The Advanced Encryption Standard (AES) is a symmetric block cipher that can operate with three different key lengths - 128 bits, 192 bits, and 256 bits, known as AES-128, AES-192, and AES-256. The AES algorithm performs encryption and decryption on blocks of 128 bits using rounds of processing that include substitution, permutation, and mixing functions. AES also supports various block cipher modes of operation like Electronic Codebook (ECB), Cipher Block Chaining (CBC), Cipher Feedback (CFB), Output Feedback (OFB), and Counter (CTR) for different applications.

1. Advanced Encryption Standard
Adri Jovin J J, M.Tech., Ph.D.
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2. AES Structure
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3. AES data structure
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4. AES Parameters
Key Size (words/bytes/bits) 4/16/128 6/24/192 8/32/256
Plaintext Block Size
(words/bytes/bits)
4/16/128 4/16/128 4/16/128
Number of Rounds 10 12 14
Round Key Size (words/bytes/bits) 4/16/128 4/16/128 4/16/128
Expanded Key Size (words/bytes) 44/176 52/208 60/240
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AES-128, AES-192, or AES-256, depending on key length

5. AES Encryption and Decryption
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6. AES Encryption
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4 distinct transformation functions (1 permutation + 3 substitutions)
• SubBytes (Substitution)
• ShiftRows (Permutation)
• MixColumns (Substitution)
• AddRoundKey (Substitution)

7. Block Cipher Modes of Operation
• Electronic Codebook (ECB)
• Cipher Block Chaining (CBC)
• Cipher Feedback (CFB)
• Output Feedback (OFB)
• Counter (CTR)
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8. Electronic Codebook (ECB)
Each block of plaintext bits is encoded independently using the same key.
Application: Secure transmission of single values (e.g., an encryption key)
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9. Electronic Codebook (ECB)
Each block of plaintext bits is encoded independently using the same key.
Application: Secure transmission of single values (e.g., an encryption key)
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10. Cipher Block Chaining (CBC)
The input to the encryption algorithm is the XOR of the next block of plaintext and the preceding block of ciphertext.
Application: General-purpose block-oriented transmission, Authentication
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11. Cipher Feedback (CFB)
Input is processed s bits at a time. Preceding ciphertext is used as input to the encryption algorithm to produce
pseudorandom output, which is XORed with plaintext to produce next unit of ciphertext.
Application: General-purpose stream-oriented transmission, Authentication
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12. Output Feedback (OFB)
Similar to CFB, except that the input to the encryption algorithm is the preceding encryption output, and full blocks are
used.
Application: Stream-oriented transmission over noisy channel (e.g., satellite communication)
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13. Output Feedback (OFB)
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14. Counter (CTR)
Each block of plaintext is XORed with an encrypted counter. The counter is incremented for each subsequent block.
Application: General-purpose block-oriented transmission, Useful for high-speed requirements
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15. Counter (CTR)
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16. References
Schneier, B. (2007). Applied cryptography: protocols, algorithms, and source code in C. John Wiley & Sons.
Stallings, W. (2014). Cryptography and network security, 6/E. Pearson Education India.
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