42 lines
2.3 KiB
Markdown
42 lines
2.3 KiB
Markdown
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# Choosing A Hash Function
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> Note: this document is still very much a work-in-progress. Currently missing:
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> - recommendations for hashed containers
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> - recommendations for a better persistent hash
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> - recommendations for a secure hash
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If a hash function with unchanging output is needed, please select from one of
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the unchanging forever options below.
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## Non-cryptographic
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name | input | output | unchanging forever | notes
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---------------------------------------------|-----------------------------|------------|--------------------|-------
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[`Hash()`][hash] | overloaded | `uint32_t` | no | This function is currently being updated to return `size_t`.
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[`PersistentHash()`][persistenthash] | overloaded | `uint32_t` | yes | Fairly weak but widely used for persisted hashes.
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[`CityHash64()`][cityhash64] | `base::span<const uint8_t>` | `uint64_t` | yes (note 1) | Version 1.0.3. Has some known weaknesses.
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[`CityHash64WithSeed()`][cityhash64withseed] | `base::span<const uint8_t>` | `uint64_t` | yes (note 1) | Version 1.0.3. Has some known weaknesses.
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## Cryptographic
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**There are no hashes in `//base` that provide cryptographic security.**
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name | input | output | unchanging forever | notes
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-------------------------------|---------------|---------------|--------------------|-------
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[`MD5String()`][md5string] | `std::string` | `std::string` | yes | **INSECURE**
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[`SHA1HashString`][sha1string] | `std::string` | `std::string` | yes | **INSECURE**
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## Deprecated
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> Note: CRC32, Murmur2, and Murmur3 will be listed here.
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Note 1: While CityHash is not guaranteed unchanging forever, the version used in
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Chrome is pinned to version 1.0.3.
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[hash]: https://cs.chromium.org/chromium/src/base/hash/hash.h?l=26
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[persistenthash]: https://cs.chromium.org/chromium/src/base/hash/hash.h?l=36
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[cityhash64]: https://cs.chromium.org/chromium/src/base/hash/city_v103.h?l=19
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[cityhash64withseed]: https://cs.chromium.org/chromium/src/base/hash/city_v103.h?l=20
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[md5string]: https://cs.chromium.org/chromium/src/base/hash/md5.h?l=74
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[sha1string]: https://cs.chromium.org/chromium/src/base/hash/sha1.h?l=22
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