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 or bytearray object giving the secret key. If msg is present, the method call update(msg) is made. digestmod is the digest name, digest constructor or module for the HMAC object to use. It supports any name suitable to hashlib.new() and defaults to the hashlib.md5 constructor.

Changed in version 3.4: Parameter key can be a bytes or bytearray object. Parameter msg can be of any type supported by hashlib.

Paramter digestmod can be the name of a hash algorithm.

Deprecated since version 3.4: MD5 as implicit default digest for digestmod is deprecated.

An HMAC object has the following methods:

HMAC.update(msg)

Update the hmac object with 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).

Changed in version 3.4: Parameter msg can be of any type supported by hashlib.

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.

A hash object has the following attributes:

HMAC.digest_size

The size of the resulting HMAC digest in bytes.

HMAC.block_size

The internal block size of the hash algorithm in bytes.

New in version 3.4.

HMAC.name

The canonical name of this HMAC, always lowercase, e.g. hmac-md5.

New in version 3.4.

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 a bytes-like object.

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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