BUD-12

Chunked blobs

draft optional

Breaking large blobs into smaller chunks for distribution

Chunking

The client MAY break large blobs into any number or size of chunks. although its recommended to use the size 1Mb or 4Mb for small and large chunks

Clients MUST NOT pad the remaining chunk, If clients need privacy they should use random chunk sizes and optionally encrypt the large blob

Clients MUST create a merkle tree using the chunk hashes as the leaf nodes

Publishing

Clients should publish a 2001 kind event after chunking the file in order to store the list of chunks

The events MUST contain an ordered list of chunk tags with the sha256 hashes of the chunks

The content field MUST be a human readable description of the chunked file

{
  "tags": [
    [ "chunk", "7e668b56a58c7891e0cf263ea3f093b75eebade23d663a45aa9920f347b3d671"],
    [ "chunk", "9b9c44a91396f19fd8700986eb0586dff2dcccf96c75bc2caefef302bcd78da1"],
    [ "chunk", "7a281548f1223664b855b10b08e59e84389ccabeb742517f6cd75eda2724a798"],
    [ "chunk", "fadeccee86b123088bbc452df10e8fbc99d4c2f22a70ef7a35605ec8e439c345"],
    [ "chunk", "5d62398419e6d136771541f3d2215e0ce31b1be45e99dbc64b43a4b734b447ca"],
  ]
}

The event MUST have an x tag with the sha256 hash of the ordered concatenated hashes of each chunk

Example using xxd and sha256sum:

#!/bin/bash
hashes=(
  "7e668b56a58c7891e0cf263ea3f093b75eebade23d663a45aa9920f347b3d671"
  "9b9c44a91396f19fd8700986eb0586dff2dcccf96c75bc2caefef302bcd78da1"
  "7a281548f1223664b855b10b08e59e84389ccabeb742517f6cd75eda2724a798"
  "fadeccee86b123088bbc452df10e8fbc99d4c2f22a70ef7a35605ec8e439c345"
  "5d62398419e6d136771541f3d2215e0ce31b1be45e99dbc64b43a4b734b447ca"
)

concatenated=""
for hex in "${hex_array[@]}"; do
  concatenated="${concatenated}${hex}"
done

echo "Chunks:"
printf '%s\n' "${hex_array[@]}"

echo -e "\nConcatenated:"
echo "$concatenated"

echo -e "\nRoot hash:"
echo "$concatenated" | xxd -r -p | sha256sum

The final chunk tags and x tag should look something like this

{
  "tags": [
    [ "chunk", "7e668b56a58c7891e0cf263ea3f093b75eebade23d663a45aa9920f347b3d671"],
    [ "chunk", "9b9c44a91396f19fd8700986eb0586dff2dcccf96c75bc2caefef302bcd78da1"],
    [ "chunk", "7a281548f1223664b855b10b08e59e84389ccabeb742517f6cd75eda2724a798"],
    [ "chunk", "fadeccee86b123088bbc452df10e8fbc99d4c2f22a70ef7a35605ec8e439c345"],
    [ "chunk", "5d62398419e6d136771541f3d2215e0ce31b1be45e99dbc64b43a4b734b447ca"],
    [ "x", "2d839865ac17d8bb10168490a88107637619f79dac21275fcec1705162581f39" ],
  ]
}

Metadata

The 2001 event MAY include additional metadata tags to help other clients know the filename, mime type or servers to download from

Metadata tags:

name Filename
mime Mime type of file
size Total size in bytes of the file
server (multiple) Recommended servers to download chunks from

Examples

Example 2001 event

{
  "id": "fd2f4da046851170576e7a09009e63f752dc469c61a24faca514b2cedfe641aa",
  "pubkey": "5328e6c743a50271745e16476590ba7ea3dc591c65f3a5e2b03430814c1dabc0",
  "created_at": 1731405194,
  "kind": 2001,
  "content": "Here is the original version",
  "tags": [
    [ "chunk", "7e668b56a58c7891e0cf263ea3f093b75eebade23d663a45aa9920f347b3d671"],
    [ "chunk", "9b9c44a91396f19fd8700986eb0586dff2dcccf96c75bc2caefef302bcd78da1"],
    [ "chunk", "7a281548f1223664b855b10b08e59e84389ccabeb742517f6cd75eda2724a798"],
    [ "chunk", "fadeccee86b123088bbc452df10e8fbc99d4c2f22a70ef7a35605ec8e439c345"],
    [ "chunk", "5d62398419e6d136771541f3d2215e0ce31b1be45e99dbc64b43a4b734b447ca"],
    [ "x", "2d839865ac17d8bb10168490a88107637619f79dac21275fcec1705162581f39" ],
    [ "name", "example.mp4" ],
    [ "mime", "video/mp4" ],
    [ "size", "4823449" ],
    [ "server", "https://cdn.example.com" ],
    [ "server", "https://nostr.download" ]
  ],
  "sig": "cd589a4ea702185c69d1fec93a7704265071b674c5708a1da04d08bcba873f5277bc0abb379cbb014852fafa2ac951ed49abbdba60028a4f02ef4e31ff2599cb"
}

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