ebmlite#

ebmlite is a lightweight, “pure Python” library for parsing EBML (Extensible Binary Markup Language) data. It is designed to crawl through EBML files quickly and efficiently, and that’s about it. ebmlite can also do basic EBML encoding, but more advanced EBML manipulation (e.g. with a proper DOM) are beyond its scope, and are better left to other libraries.

EBML Overview (the short version)#

EBML (Extensible Binary Markup Language) is a hierarchical tagged binary format, originally created for the Matroska project. The hierarchical structure of EBML bears some conceptual/functional similarity to XML, although the actual structure differs significantly.

In the raw, EBML elements consist of a numeric ID, the size of the element, and a payload. It is space-efficient; the lengths of the ID and size descriptors are variable, using prefix bits to indicate their lengths, a system similar to UTF-8. The mapping of IDs to names and payload data types is done via an external schema.

See the official specification for more information.

EBML Schemata#

An EBML file is largely meaningless without a schema that defines its elements. The schema maps element IDs to names and data types; it also describes the structure (e.g. what elements can be children of other elements) and provides additional metadata. Note: ebmlite currently uses the structure for decoding only, and does not stringently enforce it.

The Schema class is a factory used to encode and decode EBML files. When it’s initialized, it scans through the schema file and creates a new class for each element defined in the schema file; then, when encoding or decoding EBML files, it references these classes in order to encapsulate everything safely.

ebmlite schemata are defined in XML. From these XML files, a Schema instance is created; within the Schema are Element subclasses for each element defined in the XML. Since the interpretation of an EBML file is almost entirely dependent on a schema, importing of EBML files is done through a Schema instance.

Schema instances are typically created from an XML file through loadSchema(), or from a byte string using parseSchema().

from ebmlite import loadSchema
schema = loadSchema('mide_ide.xml')
doc = schema.load('test_file.ebml')

Loading an EBML file creates an instance of a Document subclass, created by the schema, which acts as the root node of the EBML tree. Document instances are typically created by reading an EBML file with Schema.load(), or a byte string via Schema.loads().

Schema Format#

ebmlite uses its own Schema definition syntax. It does not (currently) use the official schema format.

Here is an example of an ebmlite schema, showing a simplified version of the definition of the standard EBML header elements:

<?xml version="1.0" encoding="utf-8"?>
<Schema>
    <MasterElement name="EBML" id="0x1A45DFA3" mandatory="1" multiple="0">
        <UIntegerElement name="EBMLVersion" id="0x4286" multiple="0" mandatory="1" />
        <UIntegerElement name="EBMLReadVersion" id="0x42F7" multiple="0" mandatory="1"/>
        <UIntegerElement name="EBMLMaxIDLength" id="0x42F2" multiple="0" mandatory="1"/>
        <UIntegerElement name="EBMLMaxSizeLength" id="0x42F3" multiple="0" mandatory="1"/>
        <StringElement name="DocType" id="0x4282" multiple="0" mandatory="1"/>
        <UIntegerElement name="DocTypeVersion" id="0x4287" multiple="0" mandatory="1"/>
        <BinaryElement name="Void" global="1" id="0xEC" multiple="1"/>
        <BinaryElement name="CRC-32" global="1" id="0xBF" multiple="0"/>
        <MasterElement name="SignatureSlot" global="1" id="0x1B538667" multiple="1">
            <UIntegerElement name="SignatureAlgo" id="0x7E8A" multiple="0"/>
            <UIntegerElement name="SignatureHash" id="0x7E9A" multiple="0"/>
            <BinaryElement name="SignaturePublicKey" id="0x7EA5" multiple="0"/>
            <BinaryElement name="Signature" id="0x7EB5" multiple="0"/>
            <MasterElement name="SignatureElements" id="0x7E5B" multiple="0">
                <MasterElement name="SignatureElementList" id="0x7E7B" multiple="1">
                    <BinaryElement name="SignedElement" id="0x6532" multiple="1"/>
                </MasterElement>
            </MasterElement>
        </MasterElement>
    </MasterElement>
    <!-- More definitions would follow... -->
</Schema>

