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| Original file line number | Diff line number | Diff line change |
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| # DownUnderCTF 2024 - adorable encrypted animal | ||
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| - **Category:** rev | ||
| - **Solves:** 3/2194 | ||
| - **Difficulty:** ⭐️⭐️⭐️ | ||
| - **Hosting type:** file | ||
| - **Tags:** arm, macOS | ||
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| --- | ||
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| > An adorable animal was encrypted with this program! A flag too, I guess. | ||
| > | ||
| > NOTE: `aea` and `libAppleArchive.dylib` are provided for your convenience. They are taken without modification from the macOS system that ran the binary. | ||
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| Handout files: | ||
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| - [./publish/aea.tar.gz](./publish/aea.tar.gz) | ||
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| ## Solution | ||
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| Flag: `DUCTF{h0pe_y0u_enjoy3d_th3_fr33_cat_p1c_:)}` | ||
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| - [**Solver**](./solve/solv.py) | ||
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| id: ductf-2024-adorable-encrypted-animal | ||
| name: adorable encrypted animal | ||
| category: rev | ||
| ctf: DownUnderCTF 2024 | ||
| difficulty: 3 | ||
| tags: ['arm', 'macOS'] | ||
| notes: '' | ||
| description: |- | ||
| An adorable animal was encrypted with this program! A flag too, I guess. | ||
| NOTE: `aea` and `libAppleArchive.dylib` are provided for your convenience. They are taken without modification from the macOS system that ran the binary. | ||
| hosting: file | ||
| handout_files: | ||
| - ./publish/aea.tar.gz | ||
| flag: DUCTF{h0pe_y0u_enjoy3d_th3_fr33_cat_p1c_:)} | ||
| solver: ./solve/solv.py | ||
| solve_stats: | ||
| solved_teams: 3 | ||
| num_teams: 2194 |
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| from Crypto.Cipher import AES | ||
| from Crypto.Util import Counter | ||
| from Crypto.Util.strxor import strxor | ||
| from Crypto.Protocol.KDF import HKDF | ||
| from Crypto.Hash import HMAC, SHA256 | ||
| from hashlib import sha256 | ||
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| cat = open('../publish/cat.png.aea', 'rb') | ||
| key = bytes.fromhex('27b750649a0698ffcd3085f4be57b011da80be70163d4a4ff9fb883f2db5a2f1') | ||
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| cat.seek(12, 0) | ||
| salt = cat.read(0x20) | ||
| info = b'AEA_AMK\x01\x00\x00\x00' | ||
| mkey = HKDF(key, len(key), salt, SHA256, context=info) | ||
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| cat.seek(0x70, 1) | ||
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| cluster_header_ct = cat.read(0x2800) | ||
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| cluster_intmd_key = HKDF(mkey, len(mkey), b'', SHA256, context=b'AEA_CK\x00\x00\x00\x00') | ||
| cluster_keyiv_data = b''.join(HKDF(cluster_intmd_key, len(cluster_intmd_key), b'', SHA256, context=b'AEA_CHEK', num_keys=3)) | ||
| cluster_key = cluster_keyiv_data[32:64] | ||
| cluster_iv = cluster_keyiv_data[64:80] | ||
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| ch_aes = AES.new(cluster_key, AES.MODE_CTR, counter=Counter.new(nbits=128, initial_value=int.from_bytes(cluster_iv, 'big'))) | ||
| aes = AES.new(cluster_key, AES.MODE_CTR, counter=Counter.new(nbits=128, initial_value=int.from_bytes(cluster_iv, 'big'))) | ||
| pt = aes.decrypt(cluster_header_ct) | ||
| segments = [pt[i:i+40] for i in range(0, len(pt), 40) if pt[i:i+40] != b'\x00'*40] | ||
| assert len(segments) == 1 | ||
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| cat.seek(0x2020, 1) | ||
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| seg = segments[0] | ||
| seg_raw_size = int.from_bytes(seg[4:8], 'little') | ||
| ct = cat.read(seg_raw_size) | ||
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| cluster_intmd_key = HKDF(mkey, len(mkey), b'', SHA256, context=b'AEA_CK\x00\x00\x00\x00') | ||
| cluster_keyiv_data = b''.join(HKDF(cluster_intmd_key, len(cluster_intmd_key), b'', SHA256, context=b'AEA_SK\x00\x00\x00\x00', num_keys=3)) | ||
| mac_key = cluster_keyiv_data[0:32] | ||
| cluster_key = cluster_keyiv_data[32:64] | ||
| cluster_iv = cluster_keyiv_data[64:80] | ||
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| hmac = HMAC.new(mac_key, digestmod=SHA256) | ||
