After coding RC4, I thought it was time to face a more difficult challenge: DES (Data Encryption Standard). Although it has been replaced by AES (Advanced Encryption Standard) more than a decade ago, it still has some appeal to me because it was the first symmetric algorithm I learned (and also because its creation, 40 years ago, was surrounded by mystery). Before fighting the monster, I faced a lighter opponent, S-DES (Simplified DES), to get used to the awkward bit manipulation DES take advantage of. Since S-DES is just a toy cryptographic algorithm, it isn’t worthwhile to spend much time and space writing about it. For this reason, without further ado, this is the DES Python code I wrote:
#!/usr/bin/python3 # # Author: Joao H de A Franco (jhafranco@acm.org) # # Description: DES implementation in Python 3 # # Note: only single DES in ECB mode (with PKCS5 padding) # is supported # # License: Attribution-NonCommercial-ShareAlike 3.0 Unported # (CC BY-NC-SA 3.0) #=========================================================== subKeyList = 16*[[None]*8] IPtable = (58, 50, 42, 34, 26, 18, 10, 2, 60, 52, 44, 36, 28, 20, 12, 4, 62, 54, 46, 38, 30, 22, 14, 6, 64, 56, 48, 40, 32, 24, 16, 8, 57, 49, 41, 33, 25, 17, 9, 1, 59, 51, 43, 35, 27, 19, 11, 3, 61, 53, 45, 37, 29, 21, 13, 5, 63, 55, 47, 39, 31, 23, 15, 7) EPtable = (32, 1, 2, 3, 4, 5, 4, 5, 6, 7, 8, 9, 8, 9, 10, 11, 12, 13, 12, 13, 14, 15, 16, 17, 16, 17, 18, 19, 20, 21, 20, 21, 22, 23, 24, 25, 24, 25, 26, 27, 28, 29, 28, 29, 30, 31, 32, 1) PFtable = (16, 7, 20, 21, 29, 12, 28, 17, 1, 15, 23, 26, 5, 18, 31, 10, 2, 8, 24, 14, 32, 27, 3, 9, 19, 13, 30, 6, 22, 11, 4, 25) FPtable = (40, 8, 48, 16, 56, 24, 64, 32, 39, 7, 47, 15, 55, 23, 63, 31, 38, 6, 46, 14, 54, 22, 62, 30, 37, 5, 45, 13, 53, 21, 61, 29, 36, 4, 44, 12, 52, 20, 60, 28, 35, 3, 43, 11, 51, 19, 59, 27, 34, 2, 42, 10, 50, 18, 58, 26, 33, 1, 41, 9, 49, 17, 57, 25) sBox = 8*[64*[0]] sBox[0] = (14, 4, 13, 1, 2, 15, 11, 8, 3, 10, 6, 12, 5, 9, 0, 7, 0, 15, 7, 4, 14, 2, 13, 1, 10, 6, 12, 11, 9, 5, 3, 8, 4, 1, 14, 8, 13, 6, 2, 11, 15, 12, 9, 7, 3, 10, 5, 0, 15, 12, 8, 2, 4, 9, 1, 7, 5, 11, 3, 14, 10, 0, 6, 13) sBox[1] = (15, 1, 8, 14, 6, 11, 3, 4, 9, 7, 2, 13, 12, 0, 5, 10, 3, 13, 4, 7, 15, 2, 8, 14, 12, 0, 1, 10, 6, 9, 11, 5, 0, 14, 7, 11, 10, 4, 13, 1, 5, 8, 12, 6, 9, 3, 2, 15, 13, 8, 10, 1, 3, 15, 4, 2, 11, 6, 7, 12, 0, 5, 14, 9) sBox[2] = (10, 0, 9, 14, 6, 3, 15, 5, 1, 13, 12, 7, 11, 4, 2, 8, 13, 7, 0, 9, 3, 4, 6, 10, 2, 8, 5, 14, 12, 11, 15, 1, 13, 6, 4, 9, 8, 15, 3, 0, 11, 1, 2, 12, 5, 10, 14, 7, 1, 10, 13, 0, 6, 9, 8, 7, 4, 15, 14, 3, 11, 5, 2, 12) sBox[3] = ( 7, 13, 14, 3, 0, 6, 9, 10, 1, 2, 8, 5, 11, 12, 4, 15, 13, 8, 11, 5, 6, 15, 0, 3, 4, 7, 2, 12, 1, 10, 14, 9, 10, 6, 9, 0, 12, 11, 7, 13, 15, 1, 3, 14, 5, 2, 8, 4, 3, 15, 0, 6, 10, 1, 13, 8, 9, 4, 5, 11, 12, 7, 2, 14) sBox[4] = ( 2, 12, 4, 1, 7, 10, 11, 6, 8, 5, 3, 15, 13, 0, 14, 9, 14, 11, 2, 12, 4, 7, 13, 1, 5, 0, 15, 10, 3, 9, 8, 6, 4, 2, 1, 11, 10, 13, 7, 8, 15, 9, 12, 5, 6, 3, 0, 14, 11, 8, 12, 7, 1, 14, 2, 13, 6, 15, 0, 9, 10, 4, 5, 3) sBox[5] = (12, 1, 10, 15, 9, 2, 6, 8, 0, 13, 3, 4, 14, 7, 5, 11, 10, 15, 4, 2, 7, 12, 9, 5, 6, 1, 13, 14, 0, 11, 3, 8, 9, 14, 15, 5, 2, 8, 12, 3, 7, 0, 4, 10, 1, 13, 11, 6, 4, 3, 2, 12, 9, 5, 