Introduction
This paper describes the security techniques used by Catalog and the
possible attacks that might be made. It compares Actinic Catalog with
comparable SSL (Secure Socket Layer) solutions which are in current use.
Security method
Catalog allows orders to be placed and sent over the Internet.
Encryption can be disabled for non-sensitive orders e.g. requests for
further information about a house advertised for sale. If encryption
is enabled, it can happen in one of two ways : using a Java Applet or
using SSL. An alternative is where all secure payment information is
collected by an "Online Credit Card Provider" such as
NetBanx, WorldPay, Secure Trading or Datacash. In this case, the
security is provided by these companies. This particular option will
not be considered further in this paper.
Using Java Applet
Encryption occurs on the buyer's PC and decryption only occurs on the
vendor's PC. At no stage is the transaction decrypted whilst it
travels over the Internet, or whilst it is stored on a web site. In
addition, orders (including credit card details) are only stored on a
web site until the vendor downloads them to their PC. Hence there is
no large store of orders available online to invite attack.
-
The encryption is carried out by using a Java applet. The Java applet
is subject to the standard security restrictions of their
"sandbox" which restricts their ability to communicate
across the Net to only the web site that they are downloaded from.
Decryption is carried out on the vendor's PC after orders have been
downloaded from the web. The encryption technique used falls into two
parts. The first is to use Diffie-Hellman key exchange to agree a 128
bit key for use by the SAFER block cipher. The Diffie-Hellman key
currently used is 256 bits and this will be increased further in the
future up to 1024 bits, depending on performance. This encryption
method is used on the following fields only :
-
credit card number
- credit card type
- credit card expiry date
- Other fields in orders placed using the system are
also encrypted using Safer with a 128 bit key, but using a fixed key
built in to the software and common across all instances of the software.
- The following banks have approved their customers
use of Actinic Catalog - Barclays Bank, Midland Bank and The Royal
Bank of Scotland.
- Using SSL
-
Where the SSL option is used, the buyers personal details, credit
card information and other order information is sent from the browser
to the server, using industry standard SSL encryption. At the server,
the order is encrypted before being written to disk using the same
method and encrypting the same fields as is explained in the Java
encryption. Hence the order is only stored encrypted on the web site.
When the vendor downloads the orders, they are sent over the Internet
using SSL and then decrypted on their PC. Hence there is no large
store of orders available online to invite attack.
- Diffie-Hellman
-
Diffie-Hellman key exchange has been published for over 25 years and
has been proved to be strong. RSA have based their encryption method
on the same fundamental mathematics. RSA (used in SSL) is essentially
a derivation of Diffie-Hellman. Actinic chose to use Diffie-Hellman
for the following reasons :
- Cisco for their routersSafer
-
Actinic has adopted the SAFER SK-128 block encryption method
developed by Massey (the developer of IDEA which is used in PGP). The
key for use with SAFER is negotiated using Diffie-Hellman. The
algorithm has been around for some time and has stood the test of
time. It is a public algorithm and is freely available. SAFER is
briefly described in the RSA
FAQ
- Key length
-
Actinic have adopted a 128 bit Safer key, which gives a reasonable
performance whilst being several orders of magnitude beyond where
brute force methods could break the encryption. SSL offers only a
40-bit key in non-US implementations (although 56 bit key
implementations are now becoming available). To put things in
context, each additional bit of key space takes twice as long to
break. So a 41 bit key is twice as strong as a 40 bit key. The 128
bit key used in Actinic Catalog is 4,722,366,482,869,645,213,696
times as strong as the SSL 56 bit key.
Page 2 (possible attacks) 
