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From Mike Orton

The Royal Mail is apparently going to sell us an e-mail electronic stamp in
the form of a 128-bit public key encryption system. Furthermore, it isn’t
insisting on key escrow
(This Week, 27 March, p 17).

At first sight it seems a good idea, but with only a 128-digit public key,
key escrow is not required. The system is just plain insecure.

An ad hoc panel of cryptography experts has recently published a report
recommending 500 digits for minimum security for public key ciphers. Public key
ciphers are usually based on the RSA system, which relies on the time taken to
factor a composite number N (1024 digits long) into its two prime
factors, p₁ and p₂ where
N = p₁ × p₂.

With modern encryption systems, it is never appropriate to use less than the
maximum-security level. If you were stupid enough to use 128 digits for bank
transfers up to £1000, 400 digits for up to £10 000, and 1024 digits
for transactions above £100 000, you would enable an opponent to narrow
his attack by concentrating on the most potentially profitable keys.

If you have a strong system, use it for everything. Suppose that GCHQ can
break a 1024 digit key in three hours. It could cope with eight messages a day.
If you flooded it with 800 messages a day it would never keep up.

Harlech, Gwynedd