Consider a very simplified model of AES in which there will be only one P box and one S box and an 8 bit key. Use 0xAE as the key. Show, step by step, how one encrypts the string T h e (without any embedded blanks) assuming that each letter is from ISO, thus Ox54, 0x68, 0x65 as the octet (8 bit) hexadecimal values?
To encrypt the string "T h e" using a simplified model of AES with one P box, one S box, and an 8-bit key (0xAE), you will need to follow these step-by-step instructions:
1. Convert the string "T h e" to its hexadecimal representation:
- "T" = 0x54
- "h" = 0x68
- "e" = 0x65
2. XOR the first byte of the key with the first byte of the plaintext:
- 0x54 XOR 0xAE = 0xFA
3. Apply the P box permutation to the XOR result:
- Since there is only one P box, no permutation is applied.
4. Lookup the result from step 3 in the S box:
- 0xFA maps to a value, let's say 0x20.
5. XOR the second byte of the key with the second byte of the plaintext:
- 0x68 XOR 0xAE = 0xC6
6. Apply the P box permutation to the XOR result:
- No permutation is applied in this simplified model.
7. Lookup the result from step 6 in the S box:
- 0xC6 maps to a value, let's say 0x9B.
8. XOR the third byte of the key with the third byte of the plaintext:
- 0x65 XOR 0xAE = 0xCB
9. Apply the P box permutation to the XOR result:
- No permutation is applied in this simplified model.
10. Lookup the result from step 9 in the S box:
- 0xCB maps to a value, let's say 0x4D.
11. The encrypted string is the concatenation of the results from steps 4, 7, and 10:
- Encrypted string = 0x20 0x9B 0x4D
So, using the simplified AES model with one P box, one S box, and an 8-bit key, the encryption of the string "T h e" would result in the encrypted string 0x20 0x9B 0x4D.