On Thursday, October 18, 2001 4:58 AM, [EMAIL PROTECTED] wrote: > goal: a 4-16 byte 7-bit character value that somehow encodes the time > of creation such that it can be extracted if the encoding scheme/seed > is known. the encoded value should be such that it is mostly > impossible to change it so as to yield a later time of creation to be > encoded. in general, changing the encoded value may well render the > data invalid.
This is just a general idea: I'm assuming a 32-bit timestamp. You want to encode that value somewhat securely with an invertable hash into 16 * 7 = 112 bits (less if you can't use control chars). It has to be somewhat resistant to tampering (you gave no indication of the degree to which it must resist). So you can multiply your timestamp by any number < 2^(112-32) = 2^80 = 1208925819614629174706176 . and still fit in 16 7-bit chars. I would pick a prime value < 2^80 and multiply the time by it, then format it into 7-bit characters. To extract, pack it and divide. Does that work? Note that to break this, the attacker will have to search a fairly large space but not an impossible one. Some of the effectiveness of this will depend on how often you change the prime factor. Also, if it is fairly expensive for an attacker to check a single value, the search could take a while. This also doesn't take into account the value of breaking it, i.e. how big is the "prize" for finding your prime factor? Best, -=greg
