python-cbc-padding-oracle-attack
Done. Verified solution delivered.
Deliverables
- ~/cbc_padding_oracle_attack.py — runnable pure-Python padding-oracle attack
- ~/solution.md — self-contained write-up (title, root-cause analysis, exact fix, verification)
What the solution does
- Defines the oracle valid(ct_hex) exactly per spec: openssl enc -d -aes-128-cbc -K 00112233445566778899aabbccddeeff -iv 00000000000000000000000000000000 -nopad, then a manual strict PKCS#7 check (value 1..16, all pad bytes equal, count matches).
- Recovers I_i = D(C_i) block by block from the last with queries [C_0..C_{i-2}][M][C_i] (16-byte aligned; for block 0, M plays the role of the zero IV), unrolling each intermediate byte by probing all 256 values of the manipulated byte with the already-solved tail re-fixed to the current pad value per position.
- Unpads only after all 4 blocks are recovered, then prints and asserts the plaintext.
Key findings from diagnosis
- With the strict oracle described, each of the 64 byte positions yields exactly one True candidate (verified empirically), so the classic spurious-validity pitfall is structurally impossible provided the tail bytes are rebuilt with the current pad value for every position — the stale-tail bug (intermediate byte already forming valid padding → oracle True for all 256 trials) is the actual root cause when it bites. The code detects (collects all candidates, never trusts the first True) and re-probes with an alternate pad configuration as the task requires, including a demonstrated fix of the simulated stale-tail case.
- I also proved why same-position disambiguation is fundamentally limited for a strict oracle (pad2 < pad excludes position p from the padding region; pad2 > pad needs uncontrolled bytes below p), and documented that the re-probe is therefore a defensive no-op against the spec'd oracle rather than the primary mechanism.
Verification: full run recovers padding oracle attack on block three of cbc mode!! (last block de!! + 14×0x0E) in ~92 s with exit code 0 and the required assertion passing; ground truth independently confirmed by direct openssl decryption.
# Evidence - Problem class: python-cbc-padding-oracle-attack - Model: openrouter/deepseek/deepseek-v4-flash-0731 - Solved: 2026-09-05T16:19:54.417Z - Verification: solution produced by pi in sandbox; see signatures.json
{"description": "Implement a PKCS#7 CBC padding-oracle decryption attack in pure Python. Your program defines the oracle valid(ct_hex)->bool, which decrypts AES-128-CBC (key 00112233445566778899aabbccddeeff, zero IV, block size 16) by shelling out to `openssl enc -d -aes-128-cbc -K <key> -iv <iv> -nopad` (or an equivalent AES implementation) and returns True iff the final byte of the decrypted, unpadded data forms well-formed PKCS#7 padding (value 1..16, all pad bytes equal, count matches); it must then recover the plaintext of target ciphertext 584438e76e4d0be0f9bab2205cbed1c0d68703ab1d53afc414967164a278c0079fae62de7c0773ebef18e86990c1dcd951b348340c8fe8533f0a8c1b5275928a using ONLY oracle queries, working block by block from the last (4 blocks total, query ciphertexts must always be full 16-byte-aligned buffers whose final block carries the manipulated bytes). Handle the classic spurious-validity pitfall: when an intermediate byte already equals your trial pad value the oracle stays True for the wrong reason \u2014 detect this by re-probing with a different pad and adjust the search, and strip real padding only after all blocks are recovered. Print the recovered plaintext and assert it equals `padding oracle attack on block three of cbc mode!!` (50 bytes: the final plaintext block is 2 real bytes followed by fourteen 0x0E pad bytes).", "environment": "python3", "language": "python", "model": "openrouter/deepseek/deepseek-v4-flash-0731", "problem_class": "python-cbc-padding-oracle-attack", "provider": "openrouter", "solved_at": "2026-09-05T16:19:54.418Z", "version": "3.11"}Done. Verified solution delivered.
Deliverables
- ~/cbc_padding_oracle_attack.py — runnable pure-Python padding-oracle attack
- ~/solution.md — self-contained write-up (title, root-cause analysis, exact fix, verification)
What the solution does
- Defines the oracle valid(ct_hex) exactly per spec: openssl enc -d -aes-128-cbc -K 00112233445566778899aabbccddeeff -iv 00000000000000000000000000000000 -nopad, then a manual strict PKCS#7 check (value 1..16, all pad bytes equal, count matches).
- Recovers I_i = D(C_i) block by block from the last with queries [C_0..C_{i-2}][M][C_i] (16-byte aligned; for block 0, M plays the role of the zero IV), unrolling each intermediate byte by probing all 256 values of the manipulated byte with the already-solved tail re-fixed to the current pad value per position.
- Unpads only after all 4 blocks are recovered, then prints and asserts the plaintext.
Key findings from diagnosis
- With the strict oracle described, each of the 64 byte positions yields exactly one True candidate (verified empirically), so the classic spurious-validity pitfall is structurally impossible provided the tail bytes are rebuilt with the current pad value for every position — the stale-tail bug (intermediate byte already forming valid padding → oracle True for all 256 trials) is the actual root cause when it bites. The code detects (collects all candidates, never trusts the first True) and re-probes with an alternate pad configuration as the task requires, including a demonstrated fix of the simulated stale-tail case.
- I also proved why same-position disambiguation is fundamentally limited for a strict oracle (pad2 < pad excludes position p from the padding region; pad2 > pad needs uncontrolled bytes below p), and documented that the re-probe is therefore a defensive no-op against the spec'd oracle rather than the primary mechanism.
Verification: full run recovers padding oracle attack on block three of cbc mode!! (last block de!! + 14×0x0E) in ~92 s with exit code 0 and the required assertion passing; ground truth independently confirmed by direct openssl decryption.
# Evidence - Problem class: python-cbc-padding-oracle-attack - Model: openrouter/deepseek/deepseek-v4-flash-0731 - Solved: 2026-09-05T16:19:54.417Z - Verification: solution produced by pi in sandbox; see signatures.json
{"description": "Implement a PKCS#7 CBC padding-oracle decryption attack in pure Python. Your program defines the oracle valid(ct_hex)->bool, which decrypts AES-128-CBC (key 00112233445566778899aabbccddeeff, zero IV, block size 16) by shelling out to `openssl enc -d -aes-128-cbc -K <key> -iv <iv> -nopad` (or an equivalent AES implementation) and returns True iff the final byte of the decrypted, unpadded data forms well-formed PKCS#7 padding (value 1..16, all pad bytes equal, count matches); it must then recover the plaintext of target ciphertext 584438e76e4d0be0f9bab2205cbed1c0d68703ab1d53afc414967164a278c0079fae62de7c0773ebef18e86990c1dcd951b348340c8fe8533f0a8c1b5275928a using ONLY oracle queries, working block by block from the last (4 blocks total, query ciphertexts must always be full 16-byte-aligned buffers whose final block carries the manipulated bytes). Handle the classic spurious-validity pitfall: when an intermediate byte already equals your trial pad value the oracle stays True for the wrong reason \u2014 detect this by re-probing with a different pad and adjust the search, and strip real padding only after all blocks are recovered. Print the recovered plaintext and assert it equals `padding oracle attack on block three of cbc mode!!` (50 bytes: the final plaintext block is 2 real bytes followed by fourteen 0x0E pad bytes).", "environment": "python3", "language": "python", "model": "openrouter/deepseek/deepseek-v4-flash-0731", "problem_class": "python-cbc-padding-oracle-attack", "provider": "openrouter", "solved_at": "2026-09-05T16:19:54.418Z", "version": "3.11"}