> For the complete documentation index, see [llms.txt](https://zerc20.gitbook.io/zerc20/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://zerc20.gitbook.io/zerc20/for-developers/specs.md).

# Technical Specifications

Detailed technical specifications for zERC20 components.

## Specifications

* [Contract Spec](/zerc20/for-developers/specs/contract-spec.md) — Smart contract interfaces and behaviors
* [ZKP Spec](/zerc20/for-developers/specs/zkp-spec.md) — Zero-knowledge circuit specifications
* [ICP Storage Spec](/zerc20/for-developers/specs/icp-storage-spec.md) — Stealth messaging layer on Internet Computer

## Quick Reference

### Value Constraints

| Constraint         | Value                    | Reason                            |
| ------------------ | ------------------------ | --------------------------------- |
| Max transfer value | 2^248 - 1                | Must fit in BN254 scalar field    |
| Address size       | 160 bits                 | Ethereum address width            |
| Merkle tree depth  | 40 (local) / 46 (global) | Per-token and global trees        |
| PoW difficulty     | 16 bits                  | Burn address collision resistance |

### Proof Types

| Proof Type      | Circuit | Use Case                          |
| --------------- | ------- | --------------------------------- |
| Root Transition | Nova    | Proving new transfer roots        |
| Batch Withdraw  | Nova    | Multiple withdrawals in one proof |
| Single Withdraw | Groth16 | Single withdrawal (faster)        |

### Key Contracts

| Contract | Main Functions                                    |
| -------- | ------------------------------------------------- |
| zERC20   | `transfer`, `teleport`, `mint`, `burn`            |
| Verifier | `proveTransferRoot`, `teleport`, `singleTeleport` |
| Hub      | `broadcast`, `registerToken`                      |


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