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import { createHmac, timingSafeEqual } from 'node:crypto'/** * Verify a Mediflo webhook signature. * * Header shape: `t=<unix-seconds>,v1=<hex>[,v1=<hex>]` * where `v1 = HMAC_SHA256(secret, "<t>.<rawBody>")`. * * `rawBody` MUST be the exact bytes we sent. Re-serializing parsed JSON does * not reproduce the digest — in Express, use * `express.json({ verify: (req, _res, buf) => { req.rawBody = buf } })`. * * Pass BOTH secrets during a rotation window (or just the current one; the * header carries a `v1=` entry per active secret and any match is valid). */export function verifyMedifloSignature(  header: string | undefined,  rawBody: string | Buffer,  secrets: string[],  toleranceSeconds = 300): boolean {  if (!header || secrets.length === 0) return false  let timestamp: number | null = null  const presented: string[] = []  for (const part of header.split(',')) {    const trimmed = part.trim()    const eq = trimmed.indexOf('=')    if (eq <= 0) continue    const key = trimmed.slice(0, eq)    const value = trimmed.slice(eq + 1)    if (key === 't') {      if (!/^\d+$/.test(value)) return false      timestamp = Number(value)    } else if (key === 'v1') {      // More than one entry appears during a 24 h secret rotation.      presented.push(value)    }  }  if (timestamp === null || presented.length === 0) return false  // Replay protection: the timestamp is INSIDE the signed string, so an  // attacker cannot move it forward without invalidating every digest.  const now = Math.floor(Date.now() / 1000)  if (Math.abs(now - timestamp) > toleranceSeconds) return false  for (const secret of secrets) {    const expected = createHmac('sha256', secret)      .update(`${timestamp}.${rawBody}`)      .digest('hex')    for (const candidate of presented) {      // Validate the hex BEFORE comparing. Buffer.from('zz', 'hex') truncates      // silently, and timingSafeEqual throws RangeError on a length mismatch —      // so without these two guards a malformed header crashes the handler      // instead of returning false.      const normalized = candidate.trim().toLowerCase()      if (!/^[0-9a-f]+$/.test(normalized)) continue      if (normalized.length !== expected.length) continue      if (        timingSafeEqual(          Buffer.from(normalized, 'hex'),          Buffer.from(expected, 'hex')        )      ) {        return true      }    }  }  return false}