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A library implementing Recursive Length Prefixes to encode arbitray data structures

native-library rlp serialization ethereum transaction marshalling
0.1.4 Latest release released
9 1 1

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a native library implementing rlp purely for the crystal language. rlp is ethereum's recursive length prefix used to encode arbitray data structures.

this library allows for rlp-encoding of:

  • binary data (assumed encoded)
  • boolean values (true, false)
  • scalars (positive integers)
  • string literals and characters
  • arrays containing any of the above
  • nested arrays containing any of the above

this library allows for decoding of:

  • rlp-encoded data in binary format
  • rlp-encoded data in hexadecimal string format

note, that decoded data is always binary as per ethereum's design rationale:

"RLP does not attempt to define any specific data types such as booleans, floats, doubles or even integers; instead, it simply exists to store structure, in the form of nested arrays, and leaves it up to the protocol to determine the meaning of the arrays"


add the rlp library to your shard.yml

    github: q9f/
    version: "~> 0.1"


# import rlp
require "rlp"

this library exposes the following modules (in logical order):

  • Rlp: core library exposing encode and decode logic
  • Rlp::Util: a collection of utilities to ease the conversion between data types
  • Rlp::RecursiveArray: is a data type alias allowing for arrays of unknown nesting depth

basic usage:

# rlp-encode a string
rlp = Rlp.encode "A cat with a short string."
# => Bytes[154, 65, 32, 99, 97, 116, 32, 119, 105, 116, 104, 32, 97, 32, 115, 104, 111, 114, 116, 32, 115, 116, 114, 105, 110, 103, 46]

# (optionally) get a hex representation of the rlp-encoded data
hex = Rlp::Util.bin_to_hex rlp
# => "9a4120636174207769746820612073686f727420737472696e672e"

# decode the rlp data
bin = Rlp.decode hex
# => Bytes[65, 32, 99, 97, 116, 32, 119, 105, 116, 104, 32, 97, 32, 115, 104, 111, 114, 116, 32, 115, 116, 114, 105, 110, 103, 46]

# we expect a string, so we can try to convert it here
str = Rlp::Util.bin_to_str bin
# => "A cat with a short string."


the full library documentation can be found here:

generate a local copy with:

crystal docs


the library is entirely specified through tests in ./spec; run:

crystal spec --verbose


recursive length prefixes are used by the ethereum protocol to store arbitrary data structures, e.g., signed transactions, and is a fundamental serialization used by ethereum's networking protocol devp2p which implements rlpx, the recursive length prefix transfer protocol.

rlp can encode any data and data structure. the resulting data is a serialized byte-stream containing prefix bytes, header data, and actual data depending on the type and size of the encoded data.

Rlp.encode [42, "eth"]
# => Bytes[197, 42, 131, 101, 116, 104]

deserialization of rlp-encoded byte-streams allows for recovering the underlying data structure. however, rlp is kept minimalistic in its specification and is therefore agnostic to the data types used in the structures.

Rlp.decode Bytes[197, 42, 131, 101, 116, 104]
# => [Bytes[42], Bytes[101, 116, 104]]

It's up to applications using rlp to further specify protocols for decoding the actual data.

decoded = Rlp.decode Bytes[197, 42, 131, 101, 116, 104]
protocol = [] of String | Int32 | BigInt
protocol << Rlp::Util.bin_to_int decoded[0]
protocol << Rlp::Util.bin_to_str decoded[1]
# => [42, "eth"]


create a pull request, and make sure tests and linter passes.

this library with built with the help of the blog post by the mana team implementing rlp in elixir and coinmonks' annotated version of the rlp specification. ethereum classic's rlp article allows for some sweet test cases.

license: apache license v2.0

contributors: @q9f, @cserb

  github: q9f/
  version: ~> 0.1.4


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