TellIsHow's Build the Decrypt

Author:
TellIsHow

Language:
C/C++

Upload:
2026-01-31 22:53

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Platform:
Windows

Difficulty:
3.7

Quality:
4.5

Arch:
x86-64

Downloads:
459

Size:
6.63 MB

Writeups:
0

Comments:
1

Description

This crackme implements a custom hybrid cryptographic algorithm. When executed, the program generates a file named decryptme.txt in the current directory. The binary contains only the encryption logic. No decryption routine is present. Your task is to analyze the algorithm, understand its internal structure, and implement your own decryption algorithm. A fixed password is used internally: 123 If your decryption is correct, decrypting decryptme.txt will produce: a1b2-c3d4-e5f6-7890_CONGRATS_YOU_REVERSED_IT_

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deo on 2026-02-14 12:01: decryptor script: import struct import hashlib import hmac import sys # Standard AES S-box AES_SBOX = bytes([ 0x63, 0x7C, 0x77, 0x7B, 0xF2, 0x6B, 0x6F, 0xC5, 0x30, 0x01, 0x67, 0x2B, 0xFE, 0xD7, 0xAB, 0x76, 0xCA, 0x82, 0xC9, 0x7D, 0xFA, 0x59, 0x47, 0xF0, 0xAD, 0xD4, 0xA2, 0xAF, 0x9C, 0xA4, 0x72, 0xC0, 0xB7, 0xFD, 0x93, 0x26, 0x36, 0x3F, 0xF7, 0xCC, 0x34, 0xA5, 0xE5, 0xF1, 0x71, 0xD8, 0x31, 0x15, 0x04, 0xC7, 0x23, 0xC3, 0x18, 0x96, 0x05, 0x9A, 0x07, 0x12, 0x80, 0xE2, 0xEB, 0x27, 0xB2, 0x75, 0x09, 0x83, 0x2C, 0x1A, 0x1B, 0x6E, 0x5A, 0xA0, 0x52, 0x3B, 0xD6, 0xB3, 0x29, 0xE3, 0x2F, 0x84, 0x53, 0xD1, 0x00, 0xED, 0x20, 0xFC, 0xB1, 0x5B, 0x6A, 0xCB, 0xBE, 0x39, 0x4A, 0x4C, 0x58, 0xCF, 0xD0, 0xEF, 0xAA, 0xFB, 0x43, 0x4D, 0x33, 0x85, 0x45, 0xF9, 0x02, 0x7F, 0x50, 0x3C, 0x9F, 0xA8, 0x51, 0xA3, 0x40, 0x8F, 0x92, 0x9D, 0x38, 0xF5, 0xBC, 0xB6, 0xDA, 0x21, 0x10, 0xFF, 0xF3, 0xD2, 0xCD, 0x0C, 0x13, 0xEC, 0x5F, 0x97, 0x44, 0x17, 0xC4, 0xA7, 0x7E, 0x3D, 0x64, 0x5D, 0x19, 0x73, 0x60, 0x81, 0x4F, 0xDC, 0x22, 0x2A, 0x90, 0x88, 0x46, 0xEE, 0xB8, 0x14, 0xDE, 0x5E, 0x0B, 0xDB, 0xE0, 0x32, 0x3A, 0x0A, 0x49, 0x06, 0x24, 0x5C, 0xC2, 0xD3, 0xAC, 0x62, 0x91, 0x95, 0xE4, 0x79, 0xE7, 0xC8, 0x37, 0x6D, 0x8D, 0xD5, 0x4E, 0xA9, 0x6C, 0x56, 0xF4, 0xEA, 0x65, 0x7A, 0xAE, 0x08, 0xBA, 0x78, 0x25, 0x2E, 0x1C, 0xA6, 0xB4, 0xC6, 0xE8, 0xDD, 0x74, 0x1F, 0x4B, 0xBD, 0x8B, 0x8A, 0x70, 0x3E, 0xB5, 0x66, 0x48, 0x03, 0xF6, 0x0E, 0x61, 0x35, 0x57, 0xB9, 0x86, 0xC1, 0x1D, 0x9E, 0xE1, 0xF8, 0x98, 0x11, 0x69, 0xD9, 0x8E, 0x94, 0x9B, 0x1E, 0x87, 0xE9, 0xCE, 0x55, 0x28, 0xDF, 0x8C, 0xA1, 0x89, 0x0D, 0xBF, 0xE6, 0x42, 0x68, 0x41, 0x99, 0x2D, 0x0F, 0xB0, 0x54, 0xBB, 0x16 ]) def rol64(val, bits): val &= 0xFFFFFFFFFFFFFFFF return ((val << bits) | (val >> (64 - bits))) & 0xFFFFFFFFFFFFFFFF def ror64(val, bits): val &= 0xFFFFFFFFFFFFFFFF return ((val >> bits) | (val << (64 - bits))) & 0xFFFFFFFFFFFFFFFF def derive_keys(password, iv): # HKDF-ish key derivation pwd_wstr = password.encode('utf-16le') # wstring encoding pwd_hash = hashlib.sha256(pwd_wstr).digest() master_key = hmac.new(iv, pwd_hash, hashlib.sha256).digest() enc_key = hmac.new(master_key, b'enc', hashlib.sha256).digest() mac_key = hmac.new(master_key, b'mac', hashlib.sha256).digest() return enc_key, mac_key def rc4_ksa(s_box, key): # RC4 key scheduling but starting from