In today’s digital age, the security of information is paramount. Advanced Encryption Standard (AES) is a widely used encryption algorithm that ensures the confidentiality and integrity of data. This guide will walk you through the intricacies of AES encryption and decryption, including its structure, key size relationship, and the transformations involved in the process.

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Understanding AES Encryption

AES is a symmetric encryption algorithm, meaning the same key is used for both encryption and decryption. The process begins with plaintext input, which is then transformed into ciphertext through multiple rounds of encryption.

Key Components of AES Structure

1. Plaintext and State Array:

• The input plaintext is 128 bits, stored in a 16-byte state array.

1. Initial Transformation:

• The initial transformation involves XORing the plaintext with a round key, known as the round 0 key.

1. Round Transformations:

• AES performs a series of rounds, where each round includes specific transformations:
  • Substitute Bytes (SubBytes)
  • Shift Rows
  • Mix Columns
  • Add Round Key

1. Key Scheduling Algorithm:

• Generates round keys for each transformation round based on the initial key size.

Key Size and Number of Rounds

The relationship between the key size and the number of rounds in AES is crucial for its security:

• AES-128:
• Key size: 128 bits
• Rounds: 10
• AES-192:
• Key size: 192 bits
• Rounds: 12
• AES-256:
• Key size: 256 bits
• Rounds: 14

Each round uses a 128-bit round key, regardless of the initial key size.

AES Encryption Process

The encryption process in AES is a sequence of transformations applied to the plaintext:

1. Initial Transformation:

• XOR operation between the plaintext and the round 0 key.

1. Rounds 1 to n-1:

• Each round includes the following transformations:
  • SubBytes: Substitutes each byte in the state array with a corresponding byte from a fixed substitution table.
  • Shift Rows: Rows of the state array are shifted cyclically.
  • Mix Columns: Columns of the state array are mixed to produce new columns.
  • Add Round Key: The state array is XORed with the round key.

1. Final Round (Round n):

• The final round omits the Mix Columns transformation, involving only SubBytes, Shift Rows, and Add Round Key transformations.

AES Decryption Process

Decryption in AES is essentially the reverse of the encryption process:

1. Initial Reverse Transformation:

• XOR operation between the ciphertext and the last round key from encryption.

1. Rounds n to 1:

• Each round transformation is reversed, starting from the last round key and moving backward.
• The transformations include inverse operations of SubBytes, Shift Rows, Mix Columns, and Add Round Key.

1. Final Round:

• Similar to encryption, the final round in decryption involves three transformations, excluding Mix Columns.

Key Scheduling in AES

The key scheduling algorithm is vital for generating the necessary round keys:

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• AES-128:
• 44 words (each word is 32 bits).
• AES-192:
• 52 words.
• AES-256:
• 60 words.

These words are used as round keys in the encryption and decryption processes.

Conclusion

AES encryption and decryption are fundamental to modern data security, providing robust protection against unauthorized access. By understanding the structure, key size relationship, and transformation processes, you can appreciate the complexity and effectiveness of AES. Whether you are a cybersecurity professional or an enthusiast, grasping AES’s mechanics is crucial in today’s digital landscape.

For more in-depth tutorials and information on cryptography, stay tuned to our blog. Your data security is our priority!

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