Claudio Floreani
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 Sep 24 awarded Autobiographer May 3 awarded Scholar May 3 accepted Knowing a valid salted hash for an unknown secret, is it possible to compute another valid hash? May 2 comment Knowing a valid salted hash for an unknown secret, is it possible to compute another valid hash? Can you prove that if H(s,r)=d returns a different digest d for each r and R(d)=r, so there are $d_1(r_1), d_2(r_2), d_3(r_3)$ possible valid hash for the same secret s, it's generally impossible to compute a valid $d_x(r_x)$ from (one or more) $d_n(r_n)$? May 2 comment Knowing a valid salted hash for an unknown secret, is it possible to compute another valid hash? @HenrickHellström This is a good answer that shows a way to artificially construct H,G,and V that allow for any d to compute a d' so that V(d,r)=V(d',r). May 2 revised Knowing a valid salted hash for an unknown secret, is it possible to compute another valid hash? Merged some suggested changes and made a better rewording. May 1 comment Knowing a valid salted hash for an unknown secret, is it possible to compute another valid hash? I assume that by "valid hash" you mean to run the hash algorithm a second time, yielding an identical digest to a previously computed digest. Right. I believe what you are asking for is if two different messages can yield identical digest values. No, I'm really asking if knowing a digest (and its salt) of a secret, it is possible to compute another digest of the same secret. That is not a collision, since if I know the secret I can calculate a different valid digest for each salt I choose. May 1 comment Knowing a valid salted hash for an unknown secret, is it possible to compute another valid hash? Alright, V(y,s)=1 it's more appropriate. May 1 awarded Editor May 1 revised Knowing a valid salted hash for an unknown secret, is it possible to compute another valid hash? added 61 characters in body May 1 comment Knowing a valid salted hash for an unknown secret, is it possible to compute another valid hash? @fgrieu I was assuming (wrongly?) that the salt r can be always be determined by the digest d. For BCrypt implementations known to me this is true, since the digest a string composed by the concatenation of salt and the checksum computed from the secret and the salt. Also, since d=H(s,r), it can be proven that there is a V'(s,r,d)=V(d,s). So if V'(s,r,d)=1 only if H(s,r)=d, also V(d,s)=1 only if H(s,r)=d. I will edit the question if this is confusing. May 1 awarded Student May 1 asked Knowing a valid salted hash for an unknown secret, is it possible to compute another valid hash?