Extracting a Data File
a. Select file format and operation
We use here the example of extracting a text file from a PNG image file with hidden data, the text file being the file mckenna.txt used in the previous section, and the image file being the output file dome.png created in that example. This file is contained in the file stego_image_files.zip which can be downloaded from this website and used with the trial version of this program.
The procedure for extracting a data file from a BMP file is (after selecting 'BMP') exactly the same.
After selecting the file format and the operation (extract and decrypt) you have to do the following before extracting the data:
- Specify a folder and name for the file to contain the extracted data.
- Select the image file within which the data is hidden.
- Specify the stego key.
b. Specify file to contain the extracted data
Click on the "File for extracted data" button, locate a folder and enter a name for the file:
If the "Open this file" checkbox is checked, the extraction is successful and the extracted file does not have any of nearly 30 file extensions such as exe, com, dll, doc, xls, scr, sys, ini and bat then an attempt is made to open and display the file. Files with such extensions may contain executable code or macros, and such files are left to you to open or run, exercising appropriate caution.
c. Specify image file containing the hidden data
To specify the input image click on the "Stego (input) PNG [or BMP] file" button and select the file in the usual way.
d. Enter stego/encryption key
When extracting data you must specify the same key as was used to hide it. Clicking on "Specify key" brings up a window to enter a stego key:
As stated in the previous section, the stego key is any string of typeable characters from 16 to 64 characters in length. Spaces are ignored and no distinction is made between upper and lower case.
e. Extract the data
Click on the "Extract the data" button to start the operation. If all goes well then a report appears saying that the operation was successful.
Operation: Extract data Stego image file: D:\temp\stpng_test\stego_png_files\dome.png Stego image file size: 383,623 bytes Data file: D:\temp\stpng_test\extracted_data_files\mckenna.txt Data file size: 29,869 bytes The data file was successfully extracted from the PNG image file.
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