DigestService(hashAlgorithm: String)
The DigestService class is a service that offers the main crypto methods for calculating transaction hashes and building Merkle trees. The default instance is passed by default to instances of classes like TransactionBuilder and as a parameter to MerkleTree.getMerkleTree(...) method. In future the default instance can be parametrized to initialize with the network default hash algorithm or just a more secure algorithm (e.g. SHA3_256). While the SHA2_256 is vulnerable to pre-image attacks, the computeNonce and componentHash methods behaviour is defined by the hashTwiceNonce and hashTwiceComponent; with SHA2_256 they both must be set to true to ensure pre-image attack won't work (and for backward compatibility), but for other algorithms like SHA3_256 that are not affected, they can and should be set to false as hashing twice would not improve security but affect performance.
hashAlgorithm - the name of the hash algorithm to be used for the instance