Why doesn't Ethereum do double hashing like Bitcoin? What are the facts or theories on the security of single Keccak-256 hashing like Ethereum, as compared to Bitcoin's double hashing with SHA2-256? How about Bitcoin's use of a second different hash function (RIPEMD-160) in some cases?
Saw the following from a recent paper published on February 19 2016.
We have uncovered a worrying lack of defense-in-depth in Bitcoin. In most cases, the failure of a single property in one cryptographic primitive is as bad as multiple failures in several primitives at once. For future versions of Bitcoin, we recommend including various redundancies such as properly combined hash functions.
Cryptology ePrint Archive: Report 2016/167
On Bitcoin Security in the Presence of Broken Crypto Primitives
Ilias Giechaskiel and Cas Cremers and Kasper Rasmussen
It sounds like double hashing in Bitcoin does not provide as much defense-in-depth as may be surmised.