hmac – Keyed hashing for message authentication

This module implements a subset of the corresponding CPython module, as described below. For more information, refer to the original CPython documentation: cpython:hmac.

The key may be a bytes-like object or, where the port provides them, a hardwarekey.HardwareKey – so the same code works with a key in flash during development and a key held in hardware in production. The one exception: an empty message raises ValueError with a hardwarekey.HardwareKey (a hardware driver limitation), where it is valid with a bytes key.

Only "sha256" and "sha1" are supported for digestmod.

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hmac.new(key: circuitpython_typing.ReadableBuffer | hardwarekey.HardwareKey, msg: circuitpython_typing.ReadableBuffer = b'', digestmod: str = ...) → HMAC

Create a new HMAC object.

Parameters:

  • key – the secret key – a bytes-like object, or a hardwarekey.HardwareKey on ports that provide it

  • msg (ReadableBuffer) – initial data to authenticate; add more with HMAC.update()

  • digestmod (str) – the digest name, "sha256" or "sha1". Required.

hmac.digest(key: circuitpython_typing.ReadableBuffer | hardwarekey.HardwareKey, msg: circuitpython_typing.ReadableBuffer, digest: str) → bytes

Return the HMAC of msg under key for the named digest, in one call.

Equivalent to new(key, msg, digestmod=digest).digest() but does not build an intermediate object.

hmac.compare_digest(a: circuitpython_typing.ReadableBuffer, b: circuitpython_typing.ReadableBuffer) → bool

Return a == b using a constant-time comparison, to avoid leaking timing information about a MAC check.

class hmac.HMAC

An HMAC object, in progress. Created by hmac.new(); it has no user-visible constructor.

update(msg: circuitpython_typing.ReadableBuffer) → None

Feed more data into the HMAC.

Raises RuntimeError if called after digest() or hexdigest() – unlike CPython’s hmac, this cannot be resumed once a digest has been taken.

digest() → bytes

Return the HMAC of the data fed so far, as digest_size bytes.

The first call finishes the underlying MAC computation and caches the result; later calls just return the cached bytes. update() can no longer be called after this, unlike CPython’s hmac.

hexdigest() → str

Like digest() but returns the MAC as a string of hexadecimal digits.

copy() → HMAC

Not supported; always raises NotImplementedError.

PSA Crypto’s multipart MAC API has no way to duplicate an in-progress MAC operation (unlike a plain hash, which can be cloned), so this HMAC object cannot be copied.

digest_size: int

The size of the MAC in bytes (32 for sha256, 20 for sha1). (read-only)

block_size: int

The internal block size of the hash algorithm in bytes. (read-only)

name: str

The canonical name of this HMAC, e.g. "hmac-sha256". (read-only)