4 - Hash and VDF chains
A key ingredient in longest-chain blockchains are hash-chains as discussed in §4.1 below. While also uses a hash-chain (for the foliage chain ), for we use a new chaining structure called a VDF chain defined in §4.2 below.
4.1 Hash chains
For this writeup, a hash chain is a sequence of blocks, where each block contains some data value (possibly empty) and (with the exception of ) a hash value of the current data and the previous block.
Security from hash chains.
A hash chain is immutable in the following sense:
Proposition 2 (immutability of hash chains). If is a collision-resistant hash function, then it is computationally infeasible to find two distinct hash chains and where