Larry is working on an encryption algorithm based on DES.

He hasn’t worked out all the kinks yet, but he thinks it works.

Your job is to confirm that you can decrypt a message, given the algorithm and parameters used.

His system works as follows:

1. Choose a plaintext that is divisible into 12bit ‘blocks’
2. Choose a key at least 8bits in length
3. For each block from i=0 while i<N perform the following operations
4. Repeat the following operations on block i, from r=0 while r<R
5. Divide the block into 2 6bit sections Lr,Rr
6. Using Rr, “expand” the value from 6bits to 8bits.
   Do this by remapping the values using their index, e.g.
   1 2 3 4 5 6 -> 1 2 4 3 4 3 5 6
7. XOR the result of this with 8bits of the Key beginning with Key[iR+r] and wrapping back to the beginning if necessary.
8. Divide the result into 2 4bit sections S1, S2
9. Calculate the 2 3bit values using the two “S boxes” below, using S1 and S2 as input respectively.

S1	0	1	2	3	4	5	6	7
0	101	010	001	110	011	100	111	000
1	001	100	110	010	000	111	101	011

S2	0	1	2	3	4	5	6	7
0	100	000	110	101	111	001	011	010
1	101	011	000	111	110	010	001	100

10. Concatenate the results of the S-boxes into 1 6bit value
11. XOR the result with Lr
12. Use Rr as Lr and your altered Rr (result of previous step) as Rr for any further computation on block i
13. increment r

He has encryped a message using Key="Mu”, and R=2. See if you can decipher it into plaintext.

Submit your result to Larry in the format Gigem{plaintext}.

Binary of ciphertext: 01100101 00100010 10001100 01011000 00010001 10000101

Since the key and the number of iteration are known, we simply need to build a decryption function to which we will give the ciphertext. Based on Larry’s encryption algorithm, here are the different steps needed to decrypt a message :

1. Divide the encrypted text into 12-bit blocks
2. For each block from i=0 while i<N perform the following operations
3. Repeat the following operations on block i, from r=R-1 while r<=0
4. Divide the block into 2 6bit sections Rr (left) and Rr-alt (right)
5. Using Rr, “expand” the value from 6bits to 8bits using the same method as encryption (e.g. 1 2 3 4 5 6 -> 1 2 4 3 4 3 5 6)
6. XOR the result of this with 8bits of the Key beginning with Key[iR+r] and wrapping back to the beginning if necessary.
7. Divide the result into 2 4bit sections S1, S2 and calculate the 2 3bit values using the two “S boxes” above, using S1 and S2 as input respectively.
8. Concatenate the results of the S-boxes into 1 6bit value
9. XOR the result with Rr-alt to recover Lr
10. Your new block consists of Lr, the result of the XOR above (left) and Rr (right)
11. Decrement r

1
2
3

[+] Cipher: 01100101 00100010 10001100 01011000 00010001 10000101
[+] Flag:   01001101 01101001 01001110 00110000 01101110 00100001
            Gigem{MiN0n!}