Element types#

Each element defined in the schema is a subclass of one of 8 Element base classes:

MasterElement: An element containing other elements.
IntegerElement: Contains a signed integer value of variable length.
UIntegerElement: Contains an unsigned integer value of variable length.
FloatElement: Contains a 32 or 64 bit floating point value.
StringElement: Contains printable US-ASCII characters (0x20 to 0x7E).
UnicodeElement: Contains UTF-8 string data.
DateElement: Contains a timestamp, stored as nanoseconds since 2001-01-01T00:00:00 UTC as a 64 bit integer. ebmlite automatically translates this into a Python datetime.datetime object.
BinaryElement: Contains binary data.

Element definitions have several attributes:

name (string): The Element subclass’ name.
id (integer): The Element subclass’ EBML ID.
global (bool, optional): If “true” (e.g. 1 or True), the element may appear in any location in an EBML file, not just where it appears in the schema.
length (integer, optional): A fixed size to use when encoding the element, overriding the EBML variable length encoding. Use to create byte-aligned structures.
multiple (bool, optional, default=1): Indicates that the element can appear more than once within the same parent. Currently partially enforced for encoding.
mandatory (bool, optional, default=0): Indicates that the element must be present. Not currently enforced.
precache (bool, optional, default varies by type): Indicates that the element’s value should be read and cached when the element is parsed, rather than ‘lazy-loaded’ when explicitly accessed. Can be used to reduce the number of seeks when working with an EBML file after it has been imported. Simple numeric element types have this enabled by default; master, binary, and string/Unicode elements do not.

There are two additional, special-case Element subclasses which are not subclassed:

UnknownElement: Instantiated for elements with IDs that do not appear in the schema. Its payload is treated as binary data. The UnknownElement itself does not appear in the Schema. Unlike other Element subclasses, its ID can vary from instance to instance.
VoidElement: “Void” (ID 0xEC) is a standard EBML element, typically used for padding. If the Schema defines the Void element, it is replaced by this special-case element. The contents of its payload are ignored.

Schema XML Structure#

The structure of the schema’s XML defines the structure of the EBML document; children of a MasterElement in the schema are valid child element types in the EBML. An Element type can appear multiple times in a schema; i.e. if its type can appear as a child of different parent types. Only the first definition requires both name and id attributes. Successive definitions can be abbreviated to just the name and/or id; they will inherit all the other attributes of the first definition. Successive definitions must not have contradictory attributes, however.

<Schema name="Example2" version="1" readversion="1">
    ...
    <MasterElement name="Parent1" id="0x5210">
        <!-- first definition of child: has all attributes -->
        <IntegerElement name="SharedChild" id="0x5211" precache="1" length="8"/>
    </MasterElement>

    <!-- Proper reuse of a child element -->
    <MasterElement name="Parent2" id="0x5220">
        <!-- second definition of child: only name (preferred) or ID required -->
        <IntegerElement name="SharedChild"/>
    </MasterElement>
    <MasterElement name="Parent3" id="0x5230">
        <!-- third definition of child: only name (preferred) or ID required -->
        <IntegerElement id="0x5211"/>
    </MasterElement>

    <!-- A bad reuse! This will raise an exception when the schema is parsed. -->
    <MasterElement name="Parent3" id="0x5230">
        <!-- BAD REDEFINITION: attribute(s) contradict initial definition! -->
        <IntegerElement name="SharedChild" id="0xBAD1D"/>
    </MasterElement>
</Schema>

Note: As seen in the example above, ebmlite allows an EBML document to have multiple elements at its root level. Several other EBML libraries do this as well, but this is apparently counter to the official spec. Officially, an EBML document should have only a single root element, similar to an XML file.