15, 10, 11, 14, 1, 7, 6, 0, 8, 13) sBox[6] = ( 4, 11, 2, 14, 15, 0, 8, 13, 3, 12, 9, 7, 5, 10, 6, 1, 13, 0, 11, 7, 4, 9, 1, 10, 14, 3, 5, 12, 2, 15, 8, 6, 1, 4, 11, 13, 12, 3, 7, 14, 10, 15, 6, 8, 0, 5, 9, 2, 6, 11, 13, 8, 1, 4, 10, 7, 9, 5, 0, 15, 14, 2, 3, 12) sBox[7] = (13, 2, 8, 4, 6, 15, 11, 1, 10, 9, 3, 14, 5, 0, 12, 7, 1, 15, 13, 8, 10, 3, 7, 4, 12, 5, 6, 11, 0, 14, 9, 2, 7, 11, 4, 1, 9, 12, 14, 2, 0, 6, 10, 13, 15, 3, 5, 8, 2, 1, 14, 7, 4, 10, 8, 13, 15, 12, 9, 0, 3, 5, 6, 11) def bit2Byte(bitList): """Convert bit list into a byte list""" return [int("".join(map(str,bitList[i*8:i*8+8])),2) for i in range(len(bitList)//8)] def byte2Bit(byteList): """Convert byte list into a bit list""" return [(byteList[i//8]>>(7-(i%8)))&0x01 for i in range(8*len(byteList))] def permBitList(inputBitList,permTable): """Permute input bit list according to input permutation table""" return [inputBitList[e - 1] for e in permTable] def permByteList(inByteList,permTable): """Permute input byte list according to input permutation table""" outByteList = (len(permTable)>>3)*[0] for index,elem in enumerate(permTable): i = index%8 e = (elem-1)%8 if i>=e: outByteList[index>>3] |= \ (inByteList[(elem-1)>>3]&(128>>e))>>(i-e) else: outByteList[index>>3] |= \ (inByteList[(elem-1)>>3]&(128>>e))<<(e-i) return outByteList def getIndex(inBitList): """Permute bits to properly index the S-boxes""" return (inBitList[0]<<5)+(inBitList[1]<<3)+ \ (inBitList[2]<<2)+(inBitList[3]<<1)+ \ (inBitList[4]<<0)+(inBitList[5]<<4) def padData(string): """Add PKCS5 padding to plaintext""" padLength = 8-(len(string)%8) return [ord(s) for s in string]+padLength*[padLength] def unpadData(byteList): """Remove PKCS5 padding from plaintext""" return "".join(chr(e) for e in byteList[:-byteList[-1]]) def setKey(keyByteList): """Generate all sixteen round subkeys""" PC1table = (57, 49, 41, 33, 25, 17, 9, 1, 58, 50, 42, 34, 26, 18, 10, 2, 59, 51, 43, 35, 27, 19, 11, 3, 60, 52, 44, 36, 63, 55, 47, 39, 31, 23, 15, 7, 62, 54, 46, 38, 30, 22, 14, 6, 61, 53, 45, 37, 29, 21, 13, 5, 28, 20, 12, 4) PC2table= (14, 17, 11, 24, 1, 5, 3, 28, 15, 6, 21, 10, 23, 19, 12, 4, 26, 8, 16, 7, 27, 20, 13, 2, 41, 52, 31, 37, 47, 55, 30, 40, 51, 45, 33, 48, 44, 49, 39, 56, 34, 53, 46, 42, 50, 36, 29, 32) def leftShift(inKeyBitList,round): """Perform one (or two) circular left shift(s) on key""" LStable = (1, 1, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2, 2, 2, 2, 1) outKeyBitList = 56*[0] if LStable[round] == 2: outKeyBitList[:26] = inKeyBitList[2:28] outKeyBitList[26] = inKeyBitList[0] outKeyBitList[27] = inKeyBitList[1] outKeyBitList[28:54] = inKeyBitList[30:] outKeyBitList[54] = inKeyBitList[28] outKeyBitList[55] = inKeyBitList[29] else: outKeyBitList[:27] = inKeyBitList[1:28] outKeyBitList[27] = inKeyBitList[0] outKeyBitList[28:55] = inKeyBitList[29:] outKeyBitList[55] = inKeyBitList[28] return outKeyBitList permKeyBitList = permBitList(byte2Bit(keyByteList),PC1table) for round in range(16): auxBitList = leftShift(permKeyBitList,round) subKeyList[round] = bit2Byte(permBitList(auxBitList,PC2table)) permKeyBitList = auxBitList def encryptBlock(inputBlock): """Encrypt an 8-byte block with already defined key""" inputData = permByteList(inputBlock,IPtable) leftPart,rightPart = inputData[:4],inputData[4:] for round in range(16): expRightPart = permByteList(rightPart,EPtable) key = subKeyList[round] indexList = byte2Bit([i^j for i,j in zip(key,expRightPart)]) sBoxOutput = 4*[0] for nBox in range(4): nBox12 = 12*nBox leftIndex = getIndex(indexList[nBox12:nBox12+6]) rightIndex = getIndex(indexList[nBox12+6:nBox12+12]) sBoxOutput[nBox] = (sBox[nBox<<1][leftIndex]<<4)+ \ sBox[(nBox<<1)+1][rightIndex] aux = permByteList(sBoxOutput,PFtable) newRightPart = [i^j for i,j in zip(aux,leftPart)] leftPart = rightPart rightPart = newRightPart return permByteList(rightPart+leftPart,FPtable) def decryptBlock(inputBlock): """Decrypt an 8-byte block with already defined key""" inputData = permByteList(inputBlock,IPtable) leftPart,rightPart = inputData[:4],inputData[4:] for round in range(16): expRightPart = permByteList(rightPart,EPtable) key = subKeyList[15-round] indexList = byte2Bit([i^j for i,j in zip(key,expRightPart)]) sBoxOutput = 4*[0] for nBox in range(4): nBox12 = 12*nBox leftIndex = getIndex(indexList[nBox12:nBox12+6]) rightIndex = getIndex(indexList[nBox12+6:nBox12+12]) sBoxOutput[nBox] = (sBox[nBox*2][leftIndex]<<4)+ \ sBox[nBox*2+1][rightIndex] aux = permByteList(sBoxOutput,PFtable) newRightPart = [i^j for i,j in zip(aux,leftPart)] leftPart = rightPart rightPart = newRightPart return permByteList(rightPart+leftPart,FPtable) def encrypt(key, inString): """Encrypt plaintext with given key""" setKey(key) inByteList,outByteList = padData(inString),[] for i in range(0,len(inByteList),8): outByteList += encryptBlock(inByteList[i:i+8]) return outByteList def decrypt(key, inByteList): """Decrypt ciphertext with given key""" setKey(key) outByteList = [] for i in range(0,len(inByteList),8): outByteList += decryptBlock(inByteList[i:i+8]) return unpadData(outByteList)
To verify this DES implementation, I also wrote a separate Python module (shown below) containing an interesting algorithm proposed by Ron Rivest a long time ago (1985) in the paper Testing Implementation of DES. I also added a simple multi-block test.
import sys from pyDES import setKey, encryptBlock, decryptBlock, encrypt, decrypt def sanityCheck1(): """Tests single-block DES encryption & decryption using algorithm proposed by Ronald Rivest (http://people.csail.mit.edu/rivest/Destest.txt)""" x0 = [0x94, 0x74, 0xb8, 0xe8, 0xc7, 0x3b, 0xca, 0x7d] x16 = [0x1b, 0x1a, 0x2d, 0xdb, 0x4c, 0x64, 0x24, 0x38] x = x0 for i in range(16): setKey(x) if i % 2 == 0: x = encryptBlock(x) # if i is even, x[i+1] = E(x[i], x[i) else: x = decryptBlock(x) # if i is odd, x[i+1] = D(x[i], x[i) try: assert x == x16 except AssertionError: return False return True def sanityCheck2(): """Tests multi-block DES encryption and decryption""" try: key = [0x0f, 0x15, 0x71, 0xc9, 0x47, 0xd9, 0xe8, 0x59] plaintext = "The quick brown fox jumps over the lazy dog" ciphertext = encrypt(key, plaintext) assert decrypt(key, ciphertext) == plaintext except AssertionError: return False return True def main(): if sanityCheck1() and sanityCheck2(): print("All DES tests ok!") else: sys.exit(1) sys.exit() if __name__ == '__main__': main()
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can i get the cipher text by unpadding it?
Yes.