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HMAC-SHA256 Webhook Signature Verification: RFC 2104 Internals and Provider Formats

Sun, 06 Sep 2026 00:00:00 +0000

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HMAC-SHA256 Webhook Signature Verification: RFC 2104 Internals and Provider Formats

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The Problem HMAC Solves

You receive an HTTP POST claiming to be a payment confirmation. The body says an order was paid. Acting on it moves inventory and money. Two questions must be answered before your code touches the database:

Authenticity — did this come from the provider, or from anyone who found the endpoint URL?
Integrity — is the body byte-for-byte what the provider sent, or was an amount edited in transit?

A Keyed-Hash Message Authentication Code answers both. The provider holds a secret, you hold the same secret, and the provider attaches a tag that only a holder of that secret could have produced over that exact payload.

What you get from this guide: the construction, the specific attack that motivates it, working verification code for three major providers, and the four mistakes that cause almost every “signature invalid” incident.

The RFC 2104 Construction

HMAC(K, m) = H( (K' XOR opad) || H( (K' XOR ipad) || m ) )

Term	Definition
`H`	Underlying hash function (SHA-256, SHA-512, SHA-1)
`B`	Hash block size in bytes — 64 for SHA-1/SHA-224/SHA-256, 128 for SHA-384/SHA-512
`K'`	The key, normalised to exactly `B` bytes
`ipad`	Byte `0x36` repeated `B` times (inner pad)
`opad`	Byte `0x5C` repeated `B` times (outer pad)
`

Key normalisation matters: a key shorter than B is right-padded with zero bytes; a key longer than B is first hashed with H, and that digest becomes the key. This means a 200-character secret and its SHA-256 digest produce identical HMACs — a detail that occasionally explains why two “different” secrets validate the same payload.

Why not `H(secret || message)`?

Because SHA-1 and the SHA-2 family are Merkle–Damgård constructions that expose their internal state as the output digest. Given H(secret || message) and the length of secret || message, an attacker can resume the hash from that state and compute a valid digest for secret || message || padding || evil — without ever learning the secret. That is a length-extension attack, and it turns a naive “signature” into a forgery oracle.

Block sizes and digest lengths

Hash function	Block size `B`	Digest	Hex characters
MD5	64 bytes	128 bits	32
SHA-1	64 bytes	160 bits	40
SHA-256	64 bytes	256 bits	64
SHA-384	128 bytes	384 bits	96
SHA-512	128 bytes	512 bits	128

The two pads are the whole trick: ipad and opad differ in exactly half their bits, so the inner and outer passes behave like two independent keyed functions.

The double-pad nesting in HMAC means the attacker only ever sees the output of the outer hash, whose input they cannot control or extend. This is why RFC 2104 (1997) and FIPS PUB 198-1 (2008) both standardise the nested form, and why crypto.createHmac exists in every standard library.

Provider Formats Compared

Provider	Header	Encoding	Signed data	Secret
Stripe	`Stripe-Signature`	hex, inside `t=…,v1=…`	`"{timestamp}.{raw_body}"`	endpoint secret (`whsec_…`)
GitHub	`X-Hub-Signature-256`	hex, prefixed `sha256=`	raw body	webhook secret
Shopify	`X-Shopify-Hmac-SHA256`	Base64	raw body	app client secret

All three use HMAC-SHA256 and all three require the unmodified raw body.

