The other version still doesn't seem to always work. I switched it to use the C Standard Library's RAND() and SRAND() functions:

CLEAR
lcEnc = encrypt("Test string here")
? lcEnc
lcDec = decrypt(lcEnc)
? lcDec



FUNCTION encrypt
LPARAMETERS tcString
LOCAL lcString

    seedRand(0)
    lcString = SPACE(0)
    FOR lnI = 1 TO LEN(tcString)
        lnRand      = ROUND(getRand() * 255.0, 0)
        lcChar      = SUBSTR(tcString, lnI, 1)
        lcChar      = CHR((ASC(lcChar) + lnRand) % 255)
        lcString    = lcString + lcChar
    NEXT
    RETURN lcString




FUNCTION decrypt
LPARAMETERS tcString
LOCAL lcString

    seedRand(0)
    lcString = SPACE(0)
    FOR lnI = 1 TO LEN(tcString)
        lnRand      = ROUND(getRand() * 255.0, 0)
        lcChar      = SUBSTR(tcString, lnI, 1)
        lcString    = lcString + CHR((ASC(lcChar) + 255 - lnRand) % 255)
    NEXT
    RETURN lcString




FUNCTION seedRand
LPARAMETERS tnSeed

    DECLARE INTEGER srand IN msvcr71.dll as cSrand INTEGER nSeed
    DECLARE INTEGER rand  IN msvcr71.dll AS cRand
    cSrand(tnSeed)




FUNCTION getRand
LOCAL lnRand

    RETURN cRand() / 32767.0