is it possible to encrypt image file with less file size?

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Dear all,

In vb.net i tried to compress image file.using Rijndael algorithm i have encrypt the file.after encryption the file size will be the same as original..is it posible to get the file with less size compared to original?

if it is possiblel means which algorithm suits for that?

thanks in advance

Posted 1-Nov-12 21:45pm

jai_mca

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2 solutions

Solution 1

I hardly believe you can.
However, you may try a lossless compression algorithm (like the one used in ZIP files, for instance).

You might use the opposite approach: first compress and then encrypt.

Posted 1-Nov-12 22:05pm

CPallini

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jai_mca 2-Nov-12 4:11am

dear CPallini
compress and encrypt also remains the same size

CPallini 2-Nov-12 4:50am

If you have an already compressed image (like, for instance, a JPEG one) then you have few chances to compress it further.

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OriginalGriff · Accepted Answer · 2012-11-01T22:09:00

Solution 2

The chances are that you can't compress an image sifgnifcianly, unless you atart with an uncompressed image format such as a Bitmap. Most image formats already involve some form of compression, and compressed data is (understandably) difficult to compress again - or we would just repeat the compression until we got to every file occupying 16 bytes.

In fact, compressing images (and other compressed file formats) may actually increase the size of the resulting files due to the added overhead of the compression support data.

Don't you think that if you could compress images using standard techniques such as Rijndael someone would not have added it to browsers to speed display? :laugh:

Posted 1-Nov-12 22:09pm

OriginalGriff

Comments

jai_mca 2-Nov-12 4:54am

yes..i used it.but it doesn't reduce the size after encryption.so there is no possibility to attain small size after encryption?

OriginalGriff 2-Nov-12 5:05am

Almost certainly not, unless you start with an uncompressed image format in the first place.

jai_mca 2-Nov-12 5:18am

sorry to ask this,uncompressed image format in the sense?

OriginalGriff 2-Nov-12 5:24am

Bitmap files (*.BMP) are uncompressed. JPEG and GIF, and PNG, and TIF are all compressed in one way or another.
(This is one of the reasons that they are smaller than the equivalent BPM file!)
What formats are you working with?

jai_mca 2-Nov-12 5:30am

i am working with JPEG format

OriginalGriff 2-Nov-12 5:48am

Already encrypted, using a "lossy" compression - you will not reduce the size of these significantly by using other compression techniques. The only way to reduce the size of jpg files is to reduce the picture quality.

BTW: If you are actually working on images, keep the masters as BMP or similar, not a JPG - you will lose image quality every time you save otherwise as it is a "lossy" compression - it throws away information each time it is saved.

jai_mca 2-Nov-12 6:22am

oh..thanks OriginalGriff.u have provide lot of solution for me.i have learned a lot from u..i have one last question.last but not least.

With text file is it possible to reduce size using Rijndael ?

OriginalGriff 2-Nov-12 6:29am

Emphatically yes.
Text files generally waste 75% of the possible size purely by using a tiny proportion of the character set available (a-z A-Z 0-9, space and some punctuation out of a possible 256 values per byte) and they tend to have a lot of data redundancy which Rijndael can exploit.
A text file will typically reduce to 20% or so of the original size.

BTW: that doesn't mean you can rename the file as a .TXT file and it will magically compress - it's all about the data in the file, not the file name! (I know, I know - you know that already...but you'd be surprised how many people think that changing the extension from .DOC to .JPG will make a picture of the file ready to upload to a client...:laugh:)

jai_mca 2-Nov-12 6:38am

thanks Original Griff.u r ryt..lol..

jai_mca 2-Nov-12 6:41am

Dear Original Griff,

y can't u answer my last question(ie.,before this discussion).i am expecting ur answer Griff.plz provide solution for that also.

The question was how to overwrite pdf ?i have problem with adobe9.

OriginalGriff 2-Nov-12 6:50am

I hadn't seen it, so I didn't answer it. But Marcus is right - you have to make sure that all objects accessing the file release all locks. The exact manner of how to do that will depend on the package you are using, but it is probable that you can call Dispose on teh reader / write package when you have finished with it each time, and that will hopefully release the locks. If you just let the objects go out of scope at the end of the method any file locks they acquire may not be released until the Garbage Collector gets round to them. Which could be next week!

jai_mca 2-Nov-12 7:19am

oh..k Griff..thanks a lot..

jai_mca 2-Nov-12 7:20am

with adobe6 it is not a problem.adobe 9 only makes this issue.