dmdecrypt

Decrypt dmcrypt volumes using OpenSSL.

dmcrypt uses Encrypted salt-sector initialization vector (ESSIV) in which each sector of the volume is encrypted through AES-CBC, using the same key but a different IV for each sector. The IV is obtained by encrypting the padded sector number with the SHA256 hash of the key.

Usage

De(En)crypts a dmcrypt-style aes-cbc-essiv:sha256 disk volume
Usage: dmdecrypt [-e] [-y] [-q] input output key
  -e     Encrypt; if not specified, default is to decrypt.
  -y     Overwrite output file if it exists.
  -q     Quiet output. Only print errors.
  input  Input disk volume. Size in bytes must be a multiple of 512.
  output Output disk volume.
  key    Key file. Must be either 16, 24 or 32 bytes long.

Compiling

On recent Debian / Ubuntu just:

apt install gcc cmake pkg-config libssl-dev git
git clone https://github.com/francescovannini/dmdecrypt.git
cd dmdecrypt
mkdir build
cd build
cmake ..
make
./dmdecrypt

Example use case: recover data from Android Adopted Storage

This tool has been successfully used to recover data from a crashed Android phone (thanks to Nikolay Elenkov for this excellent post). The data was stored on an external SD card mounted as Android adopted storage which is essentially based on dmcrypt; but once the phone was unable to properly boot, the data in the SD card became inaccessible.

To recover it, I had to:

• Boot the phone using a recovery image
• Extract the key file /data/misc/vold/expand_xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.key
• Run this tool against the second partition of the SD card (the largest one) and the key above
• Mount the decrypted volume under Linux via loopback
• Access the files