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One time pads would be unaffected. Richard Farmbrough.
: This is briefly discussed at [[quantum computer#The power of quantum computers]]. At the present time, ''only'' factorisation and discrete log based ciphers are known to be seriously affected (if a QC of sufficient size could be built). A quantum computer could be used to attack a symmetric cipher, but it's speed up is "only" to take the square root of the number of steps. This is a huge speed up for typical block ciphers, but is trivially defeated by doubling the key size. There exists a standard, thoroughly studied method to double the key size of any block cipher, namely triple encryption. Further, the most common key size used today - 128 bits - would only be reduced to a work factor of the order of <math>2^{64}</math>, which is still quite a tough job unless the information is extremely valuable. And the AES already has 192 and 256 bit modes built in. So, at our present level of knowledge, QC poses essentially no threat to symmetric encryption. [[User:Securiger|Securiger]] 07:40, 6 Oct 2004 (UTC)
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Shouldn't it be: :<math>f(r) = a^r\ \mbox{mod}\ N</math> ? Evan Ettinger.
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