Sufficient conditions for sound tree hashing modes

We consider the general case of tree hashing modes that make use of an underlying compression function. We consider such a tree hashing mode sound if differentiating it from a random oracle, assuming the underlying compression function is a random oracle can be proven to be hard. We demonstrate two properties that such a tree hashing mode must have for such a proof to exist. For each of the two properties we show that several solutions exist to realize them. For some given solutions we demonstrate that a simple proof of indifferentiability exists and obtain an upper bound on the differentiability probability of $q^2/2^n$ with $q$ the number of queries to the underlying compression function and $n$ its output length. Finally we give two examples of hashing modes for which this proof applies: KeccakTree and Prefix-free Merkle-Damgard.