Ledger Object IDs
All objects in a ledger's state tree have a unique ID. This field is returned as the
index field in JSON, at the same level as the object's contents. The ID is derived by hashing important contents of the object, along with a namespace identifier. The ledger object type determines which namespace identifier to use and which contents to include in the hash. This ensures every ID is unique. To calculate the hash,
rippled uses SHA-512 and then truncates the result to the first 256 bytes. This algorithm, informally called SHA-512Half, provides an output that has comparable security to SHA-256, but runs faster on 64-bit processors.
For ledger basics, see Ledgers.