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Merge pull request #485 from mDuo13/hash_clarifications
Hashes - clarify hash prefixes, SHA-512Half size.
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Rome Reginelli
@@ -21,6 +21,33 @@ Each closed [Ledger](ledger-data-formats.html) has a [Ledger Index][] and a [Has
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{% include '_snippets/data_types/hash.md' %}
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<!--{#_ #}-->
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### Hash Prefixes
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[[Source]<br>](https://github.com/ripple/rippled/blob/master/src/ripple/protocol/HashPrefix.h "Source")
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In many cases, the XRP Ledger prefixes an object's binary data with a 4-byte code before calculating its hash, so that objects of different types have different hashes even if their binary formats are the same. The existing 4-byte codes are structured as three alphabetic characters, encoded as ASCII, followed by a zero byte.
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Some types of hash appear in API requests and responses. Others are only calculated as the first step of signing a certain type of data, or calculating a higher-level hash. The following table shows all 4-byte hash prefixes the XRP Ledger uses:
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| Object Type | API Fields | Hash Prefix (Hex) | Hash Prefix (Text) |
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|:--------------------------------------|:-------------------------------------|:------------------|:--|
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| Consensus proposal | N/A | `0x50525000` | `PRP\0` |
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| Ledger Version | `ledger_hash` | `0x4C575200` | `LWR\0` |
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| Ledger state data | `account_state` in [ledger header][] | `0x4D4C4E00` | `MLN\0` |
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| Ledger data inner node | N/A | `0x4D494E00` | `MIN\0` |
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| Ledger data inner node ([SHAMapv2][]) | N/A | `0x494E5200` | `INR\0` |
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| Payment Channel Claim | N/A | `0x434C4D00` | `CLM\0` |
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| Signed Transaction | `hash` of transactions | `0x54584E00` | `TXN\0` |
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| Transaction with metadata | N/A | `0x534E4400` | `SND\0` |
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| Unsigned Transaction (Single-signing) | N/A | `0x53545800` | `STX\0` |
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| Unsigned Transaction (Multi-signing) | N/A | `0x534D5400` | `SMT\0` |
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| Validation vote | N/A | `0x56414C00` | `VAL\0` |
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| Validator subkey authorization ("validator manifest") | N/A | `0x4D414E00` | `MAN\0` |
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[ledger header]: ledger-header.html
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[SHAMapv2]: known-amendments.html#shamapv2
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[Ledger objects IDs](ledger-object-ids.html) are calculated in a similar way, but they use a 2-byte prefix called a "space key" instead of a prefix in the form described here.
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## Account Sequence
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[Sequence Number]: #account-sequence
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@@ -6,7 +6,7 @@ Every ledger version has a unique header that describes the contents. You can lo
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| Field | JSON Type | [Internal Type][] | Description |
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|:-------------------------------|:----------|:------------------|:------------|
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| [`ledger_index`][Ledger Index] | String | UInt32 | The sequence number of the ledger. Some API methods display this as a quoted integer; some display it as a native JSON number. |
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| `ledger_hash` | String | Hash256 | The [SHA-512Half][] of the ledger header, excluding the `ledger_hash` itself. This serves as a unique identifier for this ledger and all its contents. |
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| `ledger_hash` | String | Hash256 | The [SHA-512Half][] of this ledger version. This serves as a unique identifier for this ledger and all its contents. |
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| `account_hash` | String | Hash256 | The [SHA-512Half][] of this ledger's state tree information. |
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| `close_time` | Number | UInt32 | The approximate time this ledger closed, as the number of seconds since the Ripple Epoch of 2000-01-01 00:00:00. This value is rounded based on the `close_time_resolution`, so later ledgers can have the same value. |
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| `closed` | Boolean | bool | If true, this ledger version is no longer accepting new transactions. (However, unless this ledger version is validated, it might be replaced by a different ledger version with a different set of transactions.) |
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@@ -1,14 +1,15 @@
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# Ledger Object IDs
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<a id="sha512half"></a>
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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#L99). 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.
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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#L99). 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 bits. This algorithm, informally called **SHA-512Half**, provides an output that has comparable security to SHA-256, but runs faster on 64-bit processors.
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## See Also
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For ledger basics, see [Ledgers](ledgers.html).
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- For more information how the XRP Ledger creates and uses hashes, see [Hashes](basic-data-types.html#hashes).
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- For ledger basics, see [Ledgers](ledgers.html).
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<!--{# common link defs #}-->
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