Stripe

The header carries a timestamp and one or more scheme-prefixed signatures:

Stripe-Signature: t=1492774577,v1=5257a869e7ecebeda32affa62cdca3fa51cad7e77a0e56ff536d0ce8e108d8bd

Verification, per Stripe’s manual procedure: split the header on , then =, build signed_payload as the timestamp, a literal ., and the raw JSON body, compute HMAC-SHA256 with the endpoint secret, compare in constant time, and reject if the timestamp is outside your tolerance. Stripe’s official libraries default to a five-minute tolerance, and Stripe advises ignoring any scheme that is not v1 to prevent downgrade attacks. During a secret roll, an endpoint can have two active secrets and the header then carries one v1 signature per secret — so compare against all v1 values, not just the first.

import crypto from "node:crypto";

export function verifyStripe(rawBody, header, secret, toleranceSec = 300) {
 const parts = Object.fromEntries(
 header.split(",").map((kv) => kv.split("=").map((s) => s.trim()))
 );
 const timestamp = Number(parts.t);
 if (!timestamp) return false;

 // Freshness: a valid signature is not a fresh one.
 if (Math.abs(Math.floor(Date.now() / 1000) - timestamp) > toleranceSec) return false;

 const expected = crypto
 .createHmac("sha256", secret)
 .update(`${timestamp}.${rawBody}`, "utf8")
 .digest("hex");

 const received = header
 .split(",")
 .filter((kv) => kv.trim().startsWith("v1="))
 .map((kv) => kv.trim().slice(3));

 return received.some((sig) => timingSafeEqualHex(expected, sig));
}

function timingSafeEqualHex(a, b) {
 const ab = Buffer.from(a, "hex");
 const bb = Buffer.from(b, "hex");
 return ab.length === bb.length && crypto.timingSafeEqual(ab, bb);
}

GitHub

export function verifyGithub(rawBody, header, secret) {
 const expected =
 "sha256=" + crypto.createHmac("sha256", secret).update(rawBody).digest("hex");
 const a = Buffer.from(expected);
 const b = Buffer.from(header ?? "");
 return a.length === b.length && crypto.timingSafeEqual(a, b);
}

Shopify

export function verifyShopify(rawBody, header, clientSecret) {
 const expected = crypto
 .createHmac("sha256", clientSecret)
 .update(rawBody, "utf8")
 .digest("base64");
 const a = Buffer.from(expected, "base64");
 const b = Buffer.from(header ?? "", "base64");
 return a.length === b.length && crypto.timingSafeEqual(a, b);
}

The Four Failure Modes

1. The body was parsed before you hashed it

The most common cause by a wide margin. express.json(), Django’s request handling, and Next.js route defaults may hand you a re-serialised object. Re-encoding {"a":1.0} as {"a":1}, reordering keys, or normalising Unicode escapes all change the byte string and therefore the digest. Stripe states the requirement plainly: the raw body must be the string it sent, in UTF-8, unchanged.

// Order matters. Mount the raw-body webhook route BEFORE the JSON parser.
app.post("/webhooks/stripe", express.raw({ type: "application/json" }), handler);
app.use(express.json());

2. Non-constant-time comparison

expected === received returns as soon as it hits a differing byte. That timing difference is measurable across enough requests and leaks the expected digest byte by byte. Use crypto.timingSafeEqual, hmac.compare_digest (Python), or hash_equals (PHP). Compare decoded bytes of equal length — timingSafeEqual throws on length mismatch, so check length first.

3. No replay protection

A valid signature says “this was authentic when created”. It says nothing about when. Sign a timestamp (Stripe’s model) and enforce a tolerance, or store processed delivery IDs and drop repeats. Stripe also warns against setting the tolerance to 0, which disables the recency check entirely.

4. Signature comparison as the only gate

Also validate that the event type is one you handle, return 2xx quickly and process asynchronously, and treat delivery as at-least-once — providers retry, and duplicate events are normal. Stripe explicitly does not guarantee ordering, so deduplicate on event ID rather than trusting timestamps.

Hex, Base64, or Base64url — Match the Provider Exactly

One 32-byte HMAC-SHA256 tag has several textual forms, and mismatched encoding is the second most common cause of failed verification after raw-body mistakes.

Encoding	Length for SHA-256	Typical users
Lowercase hex	64 characters	GitHub, Stripe, most REST APIs
Base64	44 characters (with `=` padding)	Shopify, Slack legacy, AWS headers
Base64url	43 characters (no padding)	JWT `HS256` signature segment

A digest that “looks wrong” but is the right length in a different alphabet is an encoding bug, not a secret mismatch. For the encoding mechanics themselves see the ; for the JWT variant, the .

