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ISCC Structure and Format

IEP: 0001
Title: ISCC Structure and Format
Author: Titusz Pan tp@iscc.io
Comments: #6
Status: Draft
Type: Core
License: CC-BY-4.0
Created: {{ git_creation_date_localized }}
Updated: {{ git_revision_date_localized }}

!!! note

This document is a **DRAFT** contributed as input to 
[ISO TC 46/SC 9/WG 18](https://www.iso.org/committee/48836.html). The final version is 
published under [ISO 24138:2024](https://www.iso.org/standard/77899.html).

Abstract

This document describes the coding scheme for the International Standard Content Code (ISCC).

Motivation

The ISCC is a similarity preserving identifier for all kinds of digital content. As such the ISCC requires a universal coding scheme to meet a broad set of use cases and support different media types. The coding scheme for all ISCCs should be:

  1. compact
  2. interoperable
  3. modular
  4. extensible
  5. self-describing

General structure of ISCC

  1. An ISCC shall be composed of an ISCC-HEADER and an ISCC-BODY.
  2. The ISCC-HEADER shall describe the MainType, SubType, Version, and Length of its ISCC-BODY.
  3. An ISCC-UNIT shall be an ISCC based on one specific algorithm.
  4. An ISCC-CODE shall be an ISCC composed of two or more different ISCC-UNITs.
![Figure 1 - General Structure of an ISCC](images/iscc-iep-0001-structure.png) Figure 1 - General Structure of an ISCC

ISCC-HEADER

The ISCC-HEADER is a variable sized bitstream composed of an ordered sequence of the 4 header-fields MainType, SubType, Version, Length.

Each header-field is a bitstream with a length between 4 and 16 bits and encodes an integer value between 0 and 4679 with the following encoding scheme:

  1. The total bit-length of a header-field shall be determined by its prefix-bits.
  2. The prefix-bits shall be followed by data-bits.
  3. The data-bits shall be interpreted as unsigned integer values plus the maximum value of the preceding range.
  4. If the total length of all header-fields in number of bits is not divisible by 8, the header shall be padded with 4 zero bits (0000) on the right side.

Table 1 – Variable length ISCC-HEADER field encoding {: style="text-align:center; font-size:smaller" }

Prefix bits Number of nibbles Number of data bits Integer range
0 1 3 0-7
10 2 6 8-71
110 3 9 72-583
1110 4 12 584-4679

!!! example "Header-field examples"

```
0 = 0000
1 = 0001
…
7 = 0111
8 = 1000 0000
9 = 1000 0001
```

The interpretation of the integer value of a header-field shall be context dependent:

  1. For the MainType and SubType fields it shall be an identifier for the designated type.
  2. For the Version field it shall be the literal version number.
  3. For the Length field of ISCC-UNITs it shall be a number used as a multiplier to calculate the bit length of the ISCC-BODY.
  4. For the Length field of ISCC-CODEs it shall be a bit-pattern encoding the combination of ISCC-UNITs and the bit-length of the ISCC-BODY.

MainTypes

The MainType header-field shall signify the type of the ISCC.

Backward incompatible updates to an algorithm associated with a MainType shall be indicated by incrementing the version field of the ISCC-HEADER of the respective MainType.

!!! note

The first edition of the standard specifies initial algorithms (version 0) for all reserved
MainTypes except for the SEMANTIC type which is not currently defined.

Table 2 – Reserved ISCC MainTypes {: style="text-align:center; font-size:smaller" }

ID Symbol Bits Definition
0 META 0000 An ISCC-UNIT that matches on metadata similarity
1 SEMANTIC 0001 An ISCC-UNIT that matches on semantic content similarity
2 CONTENT 0010 An ISCC-UNIT that matches on perceptual content similarity
3 DATA 0011 An ISCC-UNIT that matches on data similarity
4 INSTANCE 0100 An ISCC-UNIT that matches on data identity
5 ISCC 0101 An ISCC-CODE composed of two or more headerless ISCC-UNITs for multi-modal matching

SubTypes

The MainTypes META, DATA, and INSTANCE shall have a single default SubType NONE encoded with the bits 0000.

The MainTypes SEMANTIC, CONTENT, and ISCC shall have SubTypes that signify the perceptual mode.

Table 3 – Reserved SubTypes for MainTypes ISCC, SEMANTIC, and CONTENT {: style="text-align:center; font-size:smaller" }

ID Symbol Bits Definition
0 TEXT 0000 Match on text similarity
1 IMAGE 0001 Match on image similarity
2 AUDIO 0010 Match on audio similarity
3 VIDEO 0011 Match on video similarity
4 MIXED 0100 Match on multi-modal similarity

Table 4 – Additional Reserved SubTypes for the MainType ISCC {: style="text-align:center; font-size:smaller" }

ID Symbol Bits Definition
5 SUM 0101 Composite of ISCC-UNITs including only Data- and Instance-Code
6 NONE 0110 Composite ISCC-UNITs including Meta-, Data- and Instance-Code

Version

All ISCC-HEADERs shall have a version header-field of 0000 for the first edition of the standard.

Table 5 – Reserved ISCC Versions {: style="text-align:center; font-size:smaller" }

ID Symbol Bits Definition
0 V0 0000 Initial version of ISCC-UNITs and ISCC-CODE

Length

The encoding of the Length header-field shall be specific to the MainType.

