$1.(25$ points$)$ In this problem, you will use AES to encrypt and decrypt PGM image files. Since these files will have large sizes, you will implement the ECB mode of operation. For this problem, you need to implement the following methods in ImageCipher.java: bitsToHexString$(),$ stateToBits$(),$ readBlock$(),$ writeBlock$(),$ processHeader$(),$ encryptECB$(),$ and decryptECB$().$ To encrypt a file, you will first copy the header from the input file to the output file during the encryption$/$decryption process. Then you will encrypt the integer pixel values $($each one of which occupies a single byte$)$ sixteen at a time, since AES processes blocks of size $128$ bits. The test files for this assignment will always contain a total number of pixels that is a multiple of $16.$ Therefore, you will never have to worry about padding the last block. Once you have encrypted or decrypted a block, you will have to break it up again into $16$ bytes, convert each one to a decimal number $($between $0$ and $255$ ; no negative values!$)$ and write to the output file this integer in its ASCII representation. Make sure that each printed integer occupies EXACTLY $3$ characters $($with white space padding in front if needed$)$ and is followed by a single newline character. This is to guarantee that the output file will always have exactly the same size as the input file. In the end, the output $($encrypted or decrypted$)$ file will be a valid $.$pgm file but with different pixel values resulting from the block$-$by$-$block encryption$/$decryption dictated by AES and the mode of operation you are implementing.

Once you have implemented all of the incomplete methods listed above $($whose specification is given within the source file$),$ you can test your program with a sequence of commands like these:

$```$

$>$ ls $-$l checker$*.$pgm

$>$ java ImageCipher $-$e $-$ECB checker $00112233445566778899$AABBCCDDEEFF

$>$ ls $-$l checker$*.$pgm

$>$ display checker$\_$ECB.pgm

$>$ java ImageCipher $-$d $-$ECB checker$\_$ECB $00112233445566778899$AABBCCDDEEFF

$>$ ls $-$l checker$*.$pgm

$>$ diff checker.pgm checker$\_$ECB$\_$dec.pgm

$```$

$2.(25$ points$)$ In this problem, you will use AES to encrypt and decrypt PGM image files. Since these files will have large sizes, you will implement the CBC mode of operation. For this problem, you need to implement the following methods in ImageCipher.java: hexStringToBits$(),$ xor$(),$ encryptCBC$(),$ and decryptCBC$().$ The rest of this problem's description is identical to the one given above.

Once you have implemented all of the incomplete methods listed above $($whose specification is given within the source file$),$ you can test your program with a sequence of commands like these:

$```$

$>$ ls $-$l checker$*.$pgm

$>$ java ImageCipher $-$e $-$CBC checker $00112233445566778899$AABBCCDDEEFF $0123456789$ABCDEFFEDCBA$9876543210$

$>$ ls $-$l checker$*.$pgm

$>$ display checker$\_$CBC$.$pgm

$>$ java ImageCipher $-$d $-$CBC checker$\_$CBC $00112233445566778899$AABBCCDDEEFF $0123456789$ABCDEFFEDCBA$9876543210$

$>$ ls $-$l checker$*.$pgm

$>$ diff checker.pgm checker$\_$CBC$\_$dec.pgm

$```$

$3.(25$ points$)$ In this problem, you will use AES to encrypt and decrypt PGM image files. Since these files will have large sizes, you will implement the CTR mode of operation. For this problem, you need to implement the following methods in ImageCipher.java: encryptCTR$())$ and decryptCTR$().$ For this problem, you may want to use the BigInteger class in the Java library in order to handle the $128-$bit integer values used for the counter. The rest of this problem's description is identical to the one given above.

Once you have implemented all of the incomplete methods listed above $($whose specification is given within the source file$),$ you can test your program with a sequence of commands like these:

$```$

$>$ ls $-$l checker$*.$pgm

$>$ java ImageCipher $-$e $-$CTR checker $00112233445566778899$AABBCCDDEEFF $0123456789$ABCDEFFEDCBA$9876543210$

$>$ ls $-$l checker$*.$pgm

$>$ display checker$\_$CTR$.$pgm

$>$ java ImageCipher $-$d $-$CTR checker$\_$CTR $00112233445566778899$AABBCCDDEEFF $0123456789$ABCDEFFEDCBA$9876543210$

$>$ ls $-$l checker$*.$pgm

$```1.(25$ points$)$ In this problem, you will use AES to encrypt and decrypt PGM image files. Since these files will have large sizes, you will implement the ECB mode of operation. For this problem, you need to implement the following methods in ImageCipher.java: bitsToHexString$(),$ stateToBits$(),$ readBlock$(),$ writeBlock$(),$ processHeader$(),$ encryptECB$(),$ and decryptECB$().$ To encrypt a file, you will first copy the header from the input file to the output file during the encryption$/$decryption process. Then you will encrypt the integer pixel values $($each one of which occupies a single byte$)$ sixteen at a time, since AES processes blocks of size $128$ bits. The test files for this assignment will always contain a total number of pixels that is a multiple of $16.$ Therefore, you will never have to worry about padding the last block. Once you have encrypted or decrypted a block, you will have to break it up again into $16$ bytes, convert each one to a decimal number $($between $0$ and $255$ ; no negative values!$)$ and write to the output file this integer in its ASCII representation. Make sure that each printed integer occupies EXACTLY $3$ characters $($with w