Page History: RC4 Encryption and Decryption Functions
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Page Revision: 2020/06/30 11:53
The RC4 Encryption Algorithm, is a shared key method requiring a secure exchange of a shared key i.e. both sides of the encode/decode must possess the key. RC4 is "symmetrical" - that is the same key is used to both encrypt and decrypt. It is now considered a defunct method for really strong encryption but is still viable with a suitably complex key (at least 30 characters).
RC4$() returns a hexadecimal string of the original input expression. Note: The encrypted string has a 100% overhead in size over the original i.e. it will be twice the length of the input string.
UnRC4$() decodes a previously encrypted hexcadecimal string and returns the original string.
Syntax:
RC4$(expression)
UnRC4$(expression)
Example:
a$=RC4$(password$)
b$=UnRC4$(encrypted_data$)
Assumptions:
OPTION BASE 0
' change this key to whatever you like but it should be complex
' at least 30 characters long and don't lose it! To increase security,
' you could use Base64 encoding to obfuscate it in your code.
' It won't stop a determined snooper but why make it easy for them?
' Suitable B64 routines are available in this namespace
CONST RC4KEY$=">4!1x4q3z4+7%4{9?5\3HhH^5$9=6@1~6,7_7|1)7'3]7[9:8<3*8S9I9l7Z1eT0r1"Code:
Function RC4$(z$)
Local String outp,b
Local Integer i,j,x,y,t,temp,s(255),k(255)
For i=0 To 255: s(i)=i: Next
j=1
For i=0 To 255
if j>Len(RC4key$) Then j=1
k(i)=Asc(Mid$(RC4key$, j, 1))
j=j+1
Next
j=0
For i=0 To 255
j=(j+s(i)+k(i)) Mod 256
temp=s(i): s(i)=s(j): s(j)=temp
Next
i=0: j=0
For x=1 To Len(z$)
i=(i+1) Mod 256: j=(j+s(i)) Mod 256
temp=s(i): s(i)=s(j): s(j)=temp
t=(s(i)+(s(j) Mod 256)) Mod 256: y=s(t)
outp=outp+Chr$(Asc(Mid$(z$, x, 1)) Xor y)
Next
b="": j=0
For x=1 To Len(outp)
b=b+Hex$(Asc(Mid$(outp, x, 1)), 2): j=j+Asc(Mid$(outp, x, 1))
Next
RC4$=b
End Function
Function UnRC4$(z$)
Local String mystr,a,b
Local Integer n
mystr=""
For n=1 To Len(z$) Step 2
a=Mid$(z$, n, 2)
a=Chr$(Val("&h"+a))
mystr=mystr+a
Next
mystr=RC4$(mystr)
b=""
For n=1 To Len(mystr) Step 2
a=Mid$(mystr, n, 2)
a=Chr$(Val("&h"+a))
b=b+a
Next
UnRC4$=b
End Function