Generate a list of valid ASCII codes

Question

I have read many pages about encryption but I am really stuck on this. For access control to a building I want to print a 10 character ASCII code on a ticket (as a barcode).

The barcode reader (controlled by an Arduino or mini PC) is not connected to the PC which creates and prints the barcodes.

When both server and client have the same keys, is it possible to validate the codes?

EDIT: My first attempt was to convert the current date in to binary, and then to ASCII with base64. The problem was that I ended up with a too big code (11 characters).

Considerating everything I came to the conclusion that it would be better too convert the date and time directly into ASCII characters. ASCII characters become readable at position 33, so year 2015 becomes 15 + 33 = ASCII character 0

In this way Year + Month + Day + Hour + Second + Validtime only takes 7 characters.

EDIT2: I tried different encryptions en base encodings. Because year and month don't vary much, the generated code before encryption looks very much alike. When I use ARC4 encryption, the generated codes are also very similar.

Using DES3 the generated codes are not alike. I have included the two encryption methods in the code below. Because I use Python 2.7 I have to use the base85 module from the mom package. In Python v3 base85 is included.

Here is the code:

from datetime import datetime
from Crypto.Cipher import DES3
from Crypto.Cipher import ARC4
import base64
from mom.codec import base85


sleutel="Dsdfk3#$294ddkwlsslwEefm"
now = datetime.now()
year = chr((now.year - 2000)+33)
month = chr((now.month)+33)
day = chr((now.day)+33)
hour = chr((now.hour)+33)
minute = chr((now.minute)+33)
seconds = chr((now.second)+33)
#validtime from 0 till 9 in this case just a random 4. So 0 would be a few hours and 5 a week etc.
validtime = chr(4+33)
key = ''.join([year,month,day,hour,minute,seconds,validtime])
print key
obj1 = ARC4.new(sleutel)
obj2 = DES3.new(sleutel)
cipher_text = obj1.encrypt(key)
cipher_text1 = obj2.encrypt(key+ 'a')
print "Base64 ARC4 encrypted:", base64.b64encode(cipher_text)
print "Base64 DES3 encrypted:", base64.b64encode(cipher_text1)
print "Base85 ARC4 encrypted:", base85.b85encode(cipher_text)
print "Base85 DES3 encrypted:", base85.b85encode(cipher_text1)

Because I added seconds, every code is unique (as long as it is impossible to print two tickets in one second)

Answer 1

If you really can use all 95 printable ASCII characters, you're better off encoding everything in binary (however you want) into 64 bits, then encrypting that with your secret key (using a cipher with a 64-bit block size, such as 3DES), then turn the result of that into 10 characters using an encoding from binary into base-95. You can actually encode 64 bits into 10 characters using only 85 characters, so you could omit some that might be a problem (e.g. SPACE). Since you're encrypting only one block, and they're guaranteed to be unique, you can simply use it in ECB mode (no IV, no chaining modes).

Then just turn it into a 10 digit base-85 number (the same way you'd convert binary to base 10), then map each of the 85 possible values for a digit (0-84) into an ASCII character (either with a lookup table or something as simple as adding 0x21).

You'll want to make the data as well-structured as you can so that you can detect any tampering, including perhaps a (small) checksum of some sort, and verify all 64 bits as being valid/well-formed.