Cleanup and clarify our hashlib docs. (#105624)

Clarify and improve our hashlib docs. Now with 50% less mess!

1 files changed, 90 insertions, 47 deletions

diff --git a/Doc/library/hashlib.rst b/Doc/library/hashlib.rst
index 797870b..7f06151 100644
--- a/Doc/library/hashlib.rst
+++ b/Doc/library/hashlib.rst
@@ -11,7 +11,7 @@
 
 .. index::
    single: message digest, MD5
-   single: secure hash algorithm, SHA1, SHA224, SHA256, SHA384, SHA512
+   single: secure hash algorithm, SHA1, SHA2, SHA224, SHA256, SHA384, SHA512, SHA3, Shake, Blake2
 
 .. testsetup::
 
@@ -22,7 +22,8 @@
 
 This module implements a common interface to many different secure hash and
 message digest algorithms.  Included are the FIPS secure hash algorithms SHA1,
-SHA224, SHA256, SHA384, and SHA512 (defined in FIPS 180-2) as well as RSA's MD5
+SHA224, SHA256, SHA384, SHA512, (defined in `the FIPS 180-4 standard`_),
+the SHA-3 series (defined in `the FIPS 202 standard`_) as well as RSA's MD5
 algorithm (defined in internet :rfc:`1321`).  The terms "secure hash" and
 "message digest" are interchangeable.  Older algorithms were called message
 digests.  The modern term is secure hash.
@@ -32,11 +33,6 @@ digests.  The modern term is secure hash.
    If you want the adler32 or crc32 hash functions, they are available in
    the :mod:`zlib` module.
 
-.. warning::
-
-   Some algorithms have known hash collision weaknesses, refer to the "See
-   also" section at the end.
-
 
 .. _hash-algorithms:
 
@@ -44,38 +40,43 @@ Hash algorithms
 ---------------
 
 There is one constructor method named for each type of :dfn:`hash`.  All return
-a hash object with the same simple interface. For example: use :func:`sha256` to
-create a SHA-256 hash object. You can now feed this object with :term:`bytes-like
-objects <bytes-like object>` (normally :class:`bytes`) using the :meth:`update` method.
-At any point you can ask it for the :dfn:`digest` of the
-concatenation of the data fed to it so far using the :meth:`digest` or
-:meth:`hexdigest` methods.
-
-.. note::
-
-   For better multithreading performance, the Python :term:`GIL` is released for
-   data larger than 2047 bytes at object creation or on update.
+a hash object with the same simple interface. For example: use :func:`sha256`
+to create a SHA-256 hash object. You can now feed this object with
+:term:`bytes-like objects <bytes-like object>` (normally :class:`bytes`) using
+the :meth:`update<hash.update>` method.  At any point you can ask it for the
+:dfn:`digest` of the concatenation of the data fed to it so far using the
+:meth:`digest()<hash.digest>` or :meth:`hexdigest()<hash.hexdigest>` methods.
 
-.. note::
+To allow multithreading, the Python :term:`GIL` is released while computing a
+hash supplied more than 2047 bytes of data at once in its constructor or
+:meth:`.update<hash.update>` method.
 
-   Feeding string objects into :meth:`update` is not supported, as hashes work
-   on bytes, not on characters.
 
 .. index:: single: OpenSSL; (use in module hashlib)
 
