secp256k1.cr

a native library implementing secp256k1 purely for the crystal language. secp256k1 is the elliptic curve used in the public-private-key cryptography required by bitcoin and ethereum.

this library allows for key generation of:

private keys (from secure random within the elliptic curve field size)
public keys, prefixed, compressed (from private)
public keys, unprefixed and prefixed, uncompressed (from private)
conversion between the different public key formats

this library allows for address generation of:

bitcoin address, compressed and uncompressed (from private or public key)
any other bitcoin-based address by passing a version byte
ethereum address, checksummed and unchecksummed (from private or public key)
any other ethereum-based address

installation

add the secp256k1 library to your shard.yml

dependencies:
  secp256k1:
    github: q9f/secp256k1.cr
    version: "~> 0.1"

usage

this library exposes the following modules:

Secp256k1: the entire handling of private-public key-pairs
Crypto: implementation of various hashing algorithms
Bitcoin: for the generation of bitcoin addresses
Ethereum: for the generation of ethereum addresses

basic usage:

# import secp256k1
require "secp256k1"

# generate a keypair
private_key = Secp256k1.new_private_key
public_key = Secp256k1.public_key_from_private private_key

# display the compressed public key with prefix
puts Secp256k1.public_key_compressed_prefix public_key

generate a compressed bitcoin mainnet address:

# generate a keypair
private_key = Secp256k1.new_private_key
public_key = Secp256k1.public_key_from_private private_key
compressed = Secp256k1.public_key_compressed_prefix public_key

# display the bitcoin address (version "00" for bitcoin mainnet)
puts Bitcoin.address_from_public_key compressed, "00"

# > "1PMycacnJaSqwwJqjawXBErnLsZ7RkXUAs"

generate a checksummed ethereum address:

# generate a keypair
private_key = Secp256k1.new_private_key
public_key = Secp256k1.public_key_from_private private_key
uncompressed = Secp256k1.public_key_uncompressed public_key

# display the ethereum address
puts Ethereum.address_from_public_key uncompressed

# > "0x2Ef1f605AF5d03874eE88773f41c1382ac71C239"

testing

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

crystal spec --verbose

understand

private keys are just scalars and public keys are points with x and y coordinates.

bitcoin public keys can be uncompressed #{p}#{x}#{y} or compressed #{p}#{x}. both come with a prefix p which is useless for uncompressed keys but necessary for compressed keys to recover the y coordinate on the secp256k1 elliptic curve.

ethereum public keys are uncompressed #{x}#{y} without any prefix. the last 20 bytes slice of the y coordinate is actually used as address without any checksum. a checksum was later added in eip-55 using a keccak256 hash and indicating character capitalization.

neither bitcoin nor ethereum allow for recovering public keys from an address unless there exists a transaction with a valid signature on the blockchain.

known issues

note: this library should not be used in production without proper auditing.

this library is not constant time and might be subject to side-channel attacks. #4
this library does unnecessary big-integer math and should someday rather correctly implement the secp256k1 prime field #5

found another issue? report it: https://github.com/q9f/secp256k1.cr/issues

contribute

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

this pure crystal implementation is based on the python implementation wobine/blackboard101 which is also used as reference to write tests against. it's a complete rewrite of the abandoned packetzero/bitcoinutils for educational purposes.

honerable mention for the bitcoin wiki and the ethereum stackexchange for providing so many in-depth resources that supported this project in reimplementing everything.

license: apache license v2.0

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