Computing HMAC in the Browser

SubtleCrypto supports HMAC natively, so a signature can be produced or checked with no network exposure at all:

async function hmacSha256Hex(secret, message) {
 const enc = new TextEncoder();
 const key = await crypto.subtle.importKey(
 "raw", enc.encode(secret),
 { name: "HMAC", hash: "SHA-256" }, false, ["sign", "verify"]
 );
 const sig = await crypto.subtle.sign("HMAC", key, enc.encode(message));
 return [...new Uint8Array(sig)].map((b) => b.toString(16).padStart(2, "0")).join("");
}

For verification prefer crypto.subtle.verify("HMAC", key, signatureBytes, data) — it compares internally instead of exposing you to a naive ===.

Operational Checklist

Generate keys randomly. 32 bytes from crypto.getRandomValues or openssl rand -hex 32 — never a passphrase.
Sign the exact bytes on the wire. No re-serialisation, trimming, or charset conversion.
Compare in constant time. crypto.timingSafeEqual, Python’s hmac.compare_digest, PHP’s hash_equals, or subtle.verify.
Include a timestamp and reject stale requests. HMAC proves authenticity, not freshness.
Support two active secrets during rotation. Accept either tag while migrating, then retire the old key — this is exactly why Stripe can send one v1 signature per active secret.
Never log the secret or put it in a URL. Query strings land in access logs, proxies, and browser history.

Choosing the Hash

Algorithm	Digest	Verdict for new work
HMAC-SHA256	32 bytes	Default. Universal library support, ample margin.
HMAC-SHA512	64 bytes	Fine; faster than SHA-256 on 64-bit CPUs for large payloads.
HMAC-SHA384	48 bytes	Use when a compliance profile requires it.
HMAC-SHA1	20 bytes	Legacy interop only (older GitHub `X-Hub-Signature`, TOTP). Not broken inside HMAC, but do not choose it for new protocols.

HMAC-SHA1 remains safe as an authenticator because HMAC does not depend on collision resistance — the same reason . Even so, auditors flag it, so new designs should start at SHA-256. For plain (unkeyed) integrity digests, see the .

Step-by-Step: Debugging a Failing Signature with Toolbox

Capture the raw body from your server logs or the provider’s delivery inspector — copy the exact bytes, not a pretty-printed version.
Open the tool: visit the .
Paste the payload and the signing secret, and select SHA-256.
For Stripe, prepend the timestamp: enter 1492774577.{"id":"evt_…"} — timestamp, dot, raw body — matching the signed_payload construction.
Compare digests: place the provider’s header value next to the computed digest. A hex/Base64 mismatch means an encoding bug; a completely different digest means the body was mutated or the wrong secret is in use.

Outcome: you localise the fault to one of three causes — wrong secret, mutated body, or wrong signed-payload construction — in a couple of minutes, without redeploying to add log statements.

Related guides: · ·

FAQ

Frequently Asked Questions

Why is HMAC used instead of simply hashing the secret and the message together? ▼

SHA-1 and SHA-2 are Merkle-Damgard constructions vulnerable to length extension: given H(secret || message) and the input length, an attacker can produce a valid digest for message || padding || attacker_data without knowing the secret. HMAC's nested form, H((K XOR opad) || H((K XOR ipad) || message)), prevents that, which is why RFC 2104 and FIPS PUB 198-1 specify it.

Why does webhook signature verification fail when my framework parses JSON automatically? ▼

The digest covers the exact bytes the provider sent. Body parsers re-serialise JSON and can reorder keys or change whitespace, number, and Unicode formatting, producing a different byte string and digest. Capture the raw body before parsing middleware runs — in Express, mount the webhook route before express.json() or use express.raw() on that route.

