Metadata parsing

The metadata standard covering NFTs, NFT collections, and jettons, is outlined in TON Enhancement Proposal 64 TEP-64.

On TON, entities can have three types of metadata: on-chain, semi-chain, and off-chain.

On-chain metadata: stored inside the blockchain, including the name, attributes, and image.
Off-chain metadata: stored using a link to a metadata file hosted outside the chain.
Semi-chain metadata: a hybrid approach that allows storing small fields such as names or attributes on the blockchain while hosting the image off-chain and storing only a link to it.

Snake data encoding

The Snake encoding format allows part of the data to be stored in a standardized cell, while the remaining portion is stored in a child cell recursively. The Snake encoding format must be prefixed using the 0x00 byte. The TL-B scheme:

tail#_ {bn:#} b:(bits bn) = SnakeData ~0;
cons#_ {bn:#} {n:#} b:(bits bn) next:^(SnakeData ~n) = SnakeData ~(n + 1);

The Snake format stores additional data in a cell when the data exceeds the maximum size that a single cell can store. It does this by placing part of the data in the root cell and the remaining portion in the first child cell, continuing recursively until all data is stored.

Below is an example of Snake format encoding and decoding in TypeScript:

export function makeSnakeCell(data: Buffer): Cell {
  const chunks = bufferToChunks(data, 127)

  if (chunks.length === 0) {
    return beginCell().endCell()
  }

  if (chunks.length === 1) {
    return beginCell().storeBuffer(chunks[0]).endCell()
  }

  let curCell = beginCell()

  for (let i = chunks.length - 1; i >= 0; i--) {
    const chunk = chunks[i]

    curCell.storeBuffer(chunk)

    if (i - 1 >= 0) {
      const nextCell = beginCell()
      nextCell.storeRef(curCell)
      curCell = nextCell
    }
  }

  return curCell.endCell()
}

export function flattenSnakeCell(cell: Cell): Buffer {
  let c: Cell | null = cell;

  const bitResult = new BitBuilder();
  while (c) {
    const cs = c.beginParse();
    if (cs.remainingBits === 0) {
      break;
    }

    const data = cs.loadBits(cs.remainingBits);
    bitResult.writeBits(data);
    c = c.refs && c.refs[0];
  }

  const endBits = bitResult.build();
  const reader = new BitReader(endBits);

  return reader.loadBuffer(reader.remaining / 8);
}

The 0x00 byte prefix is not always required in the root cell when using the Snake format, such as with off-chain NFT content. Additionally, cells are filled with bytes instead of bits to simplify parsing. To prevent issues when adding a reference in a child cell after it has already been written to its parent cell, the Snake cell is constructed in reverse order.

Chunked encoding

The chunked encoding format stores data using a dictionary structure, mapping a chunk_index to a chunk. Chunked encoding must be prefixed with the 0x01 byte. The TL-B scheme:

chunked_data#_ data:(HashMapE 32 ^(SnakeData ~0)) = ChunkedData;

Below is an example of chunked data decoding in TypeScript:

interface ChunkDictValue {
  content: Buffer;
}
export const ChunkDictValueSerializer = {
  serialize(src: ChunkDictValue, builder: Builder) {},
  parse(src: Slice): ChunkDictValue {
    const snake = flattenSnakeCell(src.loadRef());
    return { content: snake };
  },
};

export function ParseChunkDict(cell: Slice): Buffer {
  const dict = cell.loadDict(
    Dictionary.Keys.Uint(32),
    ChunkDictValueSerializer
  );

  let buf = Buffer.alloc(0);
  for (const [_, v] of dict) {
    buf = Buffer.concat([buf, v.content]);
  }
  return buf;
}

NFT metadata attributes

Attribute	Type	Requirement	Description
`uri`	ASCII string	optional	A URI pointing to the JSON document with metadata used by the "Semi-chain content layout."
`name`	UTF8 string	optional	Identifies the asset.
`description`	UTF8 string	optional	Describes the asset.
`image`	ASCII string	optional	A URI pointing to a resource with a MIME type image.
`image_data`	binary*	optional	Either a binary representation of the image for the on-chain layout or base64 for the off-chain layout.

Jetton metadata attributes

uri - Optional. Used by "Semi-chain content layout". ASCII string. A URI pointing to JSON document with metadata.
name - Optional. UTF8 string. Identifies the asset.
description - Optional. UTF8 string. Describes the asset.
image - Optional. ASCII string. A URI pointing to a resource with a mime type image.
image_data - Optional. Either a binary representation of the image for onchain layout or base64 for offchain layout.
symbol - Optional. UTF8 string. The symbol of the token - e.g. "XMPL". Used in the form "You received 99 XMPL".
decimals - Optional. If not specified, 9 is used by default. UTF8 encoded string with number from 0 to 255. The number of decimals the token uses - e.g. 8, means that the token amount must be divided by 100000000 to get its custom representation.
amount_style - Optional. Necessary for external applications to understand the format of displaying the number of tokens.

"n" - Displays the number of jettons as-is. For example, if a user has 100 tokens with 0 decimals, it displays "100 tokens".
"n-of-total" - Displays the number of jettons relative to the total supply. If totalSupply = 1000 and a user holds 100 jettons, it is displayed as "100 of 1000".
"%" - Displays jettons as a percentage of the total supply. If totalSupply = 1000 and a user holds 100 jettons, it is displayed as "10%".

render_type - Optional. Required by external applications to understand which group a token belongs to and how to display it.

