CGAs are sha1 hashes truncated to a 59 bit value. Along with a 3 bit security parameter they form a IPv6 address (more precisely the interface identifier).

from wikipedia:

The Sec parameter implements a hash extension by enforcing the first 16 times Sec bits of another hash, Hash2, to be 0


For a CGA with Sec equal to 0, this means that the cost of finding a set of CGA Parameters that yield the desired 59 bits is approximately O(2^{59}) (in big O notation). A larger value of Sec, however, increases this cost by a factor of 2^{16\times Sec} to O(2^{59+16\times Sec}) because the first 16 times Sec bits of Hash2 then become relevant (i.e. it implements a hash extension by demanding those bits to be equal to 0)

Is hash extension a known name for this method? Are there other examples where this kind method is applied?


in the CGA protocol in section 7.2 RFC3972 the technique is called hash extension. It seems to be a method used particularly for CGAs and seems not to be well known in other scenarios.

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