 Constructors for hash algorithms that are always present in this module are
-:func:`sha1`, :func:`sha224`, :func:`sha256`, :func:`sha384`,
-:func:`sha512`, :func:`blake2b`, and :func:`blake2s`.
-:func:`md5` is normally available as well, though it
-may be missing or blocked if you are using a rare "FIPS compliant" build of Python.
-Additional algorithms may also be available depending upon the OpenSSL
-library that Python uses on your platform. On most platforms the
+:func:`sha1`, :func:`sha224`, :func:`sha256`, :func:`sha384`, :func:`sha512`,
 :func:`sha3_224`, :func:`sha3_256`, :func:`sha3_384`, :func:`sha3_512`,
-:func:`shake_128`, :func:`shake_256` are also available.
+:func:`shake_128`, :func:`shake_256`, :func:`blake2b`, and :func:`blake2s`.
+:func:`md5` is normally available as well, though it may be missing or blocked
+if you are using a rare "FIPS compliant" build of Python.
+These correspond to :data:`algorithms_guaranteed`.
+
+Additional algorithms may also be available if your Python distribution's
+:mod:`hashlib` was linked against a build of OpenSSL that provides others.
+Others *are not guaranteed available* on all installations and will only be
+accessible by name via :func:`new`.  See :data:`algorithms_available`.
+
+.. warning::
+
+   Some algorithms have known hash collision weaknesses (including MD5 and
+   SHA1). Refer to `Attacks on cryptographic hash algorithms`_ and the
+   `hashlib-seealso`_ section at the end of this document.
 
 .. versionadded:: 3.6
    SHA3 (Keccak) and SHAKE constructors :func:`sha3_224`, :func:`sha3_256`,
-   :func:`sha3_384`, :func:`sha3_512`, :func:`shake_128`, :func:`shake_256`.
+   :func:`sha3_384`, :func:`sha3_512`, :func:`shake_128`, :func:`shake_256`
+   were added.
 
 .. versionadded:: 3.6
    :func:`blake2b` and :func:`blake2s` were added.
@@ -89,10 +90,14 @@ library that Python uses on your platform. On most platforms the
    that the hashing algorithm is not used in a security context, e.g. as a
    non-cryptographic one-way compression function.
 
-   Hashlib now uses SHA3 and SHAKE from OpenSSL 1.1.1 and newer.
+.. versionchanged:: 3.9
+   Hashlib now uses SHA3 and SHAKE from OpenSSL if it provides it.
 
-For example, to obtain the digest of the byte string ``b"Nobody inspects the
-spammish repetition"``::
+Usage
+-----
+
+To obtain the digest of the byte string ``b"Nobody inspects the spammish
+repetition"``::
 
    >>> import hashlib
    >>> m = hashlib.sha256()
@@ -108,22 +113,42 @@ More condensed:
    >>> hashlib.sha256(b"Nobody inspects the spammish repetition").hexdigest()
    '031edd7d41651593c5fe5c006fa5752b37fddff7bc4e843aa6af0c950f4b9406'
 
-.. function:: new(name[, data], *, usedforsecurity=True)
+Constructors
+------------
+
+.. function:: new(name[, data], \*, usedforsecurity=True)
 
    Is a generic constructor that takes the string *name* of the desired
    algorithm as its first parameter.  It also exists to allow access to the
    above listed hashes as well as any other algorithms that your OpenSSL
-   library may offer.  The named constructors are much faster than :func:`new`
-   and should be preferred.
+   library may offer.
 
-Using :func:`new` with an algorithm provided by OpenSSL:
+Using :func:`new` with an algorithm name:
 
    >>> h = hashlib.new('sha256')
    >>> h.update(b"Nobody inspects the spammish repetition")
    >>> h.hexdigest()
    '031edd7d41651593c5fe5c006fa5752b37fddff7bc4e843aa6af0c950f4b9406'
 
-Hashlib provides the following constant attributes:
+
+.. function:: md5([, data], \*, usedforsecurity=True)
+.. function:: sha1([, data], \*, usedforsecurity=True)
+.. function:: sha224([, data], \*, usedforsecurity=True)
+.. function:: sha256([, data], \*, usedforsecurity=True)
+.. function:: sha384([, data], \*, usedforsecurity=True)
+.. function:: sha512([, data], \*, usedforsecurity=True)
+.. function:: sha3_224([, data], \*, usedforsecurity=True)
+.. function:: sha3_256([, data], \*, usedforsecurity=True)
+.. function:: sha3_384([, data], \*, usedforsecurity=True)
+.. function:: sha3_512([, data], \*, usedforsecurity=True)
+
+Named constructors such as these are faster than passing an algorithm name to
+:func:`new`.
+
+Attributes
+----------
+
+Hashlib provides the following constant module attributes:
 
