monero/docs/PORTABLE_STORAGE.md

Portable Storage Format

Background

Monero makes use of a set of helper classes from a small library named epee. Part of this library implements a networking protocol called Levin, which internally uses a storage format called Portable Storage. This format (amongst the rest of the epee library), is undocumented - or rather relies on the code itself to serve as the documentation. Unfortunately, whilst the rest of the library is fairly straightforward to decipher, the Portable Storage is less-so. Hence this document.

String and Integer Encoding

Integers

With few exceptions, integers serialized in epee portable storage format are serialized as little-endian.

Varints

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 (6 bits).

Byte Sizes

Lowest 2 bits	Size value	Value range
b00	1 byte	0 to 63
b01	2 bytes	64 to 16383
b10	4 bytes	16384 to 1073741823
b11	8 bytes	1073741824 to 4611686018427387903

Represenations of Example Values

Value	Byte Representation (hex)
0	00
7	1c
101	95 01
17,000	A2 09 01 00
7,942,319,744	03 BA 98 65 07 00 00 00

Strings

These are simply length (varint) prefixed char strings without a null terminator (though one can always add one if desired). There is no specific encoding enforced, and in fact, many times binary blobs are stored as these strings. This type should not be confused with the keys in sections, as those are restricted to a maximum length of 255 and do not use varints to encode the length.

"Howdy" => 14 48 6F 77 64 79

Section Keys

These are similar to strings except that they are length limited to 255 bytes, and use a single byte at the front of the string to describe the length (as opposed to a varint).

"Howdy" => 05 48 6F 77 64 79

Binary Format Specification

Header

The format must always start with the following header:

Field	Type	Value
Signature Part A	UInt32	0x01011101
Signature Part B	UInt32	0x01020101
Version	UInt8	0x01

In total, the 9 byte header will look like this (in hex): 01 11 01 01 01 01 02 01 01

Section

Next we have a root object (or section as the library calls it). This is a map of name-value pairs called entries. It starts with a count:

Section	Type
Entry count	varint

Which is followed by the section's name-value entries sequentially:

Entry

Entry	Type
Name	section key
Type	byte
Count1	varint
Value(s)	(type dependant data)

¹ Note, this is only present if the entry type has the array flag (see below).

Entry types

The types defined are:

#define SERIALIZE_TYPE_INT64                1
#define SERIALIZE_TYPE_INT32                2
#define SERIALIZE_TYPE_INT16                3
#define SERIALIZE_TYPE_INT8                 4
#define SERIALIZE_TYPE_UINT64               5
#define SERIALIZE_TYPE_UINT32               6
#define SERIALIZE_TYPE_UINT16               7
#define SERIALIZE_TYPE_UINT8                8
#define SERIALIZE_TYPE_DOUBLE               9
#define SERIALIZE_TYPE_STRING               10
#define SERIALIZE_TYPE_BOOL                 11
#define SERIALIZE_TYPE_OBJECT               12
#define SERIALIZE_TYPE_ARRAY                13

The entry type can be bitwise OR'ed with a flag:

#define SERIALIZE_FLAG_ARRAY              0x80

This signals there are multiple values for the entry. Since only one bit is reserved for specifying an array, we can not directly represent nested arrays. However, you can place each of the inner arrays inside of a section, and make 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. Finally, the all the elements are serialized in sequence with no padding and without any type information. For example:

type, count, value₁, value₂,..., value_n

Entry values

It's important to understand that entry values can be encoded any way in which an implementation chooses. For example, the integers can be in either big or little endian byte order.

Entry values which are objects (i.e. SERIALIZE_TYPE_OBJECT), are stored as sections.

Note, I have not yet seen the type SERIALIZE_TYPE_ARRAY in use. My assumption is this would be used for untyped arrays and so subsequent entries could be of any type.

Overall example

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 that most will be familiar with):

{
  "short_quote": "Give me liberty or give me death!",
  "long_quote": "Monero is more than just a technology. It's also what the technology stands for.",
  "signed_32bit_int": 20140418,
  "array_of_bools": [true, false, true, true],
  "nested_section": {
    "double": -6.9,
    "unsigned_64bit_int": 11111111111111111111
  }
}

This would translate to:

Monero specifics

Entry values

Hashes, Keys, Blobs

These are stored as strings, SERIALIZE_TYPE_STRING.

STL containers (vector, list)

These can be arrays of standard integer types, strings or SERIALIZE_TYPE_OBJECT's for structs.

Links to some Monero struct definitions

• Core RPC definitions
• CryptoNote protocol definitions