow to make sure i didn't missed anything that also occured that i didn't noticed..

[LaBBa]

hi all news:
i have looked a littel on the source code of : MobiDeDRM
and from what i have seen there is a vector like this:

def parseDRM(self, data, count, pid):
	pid = pid.ljust(16,'\0')
	keyvec1 = "\x72\x38\x33\xB0\xB4\xF2\xE3\xCA\xDF\x09\x01\xD6\xE2\xE0\x3F\x96"
	temp_key = PC1(keyvec1, pid, False)
	temp_key_sum = sum(map(ord,temp_key)) & 0xff

i looked this bytes in the Kindle for PC:
\x72\x38\x33\xB0\xB4\xF2\xE3\xCA\xDF\x09\x01\xD6\xE2\xE0\x3F\x96

and yes i found it!

currently i'm starting to debug to see when it is used...

Regards,
LaBBa.