"currency" - Displays as a currency (default value).
"game" - Displays as an NFT while also considering the amount_style.

Attribute	Type	Requirement	Description
`uri`	ASCII string	optional	A URI pointing to the JSON document with metadata used by the "Semi-chain content layout."
`name`	UTF8 string	optional	Identifies the asset
`description`	UTF8 string	optional	Describes the asset
`image`	ASCII string	optional	A URI pointing to a resource with a mime type image
`image_data`	binary*	optional	Either a binary representation of the image of an on-chain layout or base64 for off-chain layout
`symbol`	UTF8 string	optional	Token symbol - for example, "XMPL" and uses the form "You have received 99 XMPL"
`decimals`	UTF8 string	optional	The number of decimal places used by the token. If not specified, the default is 9. A UTF8-encoded string with numbers from 0 to 255. - for example, 8 means that the token amount must be divided by 100000000 to get its custom representation.
`amount_style`		optional	Required by external applications to understand the format of displaying the number of tokens. Defined using n, n-of-total, %.
`render_type`		optional	Needed by external applications to understand what group a token belongs to and how to display it. "currency" - displays as currency (default). "game" - display used for games, displays as NFT, but also displays the number of tokens and respects the amount_style value.

amount_style parameters:

n - number of jettons (default value). If the user has 100 tokens with 0 decimals, then it displays that the user has 100 tokens.
n-of-total - the number of jettons out of the total number of issued jettons. For example, if the totalSupply of jettons is 1000 and the user has 100 jettons in their wallet, it must be displayed in the user's wallet as 100 of 1000 or in another textual or graphical way to demonstrate the ratio of user tokens to the total amount of tokens available.
% - the percentage of jettons from the total number of jettons issued. For example, if the total number of tokens is 1000 and the user has 100 tokens, the percentage should be displayed as 10% of the user's wallet balance (100 ÷ 1000 = 0.1 or 10%).

render_type parameters:

currency - displayed as a currency (default value).
game - display used for games that appears as an NFT but also displays the number of tokens and takes into account the amount_style value.

Parsing metadata

To parse metadata, NFT data must first be obtained from the blockchain. To better understand this process, consider reading the Retrieving NFT Data section of our TON asset processing documentation section.

After on-chain NFT data is retrieved, it must be parsed. To carry out this process, the NFT content type must be determined by reading the first byte that makes up the inner workings of the NFT.

Off-chain

f the metadata byte string starts with 0x01, it signifies off-chain NFT content. The remaining portion of the NFT content is decoded using the Snake encoding format as an ASCII string. Once the NFT URL is obtained and the NFT identification data is retrieved, the process is complete. Here is an example of a URL using off-chain NFT content metadata parsing:: https://s.getgems.io/nft/b/c/62fba50217c3fe3cbaad9e7f/95/meta.json

URL contents (from directly above):

{
   "name": "TON Smart Challenge #2 Winners Trophy",
   "description": "TON Smart Challenge #2 Winners Trophy 1 place out of 181",
   "image": "https://s.getgems.io/nft/b/c/62fba50217c3fe3cbaad9e7f/images/943e994f91227c3fdbccbc6d8635bfaab256fbb4",
   "content_url": "https://s.getgems.io/nft/b/c/62fba50217c3fe3cbaad9e7f/content/84f7f698b337de3bfd1bc4a8118cdfd8226bbadf",
   "attributes": []
}

On-chain and semi-chain

If the metadata byte string starts with 0x00, it indicates on-chain or semi-chain NFT metadata.

The metadata is stored in a dictionary where the key is the SHA256 hash of the attribute name, and the value is the data stored using the Snake or Chunked format.

To determine the NFT type, a developer must read known NFT attributes such as uri, name, image, description, and image_data. If the uri field is present within the metadata, it indicates a semi-chain layout, requiring the off-chain content specified in uri to be downloaded and merged with the dictionary values.

Examples:

On-chain NFT: EQBq5z4N_GeJyBdvNh4tPjMpSkA08p8vWyiAX6LNbr3aLjI0

Semi-chain NFT: EQB2NJFK0H5OxJTgyQbej0fy5zuicZAXk2vFZEDrqbQ_n5YW

On-chain jetton master: EQA4pCk0yK-JCwFD4Nl5ZE4pmlg4DkK-1Ou4HAUQ6RObZNMi

On-chain NFT parser: stackblitz/ton-onchain-nft-parser

Important notes on NFT metadata

For NFT metadata, the name, description, and image(or image_data) fields are required to display the NFT.
For jetton metadata, the name, symbol, decimals and image(or image_data) fields are primary.
Anyone can create an NFT or Jetton using any name, description, or image. To prevent scams and confusion, apps should clearly distinguish NFTs from other assets.
Some items may include a video field linking to video content associated with the NFT or jetton.

References

TON Enhancement Proposal 64 (TEP-64)

Snake data encoding​

Chunked encoding​

NFT metadata attributes​

Jetton metadata attributes​

Parsing metadata​

Off-chain​

On-chain and semi-chain​

Important notes on NFT metadata​

References​

See Also​