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Flow Cryptography

The Flow crypto Go module provides the cryptography tools needed by the Flow blockchain. The primitives and protocols can be used in other projects and are not specific to Flow.

Notes:

• The package has been audited for security in January 2021 on this version. The package had a major refactor to switch all the BLS12-381 curve implementation to use BLST starting from version v0.25.0.
• The package used to live under the flow-go repository before being moved out as a separate repository. The move preserved all the module Git history.
• The module does not provide security against side channel or fault attacks.

Module import

Flow cryptography can be imported as any other Go package and does not require extra setup or pre-build (it used to require a pre-build up to version v0.24.9):

get the package

go get github.com/onflow/crypto

import the package into your Go code

import "github.com/onflow/crypto"

Build

Building your project with Flow crypto and enabling all the supported algorithms requires using cgo to compile the C code underneath. If cgo isn't enabled by default, the GCO_ENABLED environment variable should be set to 1. It is also possible to build without cgo (CGO_ENABLED=0) but this would disable some primitives (the ones related to BLS).

Build with cgo

Building with cgo is required to support all the algorithms of the module, including the algorithms based on the BLS12-381 curve.

If the test or target application crashes with a "Caught SIGILL" exception, rebuild with CGO_CFLAGS set to "-O2 -D__BLST_PORTABLE__" to disable non-portable code. The runtime error can happen if the CPU doesn't support certain instructions. Building with this flag results in a slower performance, it is therefore recommended to not use it when possible for an optimal performance.

CGO_CFLAGS="-O2 -D__BLST_PORTABLE__" go build 

If you're cross-compiling, you need to set the CC environment variable to the target C cross-compiler and set CGO_ENABLED to 1. You also need to set the GOOS and GOARCH variables.For example, to compile the test program for linux arm64:

GOOS=linux GOARCH=arm64 CC=aarch64-linux-gnu-gcc CGO_ENABLED=1 go build

Build without cgo

It is possible to build without cgo but this requires disabling all primitives based on the BLS12-381 curve (BLS signature, BLS threshold signature, BLS-based DKG, BLS-based SPoCK). Refer to algorithms and protocols to check the supported features. Calling any of the non-supported primitives would panic. In order to avoid accidental builds that result in unwanted crashes, disabling cgo must be confirmed with the no_cgo build tag.

CGO_ENABLED=0 go build -tags=no_cgo

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 the system entropy (based on https://golang.org/pkg/crypto/ecdsa/).
  • supports NIST P-256 (secp256r1) and secp256k1 curves.

• BLS (requires cgo)

  • supports BLS12-381 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.
  • aggregation of signatures, public keys and private keys.
  • verification of an aggregated signature of a single message under multiple public keys.
  • 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, using a binary tree of aggregations.
  • SPoCK scheme based on BLS: verifies two signatures have been generated from the same message that is unknown to the verifier.

PRNG

• ChaCha20-based CSPRNG

Protocols

Threshold Signature

• BLS-based threshold signature (requires cgo)
  • 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.

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 (requires cgo)
  • 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 is a reliable broadcast, where honest senders are able to reach all honest receivers, and where all honest receivers end up with the same received messages. 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 (requires cgo)
  • an extension of the simple Feldman VSS.
  • implements a complaint mechanism to qualify/disqualify the dealer.
• Joint Feldman (Pedersen) (requires cgo)
  • distributed generation.
  • based on multiple parallel instances of Feldman VSS Qual with multiple dealers.
  • same assumptions about the communication channels as in Feldman VSS.