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so , i'm using Galaxy Mega (i9152) with a CM13 ROM which i flashed through TWRP. For the sake of running PoGO , i had disabled root through the settings of the SuperSu app in CM13 (the game still didn't run , kept tripping safety net). The phone was running fine up till 2 days ago , after which it started going into bootloop.

so , i googled on how to resolve boot loop and saw that fixing file permissions through TWRP might resolve the issue. i did that BUT i also rooted the phone when TWRP prompted me to install SuperSU , citing that the phone was not rooted. After that , the phone was stuck at Boot Animation (the CM android face) , possibly because i rooted the phone through TWRP.

I had formatted my SD card as internal which meant that it would not work on any other device. All my pics and stuff is on that SD card now. I researched extensively on google to find out a way to decrypt the encrypted SD card and found out that i can decrypt it through linux. i have also recovered the .key file from /data/misc/vold folder and have converted the code into the appropriate 16 byte format through terminal commands.

I have a linux system and i have tried to decrypt it. here's what happens when i run parted -l. (i also wanted to post output of fdisk -l , but my reputation score won't let me post more than 2 links)

when i run parted -l

since the names "android_expand" and "android_meta" are still showing , i have hope and good reason to believe that my data's still intact !

but when i try to mount it ...

when i try to decrypt the SD card and mount it

i don't know what to do now , since this is all i knew , all i had :/ , i have no idea what filesystem the encrypted card is. i tried ext4 as described in the guide. maybe if there was some way i could make the fstab file cycle through ALL possible filesystems , hitting the right one and finally mounting my SD card. maybe the mount feature has such an 'auto' feature to auto detect the filesystems ?

I have tried to reflash the SAME ROM without factory resetting , but it gave me an error saying : "error executing updater binary" and also "cannot install on top of incompatible data"

i have cleared the Cache and Dalvik Cache SEVERAL times.

i have backed up the DATA portion , through TWRP , by using ANOTHER SD card in the phone . Also i have backed up SYSTEM portion in the internal storage of the phone. I am just worried about my pics and stuff on SD card. I can't see my encrypted SD card in TWRP , it just shows me O MB in the SD card option in TWRP , possibly because the SD card is encrypted. i have no idea how to access my files in the encrypted SD card through the TWRP file manager. i don't want to lose my data. just please help me recover data from this SD card that has been encrypted by Android Marshmallow. i don't know if my SD card will be detected in the OS as before if i am able to somehow reflash the EXACT SAME rom i was using prior to this disaster.

Just to clarify , i am using my Laptop's internal SD card reader for reading card with Linux (i have ubuntu 16.04 LTS), with an sd card adapter supplied with the card (it's a Kingston 16 GB , SDHC class 4)

if i can somehow access my SD card and retrieve all my data from it , my problem will be solved. please help me out.

P.S : also , at the VERY beginning of this ordeal , by mistake , i MIGHT have created partitions on the SD card , while using the fdisk. i hope that did not wipe the data off the sd card or corrupt it. i CTRL+C'ed out of the utility without saving changes ... so that might be of some hope.

  • Are you sure /dev/mmcblk0p2 exists on your Linux device? Run dmesg to see what the exact error message is. You could try running 'blockdev --getsize /dev/...' as a standalone command to get size, and see any errors. – Nikolay Elenkov Apr 3 '17 at 1:42
  • Also try running dmeset with the -v flag to get additional details about what is happening. – Nikolay Elenkov Apr 3 '17 at 1:44
  • @NikolayElenkov , thankyou for replying ! i solved the problem , thanks in part to your guide and thanks to a bit of intuition of my own. I am a Final Year CIS (Computer Engg) student and would like to stay in touch. Where can you be reached , easily ? – Muhammad Yasir Apr 3 '17 at 15:04
  • @NikolayElenkov , i have backed up the data on my PC. now the issue i face is that , i want my mobile phone to recognise the SD card as it is. i have tried this by wiping data,system,cache,dalvik and THEN reflashing the new rom. and then put the .key file in the vold folder , then rebooted the phone. AND , the phone does recognise the card (it does NOT ask for reformatting the card) but it does not recognise the data , it does not see any file , any picture inside the data , even though it shows that 1.63 GB out of 14.55 GB in the SD card is occupied. can i get some help regarding this ? – Muhammad Yasir Apr 4 '17 at 8:51
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before starting , i'd recommend making a FULL backup of your SD card , through a raw image ( google the 'dd' command in linux)

i followed the guide here ; Corrupt SD card formatted as internal storage

BUT with some IMPORTANT MODIFICATIONS of my own !

