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fc9b77d855
* More information about array entries (especially nesting) * Varint encoding examples * Expanded string and integer encoding information
199 lines
6.9 KiB
Markdown
199 lines
6.9 KiB
Markdown
# Portable Storage Format
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## Background
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Monero makes use of a set of helper classes from a small library named
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[epee](https://github.com/monero-project/monero/tree/master/contrib/epee). Part
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of this library implements a networking protocol called
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[Levin](https://github.com/monero-project/monero/blob/master/contrib/epee/include/net/levin_base.h),
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which internally uses a storage format called [Portable
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Storage](https://github.com/monero-project/monero/tree/master/contrib/epee/include/storages).
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This format (amongst the rest of the
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[epee](https://github.com/monero-project/monero/tree/master/contrib/epee)
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library), is undocumented - or rather relies on the code itself to serve as the
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documentation. Unfortunately, whilst the rest of the library is fairly
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straightforward to decipher, the Portable Storage is less-so. Hence this
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document.
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## String and Integer Encoding
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### Integers
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With few exceptions, integers serialized in epee portable storage format are serialized
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as little-endian.
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### Varints
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Varints are used to pack integers in an portable and space optimized way. Varints are stored as little-endian integers, with the lowest 2 bits storing the amount of bytes required, which means the largest value integer that can be packed into 1 byte is 63
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(6 bits).
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#### Byte Sizes
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| Lowest 2 bits | Size value | Value range |
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|---------------|---------------|-----------------------------------|
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| b00 | 1 byte | 0 to 63 |
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| b01 | 2 bytes | 64 to 16383 |
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| b10 | 4 bytes | 16384 to 1073741823 |
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| b11 | 8 bytes | 1073741824 to 4611686018427387903 |
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#### Represenations of Example Values
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| Value | Byte Representation (hex) |
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|----------------------|---------------------------|
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| 0 | 00 |
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| 7 | 1c |
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| 101 | 95 01 |
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| 17,000 | A2 09 01 00 |
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| 7,942,319,744 | 03 BA 98 65 07 00 00 00 |
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### Strings
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These are simply length (varint) prefixed char strings without a null
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terminator (though one can always add one if desired). There is no
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specific encoding enforced, and in fact, many times binary blobs are
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stored as these strings. This type should not be confused with the keys
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in sections, as those are restricted to a maximum length of 255 and
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do not use varints to encode the length.
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"Howdy" => 14 48 6F 77 64 79
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### Section Keys
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These are similar to strings except that they are length limited to 255
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bytes, and use a single byte at the front of the string to describe the
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length (as opposed to a varint).
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"Howdy" => 05 48 6F 77 64 79
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## Binary Format Specification
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### Header
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The format must always start with the following header:
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| Field | Type | Value |
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|------------------|----------|------------|
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| Signature Part A | UInt32 | 0x01011101 |
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| Signature Part B | UInt32 | 0x01020101 |
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| Version | UInt8 | 0x01 |
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In total, the 9 byte header will look like this (in hex): `01 11 01 01 01 01 02 01 01`
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### Section
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Next we have a root object (or section as the library calls it). This is a map
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of name-value pairs called [entries](#Entry). It starts with a count:
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| Section | Type |
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|---------------|-----------|
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| Entry count | varint |
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Which is followed by the section's name-value [entries](#Entry) sequentially:
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### Entry
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| Entry | Type |
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|-------------------|-----------------------|
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| Name | section key |
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| Type | byte |
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| Count<sup>1</sup> | varint |
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| Value(s) | (type dependant data) |
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<sup>1</sup> Note, this is only present if the entry type has the array flag
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(see below).
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#### Entry types
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The types defined are:
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```cpp
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#define SERIALIZE_TYPE_INT64 1
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#define SERIALIZE_TYPE_INT32 2
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#define SERIALIZE_TYPE_INT16 3
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#define SERIALIZE_TYPE_INT8 4
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#define SERIALIZE_TYPE_UINT64 5
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#define SERIALIZE_TYPE_UINT32 6
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#define SERIALIZE_TYPE_UINT16 7
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#define SERIALIZE_TYPE_UINT8 8
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#define SERIALIZE_TYPE_DOUBLE 9
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#define SERIALIZE_TYPE_STRING 10
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#define SERIALIZE_TYPE_BOOL 11
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#define SERIALIZE_TYPE_OBJECT 12
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#define SERIALIZE_TYPE_ARRAY 13
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```
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The entry type can be bitwise OR'ed with a flag:
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```cpp
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#define SERIALIZE_FLAG_ARRAY 0x80
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```
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This signals there are multiple *values* for the entry. Since only one bit is
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reserved for specifying an array, we can not directly represent nested arrays.
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However, you can place each of the inner arrays inside of a section, and make
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the outer array type `SERIALIZE_TYPE_OBJECT | SERIALIZE_FLAG_ARRAY`. Immediately following the type code byte is a varint specifying the length of the array.
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Finally, the all the elements are serialized in sequence with no padding and
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without any type information. For example:
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<p style="padding-left:1em; font:italic larger serif">type, count,
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value<sub>1</sub>, value<sub>2</sub>,..., value<sub>n</sub></p>
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#### Entry values
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It's important to understand that entry *values* can be encoded any way in which
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an implementation chooses. For example, the integers can be in either big or
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little endian byte order.
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Entry values which are objects (i.e. `SERIALIZE_TYPE_OBJECT`), are stored as
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[sections](#Section).
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Note, I have not yet seen the type `SERIALIZE_TYPE_ARRAY` in use. My assumption
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is this would be used for *untyped* arrays and so subsequent entries could be of
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any type.
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### Overall example
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Let's put it all together and see what an entire object would look like serialized. To represent our data, let's create a JSON object (since it's a format
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that most will be familiar with):
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```json
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{
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"short_quote": "Give me liberty or give me death!",
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"long_quote": "Monero is more than just a technology. It's also what the technology stands for.",
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"signed_32bit_int": 20140418,
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"array_of_bools": [true, false, true, true],
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"nested_section": {
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"double": -6.9,
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"unsigned_64bit_int": 11111111111111111111
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}
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}
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```
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This would translate to:
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## Monero specifics
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### Entry values
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#### Hashes, Keys, Blobs
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These are stored as strings, `SERIALIZE_TYPE_STRING`.
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#### STL containers (vector, list)
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These can be arrays of standard integer types, strings or
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`SERIALIZE_TYPE_OBJECT`'s for structs.
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#### Links to some Monero struct definitions
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- [Core RPC
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definitions](https://github.com/monero-project/monero/blob/master/src/rpc/core_rpc_server_commands_defs.h)
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- [CryptoNote protocol
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definitions](https://github.com/monero-project/monero/blob/master/src/cryptonote_protocol/cryptonote_protocol_defs.h)
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[//]: # ( vim: set tw=80: )
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