https://github.com/5brian/assembly-bit-stream-cipher-decoder

https://github.com/5brian/assembly-bit-stream-cipher-decoder

Last synced: about 1 month ago
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• Host: GitHub
• URL: https://github.com/5brian/assembly-bit-stream-cipher-decoder
• Owner: 5brian
• Created: 2023-12-04T20:14:18.000Z (about 1 year ago)
• Default Branch: main
• Last Pushed: 2024-07-22T04:25:34.000Z (6 months ago)
• Last Synced: 2024-12-06T04:21:02.637Z (about 2 months ago)
• Language: Assembly
• Homepage:
• Size: 2.93 KB
• Stars: 0
• Watchers: 1
• Forks: 0
• Open Issues: 0
• Metadata Files:
  • Readme: README.md

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README

        
Write a program that implements a bit stream cipher decoder. An elementary level bit stream cipher is an encryption algorithm that encrypts 1 byte of plain text at a time. This cipher uses a given 4-bit bit pattern as the key. The size of the encrypted message that we want to be able to send has a maximum length of 200 characters. The 4-bit pattern must be duplicated to an 8-bit value for this to work. (e.g., if we have the 4-bit pattern 0110, then the 8-bit key would be 0110 0110.) Here is an example: If we have the ASCII value for the letter ‘t’, then that equates to the hex value 0x74, which represents the binary value 0111 0100. If we bitwise XOR that value with a key, say 0101 0101, then

0111 0100 -> ‘t’

0101 0101 -> XOR with key encryption

0010 0001 -> encrypted value 0x21

If we XOR the encrypted value with the key, we get our ASCII value back.

0010 0001 -> encrypted value 0x21

0101 0101 -> XOR with key decryption

0111 0100 -> ‘t’

The bit stream encryption cipher that your program must decode does a little more than this. After encrypting the value, the program adds an alternating bit rotation twist to the algorithm. The alternating bit rotation algorithm is such that once you have the encrypted value (0x21, in the example above), if it is the 1st character of the encrypted message , it must be rotated one bit to the left (in our example above that would mean 0b 0010 0001 would become 0b 0100 0010 or 0x42). The encrypted value for the 2ndcharacter of the message would be rotated one bit to the right, 3rd char rotated left, 4throtated right and so forth. As a further example, if ‘t’ is rotated to the right one position, that would mean 0b 0010 0001 would become 0b 1001 0000 or 0x90.