crypto

Flow Cryptography

This Go package provides the cryptography tools needed by the Flow blockchain. Most of the primitives and protocols can be used in other projects and are not specific to Flow.

Flow is an ongoing project, which means that new features will still be added and modifications will still be made to improve security and performance of the cryptography package.

Notes:

• The package has been audited for security in January 2021 on this version. The package had a few improvements since.
• The package does not provide security against side channel or fault attacks.

Package import

Cloning Flow repository and following the installation steps builds the necessary tools to use Flow cryptography.

If you wish to only import the Flow cryptography package into your Go project, please follow the following steps:

• Get Flow cryptography package

go get github.com/onflow/flow-go/crypto

or simply import the package to your Go project

import "github.com/onflow/flow-go/crypto"

This is enough to run the package code for many functionalities. However, this isn't enough if BLS signature related functionalities are used. The BLS features rely on an extrnal C library (Relic) for lower level mathematical operations. Building your project at this stage including BLS functionalities would result in build errors related to missing "relic" files. For instance:

fatal error: 'relic.h' file not found
#include "relic.h"
         ^~~~~~~~~

An extra step is required to compile the external dependency (Relic) locally.

• Install CMake, which is used for building the package. The build also requires Git and bash scripting.
• From the Go package directory in $GOPATH/pkg/mod/github.com/onflow/flow-go/crypto@<version-tag>/, build the package dependencies. version-tag is the imported package version. For instance:

cd $GOPATH/pkg/mod/github.com/onflow/flow-go/crypto@v0.25.0/
go generate

Below is a bash script example to automate the above steps. The script can be copied into your Go project root directory. It extracts the imported pacakage version from your project's go.mod file and performs the remaining steps.

#!/bin/bash

# crypto package 
PKG_NAME="github.com/onflow/flow-go/crypto"

# go get the package
go get ${PKG_NAME}

# go.mod
MOD_FILE="./go.mod"

# the version of onflow/flow-go/crypto used in the project is read from the go.mod file
if [ -f "${MOD_FILE}" ]
then
    # extract the version from the go.mod file
    VERSION="$(grep ${PKG_NAME} < ${MOD_FILE} | cut -d' ' -f 2)"
    # using the right version, get the package directory path
    PKG_DIR="$(go env GOPATH)/pkg/mod/${PKG_NAME}@${VERSION}"
else 
   { echo "couldn't find go.mod file - make sure the script is in the project root directory"; exit 1; }
fi

# grant permissions if not existant
if [[ ! -r ${PKG_DIR}  || ! -w ${PKG_DIR} || ! -x ${PKG_DIR} ]]; then
   sudo chmod -R 755 "${PKG_DIR}"
fi

# get into the package directory and set up the external dependencies
(
    cd "${PKG_DIR}" || { echo "cd into the GOPATH package folder failed"; exit 1; }
    go generate
)

Finally, when building your project and including any BLS functionality, adding a Go build tag to include the BLS files in the build is required. The tag is not required when the package is used without BLS functions. It was introduced to avoid build errors when BLS (and therefore Relic) is not needed.

go build -tags=relic

Algorithms

Hashing and Message Authentication Code:

crypto/hash provides the hashing and MAC algorithms required for Flow. All algorithm implement the generic interface Hasher. All digests are of the generic type Hash.

• SHA-3: 256 and 384 output sizes
• Legacy Kaccak: 256 output size
• SHA-2: 256 and 384 output sizes
• KMAC: 128 variant

Signature schemes

All signature schemes use the generic interfaces of PrivateKey and PublicKey. All signatures are of the generic type Signature.

• ECDSA

  • public keys are compressed or uncompressed.
  • ephemeral key is derived from the private key, hash and an external entropy using a CSPRNG (based on https://golang.org/pkg/crypto/ecdsa/).
  • supports NIST P-256 (secp256r1) and secp256k1 curves.

• BLS

  • supports BLS 12-381 curve.
  • is implementing the minimal-signature-size variant: signatures in G1 and public keys in G2.
  • default set-up uses compressed G1/G2 points, but uncompressed format is also supported.
  • hashing to curve uses the Simplified SWU map-to-curve.
  • expanding the message in hash-to-curve uses a cSHAKE-based KMAC128 with a domain separation tag. KMAC128 serves as an expand_message_xof function.
  • this results in the full ciphersuite BLS_SIG_BLS12381G1_XOF:KMAC128_SSWU_RO_POP_ for signatures and BLS_POP_BLS12381G1_XOF:KMAC128_SSWU_RO_POP_ for proofs of possession.
  • signature verification includes the signature membership check in G1.
  • public key membership check in G2 is provided outside of the signature verification.
  • membership check in G1 is using Bowe's fast check, while membership check in G2 is using a simple scalar multiplication by the group order (both will be updated to use Scott's method)
  • non-interactive aggregation of signatures, public keys and private keys.
  • multi-signature verification of an aggregated signature of a single message under multiple public keys.
  • multi-signature verification of an aggregated signature of multiple messages under multiple public keys.
  • batch verification of multiple signatures of a single message under multiple public keys: use a binary tree of aggregations to find the invalid signatures.
  • SPoCK scheme based on BLS: verifies two signatures have been generated from the same message that is unknown to the verifier.

• Future features:

  • membership checks in G1/G2 using Scotts's method.
  • support minimal-pubkey-size variant

PRNG

• ChaCha20-based CSPRNG

Protocols

Threshold Signature

• BLS-based threshold signature

  • non interactive threshold signature reconstruction.
  • supports only BLS 12-381 curve with the same features above.
  • (t+1) signatures are required to reconstruct the threshold signature.
  • key generation (single dealer) to provide the set of keys.
  • provides a stateless api and a stateful api.

• Future features:

  • support a partial signature reconstruction in the stateful api to avoid a long final reconstruction.

Discrete-Log based distributed key generation

All supported Distributed Key Generation protocols are discrete log based and are implemented for the same BLS setup on the BLS 12-381 curve. The protocols generate key sets for the BLS-based threshold signature.

• Feldman VSS
  • simple verifiable secret sharing with a single dealer.
  • the library does not implement the communication channels between participants. The caller should implement the methods PrivateSend (1-to-1 messaging) and Broadcast (1-to-n messaging)
  • 1-to-1 messaging must be a private channel, the caller must make sure the channel preserves confidentialiy and authenticates the sender.
  • 1-to-n broadcasting assume all destination participants receive the same copy of the message. The channel should also authenticate the broadcaster.
  • It is recommended that both communication channels are unique per protocol instance. This could be achieved by prepending the messages to send/broadcast by a unique protocol instance ID.
• Feldman VSS Qual.
  • an extension of the simple Feldman VSS.
  • implements a complaint mechanism to qualify/disqualify the dealer.
• Joint Feldman (Pedersen)
  • distributed generation.
  • based on multiple parallel instances of Feldman VSS Qual with multiple dealers.
  • same assumptions about the communication channels as in Feldman VSS.