BinData - Parsing Binary Data in Crystal Lang

BinData provides a declarative way to read and write structured binary data.

This means the programmer specifies what the format of the binary data is, and BinData works out how to read and write data in this format. It is an easier (and more readable) alternative.

Usage

Firstly, it's recommended that you specify the datas endian.

class Header < BinData
  endian big
end

Then you can specify the structures fields. There are a few different field types:

1. Core types
   • UInt8 to UInt128 values respectively
   • You can use endian-aware types to mix endianess: , endian: IO::ByteFormat::LittleEndian
2. Custom types
   • anything that is io serialisable
3. Bit Fields
   • These are a group of fields who values are defined by the number of bits used to represent their value
   • The total number of bits in a bit field must be divisible by 8
4. Groups
   • These are embedded BinData class with access to the parent fields
   • Useful when a group of fields are related or optional
5. Enums
6. Bools
7. Arrays and Sets (fixed size and dynamic)

Examples

see the spec helper for all possible manipulations

  enum Inputs
    VGA
    HDMI
    HDMI2
  end

  class Packet < BinData
    endian big

    # Default sets the value at initialisation.
    field start : UInt8 = 0xFF_u8

    # Value procs assign these values before writing to an IO, overwriting any
    # existing value
    field size : UInt16, value: ->{ text.bytesize + 1 }

    # String fields without a length use `\0` null byte termination
    # Length is being calculated by the size field above
    field text : String, length: ->{ size - 1 }

    # Bit fields should only be used when one or more fields are not byte aligned
    # The sum of the bits in a bit field must be divisible by 8
    bit_field do
      # a bits value can be between 1 and 128 bits long
      bits 5, reserved

      # Bool values are a single bit
      bool set_input = false

      # This enum is represented by 2 bits
      bits 2, input : Inputs = Inputs::HDMI2
    end

    # isolated namespace
    group :extended, onlyif: ->{ start == 0xFF } do
      field start : UInt8 = 0xFF_u8

      # Supports custom objects as long as they implement `from_io`
      field header : ExtHeader = ExtHeader.new
    end

    # optionally read the remaining bytes out of io
    remaining_bytes :rest
  end

The object above can then be accessed like any other object

  pack = io.read_bytes(Packet)
  pack.size # => 12
  pack.text # => "hello world"
  pack.input # => Inputs::HDMI
  pack.set_input # => true
  pack.extended.start # => 255

Additionally, BinData fields support a verify proc, which allows data to be verified while reading and writing io.

class VerifyData < BinData
  endian big

  field size : UInt8
  field bytes : Bytes, length: ->{ size }
  field checksum : UInt8, verify: ->{ checksum == bytes.reduce(0) { |acc, i| acc + i } }
end

If the verify proc returns false, a BinData::VerificationException is raised with a message matching the following format.

Failed to verify reading basic at VerifyData.checksum

Inheritance is also supported

ASN.1 Helpers

Included in this library are helpers for decoding and writing ASN.1 data, such as those used in SNMP and LDAP

require "bindata/asn1"

# Build an object
ber = ASN1::BER.new
ber.tag_number = ASN1::BER::UniversalTags::Integer
ber.payload = Bytes[1]

# Write it to an IO:
io.write_bytes(ber)

# Read data out of an IO:
ber = io.read_bytes(ASN1::BER)
ber.tag_class # => ASN1::BER::TagClass::Universal

Real World Examples

• ASN.1
  • https://github.com/crystal-community/jwt/blob/master/src/jwt.cr#L251
  • https://github.com/spider-gazelle/crystal-ldap
• enums and bit fields
  • https://github.com/spider-gazelle/knx/blob/master/src/knx/cemi.cr#L195
• variable sized arrays
  • https://github.com/spider-gazelle/crystal-bacnet/blob/master/src/bacnet/virtual_link_control/secure_bvlci.cr#L54