4.9 Code Practice: Question 4 Edhisive
2 answers:
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Answer:
There are two better techniques for encryption as compared to shift cipher. One is block cipher. Such techniques use one key for both encryption and decryption. For example Advanced Encryption Standard (AES). This is based on symmetric key encryption.
The other way is to use asymmetric key encryption technique which based on two keys i.e. public and private key for encryption and decryption. For example, Rivest-Shamir-Adleman (RSA).
Explanation:
What is AES?
Advanced Encryption Standard (AES) is a symmetric key block cipher which uses 128, 192 or 256-bit keys length to encrypt and decrypt a block of plain text or message. It is stronger and faster than Shift Cipher encryption technique. It can encrypt data blocks of 128 bit using the above mentioned bit key lengths. The greater then key length the greater the security. the number of rounds in AES is variable and depends on the length of the key. The length of these keys determines the number of rounds for example 128 bit keys has 10 rounds, 12 rounds for 192 bit keys and 14 rounds for 256 bit keys. Each rounds involves some processing. This process is explained below:
How AES works?
The data is divided into each of size 128 bits makes a matrix of 4x4 columns of 16 bytes. (bytes because AES takes 128 bit plain text block as 16 bytes)
Substitution: First the data is substituted using a fixed substitution table which is predetermined. This makes a matrix of rows and columns.
Shifting: Each rows of the matrix is shifted to the left and the dropped entries in row are inserted on the right. Shift is carried out as follows −
Mixing columns: Use a mathematical method/equation to transform the columns where the input of the method is each column of data which is then replaced by this function into a different new matrix of data(bytes). This is basically the first round to transform plain text to cipher text. Now consider these bytes as 128 bits, the first round key is added to these resultant column bits by XOR. A separate portion of the encryption key length is used to perform the transformation on each column. The number of rounds depend on the the key length.
After the first round key comes the second round key and so on and when the last round key is added then this whole process goes back to Substitution phase, then shifting rows phase, then mixing columns and another round key is added after this. For example, if 128 bit key is used this means there will be nine such rounds.
Advantages
AES supports large key sizes which makes it more secure and stronger than shift cipher encryption techniques which can be cracked because of their small key space. If someone wants to decrypt a message and knows it's encrypted by a Shift Cipher technique then the key space of all possible keys is only the size of the alphabet. This is a very small scale. An attack such brute force attack attempting all 26 keys or even using exhaustive technique will easily crack this. So AES having large key size is more robust against cracking. For example in AES in order to crack an encryption for 128 bit around 2128 attempts are needed. Also for a 256 bit key, 2256 different combinations attempts are needed to ensure the right one is included which makes it harder to crack and makes is more secure.
I have used an internet solver to encrypt a string and i am attaching the result of the encryption. The plain text is: Two One Nine Two
The key is Thats my Kung Fu
and the cipher text in hexadecimal after using AES is:
29 c3 50 5f 57 14 20 f6 40 22 99 b3 1a 02 d7 3a
