inc'd feedback

content/references/rippled-api/ledger-data-formats/ledger-object-ids.md

@@ -0,0 +1,17 @@
+# Ledger Object IDs
+<a id="sha512half"></a>
+
+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](https://github.com/ripple/rippled/blob/master/src/ripple/protocol/LedgerFormats.h#L97). The [ledger object type](ledger-object-types.html) 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.
+
+![Diagram: rippled uses SHA-512Half to generate IDs for ledger objects. The space key prevents IDs for different object types from colliding.](img/ledger-indexes.png)
+
+
+## See Also
+
+For ledger basics, see [Ledgers](ledgers.html).
+
+
+<!--{# common link defs #}-->
+{% include '_snippets/rippled-api-links.md' %}
+{% include '_snippets/tx-type-links.md' %}
+{% include '_snippets/rippled_versions.md' %}