How to encrypt Android apk and Androidapk

How to encrypt Android apk and Androidapk

After a busy week, I finally had time to wait and write something. We will continue to introduce another method to prevent decompilation of android apk. The first two articles we talked about shelling Technology (http://my.oschina.net/u/2323218/blog/393372) and runtime modification bytecode (http://my.oschina.net/u/2323218/blog/396203 ), if you do not understand, check the introduction of these two technologies in the first two articles of my blog. Next we will introduce another simple and applicable technology to prevent apk decompilation-pseudo encryption.

 

I. Principles of pseudo-Encryption

We know that android apkis essentially a zip package. We can change the android app extension .apkto. Zip to easily decompress the android app with the decompression software. In daily life or work, we usually encrypt our files to protect them in a compressed manner. This method is also applicable to android apk. The principle is very simple. There is a bit in the zip file format to indicate whether the files in the zip file are encrypted. We only need to set this flag bit to 1 to achieve our goal. Android PackageManagerService does not care about this encrypted Bit During apk installation (we will call it this for the time being) it can be installed normally without affecting the running of apk.

 

Ii. zip file format

The zip file format consists of three parts: compressed file source data, compressed directory source data, and directory end ID. The three parts are related to the encrypted bit, which is the compressed directory source data section. Next we will introduce this part in detail.

The compressed directory source data records all the compressed directory source data. Its structure is as follows:

Central directory file header
Offset	Bytes	Description [18]	Translation
Limit 0	4	Central directory file header signature = 0x02014b50	Core directory file header id = (0x02014b50)
Limit 4	2	Version made	Pkware version used for compression
Limit 6	2	Version needed to extract (minimum)	Extract the minimum pkware version
Listen 8	2	General purpose bit flag	Common bits
10	2	Compression method	Compression Method
12	2	File last modification time	Last file modification time
14	2	File last modification date	Last file modification date
16	4	CRC-32	6) CRC-32 Algorithm
20	4	Compressed size	Size after compression
24	4	Uncompressed size	Uncompressed size
28	2	File name length (n)	File Name Length
30	2	Extra field length (m)	Extended domain Length
32	2	File comment length (k)	File comment Length
34	2	Disk number where file starts	Disk number at the beginning of the file
36	2	Internal file attributes	Internal file attributes
38	4	External file attributes	External file attributes
42	4	Relative offset of local file header. this is the number of bytes between the start of the first disk on which the file occurs, and the start of the local file header. this allows software reading the central directory to locate the position of the file inside the ZIP file.	The relative displacement of the header of the local file.
46	N	File name	Directory file name
46 + n	M	Extra field	Extended domain
46 + n + m	K	File comment	File comment content

 

If the first bits of the General purpose bit flag part in this structure are set to 1, the compressed package is encrypted; if it is set to 0, the compressed package is not encrypted.

 

Iii. Implementation

We can use the jar package ZipCenOp. jar to encrypt and decrypt the apk (or use python to perform this operation. We will not introduce it here ).

(1) encrypt the apk

After encryption, we use the decompression software for decryption and will see the following prompt information:

 

If you use apktool for decompilation, the following error message is displayed:

The encrypted apk can be installed and run normally.

 

(2) decrypt the apk

 

Decryption is not very useful for us, just to understand.

 

I believe that you have some knowledge about the pseudo-encryption technology of apk, but this method is not applicable to android 4.2.x and later versions. It will reject the installation of this encrypted apk. In the next article, we will introduce another android apk anti-decompilation technology. We are looking forward to your support.

Thank you for reading this article. We look forward to seeing you next time:

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From: http://my.oschina.net/u/2323218/blog/399326