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

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

`hmac.``new`(key[, msg[, digestmod]])

  Return a new hmac object. 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.

  Note

  The md5 hash has known weaknesses but remains the default for backwards compatibility. Choose a better one for your application.

An HMAC object has the following methods:

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

`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.

`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.

See also

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

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