Question

Show that AES decryption is, in fact, the inverse of AES encryption

Answer 1

• The AES algorithm has a 128-bit block size, regardless of whether you key length is 256, 192 or 128 bits. When a symmetric cipher mode requires an IV, the length of the IV must be equal to the block size of the cipher.
• AES provides 128 bit, 192 bit and 256 bit of secret key size for encryption.
• AES decryption has also the same process. By default it assumes the entered text be in Base64. The input can be Base64 encoded or Hex encoded image and .txt file too. And the final decrypted output will be Base64 string.
• The Advanced Encryption Standard (AES), also known by its original name Rijndael is a specification for the encryption of electronic data. It describes a symmetric-key algorithm using the same key for both encrypting and decrypting.

USE OF AES:

The result of the encryption will appear in base64 encoded to prevent character encoding problems.
To decrypt a text be sure it is in base64 encoded and is encrypted with AES algorithm.
Put the encrypted text in the white textarea, set the key and push the Decrypt button.

ENCRYPTION WITH AES:

The encryption phase of AES can be broken into three phases: the initial round, the main rounds, and the final round.

Initial Round

• AddRoundKey

Main Rounds

• SubBytes
• ShiftRows
• MixColumns
• AddRoundKey

Final Round

• SubBytes
• ShiftRows
• AddRoundKey

AddRoundKey

• The AddRoundKey operation is the only phase of AES encryption that directly operates on the AES round key. In this operation, the input to the round is exclusive-ored with the round key.

SubBytes

• The SubBytes phase of AES involves splitting the input into bytes and passing each through a Substitution Box

ShiftRows

• In the ShiftRows phase of AES, each row of the 128-bit internal state of the cipher is shifted.

MixColumns

• Like the ShiftRows phase of AES, the MixColumns phase provides diffusion by mixing the input around.

The AES Key Schedule

• The AES Key Schedule is used to produce a set number of round keys from the initial key.

DECRYPTION IN  AES
To decrypt an AES-encrypted ciphertext, it is necessary to undo each stage of the encryption operation in the reverse order.

Inverse Final Round

• AddRoundKey
• ShiftRows
• SubBytes

Inverse Main Round

• AddRoundKey
• MixColumns
• ShiftRows
• SubBytes

Inverse Initial Round

• AddRoundKey

Of the four operations in AES encryption, only the AddRoundKey operation is its own inverse (since it is an exclusive-or). To undo AddRoundKey, it is only necessary to expand the entire AES key schedule.

Advantages of AES
One of the primary advantages of AES is its ubiquity. it is supported by most vendors. Also, it is relatively fast in both hardware and software.

Disadvantages of AES
AES has a very simple key schedule and simple encryption operations. Many AES attacks are based upon the simplicity of this key schedule and it is possible that one day an attack will be created to break AES encryption.