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+- **TEP**: [555](https://github.com/ton-blockchain/TEPs/pull/555)
+- **title**: TON Sidechain Addresses
+- **status**: Draft
+- **type**: Core
+- **authors**: thekiba, TrueCarry
+- **created**: 08.12.2024
+- **replaces**: -
+- **replaced by**: -
+
+# Summary
+
+This document extends [TEP-2](./0002-address.md) to support sidechain and L2 addresses by introducing a chain ID field in the user-friendly address format.
+
+# Motivation
+
+As TON ecosystem grows with sidechains and L2 solutions, users need a way to:
+- Distinguish addresses across different chains
+- Transfer assets between mainnet and sidechains seamlessly
+- Prevent accidental cross-chain address confusion
+
+The new format intentionally breaks compatibility with existing libraries, ensuring users cannot accidentally use sidechain addresses in mainnet-only wallets.
+
+# Specification
+
+## Network Chain IDs
+
+Reserved chain IDs for existing networks:
+
+| Network | Chain ID |
+|---------|----------|
+| Mainnet | -239 |
+| Testnet | -3 |
+
+These values are returned when parsing mainnet/testnet addresses, but are NOT encoded in the address bytes. Sidechains use explicit chain IDs encoded in the address.
+
+## Sidechain Flag
+
+A new flag `0x20` (bit 5) is added to the tag byte, following the same pattern as the testnet flag (`0x80`).
+
+| Flag | Value | Description |
+|------|-------|-------------|
+| Bounceable | 0x11 | Base tag for bounceable addresses |
+| Non-bounceable | 0x51 | Base tag for non-bounceable addresses |
+| Testnet | +0x80 | Added for testnet addresses |
+| Sidechain | +0x20 | Added for sidechain addresses |
+
+Sidechain flag and testnet flag are **mutually exclusive**. A sidechain address MUST NOT have the testnet flag set.
+
+Tag byte examples:
+- `0x11` = bounceable, mainnet
+- `0x31` = bounceable, sidechain (0x11 | 0x20)
+- `0x51` = non-bounceable, mainnet
+- `0x71` = non-bounceable, sidechain (0x51 | 0x20)
+- `0x91` = bounceable, testnet (0x11 | 0x80)
+- `0xD1` = non-bounceable, testnet (0x51 | 0x80)
+
+## Chain ID
+
+The chain ID is a **32-bit signed integer** identifying the specific 
+sidechain or L2 network. It is encoded in big-endian format.
+
+Note: Chain ID is distinct from workchain ID. Workchain ID identifies 
+a workchain within a single chain (mainnet or sidechain), while chain 
+ID identifies the chain itself.
+
+## Sidechain address format
+
+When the sidechain flag is set, the user-friendly address contains an additional chain_id field:
+
+- 1 byte: tag (with sidechain flag `0x20` set)
+- 1 byte: workchain_id (signed 8-bit integer)
+- 4 bytes: chain_id (signed 32-bit integer, big-endian)
+- 32 bytes: address hash (256 bits, big-endian)
+- 2 bytes: CRC16-CCITT of the previous 38 bytes
+
+The 40 bytes are encoded using base64url (with `_` and `-` instead of `/` and `+`), yielding 54 printable characters.
+
+The chain_id identifies the specific sidechain or L2 network. It is distinct from workchain_id, which identifies a workchain within that chain.
+
+### Raw Format (unchanged)
+
+Raw format remains unchanged for all address types:
+
+```
+<workchain_id>:<64 hexadecimal digits>
+```
+
+The raw format does NOT include chain ID. Chain information is only 
+available in the user-friendly format.
+
+## Example
+
+Consider an address with the following parameters:
+
+| Parameter | Value |
+|-----------|-------|
+| chain_id | 1 |
+| workchain_id | 0 |
+| hash | `9da971af38d2f03abdf308d5f91636a97e5a2b07a66c39d71d7cbae3b032eddc` |
+
+### Address Formats Comparison
+
+| Network | Type | Length | Address |
+|---------|------|--------|---------|
+| Mainnet | non-bounceable | 48 | `UQCdqXGvONLwOr3zCNX5FjapflorB6ZsOdcdfLrjsDLt3AF4` |
+| Mainnet | bounceable | 48 | `EQCdqXGvONLwOr3zCNX5FjapflorB6ZsOdcdfLrjsDLt3Fy9` |
+| Sidechain (chain_id=1) | non-bounceable | 54 | `cQAAAAABnalxrzjS8Dq98wjV-RY2qX5aKwembDnXHXy647Ay7dyTvw` |
+| Sidechain (chain_id=1) | bounceable | 54 | `MQAAAAABnalxrzjS8Dq98wjV-RY2qX5aKwembDnXHXy647Ay7dwSBQ` |
+
+The sidechain addresses are 54 characters (vs 48 for mainnet) because they include the 4-byte chain_id field.
+
+## Wallet applications
+
+When sending:
+
+1. The wallet app parses the tag byte to check for the sidechain flag (`0x20`).
+
+2. If the sidechain flag is set:
+   - The wallet extracts the chain_id from bytes 2-5
+   - If the wallet is connected to a different chain, it may offer to bridge assets to the target chain
+
+3. If the address has both sidechain flag and testnet flag set, the address is invalid.
+
+# Drawbacks
+
+- Longer addresses (54 vs 48 characters) may be less convenient for manual entry
+- Requires wallet updates to support the new format, but they won't support new chains without updates anyway
+
+# Rationale and alternatives
+
+- Different address length ensures old libraries fail explicitly rather than silently
+- Sidechain and testnet flags are mutually exclusive because sidechains manage their own test environments