What is the difference between HMAC and a digital signature? ▼

HMAC is symmetric: the same secret creates and verifies the tag, so any verifier can also forge. It proves the sender holds the shared secret. RSA or ECDSA signatures are asymmetric — the private key signs, the public key only verifies — adding non-repudiation. Webhooks use HMAC because a shared secret already exists and HMAC is far faster.

Do I still need a timestamp check if the signature is valid? ▼

Yes. A valid signature proves authenticity and integrity, not freshness. Without a signed timestamp an intercepted request can be replayed indefinitely. Stripe signs a timestamp and its libraries default to a five-minute tolerance; where no timestamp is signed, deduplicate on the delivery or event ID.

How TOTP 2FA Codes Work: RFC 6238 Math, Base32 Secrets, and Clock Drift

Sun, 06 Sep 2026 00:00:00 +0000

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How TOTP 2FA Codes Work: RFC 6238 Math, Base32 Secrets, and Clock Drift

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What TOTP Actually Is

Time-based One-Time Password (TOTP) is defined in RFC 6238. It is not a new algorithm — it is a thin wrapper around HMAC-based One-Time Password (HOTP), specified in RFC 4226. HOTP derives a code from a shared secret and a moving counter. TOTP simply says: use the clock as the counter.

That one substitution is what makes authenticator apps work offline. There is no request to Google, Authy, or your identity provider when a code appears. The phone holds the secret, reads its own clock, and computes the answer. The server does the identical computation and compares.

What you get from this guide: the exact formula, the truncation step almost every explanation skips, the enrolment URI format, and the three failure modes that generate support tickets.

The RFC 6238 Formula, Step by Step

Step 1: Derive the time counter

T = floor((Current Unix Time - T0) / X)

Symbol	Meaning	Default
`Current Unix Time`	Seconds since 1970-01-01T00:00:00Z, UTC	—
`T0`	Epoch offset from which counting starts	`0`
`X`	Time step in seconds	`30`

At Unix time 1789000000, with defaults, T = floor(1789000000 / 30) = 59633333.

Step 2: Compute the HMAC

T is encoded as an 8-byte big-endian unsigned integer and passed as the message to HMAC, keyed with the raw secret:

HS = HMAC-SHA1(K, T) // 20-byte digest

The default hash is SHA-1. RFC 6238 explicitly permits HMAC-SHA256 and HMAC-SHA512. Using SHA-1 here is not the weakness people assume — HMAC’s security relies on the pseudorandomness of the compression function, not on collision resistance, so the known SHA-1 collision attacks do not translate into TOTP forgery. Read the underlying construction in the .

Step 3: Dynamic truncation

This is the step most tutorials omit, and the reason a naive implementation produces codes the server rejects.

offset = HS[19] & 0x0F // low 4 bits of the last byte
P = HS[offset..offset+3] // 4 consecutive bytes
binCode = P & 0x7FFFFFFF // clear the most significant bit

Masking the high bit removes any signed-integer ambiguity across language implementations. The result is a 31-bit integer.

Step 4: Reduce to digits

Code = binCode mod 10^Digits

With Digits = 6, binCode = 1284755224 yields 755224 — the canonical RFC 4226 test vector. Codes are zero-padded to the full width: 000042 is a valid code, 42 is not.

Reference implementation

// Browser-native TOTP with Web Crypto. rawKey = Base32-decoded secret bytes.
async function totp(rawKey, { digits = 6, step = 30, algo = "SHA-1" } = {}) {
 const counter = Math.floor(Date.now() / 1000 / step);

 // 8-byte big-endian counter
 const msg = new DataView(new ArrayBuffer(8));
 msg.setUint32(0, Math.floor(counter / 2 ** 32));
 msg.setUint32(4, counter >>> 0);

 const key = await crypto.subtle.importKey(
 "raw", rawKey, { name: "HMAC", hash: algo }, false, ["sign"]
 );
 const hs = new Uint8Array(await crypto.subtle.sign("HMAC", key, msg.buffer));

 const offset = hs[hs.length - 1] & 0x0f;
 const bin =
 ((hs[offset] & 0x7f) << 24) |
 (hs[offset + 1] << 16) |
 (hs[offset + 2] << 8) |
 hs[offset + 3];

 return String(bin % 10 ** digits).padStart(digits, "0");
}

Base32 Secrets: Why Not Base64?