AES S-box instead of [0..255] # This was the main trick! S = list(s_box) j = 0 for i in range(256): j = (j + S[i] + key[i & 0x1F]) & 0xFF S[i], S[j] = S[j], S[i] return bytes(S) def reverse_substitution_layer(ciphertext, S, key): result = bytearray(ciphertext) n = len(result) if n == 0: return bytes(result) # Build inverse S-box inv_S = bytearray(256) for i in range(256): inv_S[S[i]] = i # Decrypt with state tracking v42 = key[0] v43 = key[31] for j in range(n): v45 = key[j & 0x1F] encrypted_byte = result[j] # Reverse the substitution formula plaintext_byte = inv_S[(encrypted_byte - v43) & 0xFF] ^ v42 result[j] = plaintext_byte # Update states for next iteration v42 = v45 ^ plaintext_byte v43 = encrypted_byte ^ v45 return bytes(result) def reverse_chacha_stream(ciphertext, enc_key): # XOR-based stream cipher with ChaCha-inspired mixing result = bytearray(ciphertext) n = len(result) if n == 0: return bytes(result) # Initialize state from key v34 = struct.unpack('<Q', enc_key[0:8])[0] ^ 0x6A09E667F3BCC908 v35 = struct.unpack('<Q', enc_key[8:16])[0] ^ 0xBB67AE8584CAA73B v36 = struct.unpack('<Q', enc_key[16:24])[0] ^ 0x3C6EF372FE94F82B v37 = struct.unpack('<Q', enc_key[24:32])[0] ^ 0xA54FF53A5F1D36F1 S = rc4_ksa(AES_SBOX, enc_key) v68 = 0 for pos in range(n): if (pos & 7) == 0: # Full round every 8 bytes v38 = (v35 + v34) & 0xFFFFFFFFFFFFFFFF v37 = rol64(v38 ^ v37, 23) v36 = (v36 + v37) & 0xFFFFFFFFFFFFFFFF v39 = (v38 ^ S[v38 & 0xFF]) & 0xFFFFFFFFFFFFFFFF v35 = ror64(v36 ^ v35, 23) v68 = (v37 ^ v36 ^ v39 ^ v35) & 0xFFFFFFFFFFFFFFFF keystream_byte = v68 & 0xFF plaintext_byte = result[pos] ^ keystream_byte result[pos] = plaintext_byte v34 = (v39 + plaintext_byte) & 0xFFFFFFFFFFFFFFFF else: # Reuse keystream for next 7 bytes v68 = (v37 ^ v36 ^ v34 ^ v35) & 0xFFFFFFFFFFFFFFFF keystream_byte = (v68 >> ((pos & 7) * 8)) & 0xFF ciphertext_byte = result[pos] plaintext_byte = ciphertext_byte ^ keystream_byte result[pos] = plaintext_byte v34 = (v34 + plaintext_byte) & 0xFFFFFFFFFFFFFFFF return bytes(result) def decrypt_file(filepath, password): with open(filepath, 'rb') as f: data = f.read() # Parse file format magic = data[0:3] version = struct.unpack('<H', data[3:5])[0] iv = data[5:21] ciphertext = data[21:-32] stored_mac = data[-32:] print(f"[*] File: {magic}, version {version}") print(f"[*] IV: {iv.hex()}") print(f"[*] Ciphertext: {len(ciphertext)} bytes") # Derive keys from password enc_key, mac_key = derive_keys(password, iv) # Verify integrity computed_mac = hmac.new(mac_key, ciphertext, hashlib.sha256).digest() if computed_mac != stored_mac: print("[!] MAC verification failed!") return None print("[+] MAC verified") # Decrypt: reverse substitution, then reverse stream cipher S = rc4_ksa(AES_SBOX, enc_key) temp = reverse_substitution_layer(ciphertext, S, enc_key) plaintext = reverse_chacha_stream(temp, enc_key) print(f"\n[+] Decrypted: {plaintext.decode('utf-8')}") return plaintext if __name__ == "__main__": filepath = sys.argv[1] if len(sys.argv) > 1 else "decryptme.txt" password = "123" print(f"Decrypting {filepath}...\n") decrypt_file(filepath, password)

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TellIsHow's Build the Decrypt