a05b763b01
SHA3 is still waiting for final standardization. Although this is looking less likely given https://www.cdt.org/blogs/joseph-lorenzo-hall/2409-nist-sha-3 In the meantime, cryptohash implements skein, and it's used by some of the haskell ecosystem (for yesod sessions, IIRC), so this implementation is likely to continue working. Also, I've talked with the cryprohash author and he's a reasonable guy. It makes sense to have an alternate high security hash, in case some horrible attack is found against SHA2 tomorrow, or in case SHA3 comes out and worst fears are realized. I'd also like to support using skein for HMAC. But no hurry there and a new version of cryptohash has much nicer HMAC code, so I will probably wait until I can use that version.
42 lines
2 KiB
Markdown
42 lines
2 KiB
Markdown
When a file is annexed, a key is generated from its content and/or metadata.
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The file checked into git symlinks to the key. This key can later be used
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to retrieve the file's content (its value).
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Multiple pluggable key-value backends are supported, and a single repository
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can use different ones for different files.
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* `SHA256E` -- The default backend for new files, combines a SHA256 hash of
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the file's content with the file's extension. This allows
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verifying that the file content is right, and can avoid duplicates of
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files with the same content. Its need to generate checksums
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can make it slower for large files.
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* `SHA256` -- Does not include the file extension in the key, which can
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lead to better deduplication but can confuse some programs.
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* `WORM` ("Write Once, Read Many") This assumes that any file with
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the same basename, size, and modification time has the same content.
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This is the least expensive backend, recommended for really large
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files or slow systems.
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* `SHA512`, `SHA512E` -- Best currently available hash, for the very paranoid.
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* `SHA1`, `SHA1E` -- Smaller hash than `SHA256` for those who want a checksum
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but are not concerned about security.
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* `SHA384`, `SHA384E`, `SHA224`, `SHA224E` -- Hashes for people who like
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unusual sizes.
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* `SKEIN512`, `SKEIN256` -- [Skein hash](http://en.wikipedia.org/wiki/Skein_hash),
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a well-regarded SHA3 hash competition finalist.
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The `annex.backends` git-config setting can be used to list the backends
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git-annex should use. The first one listed will be used by default when
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new files are added.
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For finer control of what backend is used when adding different types of
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files, the `.gitattributes` file can be used. The `annex.backend`
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attribute can be set to the name of the backend to use for matching files.
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For example, to use the SHA256E backend for sound files, which tend to be
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smallish and might be modified or copied over time,
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while using the WORM backend for everything else, you could set
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in `.gitattributes`:
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* annex.backend=WORM
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*.mp3 annex.backend=SHA256E
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*.ogg annex.backend=SHA256E
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