The shared secret is transported as Base32 (RFC 4648, the same document that defines Base64 — see the for the shared 5-bit and 6-bit chunking mechanics).

Base32 is the deliberate choice for three reasons:

Case-insensitive alphabet (A-Z and 2-7) — a user can retype it from a printed backup without shift-key errors.
No visually ambiguous characters — 0, 1, and 8 are excluded, so O/0 and l/1 confusion disappears.
Safe in URIs and QR codes without percent-encoding.

Practical detail that breaks implementations: Base32 padding (=) is optional in authenticator secrets, and most apps strip whitespace and lowercase before decoding. A 160-bit secret encodes to 32 Base32 characters. RFC 4226 requires a minimum of 128 bits and recommends 160 bits.

The Enrolment QR Code: `otpauth://` URI

The QR code you scan is a plain text URI in the Key Uri Format:

otpauth://totp/Acme%20Inc:alice@example.com?secret=JBSWY3DPEHPK3PXP&issuer=Acme%20Inc&algorithm=SHA1&digits=6&period=30

Parameter	Purpose	Practical note
`secret`	Base32 shared key	Required. Everything else has a default.
`issuer`	Service name shown in the app	Set it in both the label prefix and the query parameter — older apps read only one.
`algorithm`	`SHA1`, `SHA256`, `SHA512`	Widely ignored by popular apps, which assume SHA-1.
`digits`	`6` or `8`	Also frequently ignored.
`period`	Time step in seconds	Also frequently ignored.

Design consequence: if your server issues algorithm=SHA256&digits=8 and your users authenticate with an app that silently assumes SHA-1 and 6 digits, every code fails and the failure is indistinguishable from a wrong password. Unless you control the client, stay on the SHA-1 / 6-digit / 30-second defaults.

Clock Drift, Validation Windows, and Replay

TOTP has no synchronisation channel. The protocol’s entire tolerance mechanism is the server checking neighbouring time steps.

Server window	Effective validity	Trade-off
`T` only	0-30 s	Rejects codes typed slowly; poor UX
`T-1, T, T+1`	Up to 90 s	Common default; absorbs drift and typing delay
`T-2 … T+2`	Up to 150 s	Use only with strict rate limiting

Three rules that separate a correct deployment from a fragile one:

Run NTP on the server. Client drift is the user’s problem; server drift breaks every user at once.
Burn the code after use. RFC 6238 § 5.2 is explicit: accept each code once per user per time step. Without this, a code intercepted at second 1 stays replayable for the rest of the window.
Rate-limit attempts. A 6-digit code is 1,000,000 possibilities. At an unlimited request rate, an attacker inside a 90-second window has real odds. Cap at roughly 5 failures, then lock or back off exponentially.

Where TOTP Fits in 2026

Factor	SMS OTP	TOTP (RFC 6238)	FIDO2 / Passkey
Works offline	❌	✅	✅
Resists SIM swap	❌	✅	✅
Resists real-time phishing	❌	❌	✅ (origin-bound)
Server holds a reusable secret	✅ (risk)	✅ (risk)	❌ (public key only)
Recovery burden	Low	Medium (backup codes)	Medium (multi-device sync)

TOTP’s structural weakness is the shared secret at rest. The server must store the key in a form it can compute with, so a database breach exposes every enrolled seed unless the seeds are encrypted with a key held outside the database. Store them encrypted, and treat the enrolment QR code as a credential — a screenshot in a chat thread is a bypassed second factor.

