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# 15.2. `hmac` — Keyed-Hashing for Message Authentication

Source code: Lib/hmac.py

This module implements the HMAC algorithm as described by RFC 2104.

`hmac.``new`(key, msg=None, digestmod=None)

  Return a new hmac object. key is a bytes object giving the secret key. If msg is present, the method call `update(msg)` is made. digestmod is the digest constructor or module for the HMAC object to use. It defaults to the `hashlib.md5()` constructor.

An HMAC object has the following methods:

`HMAC.``update`(msg)

  Update the hmac object with the bytes object msg. Repeated calls are equivalent to a single call with the concatenation of all the arguments: `m.update(a); m.update(b)` is equivalent to `m.update(a + b)`.

`HMAC.``digest`()

  Return the digest of the bytes passed to the `update()` method so far. This bytes object will be the same length as the digest_size of the digest given to the constructor. It may contain non-ASCII bytes, including NUL bytes.

  Warning

  When comparing the output of `digest()` to an externally-supplied digest during a verification routine, it is recommended to use the `compare_digest()` function instead of the `==` operator to reduce the vulnerability to timing attacks.

`HMAC.``hexdigest`()

  Like `digest()` except the digest is returned as a string twice the length containing only hexadecimal digits. This may be used to exchange the value safely in email or other non-binary environments.

  Warning

  When comparing the output of `hexdigest()` to an externally-supplied digest during a verification routine, it is recommended to use the `compare_digest()` function instead of the `==` operator to reduce the vulnerability to timing attacks.

`HMAC.``copy`()

  Return a copy (“clone”) of the hmac object. This can be used to efficiently compute the digests of strings that share a common initial substring.

This module also provides the following helper function:

`hmac.``compare_digest`(a, b)

  Return `a == b`. This function uses an approach designed to prevent timing analysis by avoiding content-based short circuiting behaviour, making it appropriate for cryptography. a and b must both be of the same type: either `str` (ASCII only, as e.g. returned by `HMAC.hexdigest()`), or any type that supports the buffer protocol (e.g. `bytes`).

  Note

  If a and b are of different lengths, or if an error occurs, a timing attack could theoretically reveal information about the types and lengths of a and b–but not their values.

  New in version 3.3.

See also

Module `hashlib`
  The Python module providing secure hash functions.

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