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Created K-Anonymity (markdown)

Ajay Ramachandran 2021-07-01 01:46:43 -04:00
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K-Anonymity.md Normal file

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Segments are fetched from the server using a k-anonymity system [originally proposed](https://github.com/ajayyy/SponsorBlockServer/issues/25) by [Girst](https://github.com/girst) and [implemented](https://github.com/ajayyy/SponsorBlockServer/pull/127) by [Zegnat](https://github.com/Zegnat) and [Joe-Dowd](https://github.com/Joe-Dowd).
Every time you watch a video, it takes the videoID and hashes it, producing a value that looks like `5f6b0b4e201f2a7e66927abb5cadeec81624dcc8efe6644b78aa182213f653a2`. Instead of sending the videoID to the server, it sends only the first 4 characters of that hash, `5f6b`.
```
https://sponsor.ajay.app/api/skipSegments/5f6b
```
The client then gets a list of possible videos that have that prefix. This list may or may not contain the videoID
<details>
<summary> Click to see the result </summary>
```json
[
{
"videoID":"ifI_fwg55k8",
"hash":"5f6b9697b907622242a43ac766a0735840efe62a93249dd00e4cd2bd3e29c6c2",
"segments":[
{
"category":"intro",
"segment":[
0,
2.811777
],
"UUID":"1db91efbce60d754d1cc86687202701741c8f31ee239f75d88979e29ecd9be03",
"videoDuration":1163
},
{
"category":"intro",
"segment":[
24.182026,
39.86125
],
"UUID":"966d6ee4f37811e790bc902eff1650c912e6e148e85a150730c15a22bffa8878",
"videoDuration":1163
}
]
},
{
"videoID":"rPA_nqPHjXo",
"hash":"5f6b2a774993bd13c38ea79d53d341f9246e1fc564d2e33c08014262690746de",
"segments":[
{
"category":"outro",
"segment":[
0,
145
],
"UUID":"5d5184386913bd10021f5bccff54c26441cc88479caf33c087710b12fcc887a7",
"videoDuration":1828
},
{
"category":"outro",
"segment":[
1705.593,
1827.661
],
"UUID":"1829807614e13b0cb81e8aa9ac9ed23892ce66dd300c64c40e2b4f77f5a14070",
"videoDuration":1828
}
]
},
{
"videoID":"6qc2GsH_ii4",
"hash":"5f6b3279fbf3a40cd1eefe71aa9e1812a3957b44d0d88f6f23b8bb7d8db857a0",
"segments":[
{
"category":"intro",
"segment":[
0,
27.439728
],
"UUID":"9ee28368f6f450d14fd9cede79dcf6b6ceff04ddb7eb48ca6f12120e19304139",
"videoDuration":1790
}
]
},
{
"videoID":"dQw4w9WgXcQ",
"hash":"5f6b0b4e201f2a7e66927abb5cadeec81624dcc8efe6644b78aa182213f653a2",
"segments":[
]
},
{
"videoID":"auBq3fZ-WAM",
"hash":"5f6b5627fa138974690be4333b8f53fb3e76e2dc70dd661e9d82c7c5854aaceb",
"segments":[
{
"category":"intro",
"segment":[
0,
6.9
],
"UUID":"e0a750b81efc389b2a69f8a4a62926f4f8d2cb7acc37aff110b5b43370706b11",
"videoDuration":0
}
]
},
{
"videoID":"YRQuvRjXXPc",
"hash":"5f6b3d10cc6f79f099014ac4dc4aad4a43f648f25397a1a481ccae9a37cca644",
"segments":[
{
"category":"intro",
"segment":[
0,
12
],
"UUID":"7b590105e87bfcecfb3c9524c7ec889bde92a1486b7fafd08309531bec408460",
"videoDuration":0
}
]
},
{
"videoID":"5GKIUKsrnKo",
"hash":"5f6bbb7cc756d761ab675469743142c0b27d0e1b2436309410201cc3c86ba2c9",
"segments":[
{
"category":"intro",
"segment":[
0,
4.177
],
"UUID":"975f0840028e7ce21e9608a9dcc2a2a529a079b68eca636da87259d7e3cba80a",
"videoDuration":0
},
{
"category":"sponsor",
"segment":[
80.93964,
145.15292
],
"UUID":"6bb6d1cbf85c7fa65ad5c08acdceb6c4ebb7720f2f794e51e38caf20fad83853",
"videoDuration":0
}
]
},
{
"videoID":"aQp-iM0bCs4",
"hash":"5f6bcdd946838ebf572b0526cb29dc557918674e7ff650e33d8a78893f4aabb2",
"segments":[
{
"category":"sponsor",
"segment":[
0,
31.861
],
"UUID":"b772d4b75ca73ce63940fa982d051c031964160941efd93bd6ed1bbe9327a46e",
"videoDuration":361
}
]
},
{
"videoID":"thODYqd9G9U",
"hash":"5f6bb87827fd5c93e44650defe57247c5965fac3c2d26068c3aae770436b83a7",
"segments":[
{
"category":"sponsor",
"segment":[
0.332106,
22.484015
],
"UUID":"5f498eade41a1fbb46aee46d94fdc6fff5ad3b1d9f5046720c75ad6a28bdfd8b",
"videoDuration":0
}
]
},
{
"videoID":"5t6saOlOTZE",
"hash":"5f6bb71ba929d4395f0c25f964f010e103de517f3adb269b22d0cb939bd06550",
"segments":[
{
"category":"sponsor",
"segment":[
171.933,
255.06
],
"UUID":"33ef5c921624914930ffb0b37e1182abe938f2ff44a7fdeeafd7e8e5ba6f608b",
"videoDuration":0
},
{
"category":"selfpromo",
"segment":[
438.051,
476.568
],
"UUID":"e0f8c2416494211200774f74dea45e0dcad62eead20217a7582fbb3ece395f3d",
"videoDuration":0
}
]
},
{
"videoID":"HXzMwe8PZBA",
"hash":"5f6b33fd117ef95e62f7fe07c9fa1a7667147994081a70cb933941a8622d589f",
"segments":[
{
"category":"outro",
"segment":[
449.84006,
459.581
],
"UUID":"cbd9fd2701548277e43cbb83fc839b5679a525a30ee794282958ef3036a8741e",
"videoDuration":460
}
]
}
]
```
</details>
The client then gets back these 11 videos (in this case) and checks to see if the video it is looking for is in the list. If it is not, it treats it as not found. Since the list may, or may not contain the video, this makes it even more secure than a 1 / 11 chance.
This system allows the client to be lightweight and always get the latest segments on every video while ensuring the server cannot predict which videos they are watching.