Step-by-Step: Testing TOTP Codes with Toolbox

Open the tool: visit the .
Paste the Base32 secret produced by your enrolment endpoint (spacing and case are normalised for you).
Set algorithm, digits, and period to match your server configuration — this is where SHA-256 or 8-digit mismatches surface immediately.
Compare against your backend: call your verification endpoint with the displayed code while the countdown ring is above roughly 5 seconds, so a step boundary does not confuse the result.
Reproduce drift deliberately: verify a code just as the ring empties to confirm your server actually accepts T-1 and does not silently reject late submissions.

Outcome: you can prove whether a failing 2FA flow is a secret-encoding bug, an algorithm mismatch, or a clock/window problem — without installing a mobile app or exposing the seed to a third-party service.

Related guides: · ·

FAQ

Frequently Asked Questions

How does an authenticator app generate a 6-digit code without internet access? ▼

The app stores a shared secret exchanged once during enrolment (usually via a QR code). Every 30 seconds it computes HMAC-SHA1 over the current time step (Unix time divided by 30), truncates the 20-byte digest to a 31-bit integer, and takes that value modulo 1,000,000 for 6 digits. Both phone and server hold the same secret and read the same clock, so they derive the same code with no network round trip.

Why do TOTP codes change every 30 seconds? ▼

RFC 6238 defines a time step X (default 30 seconds) and computes T = floor((current Unix time - T0) / X), with T0 normally 0. Each time T increments, the HMAC input changes and a new code appears. Thirty seconds balances usability against how long a stolen code stays valid.

What happens if my phone's clock is wrong? ▼

TOTP has no synchronisation protocol, so a device off by more than one time step produces codes the server rejects. Most servers accept the previous and next step (roughly a 90-second window) to absorb drift and typing delay. Persistent failures are fixed by enabling automatic network time on the device, not by retyping codes.

Is TOTP still secure in 2026, or should I move to passkeys? ▼

TOTP is far stronger than SMS one-time passwords and satisfies most compliance baselines, but it is phishable: a real-time phishing proxy can relay the digits inside the validity window, and the shared seed can leak from a breached database or a screenshotted QR code. Where phishing is in scope, FIDO2 or WebAuthn passkeys are stronger because the signed challenge is bound to the origin.

Hmac | Toolbox Guides & Articles

HMAC-SHA256 Webhook Signature Verification: RFC 2104 Internals and Provider Formats

HMAC-SHA256 Webhook Signature Verification: RFC 2104 Internals and Provider Formats

HMAC Signature Generator & Verifier

The Problem HMAC Solves

The RFC 2104 Construction

Why not H(secret || message)?

Block sizes and digest lengths

Provider Formats Compared

Stripe

GitHub

Shopify

The Four Failure Modes

1. The body was parsed before you hashed it

2. Non-constant-time comparison

3. No replay protection

4. Signature comparison as the only gate

Hex, Base64, or Base64url — Match the Provider Exactly

Computing HMAC in the Browser

Operational Checklist

Choosing the Hash

Step-by-Step: Debugging a Failing Signature with Toolbox

Frequently Asked Questions

How TOTP 2FA Codes Work: RFC 6238 Math, Base32 Secrets, and Clock Drift

How TOTP 2FA Codes Work: RFC 6238 Math, Base32 Secrets, and Clock Drift

TOTP 2FA Authenticator Simulator

What TOTP Actually Is

The RFC 6238 Formula, Step by Step

Step 1: Derive the time counter

Step 2: Compute the HMAC

Step 3: Dynamic truncation

Step 4: Reduce to digits

Reference implementation

Base32 Secrets: Why Not Base64?

The Enrolment QR Code: otpauth:// URI

Clock Drift, Validation Windows, and Replay

Where TOTP Fits in 2026

Step-by-Step: Testing TOTP Codes with Toolbox

Frequently Asked Questions

Why not `H(secret || message)`?

The Enrolment QR Code: `otpauth://` URI