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hashutil.py
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# Copyright (C) 2015-2017 The Software Heritage developers
# See the AUTHORS file at the top-level directory of this distribution
# License: GNU General Public License version 3, or any later version
# See top-level LICENSE file for more information
"""Module in charge of hashing function definitions. This is the base
module use to compute swh's hashes.
Only a subset of hashing algorithms is supported as defined in the
ALGORITHMS set. Any provided algorithms not in that list will result
in a ValueError explaining the error.
This modules defines the following hashing functions:
- hash_file: Hash the contents of the given file object with the given
algorithms (defaulting to DEFAULT_ALGORITHMS if none provided).
- hash_data: Hash the given binary blob with the given algorithms
(defaulting to DEFAULT_ALGORITHMS if none provided).
- hash_path: Hash the contents of the file at the given path with the
given algorithms (defaulting to DEFAULT_ALGORITHMS if none
provided).
"""
import binascii
import functools
import hashlib
import os
from io import BytesIO
# Supported algorithms
ALGORITHMS = set(['sha1', 'sha256', 'sha1_git', 'blake2s256', 'blake2b512'])
# Default algorithms used
DEFAULT_ALGORITHMS = set(['sha1', 'sha256', 'sha1_git'])
# should be a multiple of 64 (sha1/sha256's block size)
# FWIW coreutils' sha1sum uses 32768
HASH_BLOCK_SIZE = 32768
def _new_git_hash(base_algo, git_type, length):
"""Initialize a digest object (as returned by python's hashlib) for the
requested algorithm, and feed it with the header for a git object of the
given type and length.
The header for hashing a git object consists of:
- The type of the object (encoded in ASCII)
- One ASCII space (\x20)
- The length of the object (decimal encoded in ASCII)
- One NUL byte
Args:
base_algo: a hashlib-supported algorithm
git_type: the type of the git object (supposedly one of 'blob',
'commit', 'tag', 'tree')
length: the length of the git object you're encoding
Returns:
a hashutil.hash object
"""
h = hashlib.new(base_algo)
git_header = '%s %d\0' % (git_type, length)
h.update(git_header.encode('ascii'))
return h
def _new_hash(algo, length=None):
"""Initialize a digest object (as returned by python's hashlib) for
the requested algorithm. See the constant ALGORITHMS for the list
of supported algorithms. If a git-specific hashing algorithm is
requested (e.g., "sha1_git"), the hashing object will be pre-fed
with the needed header; for this to work, length must be given.
Args:
algo (str): a hashing algorithm (one of ALGORITHMS)
length (int): the length of the hashed payload (needed for
git-specific algorithms)
Returns:
a hashutil.hash object
Raises:
ValueError if algo is unknown, or length is missing for a git-specific
hash.
"""
if algo not in ALGORITHMS:
raise ValueError(
'Unexpected hashing algorithm %s, expected one of %s' %
(algo, ', '.join(sorted(ALGORITHMS))))
if algo.endswith('_git'):
if length is None:
raise ValueError('Missing length for git hashing algorithm')
base_algo = algo[:-4]
return _new_git_hash(base_algo, 'blob', length)
return hashlib.new(algo)
def hash_file(fobj, length=None, algorithms=DEFAULT_ALGORITHMS, chunk_cb=None):
"""Hash the contents of the given file object with the given algorithms.
Args:
fobj: a file-like object
length: the length of the contents of the file-like object (for the
git-specific algorithms)
algorithms: the hashing algorithms used
Returns: a dict mapping each algorithm to a bytes digest.
Raises:
ValueError if algorithms contains an unknown hash algorithm.
"""
hashes = {algo: _new_hash(algo, length) for algo in algorithms}
while True:
chunk = fobj.read(HASH_BLOCK_SIZE)
if not chunk:
break
for hash in hashes.values():
hash.update(chunk)
if chunk_cb:
chunk_cb(chunk)
return {algo: hash.digest() for algo, hash in hashes.items()}
def hash_path(path, algorithms=DEFAULT_ALGORITHMS, chunk_cb=None):
"""Hash the contents of the file at the given path with the given
algorithms.
Args:
path: the path of the file to hash
algorithms: the hashing algorithms used
chunk_cb: a callback
Returns: a dict mapping each algorithm to a bytes digest.
Raises:
ValueError if algorithms contains an unknown hash algorithm.
OSError on file access error
"""
length = os.path.getsize(path)
with open(path, 'rb') as fobj:
hash = hash_file(fobj, length, algorithms, chunk_cb)
hash['length'] = length
return hash
def hash_data(data, algorithms=DEFAULT_ALGORITHMS):
"""Hash the given binary blob with the given algorithms.
Args:
data: a bytes object
algorithms: the hashing algorithms used
Returns: a dict mapping each algorithm to a bytes digest
Raises:
TypeError if data does not support the buffer interface.
ValueError if algorithms contains an unknown hash algorithm.
"""
fobj = BytesIO(data)
return hash_file(fobj, len(data), algorithms)
def hash_git_data(data, git_type, base_algo='sha1'):
"""Hash the given data as a git object of type git_type.
Args:
data: a bytes object
git_type: the git object type
base_algo: the base hashing algorithm used (default: sha1)
Returns: a dict mapping each algorithm to a bytes digest
Raises:
ValueError if the git_type is unexpected.
"""
git_object_types = {'blob', 'tree', 'commit', 'tag'}
if git_type not in git_object_types:
raise ValueError('Unexpected git object type %s, expected one of %s' %
(git_type, ', '.join(sorted(git_object_types))))
h = _new_git_hash(base_algo, git_type, len(data))
h.update(data)
return h.digest()
@functools.lru_cache()
def hash_to_hex(hash):
"""Converts a hash (in hex or bytes form) to its hexadecimal ascii form"""
if isinstance(hash, str):
return hash
return binascii.hexlify(hash).decode('ascii')
@functools.lru_cache()
def hash_to_bytehex(hash):
"""Converts a hash to its hexadecimal bytes representation"""
return binascii.hexlify(hash)
@functools.lru_cache()
def hash_to_bytes(hash):
"""Converts a hash (in hex or bytes form) to its raw bytes form"""
if isinstance(hash, bytes):
return hash
return bytes.fromhex(hash)
@functools.lru_cache()
def bytehex_to_hash(hex):
"""Converts a hexadecimal bytes representation of a hash to that hash"""
return hash_to_bytes(hex.decode())

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