"Yes it's interesting application of cryptography" Only as a way to illustrate that using cryptographic primitives like hashes and digital signatures does not

"Yes it's interesting application of cryptography" Only as a way to illustrate that using cryptographic primitives like hashes and digital signatures does not make a system secure. If Bitcoin were a security system, the effort required to successfully double-spend the currency would be exponential in the effort required to use Bitcoin; as things are, the required effort scales linearly with the number of parties. Bitcoin may utilize secure cryptosystems, but as a cryptosystem Bitcoin is not secure under any accepted notion of "security."