"terminal capture". This makes actions 2, 5, and 6 internal to the
merchant but requires messages to perform action 3. Such batch
clearance messages will be included in future versions of the
CyberCash merchant and server software.
5.2 Lessons Learned
The continuing rapid development of the CyberCash system is an
interesting experience. The system must deal with many existing
browsers and legacy banking systems. Existing credit card processors
that convey transactions to acquiring banks have complex and varied
interfaces. The sophistication of security attacks on the Internet
is growing rapidly.
In the face of such a rapidly changing environment, it was essential
to adopt a general message framework so that messages and fields
could be added as they were found necessary. Any attempt to reduce
the system to a small number of perfectly opimized messages in
advance would have doomed the system to failure. (As of mid-October
1995, the total number of CyberCash messages defined, including those
planned for cash and microcash, enhancements to the credit card
system, and some old messages being phased out in favor of improved
replacements, is just over a hundred.)
Flexible operational and error handing facilities are also, as usual,
the bulk of the system. Version numbering and tracking has proved to
be quite important and merchant versioning is being added.
Use of text for messages has proven very beneficial. This makes it
possible to easily deal with messages using common everyday tools
such as text editors and spead sheets. Use of binary TLV (type,
length, value) encoding or the like is certainly possible but imposes
a significantly higher level of complexity on every tool that has to
deal with the messages.
Encryption and decryption impose some difficulties in development.
Any confusion about decryption keys or algorithms will render
encrypted material meaningless and tools are needed to provide
decyrption for debugging outside of normal program operation. But
this pales compared with the stringencies imposed by signatures. All
parts of the system must have absolutely identical ideas as to the
exact bit patterns to be hashed or signed and their exact order.
Seemingly trivial differences in capitalization, punctuation,
framing, order, or the like, in addition to any disagreement about
keys or algorithms, will lead to frustrating failures of signatures
to match. Passing signatures through an intermediate system and
checking them at a third system, as is done when a customer's
signature is passed through a merchant and checked at the CyberCash
server, compounds the problem.
6. Security Considerations
The CyberCash Version 0.8 Credit Card system provides substantial
protection to payment messages as described above in sections 1.2,
2.2.4, and 2.2.5. However, it provides no privacy to the shopping
interaction which is essentially outside of its purview. It also
provides no protection against dishonest merchants other than those
normally available with credit card purchases. Care must be taken to
avoid loss of control of the machines on which parts of this system
runs or security may be compromised.
Current credit card dispute resolution systems require deliberate
bypasses be implemented for some of the security normally established
by CyberCash as described in section 3.4.
References
[ISO 4217] - Codes for the representation of currencies and funds
[ISO 8583] - Financial transaction card originated messages -
Interchange message specifications, 1993-12-15.
[RFC822] - Crocker, D., "Standard for the format of ARPA Internet
text messages", STD 11, RFC822, UDEL, August 1982.
[RFC1521] - Borenstein, N., and N. Freed, "MIME (Multipurpose
Internet Mail Extensions) Part One: Mechanisms for Specifying and
Describing the Format of Internet Message Bodies", RFC1521,
Bellcore, Innosoft, September 1993.
[RFC1766] - Alvestrand, H., "Tags for the Identification of
Languages", UNINETT, March 1995.
Authors' Addresses
Donald E. Eastlake 3rd
CyberCash, Inc.
318 Acton Street
Carlisle, MA 01741 USA
Phone: +1 508 287 4877
EMail: dee@cybercash.com
Brian Boesch
CyberCash, Inc.
2100 Reston Parkway, Suite 430
Reston, VA 22091 USA
Phone: +1 703-620-4200
EMail: boesch@cybercash.com
Steve Crocker
CyberCash, Inc.
2100 Reston Parkway, Suite 430
Reston, VA 22091 USA
Phone: +1 703-620-4200
EMail: crocker@cybercash.com
Magdalena Yesil
CyberCash, Inc.
555 Twin Dolphin Drive, Suite 570
Redwood City, CA 94065 USA
Phone: +1 415-594-0800