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Apple patents OTP reuse!
Like a lot of people, I often get angry when I read US patents. At
best, I'll get confused or bored.
Tonight, probably for the first time ever, I found comic relief in the
otherwise sterile pages of a patent.
Check out US Patent 5,345,508, "Method and Apparatus for
Variable-Overhead Cached Encryption", by Lynn, Zweig and Mincher,
assigned to Apple Computer, issued September 6, 1994.
These guys have patented the notion of reusing one-time pads. That's
right, reusing one-time pads! To "cut the computational overhead",
they say, they generate a PN sequence once and then reuse it by XOR
with successive packets until a "use limit" is reached. Quoting:
"When the maximum count value specifies that the PN sequence is to be
used only once, the security afforded by the present invention will be
high, but a new PN sequence must be generated for each message
sequence transmitted and so the computational overhead will also be
high. If the maximum count value specifies a maximum count value [sic]
greater than one, the PN sequence stored in the cache will be reused
to encrypt the maximum count number of message sequences. The
resulting ciphertext messages will be more vulnerable to statistical
cryptoanalytic attack as the maximum count value increases..."
No shit! Talk about missing out on some absolutely fundamental
There may actually be a silver lining to this utterly silly
patent. Although I've only looked it over briefly, it seems to
unintentionally cover the TIA's so-called voice "privacy" technique
for TDMA digital cellular -- i.e., generating a pseudorandom sequence
at the beginning of a call and then reusing it for every frame. Maybe
Apple could be persuaded to agressively sue everybody so cellular will
switch to *real* encryption... :-)