 .. data:: algorithms_guaranteed
 
@@ -144,10 +169,12 @@ Hashlib provides the following constant attributes:
 
    .. versionadded:: 3.2
 
+Hash Objects
+------------
+
 The following values are provided as constant attributes of the hash objects
 returned by the constructors:
 
-
 .. data:: hash.digest_size
 
    The size of the resulting hash in bytes.
@@ -207,6 +234,9 @@ A hash object has the following methods:
 SHAKE variable length digests
 -----------------------------
 
+.. function:: shake_128([, data], \*, usedforsecurity=True)
+.. function:: shake_256([, data], \*, usedforsecurity=True)
+
 The :func:`shake_128` and :func:`shake_256` algorithms provide variable
 length digests with length_in_bits//2 up to 128 or 256 bits of security.
 As such, their digest methods require a length. Maximum length is not limited
@@ -223,8 +253,13 @@ by the SHAKE algorithm.
 
    Like :meth:`digest` except the digest is returned as a string object of
    double length, containing only hexadecimal digits.  This may be used to
-   exchange the value safely in email or other non-binary environments.
+   exchange the value in email or other non-binary environments.
 
+Example use:
+
+   >>> h = hashlib.shake_256(b'Nobody inspects the spammish repetition')
+   >>> h.hexdigest(20)
+   '44709d6fcb83d92a76dcb0b668c98e1b1d3dafe7'
 
 File hashing
 ------------
@@ -768,12 +803,17 @@ Domain Dedication 1.0 Universal:
 .. _BLAKE2: https://www.blake2.net
 .. _HMAC: https://en.wikipedia.org/wiki/Hash-based_message_authentication_code
 .. _BLAKE: https://web.archive.org/web/20200918190133/https://131002.net/blake/
-.. _SHA-3: https://en.wikipedia.org/wiki/NIST_hash_function_competition
+.. _SHA-3: https://en.wikipedia.org/wiki/Secure_Hash_Algorithms
 .. _ChaCha: https://cr.yp.to/chacha.html
 .. _pyblake2: https://pythonhosted.org/pyblake2/
 .. _NIST-SP-800-132: https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-132.pdf
 .. _stackexchange pbkdf2 iterations question: https://security.stackexchange.com/questions/3959/recommended-of-iterations-when-using-pbkdf2-sha256/
+.. _Attacks on cryptographic hash algorithms: https://en.wikipedia.org/wiki/Cryptographic_hash_function#Attacks_on_cryptographic_hash_algorithms
+.. _the FIPS 180-4 standard: https://csrc.nist.gov/publications/detail/fips/180/4/final
+.. _the FIPS 202 standard: https://csrc.nist.gov/publications/detail/fips/202/final
+
 
+.. _hashlib-seealso:
 
 .. seealso::
 
@@ -783,15 +823,18 @@ Domain Dedication 1.0 Universal:
    Module :mod:`base64`
       Another way to encode binary hashes for non-binary environments.
 
-   https://www.blake2.net
-      Official BLAKE2 website.
+   https://nvlpubs.nist.gov/nistpubs/fips/nist.fips.180-4.pdf
+      The FIPS 180-4 publication on Secure Hash Algorithms.
+
+   https://csrc.nist.gov/publications/detail/fips/202/final
+      The FIPS 202 publication on the SHA-3 Standard.
 
-   https://csrc.nist.gov/csrc/media/publications/fips/180/2/archive/2002-08-01/documents/fips180-2.pdf
-      The FIPS 180-2 publication on Secure Hash Algorithms.
+   https://www.blake2.net/
+      Official BLAKE2 website.
 
-   https://en.wikipedia.org/wiki/Cryptographic_hash_function#Cryptographic_hash_algorithms
-      Wikipedia article with information on which algorithms have known issues and
-      what that means regarding their use.
+   https://en.wikipedia.org/wiki/Cryptographic_hash_function
+      Wikipedia article with information on which algorithms have known issues
+      and what that means regarding their use.
 
    https://www.ietf.org/rfc/rfc8018.txt
       PKCS #5: Password-Based Cryptography Specification Version 2.1