Add support for generating blake2 256 digests

Warehouse added support for receiving and validating blake2 256 digests
for a more secure digest computation. If the user installs twine with the
with-blake2 extension, we can generate the hash for them and upload the
package.

Closes pypa#171

docs/changelog.rst

@@ -20,6 +20,12 @@ Changelog
     (``--repository-url``) or via an environment variable
     (``TWINE_REPOSITORY_URL``).
 
+  * Generate SHA256 digest for all packages by default.
+
+  * :feature:`171` Generate Blake2b 256 digests for packages *if* ``pyblake2``
+    is installed. Users can use ``python -m pip install twine[with-blake2]``
+    to have ``pyblake2`` installed with Twine.
+
 * :release:`1.7.4 <2016-07-09>`
 
   * Correct a packaging error.

setup.cfg

@@ -14,3 +14,4 @@ requires-dist =
     pkginfo >= 1.0
     setuptools >= 0.7.0
     argparse; python_version == '2.6'
+    pyblake2; extra == 'with-blake2'

setup.py

@@ -79,4 +79,9 @@
     },
 
     install_requires=install_requires,
+    extras_require={
+        'with-blake2': [
+            'pyblake2',
+        ]
+    },
 )

twine/package.py

@@ -20,6 +20,11 @@
 import pkginfo
 import pkg_resources
 
+try:
+    import pyblake2
+except ImportError:
+    pyblake2 = None
+
 from twine.wheel import Wheel
 from twine.wininst import WinInst
 
@@ -53,15 +58,24 @@ def __init__(self, filename, comment, metadata, python_version, filetype):
         self.signed_basefilename = self.basefilename + '.asc'
         self.gpg_signature = None
 
+        blake2_256_hash = None
+        if pyblake2 is not None:
+            blake2_256_hash = pyblake2.blake2b(digest_size=256 // 8)
+        # NOTE(sigmavirus24): We may or may not be able to use blake2 so let's
+        # either use the methods or lambdas to do nothing.
+        blake_update = getattr(blake2_256_hash, 'update', lambda *args: None)
+        blake_hexdigest = getattr(blake2_256_hash, 'hexdigest', lambda: None)
         md5_hash = hashlib.md5()
         sha2_hash = hashlib.sha256()
         with open(filename, "rb") as fp:
             for content in iter(lambda: fp.read(io.DEFAULT_BUFFER_SIZE), b''):
                 md5_hash.update(content)
                 sha2_hash.update(content)
+                blake_update(content)
 
         self.md5_digest = md5_hash.hexdigest()
         self.sha2_digest = sha2_hash.hexdigest()
+        self.blake2_256_digest = blake_hexdigest()
 
     @classmethod
     def from_filename(cls, filename, comment):
@@ -117,6 +131,7 @@ def metadata_dictionary(self):
             "comment": self.comment,
             "md5_digest": self.md5_digest,
             "sha256_digest": self.sha256_digest,
+            "blake2_256_digest": self.blake2_256_digest,
 
             # PEP 314
             "provides": meta.provides,