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by mbp at sourcefrog
import all docs from arch |
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Use of hashes in Bazaar-NG |
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* http://infohost.nmt.edu/~val/review/hash.html |
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* http://infohost.nmt.edu/~val/review/hash2.html |
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The main attraction of hashes in bazaar-ng is as an easy way to get |
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universally-unique IDs, or at least with a low chance of collision: |
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The first paper is a bit paranoid; the second has some sensible |
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advice: |
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1. Will compare-by-hash provide significant benefit -- save time, |
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bandwidth, etc? |
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2. Is the system usable if hash collisions can be generated at will? |
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3. Can the hashes be regenerated with a different algorithm at any |
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time? |
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We should try to abide by these rules. I think they are possibly too |
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paranoid -- a real break of SHA-1 would have much wider security |
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implications -- but if a design that respects them is practical, it |
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should be preferred. |
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The first is probably true; the third is just a matter of making sure |
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we allow for the choice of hash to be varied in the format. |
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There are actually two variations on the second: |
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2a. Is the system safe if an attacker can generate hash collisions? |
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2b. Is the system safe if a user's own files contain collisions. |
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Regardless of cryptographic weakness, SHA-1 is unlikely to |
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"accidentally" collide, but it's possible that someone will |
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intentionally generate collisions (in research on SHA) and then want |
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to store them. It would be unfortunate if that did not work. |
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An advantage of naming by hash is that it lets us store only a single |
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copy of identical files, but we have already decided__ that disk space |
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is pretty cheap. It is perhaps enough to have a single copy of files |
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that do not change from one tree revision to the next. |
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__ costs.html |
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As far as an attacker: we will not automatically trust that ids from |
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one branch have the same value in another. It is possible for a |
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branch to contain "lies" about its history or contents, but that |
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doesn't corrupt anything else. It may confuse or mislead someone who |
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looks at the branch, but there is no substitute for human review |
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anyhow. |
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------- |
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The safest position may be to never rely on identifying content by |
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hash. Rather, things which need a universally unique ID should get a |
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UUID instead. |
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This has a slight advantage that the id can be stored directly in the |
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object it refers to, when that's useful. |
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So a `Revision` holds a UUID for the `Inventory`. |
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An inventory holds `InventoryEntry` objects, each with |
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* file-id |
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* filename (location in tree) |
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* type (file, dir, etc) |
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* text-id (uuid identifying the text) |
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* text-sha1 |
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* text-length (for catching bugs) |
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* parent-file-id |