by Patterson Programming (5 Submissions)
Category: Encryption
Compatability: Visual Basic 3.0
Difficulty: Unknown Difficulty
Originally Published: Sat 22nd July 2000
Date Added: Mon 8th February 2021
Rating: 
(1 Votes)
OurSeeFour stream cipher. Based on the alleged RC4(tm) cipher. Code snippet.
VBcoders
Option Explicit
Dim DataLen&, DataBuffer() As Byte
'* assume 128-bit key
Const SessionKeyLen = 16
Dim SessionKey(15) As Byte
'* use static numeric arrays
'* easier to clean up
Dim Permutation%(255), LongKey%(255)
Dim ii%, jj%
Sub main ()
'SessionKey(0..15) = ?
'* make the permutation
OurSeeFourKey
'DataLen& = ?
ReDim DataBuffer(DataLen& - 1) As Byte
'DataBuffer(0..?) = ? (COPY IN THE PLAINTEXT)
'* do the encryption
ii% = 0: jj% = 0
OurSeeFourCipher
End Sub
Sub OurSeeFourKey()
Dim i%, j%
'* initialize Permutation vector array *
For i% = 0 To 255
Permutation%(i%) = i%
Next
'* replicate session key into the LongKey array *
j% = 0
For i% = 0 To 255
LongKey%(i%) = SessionKey(j%)
j% = (j% + 1) Mod SessionKeyLen
Next
'* randomize Permutation vector *
j% = 0
For i% = 0 To 255
'* for efficiency, use &HFF rather than Mod *
j% = (j% + Permutation%(i%) + LongKey%(i%)) And &HFF
SWAP Permutation%(i%), Permutation%(j%)
Next
'* this is added to the original RC4 (tm) algorithm
'* to prevent some attacks
For i% = 0 To 255
j% = (j% + Permutation%(i%)) And &HFF
SWAP Permutation%(i%), Permutation%(j%)
Next
End Sub
Static Sub OurSeeFourCipher()
Dim x&, IndexByte As Byte, CipherByte As Byte
For x& = 0 To DataLen& - 1
ii% = (ii% + 1) And &HFF
jj% = (jj% + Permutation%(ii%)) And &HFF
SWAP Permutation%(ii%), Permutation%(jj%)
IndexByte = (Permutation%(ii%) + Permutation%(jj%)) And &HFF
CipherByte = Permutation%(IndexByte) And &HFF
DataBuffer(x&) = DataBuffer(x&) Xor CipherByte
Next
End Sub
Private Sub SWAP(a%, b%)
Dim y%
y% = a%: a% = b%: b% = y%
End Sub
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