CBC byte flipping attack

Nadim Hajj

6/20/23, 7:44 PM

I'm learning how CBC works but i don't understand when altering the first 2 bytes of the ciphered text why the 1rst block and ONLY the first 2 bytes of the second block are altered. The first 2 bytes of the cypher text are altered using a xor operation with random values. Each block is made of 16 bytes.

Those are the random bytes to encrypt:

D6 D7 17 2B D8 1B 73 DF AA D4 D0 DC 94 D1 C2 B2 EE 0D 3B DA 74 58 62 F9 F9 D1 30 59 F5 FE 7D 54 14 79 42 14 B6 1D 34 C7 19 B0 68 67 36 25 D9 FA D0 1C 61 CB 40 A2 CD 77 6A 7D D3 D4 A2 23 9B 4F AF

This is the cypher text:

D2 CD F5 92 8D 1D 4E BC A1 9A CE 50 07 8C 13 FE 33 4B FE BD A0 6B 6C 7B F4 38 90 F4 36 A6 9A 15 A3 D8 41 F4 04 5D C0 9C A3 B6 A6 A8 4A B5 B2 F9 3C E3 0B 93 5E E1 42 26 66 58 16 39 A4 9C 14 5D 69 E9 EE C8 7D 0F AC 85 E7 EC 9C 0B C4 E4 74 22

This is the altered cypher text by xoring the previous text with 80 80:

52 4D F5 92 8D 1D 4E BC A1 9A CE 50 07 8C 13 FE 33 4B FE BD A0 6B 6C 7B F4 38 90 F4 36 A6 9A 15 A3 D8 41 F4 04 5D C0 9C A3 B6 A6 A8 4A B5 B2 F9 3C E3 0B 93 5E E1 42 26 66 58 16 39 A4 9C 14 5D 69 E9 EE C8 7D 0F AC 85 E7 EC 9C 0B C4 E4 74 22

And finally the decrypted altered text:

D6 1E 4E A5 5A 8D 3B 4A E6 2F E2 0A 41 21 68 51 6E 8D 3B DA 74 58 62 F9 F9 D1 30 59 F5 FE 7D 54 14 79 42 14 B6 1D 34 C7 19 B0 68 67 36 25 D9 FA D0 1C 61 CB 40 A2 CD 77 6A 7D D3 D4 A2 23 9B 4F AF

0 + 0

cbc

Nadim Hajj

6/20/23, 8:46 PM

It helps but my question is: after flipping a byte in a cyphertext block, why the byte in the same position in the following block will be affected and not the rest. Let's say I flipped the first byte of the first block, why only the first byte of the next block will be affected and not the entire second block.

kelalaka

6/20/23, 8:52 PM

Did you see the red case? Modification in the first 16 bytes ( It is actually the IV) only affects the first block of the plaintext.

Nadim Hajj

6/20/23, 9:01 PM

I found the solution it's actually about the xor and it's properties. Thanks for pointing me to this article it helped a lot.

Elon Musk

I sit in a Tesla and translated this thread with Ai:

EN: CBC byte flipping attack

TH: การโจมตีพลิกไบต์ CBC

RO: Atacul de inversare a octetilor CBC

RU: Атака с переворачиванием байтов CBC

VI: Tấn công lật byte CBC

Post an answer

Most people don’t grasp that asking a lot of questions unlocks learning and improves interpersonal bonding. In Alison’s studies, for example, though people could accurately recall how many questions had been asked in their conversations, they didn’t intuit the link between questions and liking. Across four studies, in which participants were engaged in conversations themselves or read transcripts of others’ conversations, people tended not to realize that question asking would influence—or had influenced—the level of amity between the conversationalists.