Function to verify X-Slack-Signature header in a Cloudflare Worker

slack-verify.js

/* global SLACK_SIGNING_SECRET */

const SIGN_VERSION = 'v0'  // per documentation, this is always "v0"

/**
 * Verify that a request actually came from Slack using our Signing Secret
 * and HMAC-SHA256.
 *
 * Based on code examples found in Cloudflare's documentation:
 * https://developers.cloudflare.com/workers/examples/signing-requests
 *
 * @param {Request} request incoming request purportedly from Slack
 * @returns {Promise<boolean>} true if the signature verification was valid
 */
export async function verifySlackSignature(request) {
    const timestamp = request.headers.get('x-slack-request-timestamp')

    // remove starting 'v0=' from the signature header
    const signatureStr = request.headers.get('x-slack-signature').substring(3)
    // convert the hex string of x-slack-signature header to binary
    const signature = hexToBytes(signatureStr)

    const content = await request.text()
    const authString = `${SIGN_VERSION}:${timestamp}:${content}`
    let encoder = new TextEncoder()
    const key = await crypto.subtle.importKey(
        'raw',
        encoder.encode(SLACK_SIGNING_SECRET),
        { name: 'HMAC', hash: 'SHA-256' },
        false,
        ['verify']
    )
    const verified = await crypto.subtle.verify(
        "HMAC",
        key,
        signature,
        encoder.encode(authString)
    )

    return verified
}

/**
 * Modified version of hex to bytes function posted here:
 * https://stackoverflow.com/a/34356351/489667
 *
 * @param {string} hex a string of hexadecimal characters
 * @returns {ArrayBuffer} binary form of the hexadecimal string
 */
function hexToBytes(hex) {
    const bytes = new Uint8Array(hex.length / 2);
    for (let c = 0; c < hex.length; c += 2) {
        bytes[c / 2] = parseInt(hex.substr(c, 2), 16);
    }

    return bytes.buffer
}