Length of ISCC-UNITs

For ISCC-UNITs of the MainTypes META, SEMANTIC, CONTENT, DATA, and INSTANCE the length value shall be encoded as the number of 32-bit blocks of the ISCC-BODY in addition to the minimum length of 32 bits.

Table 6 – Reserved length field values (multiples of 32 bit) {: style="text-align:center; font-size:smaller" }

ID Symbol Bits Definition
0 L32 0000 Length of body is 32 bits (minimum length)
1 L64 0001 Length of body is 64 bits (default length)
2 L96 0010 Length of body is 96 bits
3 L128 0011 Length of body is 128 bits
4 L160 0100 Length of body is 160 bits
5 L192 0101 Length of body is 192 bits
6 L224 0110 Length of body is 224 bits
7 L256 0111 Length of body is 256 bits

Length of ISCC-CODEs

  1. For ISCC-CODEs the length value shall designate the composition of ISCC-UNITs.

  2. The Data-Code and Instance-Code shall be mandatory 64-bit components of an ISCC-CODE.

  3. The first data-bit shall designate the presence of a 64-bit Meta-Code.

  4. The second data-bit shall designate the presence of a 64-bit Semantic-Code.

  5. The third data-bit shall designate the presence of a 64-bit Content-Code.

  6. The length of an ISCC-CODE shall be calculated as the number of active data-bits times 64 plus 128 bits of mandatory data.

Table 7 – Reserved length field values (for MainType ISCC) {: style="text-align:center; font-size:smaller" }

ID Symbol Bits Definition
0 SUM 0000 No optional ISCC-UNITs. Length of body is 128 bits.
1 CDI 0001 Includes Content-Code. Length of body is 192 bits
2 SDI 0010 Includes Semantic-Code. Length of body is 192 bits
3 SCDI 0011 Includes Semantic- and Content-Code. Length of body is 256 bits
4 MDI 0100 Includes Meta-Code. Length of body is 192 bits
5 MCDI 0101 Includes Meta-Code and Content-Code. Length of body is 256 bits
6 MSDI 0110 Includes Meta-Code and Semantic-Code. Length of body is 256 bits
7 MSCDI 0111 Includes Meta-, Semantic-, and Content-Code. Length is 320 bits

ISCC-BODY

  1. The preceding MainType, SubType, and Version fields shall qualify the semantics of an ISCC-BODY.
  2. The Length field shall determine the number of bits of an ISCC-BODY.

Encoding of ISCC

Canonical form

The printable canonical form of an ISCC shall be its RFC 4648 Base32 encoded representation without padding and prefixed with “ISCC:”.

!!! example "Canonical ISCC-CODE example"

ISCC:KEC43HJLPUSHVAZT66YLPUWNVACWYPIV533TRQMWF2IUQYSP5LA4CTY

URI encoding

  1. An ISCC shall be encodable as a Uniform Resource Identifier (URI) according to RFC 3986.
  2. The URI representation shall have the format <scheme>:<path>.
  3. The URI scheme shall be the string “iscc”.
  4. The URI path shall be the lower-cased base32 representation of an ISCC without padding.

!!! example "URI encoded ISCC-CODE example"

iscc:kec43hjlpushvazt66ylpuwnvacwypiv533trqmwf2iuqysp5la4cty

Multiformats encoding

  1. The ISCC shall be encodable as a multibase string.
  2. The multicodec identifier of an ISCC shall be 0xcc01.
  3. A Multiformat representation of an ISCC shall be prefixed with a Multibase code.
  4. The encoding scheme shall be <multibase><multicodec><iscc-header><iscc-body>.

ISCC shall support the following multibase encodings:

Table 8 – Supported multibase encodings {: style="text-align:center; font-size:smaller" }

Encoding Code Definition
base16 f hexadecimal
base32 b RFC4648 case-insensitive - no padding
base32hex v RFC4648 case-insensitive - no padding - highest char
base58btc z base58 bitcoin
base64url u RFC4648 no padding

Table 9 – Examples of ISCCs in multiformats encoding {: style="text-align:center; font-size:smaller" }

Encoding Example
MF base16 fcc015105cd9d2b7d247a8333f7b0b7d2cda8056c3d15eef738c1962e9148624feac1c14f
MF base32 bzqavcbontuvx2jd2qmz7pmfx2lg2qblmhuk655zyyglc5ekimjh6vqobj4
MF base32hex vpg0l21edjklnq93qgcpvfc5nqb6qg1bc7kauttpoo6b2t4a8c97ulge19s
MF base58btc z2Yr3BMx3Rj56fyYkNvfa19PCk4SjspQhpVWoLSGg9yXr4vUGsx
MF base64url uzAFRBc2dK30keoMz97C30s2oBWw9Fe73OMGWLpFIYk_qwcFP

Readable encoding

  1. The ISCC shall be encodable in human-readable representation.
  2. The readable representation shall encode the header-fields with their symbols and the ISCC-BODY in base16 lower-case.
  3. The header-fields and the ISCC-BODY shall be separated with hyphens.

!!! example "Example of human-readable ISCC-CODE"

ISCC-IMAGE-V0-MCDI-cd9d2b7d247a8333f7b0b7d2cda8056c3d15eef738c1962e9148624feac1c14f

Reference implementation

The reference implementation of this coding scheme is published in the iscc-core python package in the codec.py module.