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+..
+ This work is licensed under a Creative Commons Attribution 3.0 Unported
+ License.
+
+ http://creativecommons.org/licenses/by/3.0/legalcode
+
+=================
+Multihash Support
+=================
+
+https://blueprints.launchpad.net/glance/+spec/multihash
+
+This spec provides a future-proof mechanism for providing a
+self-describing hash for each image.
+
+Problem description
+===================
+
+The hash of an image can be obtained from the ``checksum`` field in the
+image metadata. Currently, this checksum field holds a md5 hexdigest
+value. While md5 can still be used to verify data integrity against
+unintentional corruption, it should not be used to verify data integrity
+against tampering. A more collision-resistant hashing algorithm should
+be used for broader data integrity coverage.
+
+Proposed change
+===============
+
+This spec proposes adding a new field to the image metadata. The
+proposed key name for this field is ``multihash``. The proposed value is
+a multihash [0]_ formatted sha512 hexdigest (codec:sha2-512, code:0x13).
+The hexdigest will be obtained using the current hashlib library. The
+digest length and hash function type will be prepended as described in
+the documentation [0]_ for the multihash format.
+
+Adding a new image will set both the ``checksum`` and ``multihash``
+fields.
+
+Requesting the image metadata of an image without the ``multihash``
+value set, will result in a null for the ``multihash`` field in the
+metadata response.
+
+Downloading an image without the ``multihash`` value set, will result in
+the value being populated upon completion.
+
+Uploading an image to an existing image location will result in the
+``multihash`` value being set / updated.
+
+Alternatives
+------------
+
+1. Update the value of the ``checksum`` field with the hexdigest of a
+   different hashing algorithm. This will likely break client-side
+   checksum verification code.
+
+2. Add a new non-multihash checksum field. This is a less future proof
+   approach that will likely result in a new spec in a few years to add
+   yet another checksum field. Using the multihash format allows
+   changing the algorithm without adding a new field or breaking the API
+   contract.
+
+3. Use the hexdigest of a different algorithm. Tests using hashlib's
+   sha256 and sha512 algorithms consistently yielded faster times for
+   sha512 (SEE: Performance impact). The implementation of the sha512
+   algorithm within hashlib demonstrates reasonable performance and
+   should be considered collision-resistant for many years.
+
+Data model impact
+-----------------
+
+* Triggers: None
+* Expand: A new column (``multihash``) with type string/varchar
+  (defaulting to null) will be added to the images table along with an
+  index similar to the one on ``checksum``
+* Migrate: None
+* Contract: None
+* Conflicts: None
+
+REST API impact
+---------------
+
+A new field (``multihash``) will exist in requests for the image
+metadata.
+
+Security impact
+---------------
+
+A new more collision-resistant hash will be returned in addition to the
+current checksum.
+
+Notification impact
+-------------------
+
+None
+
+Other end user impact
+---------------------
+
+None
+
+Performance impact
+------------------
+
+New image uploads (and the first download of previously uploaded images)
+will take an additional amount of time to calculate the sha512 checksum:
+
+5G binary blob of random data:
+* md5: ~9s
+* sha256: ~22s
+* sha512: ~14s
+* 1Gbps line speed upload: 42s
+
+1.5G Ubuntu 16.04 cloud image:
+* md5: ~2.9s
+* sha256: ~7.2s
+* sha512: ~4.6s
+* 1Gbps line speed upload: 12s
+
+555M Debian 8 cloud image:
+* md5: ~1.0
+* sha256: ~2.5
+* sha512: ~1.6
+* 1Gbps line speed upload: 4.5s
+
+Test Code:
+
+.. code:: python
+
+    #!/usr/bin/env python3
+
+    import hashlib
+    import time
+
+
+    def runtime(f):
+        def wrapper(*args, **kwargs):
+            start = time.time()
+            f(*args, **kwargs)
+            print("Time elapsed: %s" % (time.time() - start))
+        return wrapper
+
+
+    @runtime
+    def checksum(filename, algorithm):
+        algorithms = {"md5": hashlib.md5,
+                      "256": hashlib.sha256,
+                      "512": hashlib.sha512,
+                      }
+        with open(filename, "rb") as f:
+            m = algorithms[algorithm]()
+            for chunk in iter(lambda: f.read(65536), ''):
+                m.update(chunk)
+        print("%s: %s" % (algorithm, m.hexdigest()))
+
+    checksum("fake.img", "512")
+    checksum("fake.img", "256")
+    checksum("fake.img", "md5")
+    checksum("fake.img", "256")
+    checksum("fake.img", "md5")
+    checksum("fake.img", "512")
+
+
+Developer impact
+----------------
+
+Any future checksum verification code should use the ``multihash`` field.
+
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee: unassigned
+
+
+Other contributors:
+
+Work Items
+----------
+
+* Add tests
+
+* Update the db to add ``multihash`` column to the images table (including
+  expand, migrate, contract, and monolith code)
+
+* Update the sections of code that calculate the ``checksum`` to also
+  calculate ``multihash`` (includes calculation on upload)
+
+* Update ``multihash`` on image download when value is null
+
+* Update internal checksum verification code to use the ``multihash``
+  field and fallback to ``checksum`` field when not present
+
+* Update glance client
+
+* Update docs
+
+Dependencies
+============
+
+The multihash specification [0]_
+
+Testing
+=======
+
+Update the tests to verify multihash
+
+
+References
+==========
+
+.. [0] http://multiformats.io/multihash/