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23 May 2019CPOL
Conversion of image to byte and Encryption using 128 bit key than Decryption using the same key and re-conversion from byte to image
Introduction
This article explains two things:
- The working of
TripleDESCryptoServiceProvider()
cryptographic function - The conversion of Image to Byte and vice versa
Cryptography is a Greek word which means hidden. Cryptography in computers means secure and protected communication between the authorized ones. It was initially used by the army to exchange confidential information and earlier, it was used by kings to send secret messages. Modern cryptography aims at achieving the following goals:
- Confidentiality
- Integrity
- Authentication
- Non-repudiation
- Availability
There are three types of cryptography:
- Symmetric CryptographySingle key/ Secret key/ Private Key - same key and algorithm used to encrypt/decrypt
- Asymmetric CryptographyPublic and Private keys/ two keys one for encryption and other for decryption in any order used
- Hybrid CryptographyCombination of both above, adopts good qualities of both and discards weaknesses of both
Now we will understand symmetric cryptography by an example of encoding an image to cipher text using 128 bit key and sending to other user and then decoding using same key.
Understanding the Code
First, we generate a key using any characters or numbers or symbols, validate it as 128 bit and convert to binary.
Length = 16 means 16 characters or symbols each of 8 bits so making 128 bits in total.
Here, a
StringToBinary()
function is used which converts a string to binary.Now to open image using browse button:
Reads image from
FileDialog
and shows in a picture box.Now the real working gets started and we start using
TripleDESCryptoServiceProvider()
function:Firstly, image is converted to byte using
imageconverter
then by using self made function beforeEnc()
the bytes are converted to binary to string and shown in a multiline textbox.Now a wrapper object for
TripleDESCryptoServiceProvider()
function is created as tripleDES
. Then as we defined, our key size should be exactly 128 bits so it is set for tripleDES
too. After converting key into bytes, cipher algorithm is selected ECB.Electronic Code Book(ECB)
It is an operation mode for creating block cipher, which means the bytes or bits will be encrypted using block cipher. Cipher is the encrypted plain text using any algorithm. There are two types of cipher:
- Stream cipher
- Block cipher
ECB is a block cipher algorithm which will convert the repeated plain text to same repeated cipher text. Means if '
a
' means 0011
then every time when 'a
' strikes, it will encode it as 0011
. So it's not good for small block sizes, that is why we use 128 bits block.Public Key Cryptography Standards(PKCS) # 7
These are the de-facto standard formats for public key encryption by RSA Data Security Inc. It deals with Digital Signatures and Certificates. And PKCS #7 encrypts a message using a certificate's PK and decrypted only with that certificate's private key. Some extended features of PKCS #7 includes:
- It can encrypt using multiple certificates at the same time
- Recursive, means a digital envelop is wrapped in a digital envelop and further in another digital envelop and so on
- Time stamps for encrypted message and digital signatures
- Counter signatures and user defined characteristics
Then
ICryptoTransform
defines the basic encryption operations and converts the encypted message to byte array and assigns to a textbox to compare the message before and after encryption.Now encryption is done!
And the turn to Decrypt the message.
As both the Key and Encrypted message are sent together, so here is the code:
The same code is used to decrypt the message and the original images will be generated in
bytes[]
. This time, we used ICryptoTransform
to create Decryptor()
. And then, using a memory stream converted bytes array to image.Just remember one thing that cipher mode and padding mode should always be the same on both sides (Encryption/Decryption).
History
- 24th May, 2019: Initial version
Active11 months ago
I just start learning Bouncy Castle for AES encryption/decryption. I am using
AES/CBC/PKCS7PADDING
with 256-bit key.BC can encrypt and decrypt text successfully, however after decryption I notice that there are always a few padding of null (0x00), which therefore fails my hash comparison. For example, suppose original input string is
“1234567890”
, the decrypted byte array is always:Why the padding is not
jww0x06,0x06,0x06,0x06,0x06,0x06
? And is there any way to deterministically tell the padding length (could be 0) after encryption so that I can get exactly the same string before encryption?58.2k4545 gold badges261261 silver badges560560 bronze badges
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2 Answers
When you specify PKCS7, BC will add the padding to the data before encrypting, and remove it again when decrypting. PKCS7 with AES would always add at least 1 byte of padding, and will add enough data to make the input a multiple of the AES block size. When decrypting the padding is verified to be correct, and in the case of PKCS7 also serve as an indicator of how much of the last block of decrypted data is padding, and how much is real data.
If you try decrypting the encrypted and padded data without specifying PKCS7 in the decrypt step, the padding would still be in the decrypted data.
Edit:
To illustrate my point .. here is some Java code that encrypts '1234567890' with AES/CBC/PKCS7, and then decrypts it again both with and without the PKCS7 padding:
Output:
When decrypting with the padding option, the output have been striped of the padding - and the cipher indicates 10 bytes of decrypted data - the rest of the buffer is 0 filled. Decrypting without the padding option, results in the padding now being part of the decrypted data.
Edit2:
Now seeing the original code, confirms my hunch. The methode
GetOutputSize
don't return the output size of the decrypted string, but only the maximum needed space in an output buffer. The methode have the following documentation in the BC code:DoFinal returns the actual length of the decrypted data put in the buffer.
So in
The
Andrew OrobatorplainTextBuffer
would be slightly larger than the actual decrypted data - the actual length of data would be in length
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i am using c# from bouncycastle. looks to me this might be a bug from bouncycastle, or at least bouncycastle c# implementation does not follow pkcs7 spec exactly.
my solution is to chop off the trailing bytes that are not included in the return length of DoFinal. still not very sure why there are padding of 0x00, which as said should not exist at all.
below is the code. i used AES/CBC/PKCS7PADDING for both encryption and decryption.
encryption --->
decryption --->
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