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To implement HMAC authentication in ASP.NET Core, define a request-signing format shared by client and server, then verify each request in a custom authentication handler. Use HMAC-SHA-256, compare signatures safely, and add timestamp and replay checks: HMAC proves possession of a shared secret and protects integrity, but it does not encrypt the request.
What HMAC authentication does—and does not do
HMAC is a message authentication code made with a shared secret. Microsoft describes it as a way to determine whether a message sent over an insecure channel has been tampered with, provided sender and receiver share a secret. It can authenticate message integrity and possession of that secret; it does not conceal the message contents. Use HTTPS as well. Microsoft Learn: HMAC class
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For new designs, use HMAC-SHA-256 or stronger. Microsoft recommends SHA-256 or better because of collision problems with MD5 and SHA-1. HMACSHA256 produces a 256-bit output. Microsoft Learn: HMACSHA256 class
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HMAC does not define an HTTP request format. Client and server must agree on exactly which bytes are signed and how those bytes are constructed. A defensible application-specific format might include the HTTP method, normalized path and query, timestamp, unique request ID, and body digest. This is a design example, not a standard format defined by ASP.NET Core or Microsoft’s HMAC APIs.
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Write down these details as protocol requirements before implementing the handler:
- Which request components are included, and in what order.
- How fields are separated, including exact newline behavior.
- Character encoding and any normalization for methods, paths, query strings, and header names.
- Whether the body itself or a digest of its exact bytes is signed, and the digest format.
- How the signature is encoded for transport, such as Base64 or hexadecimal.
- Which timestamp format, freshness window, and request-ID rules apply.
Differences as small as URL escaping, query ordering, or a trailing newline can produce different HMAC values. Do not rely on vague instructions such as “sign the URL and body”; specify byte-for-byte behavior for both implementations.
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Implement verification as an ASP.NET Core authentication scheme
ASP.NET Core supports custom authentication through schemes and handlers. A handler implements a scheme and, on successful authentication, creates an authentication ticket. The request validation logic belongs in that authentication flow rather than in ad hoc endpoint code. Microsoft Learn: Overview of ASP.NET Core Authentication
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- Register a named scheme. Configure an authentication handler derived from
AuthenticationHandler<TOptions>and register it with the application’s authentication setup. - Read and validate the protocol fields. Require the key ID, timestamp, request ID, and signature headers your protocol defines. Reject missing or malformed values before attempting verification.
- Resolve the key by key ID. Look up the shared secret associated with the presented identifier. The key ID is not secret; never send the secret itself in a header.
- Reconstruct canonical bytes. Apply the exact normalization, encoding, field ordering, and body-digest rules used by clients.
- Compute and check the HMAC. Compute HMAC-SHA-256 from the resolved key and canonical bytes, decode the submitted signature, then compare the byte arrays using a fixed-time comparison.
- Apply freshness and replay checks. Reject timestamps outside the allowed window and request IDs already accepted for that key and time window.
- Create the authenticated identity. Only after all checks succeed should the handler return a successful authentication result with a
ClaimsPrincipalandAuthenticationTicket.
Use a fixed-time signature comparison
Do not compare signature strings with ordinary equality. Decode the supplied signature, ensure it has the same byte length as the expected value, and use CryptographicOperations.FixedTimeEquals. Microsoft documents that this method avoids timing leakage from the compared values when lengths match; its runtime may still depend on input length. Microsoft Learn: FixedTimeEquals
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using System.Security.Cryptography;
static byte[] ComputeSignature(byte[] secret, byte[] canonicalRequest)
{
return HMACSHA256.HashData(secret, canonicalRequest);
}
static bool SignatureMatches(byte[] expected, byte[] supplied)
{
return expected.Length == supplied.Length &&
CryptographicOperations.FixedTimeEquals(expected, supplied);
}This is only the cryptographic core, not a complete handler. Key lookup and rotation, configuration, header parsing, canonicalization, body buffering, claims, failure responses, and replay protection still require application-specific implementation. Check API availability against the project’s target framework: the cited documentation spans .NET 10 API pages and a .NET Framework 4.8.1 HMACSHA256 page, so the snippet should not be assumed to compile unchanged for every ASP.NET Core target.
Prevent replay and bind signatures to the operation
A valid HMAC can be replayed: an attacker who captures a request can resend the same signed bytes. HMAC alone does not provide freshness or uniqueness. Include a timestamp and unique request identifier in the signed canonical data, enforce a narrow freshness window, and record accepted identifiers until they expire.
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For a multi-instance service, replay records need shared storage and an atomic check-and-insert operation; otherwise concurrent copies could pass checks on separate instances. Bind the signature to the HTTP method, path, query, and relevant content metadata so it cannot be reused for another operation. These are protocol and application responsibilities, not replay features supplied by HMAC itself.
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- Body handling: If verification reads the ASP.NET Core request stream to compute a body digest, account for buffering, stream position, and request-size limits. Ensure downstream endpoint code can still read the body as intended.
- Secret storage: Keep shared keys out of source control and logs. Supply them through a managed secret-configuration path appropriate to the deployment.
- Rotation: Use a key ID and maintain a controlled set of active keys so clients can transition during rotation. Plan how updated keys reach every service instance.
- Failure behavior: Treat invalid, expired, and replayed requests as authentication failures. Avoid responses that reveal which secret-dependent check failed, and do not log secrets or sensitive full request bodies.
Require the scheme on the intended endpoints
Registering a handler does not by itself ensure every endpoint requires HMAC authentication. Configure authorization so the intended endpoints use the HMAC scheme or an authorization policy that requires it. If an application supports multiple authentication mechanisms, ASP.NET Core policy schemes can dynamically forward authentication to a selected scheme based on the request. Microsoft Learn: Policy schemes in ASP.NET Core
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When HMAC is a suitable choice
HMAC fits systems where the client can securely hold a shared secret and the server needs to verify both the client’s possession of that secret and the integrity of request components such as method, path, and body. It also brings protocol design, secret provisioning, rotation, and replay-cache responsibilities. If clients cannot keep a shared secret safe, that assumption is not met; choose an authentication design suited to that client environment rather than treating HMAC as encryption or as a complete API security system.
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