| hmac.update(ct) | ||
| hmac.update(b'\x00'*8) | ||
| k2 = hmac.digest() | ||
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| aes = AES.new(cluster_key, AES.MODE_CTR, counter=Counter.new(nbits=128, initial_value=int.from_bytes(cluster_iv, 'big'))) | ||
| catpng = aes.decrypt(ct) | ||
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| k1 = ch_aes.encrypt(b'x' * 8 + sha256(catpng).digest())[8:] | ||
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| print('k1:', k1.hex()) | ||
| print('k2:', k2.hex()) | ||
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| # open('cat-dec.png', 'wb').write(catpng) | ||
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| flag = open('../publish/flag.txt.aea', 'rb') | ||
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| flag.seek(12, 0) | ||
| salt = flag.read(0x20) | ||
| info = b'AEA_AMK\x01\x00\x00\x00' | ||
| key = strxor(k1, k2) | ||
| print('flag key:', key.hex()) | ||
| mkey = HKDF(key, len(key), salt, SHA256, context=info) | ||
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| flag.seek(0x70, 1) | ||
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| cluster_header_ct = flag.read(0x2800) | ||
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| cluster_intmd_key = HKDF(mkey, len(mkey), b'', SHA256, context=b'AEA_CK\x00\x00\x00\x00') | ||
| cluster_keyiv_data = b''.join(HKDF(cluster_intmd_key, len(cluster_intmd_key), b'', SHA256, context=b'AEA_CHEK', num_keys=3)) | ||
| cluster_key = cluster_keyiv_data[32:64] | ||
| cluster_iv = cluster_keyiv_data[64:80] | ||
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| aes = AES.new(cluster_key, AES.MODE_CTR, counter=Counter.new(nbits=128, initial_value=int.from_bytes(cluster_iv, 'big'))) | ||
| pt = aes.decrypt(cluster_header_ct) | ||
| segments = [pt[i:i+40] for i in range(0, len(pt), 40) if pt[i:i+40] != b'\x00'*40] | ||
| assert len(segments) == 1 | ||
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| flag.seek(0x2020, 1) | ||
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| seg = segments[0] | ||
| seg_raw_size = int.from_bytes(seg[4:8], 'little') | ||
| ct = flag.read(seg_raw_size) | ||
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| cluster_intmd_key = HKDF(mkey, len(mkey), b'', SHA256, context=b'AEA_CK\x00\x00\x00\x00') | ||
| cluster_keyiv_data = b''.join(HKDF(cluster_intmd_key, len(cluster_intmd_key), b'', SHA256, context=b'AEA_SK\x00\x00\x00\x00', num_keys=3)) | ||
| cluster_key = cluster_keyiv_data[32:64] | ||
| cluster_iv = cluster_keyiv_data[64:80] | ||
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| aes = AES.new(cluster_key, AES.MODE_CTR, counter=Counter.new(nbits=128, initial_value=int.from_bytes(cluster_iv, 'big'))) | ||
| flag = aes.decrypt(ct) | ||
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| print(flag.decode()) |
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| #include <stdio.h> | ||
| #include <stdlib.h> | ||
| #include <string.h> | ||
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| const unsigned char KEY[32] = {39, 183, 80, 100, 154, 6, 152, 255, 205, 48, 133, 244, 190, 87, 176, 17, 218, 128, 190, 112, 22, 61, 74, 79, 249, 251, 136, 63, 45, 181, 162, 241}; | ||
| const unsigned char Z[32] = "ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ"; | ||
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| int main() { | ||
| char cmd[0x1000]; | ||
| char hexKey[65] = {0}; | ||
| char* tmp; | ||
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| tmp = hexKey; | ||
| for(int i = 0; i < 32; i++) { | ||
| tmp += sprintf(tmp, "%02x", KEY[i]); | ||
| } | ||
| sprintf(cmd, "/usr/bin/aea encrypt -i cat.png -key-value hex:%s -o cat.png.aea", hexKey); | ||
| system(cmd); | ||
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| FILE* f = fopen("cat.png.aea", "r"); | ||
| fseek(f, 0, SEEK_END); | ||
| size_t fs = ftell(f); | ||
| fseek(f, 0, SEEK_SET); | ||
| tmp = malloc(fs); | ||
| fread(tmp, 1, fs, f); | ||
| fclose(f); | ||
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| unsigned char k1[32] = {0}; | ||
| unsigned char k2[32] = {0}; | ||
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| memcpy(k1, &tmp[0xa4], 0x20); | ||
| memcpy(k2, &tmp[0x28bc], 0x20); | ||
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| f = fopen("cat.png.aea", "w"); | ||
| memcpy(&tmp[0xa4], Z, 0x20); | ||
| memcpy(&tmp[0x28bc], Z, 0x20); | ||
| fwrite(tmp, 1, fs, f); | ||
| fclose(f); | ||
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| tmp = hexKey; | ||
