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HMAC verification fails when the verifier receives different key bytes, algorithm settings, signature bytes, or message bytes from those used to create the signature. With JSON, a common debugging trap is parsing a request and serializing it again: the object may look unchanged while its whitespace, escaping, property order, or number representation—and therefore its authenticated bytes—differ.
TypeScript itself is not established as a cause. To find the fault, compare the sender’s signing contract with the exact bytes, decoding steps, and crypto configuration used by the deployed verifier.
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What HMAC verification actually checks
HMAC authenticates bytes using keying material and a configured hash algorithm. The W3C Web Cryptography specification describes HMAC as calculating and verifying hash-based message authentication codes using the SHA hash functions defined in that specification (Web Cryptography specification, HMAC).
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Why equivalent-looking JSON can fail
HMAC operates on a byte sequence, not on an abstract JavaScript object. If the sender signs the original HTTP body, parsing that body and then serializing the resulting object can alter the authenticated bytes. Differences can include whitespace, escaping, property order, or how numbers are represented.
RFC 8785 explains the underlying requirement: “Cryptographic operations like hashing and signing need the data to be expressed in an invariant format so that the operations are reliably repeatable.” Its JSON Canonicalization Scheme (JCS) defines deterministic serialization, including sorting object properties (RFC 8785, JSON Canonicalization Scheme).
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Use JCS only if the sender’s contract specifies it or both parties have agreed to it. Applying canonicalization only on the receiving side does not repair a mismatch if the sender signed the original body or used a different serialization rule.
Compare the signing contract before changing code
Write down the sender’s requirements and the verifier’s actual inputs side by side. Generic HMAC examples cannot establish the conventions of a particular integration.
- Algorithm: the precise HMAC hash required by the sender.
- Key: the exact key bytes, including how any text, hex, or Base64 representation is decoded.
- Message: whether the signature covers the original request body or a JSON value serialized by an agreed method.
- Signature format: the required output encoding, such as hex or Base64, and the corresponding decoding procedure.
- Runtime and API: the key-import and crypto configuration used in the deployed environment.
These are explicit inputs to Web Crypto verification: algorithm, key, signature, and data. Confirm each from the provider’s signing specification and your implementation rather than inferring it from a sample built for another integration.
Trace the exact bytes through the verifier
Preserve the body if the contract signs the body
If the provider signs the raw request body, verify against those original bytes before JSON parsing changes their representation. Capture the body at the point it enters the verifier and compare it with the byte sequence the sender says it signed. Logging only a parsed object is not enough to establish that the original bytes match.
Make serialization an agreed rule if signing JSON values
If both parties sign a JSON value rather than preserving the transmitted body, establish the exact deterministic serialization each side uses. JCS is one defined option, but it must be part of the shared contract; do not silently add it to only one implementation.
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Check character encoding
Strings must be converted into the same bytes on both sides. Node.js uses UTF-8 when a string is passed to a crypto update operation without an explicit encoding; a Buffer is passed as bytes (Node.js crypto API). Inspect the byte representation rather than assuming two visually identical strings have the same encoding.
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Decode the signature into bytes
Keep HMAC output as bytes until converting it to the format the integration requires. Decode incoming hex or Base64 according to the sender’s stated format, then supply the resulting bytes to verification. Do not compare a textual encoding directly with raw digest bytes or interpret binary output as Unicode: Node.js cautions that cryptographic outputs are pseudorandom byte sequences, not ordinary text (Node.js crypto API).
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- Record the contract. Confirm the HMAC hash, key bytes and decoding, signed-message definition, and signature encoding with the sender’s specification.
- Capture verifier inputs. At the verification boundary, inspect the data bytes, decoded signature bytes, selected algorithm, and imported key configuration. Protect secrets: do not expose key material in logs.
- Compare raw and rebuilt payloads. Determine whether the sender signs the original body or a serialized JSON value. If the receiver parses and rebuilds the body, check for changes to whitespace, escaping, property order, and numbers.
- Verify encoding and decoding. Confirm the character-to-byte encoding for the message and the exact decoding used for the signature representation.
- Compare working and failing environments. Check the same seven items in each: signed bytes, original versus reserialized body, canonicalization and property order, text encoding, hash and key bytes, signature encoding, and runtime/API and key-import configuration.
- Classify the failure. Treat a returned
falseas a completed comparison that did not match. Treat a thrown error as an API, algorithm, or key setup problem and diagnose it separately.
What the evidence does—and does not—establish
The standards and API documentation establish that HMAC verification depends on the configured algorithm, key, signature, and message data, and that JSON needs a stable representation when cryptographic operations are expected to be repeatable. They do not identify a universal production root cause or show that TypeScript itself causes failures. The incident-specific answer depends on the provider’s signing specification, a representative payload and signature, the implementation, and runtime details.
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