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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

and...

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?

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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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