| for(int i = 0; i < 32; i++) { | ||
| tmp += sprintf(tmp, "%02x", k1[i] ^ k2[i]); | ||
| } | ||
| memset(cmd, 0, 0x1000); | ||
| sprintf(cmd, "/usr/bin/aea encrypt -i flag.txt -key-value hex:%s -o flag.txt.aea", hexKey); | ||
| system(cmd); | ||
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| } |
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| @@ -0,0 +1 @@ | ||
| DUCTF{h0pe_y0u_enjoy3d_th3_fr33_cat_p1c_:)} |
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| # DownUnderCTF 2024 - average assembly assignment | ||
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| - **Category:** rev | ||
| - **Solves:** 13/2194 | ||
| - **Difficulty:** ⭐️⭐️ | ||
| - **Hosting type:** tcp | ||
| - **Tags:** Puzzle, Misc | ||
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| --- | ||
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| > This is just your average assembly assignment. | ||
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| Handout files: | ||
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| - [./publish/aaa](./publish/aaa) | ||
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| ## Solution | ||
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| Flag: `DUCTF{y0u_pass!_dbfae0837abc0a239fd7abf93be8a01dfec}` | ||
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| - [**Solver**](./solve/solv.py) | ||
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downunderctf-2024/average-assembly-assignment/details.yaml
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| id: ductf-2024-average-assembly-assignment | ||
| name: average assembly assignment | ||
| category: rev | ||
| ctf: DownUnderCTF 2024 | ||
| difficulty: 2 | ||
| tags: ['Puzzle', 'Misc'] | ||
| notes: '' | ||
| description: This is just your average assembly assignment. | ||
| hosting: tcp | ||
| handout_files: | ||
| - ./publish/aaa | ||
| flag: DUCTF{y0u_pass!_dbfae0837abc0a239fd7abf93be8a01dfec} | ||
| solver: ./solve/solv.py | ||
| solve_stats: | ||
| solved_teams: 13 | ||
| num_teams: 2194 |
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downunderctf-2024/average-assembly-assignment/solve/solv.py
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| sol = ''' | ||
| # program to find the average of given inputs, where 0 indicates end of input | ||
| # | ||
| # read all input elements while summing them and counting how many there are | ||
| # INP reads into R0, so we need to store this into ACC to check if it is 0 | ||
| # we will store the running sum in BAK and the count in R1 | ||
| read_all_loop: | ||
| INP # read input into R0 | ||
| MOV R0 ACC | ||
| JZ read_all_loop_break | ||
| SWP | ||
| ADD R0 # add input to running sum | ||
| SWP | ||
| MOV R1 ACC | ||
| ADD 1 | ||
| MOV ACC R1 | ||
| JMP read_all_loop | ||
| # now we will do a nested for loop, with the inner loop iterating R1 times and | ||
| # subtracting one each time, and checking if the result is 0 or not. if it's 0, | ||
| # we are finished and should put the result in ACC, otherwise add 1 to the | ||
| # result. in the case where both loops hit 0 at the same time, it's a clean | ||
| # division, so we should add an extra 1 to the result | ||
| read_all_loop_break: | ||
| MOV R1 ACC | ||
| inner_loop: | ||
| SUB 1 | ||
| SWP | ||
| SUB 1 | ||
| JZ done | ||
| SWP | ||
| JZ inc | ||
| JMP inner_loop | ||
| inc: | ||
| MOV R0 ACC | ||
| ADD 1 | ||
| MOV ACC R0 | ||
| JMP read_all_loop_break | ||
| done: | ||
| SWP | ||
| JZ plusone | ||
| MOV R0 ACC | ||
| JMP exit | ||
| plusone: | ||
| MOV R0 ACC | ||
| ADD 1 | ||
| exit: | ||
| ''' | ||
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| op_map = { | ||
| 'MOV': 'OWO', | ||
| 'ACC': 'AAA', | ||
| 'BAK': 'BBB', | ||
| 'INP': 'INP', | ||
| 'ADD': 'UWU', | ||
| 'SUB': 'QAQ', | ||
| 'SAV': 'TVT', | ||
| 'SWP': 'TOT', | ||
| 'JMP': 'WOW', | ||
| 'JZ': 'WEW', | ||
| 'JNZ': 'WAW', | ||
| 'LABEL': 'LOL', | ||
| 'NOP': 'NOP' | ||
| } | ||
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| for op in op_map: | ||
| sol = sol.replace(op, op_map[op]) | ||
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| print(sol) | ||
| print('EOF') |
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| FROM ghcr.io/downunderctf/docker-vendor/nsjail:ubuntu-22.04 | ||
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| ENV JAIL_CWD=/chal | ||
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| COPY ./flag.txt /home/ctf/chal | ||
| COPY ./aaa /home/ctf/chal/pwn |
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