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If nothing else, it makes the output of the pool irrecoverable. One of Fortuna's goals is to make prior Fortuna outputs safe from a compromise (the discovery of all of Fortuna's current data by an adversary). If the pool continued on without a reset, with little or no entropy added before the compromise took place, the adversary could more easily calculate ...


2

As noted by izaera, that reset of the pool is explicitly specified in Fortuna, and not an implementation artifact. The pools in Fortuna are SHA-256 hashes. By definition, in order to obtain a pool's result, the SHA-256 hash must be obtained (in the present code, that's the job of sha256_done). With a standard SHA-256 implementation, there is no way to ...


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Since Blum-Micali is cryptographically secure, you know that there won't be a short cycle (not one short enough to detect in polynomial time), because a short cycle would violate cryptographic security. Therefore, you don't need to worry about this. It's not worth your time worrying about it: it's very unlikely you'll run into a short cycle. That said, ...


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Ferguson and Schneier define SHAd-256 in their book Practical Cryptography in Chapter 6.3.1 Length Extensions. For any hash function SHA-X, where X is 1, 256, 384 or 512 we define SHAd-X as the function that maps m to SHA-X(SHA-X(m)). In particular, SHAd-256 is just the function m ↦ SHA-256(SHA-256(m)). They clearly defined SHAd-256 to prevent length ...



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