Encrypt file with WriteEncryptedObject

Erel

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• Mar 20, 2012

• #2

You should first read the input stream:

B4X:

Dim output As RandomAccessFile
Dim text As String
text = File.ReadString(File.DirRootExternal, "normal.txt")
output.Initialize(File.DirRootExternal, "secret.dat", False)

output.WriteEncryptedObject(text, "some secret password", output.CurrentPosition)
output.Close

 

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susu

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• Mar 20, 2012

• #3

Thank you Erel. What if the file is not text (jpeg, png...) ?

 

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Erel

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• Mar 21, 2012

• #4

Then you should first read the file to an array of bytes. You can use a temporary memory stream by using output2.InitializeToBytesArray. You will then be able to read the file with File.Copy2.
The last step is to write the array to 'output'.

 

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susu

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• Mar 21, 2012

• #5

Thank you Erel. Now I can encrypt/decrypt a jpg file but the speed is really slow. It takes about 7 seconds to decrypt a 130KB file on dual-core CPU device. My code below:

B4X:

Dim input As RandomAccessFile
Dim Buffer() As Byte
input.Initialize(File.DirRootExternal, "encrypted.dat", True)
Buffer = input.ReadEncryptedObject("password", input.CurrentPosition)
   
Dim InputStream1 As InputStream
InputStream1.InitializeFromBytesArray(Buffer, 0, Buffer.Length)
   
InputStream1.Close
input.Close
   
Dim bm As Bitmap
bm.Initialize2(InputStream1)
   
Activity.SetBackgroundImage(bm)

Do you have any idea to speed up? Thank you.

 

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Erel

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• Mar 22, 2012

• #6

Can you create and upload a small project? I will try to optimize it.

 

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susu

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• Mar 22, 2012

• #7

Here my project, please take a look. Thank you

 

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Erel

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• Mar 23, 2012

• #8

I tried several things however I got similar results. You may want to try to encrypt it with the Encryption library.

RandomAccessFile.WriteEncryptedObject is optimized for text based objects.

 

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susu

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• Mar 23, 2012

• #9

Thank you Erel. How about this idea:
Add 10 random bytes into the beginning of jpg file. When loading, just read the content after first 10 bytes. My purpose is make it a bit harder to read these jpg files. Is it possible?

 

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Erel

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• Mar 23, 2012

• #10

Yes. It should work.

 

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susu

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• Mar 28, 2012

• #11

I tried to write 56 bytes into jpeg file then save as a new file => It's OK.
But I got problem when read it again. Please take a look at attached file. Thank you Erel.

 

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Erel

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• Mar 28, 2012

• #12

Please start a new thread for this question. It is not really related to this tutorial.

 

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