Here are my steps.
At first, I took a look to RK30xxLoader(L)_V1.18.bin. This file appears in update.img for my device. So, I unpacked update.img using rk29Kitchen.
strings on that file returns nothing interesting, so I assumed the file is crypted
alanyih
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| get Android source code: http://source.android.com/source/downloading.html | |
| $ cd /path/to/android-src | |
| $ cd system/core/libmincrypt/ | |
| $ gcc -c *.c -I../include | |
| $ ar rcs libmincrypt.a *.o | |
| $ cd ../mkbootimg | |
| $ gcc mkbootimg.c -o mkbootimg -I../include ../libmincrypt/libmincrypt.a | |
| $ cd ../cpio | |
| $ gcc mkbootfs.c -o mkbootfs -I../include |