# HardGuard25

> An open standard for human-safe identifiers.

HardGuard25 is a 25-character alphabet designed so every symbol is visually distinct in any typeface, at any size, for any reader — including those with dyslexia. Use it anywhere humans read, type, print, or say an identifier out loud.

## Alphabet

0 1 2 3 4 5 6 7 8 9 A C D F G H J K M N P R U W Y

25 characters: 10 digits + 15 letters. Regex: ^[0-9ACDFGHJKMNPRUWY]+$

## Design principle

When a letter and a digit compete for the same visual slot, the digit always wins.

## Excluded characters

- O → confusable with 0
- I → confusable with 1 and lowercase L
- L → confusable with 1 and uppercase I
- B → confusable with 8; dyslexia mirror pair with D
- S → confusable with 5
- Z → confusable with 2
- E → confusable with 3 for dyslexic readers
- Q → confusable with O; dyslexia mirror pair with P
- V → confusable with U
- T → resembles + in some contexts
- X → collides with multiplication operator

## Entropy

Each character provides 4.64 bits of entropy (log2 25).

- 4 chars: 390,625 unique IDs (small inventory, tickets)
- 6 chars: 244 million (medium businesses)
- 8 chars: 152 billion (large systems)
- 12 chars: 59.6 trillion (internal tokens)
- 16 chars: 3.55 × 10^22 (cross-system identifiers)
- 20 chars: 2.11 × 10^27 (public tokens)

## Normalization

1. Trim whitespace
2. Remove separators (hyphens, spaces, underscores, dots)
3. Uppercase all letters
4. Reject characters outside the alphabet

## Check digit (optional)

Mod-25 weighted checksum (ISO 7064 style). Catches all single-character substitution errors and most transpositions.

Algorithm: sum = Σ(index[i] × (i+1)), check = alphabet[sum % 25]

## Install

- JavaScript: npm install @snapsynapse/hardguard25
- Python: pip install hardguard25
- Go: import "github.com/snapsynapse/hardguard25/go"

## Links

- Specification: https://hardguard25.com/
- GitHub: https://github.com/snapsynapse/hardguard25
- Full spec (SPEC.md): https://github.com/snapsynapse/hardguard25/blob/main/SPEC.md
- Interactive generator: https://hardguard25.com/generator/
- npm: https://www.npmjs.com/package/@snapsynapse/hardguard25
- PyPI: https://pypi.org/project/hardguard25/

## License

- Specification: CC BY 4.0
- Code: MIT

## Author

Sam Rogers, PAICE.work PBC (paice.work)