Signing and Verifying Arbitrary Data

Signing Arbitrary Data

Cryptographic signatures are a key part of the blockchain. They are used to prove ownership of an address without exposing its private key. While primarily used for signing transactions, cryptographic signatures can also be used to sign arbitrary messages.

FCL has a feature that lets you send arbitrary data to a configured wallet/service where the user may approve signing it with their private key/s.

Verifying User Signatures

What makes message signatures more interesting is that we can use Flow blockchain to verify the signatures. Cadence has a built-in function publicKey.verify that will verify a signature against a Flow account given the account address.

FCL includes a utility function, AppUtils.verifyUserSignatures, for verifying one or more signatures against an account's public key on the Flow blockchain.

You can use both in tandem to prove a user is in control of a private key or keys.

This enables cryptographically-secure login flow using a message-signing-based authentication mechanism with a user’s public address as their identifier.

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currentUser.signUserMessage()

A method to use allowing the user to personally sign data via FCL Compatible Wallets/Services.

:Note: Requires authentication/configuration with an authorized signing service.

Arguments

Name	Type	Description
message	string	A hexadecimal string to be signed

Returns

Type	Description
Array	An Array of CompositeSignatures: signature

Usage

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import * as fcl from "@onflow/fcl"

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const signMessage = async () => {

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const MSG = Buffer.from("FOO").toString("hex")

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try {

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return await fcl.currentUser.signUserMessage(MSG)

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} catch (error) {

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console.log(error)

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}

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}

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AppUtils.verifyUserSignatures

Note

⚠️ fcl.config.flow.network or options override is required to use this API. See FCL Configuration.

A method allowing applications to cryptographically verify the ownership of a Flow account by verifying a message was signed by a user's private key/s. This is typically used with the response from currentUser.signUserMessage.

Arguments

Name	Type	Description
message	string (required)	A hexadecimal string
compositeSignatures	Array (required)	An Array of CompositeSignatures
opts	Object (optional)	opts.fclCryptoContract can be provided to override FCLCryptoContract address for local development

Returns

Type	Description
Boolean	true if verified or false

Usage

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/**

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* Verify a valid signature/s for an account on Flow.

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*

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* @param {string} msg - A message string in hexadecimal format

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* @param {Array} compSigs - An array of Composite Signatures

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* @param {string} compSigs[].addr - The account address

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* @param {number} compSigs[].keyId - The account keyId

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* @param {string} compSigs[].signature - The signature to verify

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* @param {Object} [opts={}] - Options object

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* @param {string} opts.fclCryptoContract - An optional override of Flow account address where the FCLCrypto contract is deployed

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* @return {bool}

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*

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* @example

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*

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* const isValid = await fcl.AppUtils.verifyUserSignatures(

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* Buffer.from('FOO').toString("hex"),

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* [{f_type: "CompositeSignature", f_vsn: "1.0.0", addr: "0x123", keyId: 0, signature: "abc123"}],

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* {fclCryptoContract}

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* )

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*/

Examples

Use cases include cryptographic login, message validation, verifiable credentials, and others.

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