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Low-level AES usage

Also by EldoS: CallbackFilter
A component to monitor and control disk activity, track file and directory operations (create, read, write, rename etc.), alter file data, encrypt files, create virtual files.
#8147
Posted: 11/10/2008 07:43:29
by Scratch  (Standard support level)
Joined: 02/07/2008
Posts: 34

I'm going to use only SBAES unit to encrypt/decrypt a block of memory. just don't want to use classes\etc.
As far as I understood PKCS#5 padding scheme (for 16 byte AES256 block)- if data is for example 3 bytes long than for last block I should use data+13 bytes of "13". and remove 13 bytes after decryption. But what if blocksize equals 16 and data looks like (3 bytes smth)+ 13 bytes of 13?

Tried to dig in SBCryptoProvBuiltInSym unit but got my brain broken )
#8148
Posted: 11/10/2008 07:53:39
by Mykola Olshevsky (Basic support level)
Joined: 07/07/2005
Posts: 450

Hi. The data is also padded with 0xBlockSize, if it's size is a multiple of the block size.
#8150
Posted: 11/10/2008 08:06:25
by Scratch  (Standard support level)
Joined: 02/07/2008
Posts: 34

So, we always delete the last block after decryption if it is filled by bytes of its size.
Ok, I've got it ) thanks
#8152
Posted: 11/10/2008 08:31:23
by Mykola Olshevsky (Basic support level)
Joined: 07/07/2005
Posts: 450

To be more precise, you should look at the last byte of decrypted data, and drop <value-of-the-last-byte> bytes from the end.
#8154
Posted: 11/10/2008 08:41:48
by Ken Ivanov (EldoS Corp.)

...remembering to check that the values of <value-of-the-last-byte> bytes from the end of the last block are equal to the <value-of-the-last-byte>.
#8166
Posted: 11/11/2008 23:02:37
by Scratch  (Standard support level)
Joined: 02/07/2008
Posts: 34

yep ) thanks so much for help
Also by EldoS: CallbackProcess
A component to control process creation and termination in Windows and .NET applications.

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