'BasEncrypt:  Encryption/decryption routine

'Nelson Ford, Public (software) Library
'P.O.Box 35705, Houston, TX 77235-5705
'Call for free newsletter: 800-2424-PSL
'Information: 713-524-6394.  CIS#: 71355,470

'This file can be freely used and copied for others so long as the file
'is not changed in any way.

'Purpose:
'''''''''
'Someone asked on the MSSYS forum on CompuServe for a simple encryption routine
'and the old XOR method was recommended.

'The accompanying routine is a lot more secure and just about as easy.

'Algorithm:
'''''''''''
'The idea is to set up a look-up substitution string for the ASCII character
'set, and replace each character in your text with the character assigned to
'that character's ASCII number position in the look-up string.

'In the sample code, the ASCII character set is simply reversed in the
'encryption string ("e$"). In actual use, you should assign the characters to
'the string more randomly. You should probably NOT change control characters
'[eg: leave the 13th character in the substitution string as CHR$(13)]
'so that you do not lose the ability to do INPUTs.

DEFINT A -Z: E$ = SPACE$(255) : D$ = SPACE$(255)

FOR I = 1 TO 255: MID$(E$, 256-I, 1) = CHR$(I): NEXT
FOR I = 1 TO 255: MID$(D$, ASC( MID$(E$,I))) = CHR$(I): NEXT

LINE INPUT "Enter text to encrypt: "; T$
FOR I = 1 TO LEN(T$)
  MID$(T$, I, 1) = MID$(E$, ASC( MID$(T$, I)))
NEXT
PRINT T$

INPUT "Ready to decrypt"; x$
FOR I = 1 TO LEN(T$)
  MID$(T$, I, 1) = MID$(D$, ASC( MID$(T$, I)))
NEXT
PRINT T$