  1. Phone was stuck in BOOTLOOP and later stuck at BOOT Animation, no access to any usual file manager utility. Mind you , this is a VERY Grievous situation , since there's no trivial method of gaining access to your sd card (it shows as 0mb in TWRP file manager) for backing up your precious data , pics and memories. Also , this is the only part which is a bit shaky since i am not completely sure if my phone was rooted or not (since my problem changed from bootloop to boot animation when i tried to install to SuperSu through TWRP) and retrieving the encryption key is central to solving this. I had to get the .key file , since that was pivotal to breaking the SD card's encryption. So amidst all the panic , i was able to browse , through TWRP file manager , to the /data/misc/vold folder and found the .key file. This was a MAJOR breakthrough !

  2. Next , to extract the file. i used another , UNENCRYPTED SD card and transferred the .key file to that card , through TWRP file manager , read that SD card into the laptop and got the .key file onto Ubuntu (16.04) . Decoded the .key file and got the 16 byte key. there's 2 ways to do this ... either open the file through a hex editor (i used BLESS) and copy off the 16 pairs of hex digits you see OR use this command ;

hexdump -e '1/1 "%.2x"' expand_8838e738a18746b6e435bb0d04c15ccd.key

make sure you're in the SAME directory as the .key file when executing this command (advanced linux users will work their way around this but i want it to be as beginner friendly as possible)

so , i got the 32 digit / 16 byte key.

  1. NEXT , decrypting the SD card AND making Linux recognise it. when i read it in , through the card reader , it wouldn't show in Nautilus (the Ubuntu filemanager) but upon running fdisk-l and parted -l , i saw

"parted -l output"

http://pasteboard.co/5klviklR8.jpg

"fdisk -l output"

http://pasteboard.co/POGvyBOxO.png

notice the android_meta and android_expand , those names gave me hope , that my data was still intact. ESPECIALLY , since at the beginning of the problem , i had created partitions and partition tables in the sd card through the fdisk utility (but i ctrl+c'ed out so it's possible that the changed were not written , PHEW! )

the unknown filesystem was the biggest pain in this whole fiasco ... since according to the few guides and discussions regarding this issue , SD cards formatted as internal by marshmallow were supposed to be ext4 filesystem. but that was not the case , at least not here.

  1. So , to decrypt the card , i used
dmsetup create crypt1 --table "0 `blockdev --getsize /dev/mmcblk0p2` crypt aes-cbc-essiv:sha256 "insert your 16-byte key here w/out double quotes" 0 /dev/mmcblk0p2 0"

if you get some ioctl related error use this format instead (this is the EXACT format i used today , with my key and everything)

dmsetup create crypt7 --table "0 `blockdev --getsize /dev/mmcblk0p2` crypt aes-cbc-essiv:sha256 07147CFFB77F249A5DBD2AD204610E7D  0 /dev/mmcblk0p2 0"

please note that the dmsetup utility should be installed in linux. (google it ) . also , note that i went for the parition 'mmcblk0p2' , since that was the bigger one in size and had the obvious inference of containing all the data !

this went well , completed without error. the next bit , though , ABSOLUTELY drove me crazy and took around 20 hours (including my sleep time ) to be properly resolved.

  1. the GADDAMN file system ! it was written EVERYWHERE that adoptive storage is formatted as ext4 , so when i ran the command

# mount -t ext4 /dev/mapper/crypt1 /mnt/1/

(please note that mnt/1/ should exist as directory in your root folder , if not , use mkdir to create it)

it yielded the damn error

wrong fs or bad option.

and this got me thinking ... if the sd card is NOT ext4 type , then HOW ON EARTH am i supposed to know what file system it is out of all the possible ones !

i searched and searched for a command that could force cycling through ALL possibilities in a brute-force method or any entry i could make in the /etc/fstab file that could AUTOMATICALLY detect the filesystem somehow, but to no avail...

5 . In the end , i recalled ... that maybe , just MAYBE the filesystem was f2fs (flash-friendly file system). And that proved to be a telling recall ...

for this code ;

mount -t f2fs /dev/mapper/crypt1 /mnt/1

proved to be the WINNING GOAL !

i was then able to cd into /mnt/1 and saw some very familiar android-esque files and from there on , it was Eternal BLISS !

  • 1
    both ext4 and f2fs have a superblock, which identifies the filesystem. if you look at the first bytes of the partition, you can easily identify the used filesystem. – Nikolay Elenkov Apr 4 '17 at 9:33
  • @NikolayElenkov ah .. thankyou ! that adds to my knowledge and will keep in mind the next time i'm dealing with filesystems ! in the meanwhile , can you please take a look here ; android.stackexchange.com/questions/172347/… also , i made a detailed post regarding this , here : forum.xda-developers.com/… <-- here , i have described the problem in detail and step by step. i'd appreciate if you take a look at it and bail me out ! – Muhammad Yasir Apr 4 '17 at 10:29

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