We were given source code of encryptor and an encrypted file

[*] Generates a 4x4 matrix as key using randint() function. An secret password is used as seed, which we dont know

[*] File is padded with NUL such that size(file) % 16 == 0

[*] Each 16 bytes of file is converted to 4x4 matrix and multiplied with key

[*] Each element of resultant 4x4 matrix is packed into 2 bytes of data and written to file

First we should find the key to perform decryption. The file extension leaves a clue

*flag.wmv.out*. The encryption algorithm performs the following operation[*] Generates a 4x4 matrix as key using randint() function. An secret password is used as seed, which we dont know

[*] File is padded with NUL such that size(file) % 16 == 0

[*] Each 16 bytes of file is converted to 4x4 matrix and multiplied with key

[*] Each element of resultant 4x4 matrix is packed into 2 bytes of data and written to file

First we should find the key to perform decryption. The file extension leaves a clue

*wmv*. Using wmv header as known plain text and encrypted header as cipher text, the key could be found as wmv_header.inverse() * enc_header. Once the key found, we have to write decryption routine such that encrypted file is processed in blocks for 32 bytes in a 4x4 matrix. Below is the code to solve this#!/usr/bin/env python from sage.all import * import struct wmv_header = [[0x30, 0x26, 0xB2, 0x75], [0x8E, 0x66, 0xCF, 0x11], [0xA6, 0xD9, 0x00, 0xAA], [0x00, 0x62, 0xCE, 0x6C]] def Str2matrix(s): #convert string to 4x4 matrix return [map(lambda x : ord(x), list(s[i:i+4])) for i in xrange(0, len(s), 4)] def Matrix2str(m): #convert matrix to string return ''.join(map(lambda x : ''.join(map(lambda y : struct.pack('!B', y), x)), m)) def PackedStr2matrix(s): matrix = [0] * 16 for i in range(0, len(s), 2): matrix[i/2] = struct.unpack('!H', s[i:i+2])[0] matrix = [matrix[i:i+4] for i in range(0,len(matrix), 4)] return matrix def Multiply(A,B): #multiply two 4x4 matrix C = [[0 for i in xrange(4)] for j in xrange(4)] for i in xrange(4): for j in xrange(4): for k in xrange(4): C[i][j] += A[i][k] * B[k][j] return C header = matrix(wmv_header) encrypted_wmv = open('flag.wmv.out','rb').read() size = struct.unpack('!I', encrypted_wmv[:4]) enc_header = encrypted_wmv[4:36] enc_header = matrix(PackedStr2matrix(enc_header)) # sage: header = matrix([[48, 38, 178, 117], [142, 102, 207, 17], [166, 217, 0, 170], [0, 98, 206, 108]]) # sage: enc_header = matrix([[6025, 10758, 8274, 14059], [10718, 11769, 5025, 15260], [19537, 18796, 14142, 15035], [7648, 8842, 7254, 17852]]) # sage: header.inverse() * enc_header # [31 51 20 0] # [53 10 6 45] # [ 3 13 3 49] # [17 48 56 31] # sage: key.inverse() # [ 5732/2519421 96221/5038842 -41017/2519421 -10009/5038842] # [ 65399/2519421 -67381/5038842 44957/2519421 -44311/5038842] # [-49681/2519421 22679/5038842 -51064/2519421 128507/5038842] # [-14660/2519421 10597/5038842 45127/2519421 4501/5038842] key = header.inverse() * enc_header key_inverse = key.inverse() out = open('flag.wmv','wb') encrypted_wmv = encrypted_wmv[4:] for i in xrange(0, len(encrypted_wmv), 32): unpacked_data = matrix(PackedStr2matrix(encrypted_wmv[i:i+32])) decrypted = unpacked_data * key_inverse out.write(Matrix2str(decrypted)) out.close() # CTF{b699a72e2692d16f65ec9626055aa740}We get a decrypted wmv file which has the flag.

root@sagepc:~ $file flag.wmv flag.wmv: Microsoft ASFFlag for the challenege is

**CTF{b699a72e2692d16f65ec9626055aa740}**
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