Puedes hacer algo así.
saludos
Código (python) [Seleccionar]
def rc4crypt(data, key):
x = 0
box = range(256)
for i in range(256):
x = (x + box[i] + ord(key[i % len(key)])) % 256
box[i], box[x] = box[x], box[i]
x = 0
y = 0
out = []
for char in data:
x = (x + 1) % 256
y = (y + box[x]) % 256
box[x], box[y] = box[y], box[x]
out.append(chr(ord(char) ^ box[(box[x] + box[y]) % 256]))
return ''.join(out)
f = open('Oiginal.txt', "r")
lines = f.readlines()
f.close()
f = open('Cifrado.txt', "w")
for line in lines:
print line
split=line.split("|")
print split[4]
line=line.replace(split[4],rc4crypt(split[4],"Hola"))
print line
f.write(line)
f.close()
saludos
de las reglas leíste el titulo 
