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+---
+tip: 51
+title: ISC request Output metadata encoding
+description: describes the encoding to send request outputs to an ISC chain
+author: Jorge Silva (@jorgemmsilva) <jorge.silva@iota.org>
+status: Draft
+type: Standards
+layer: L2 Smart Contracts
+created: 2023-09-27
+requires: None
+replaces: None
+---
+
+## Summary
+
+This document describes the encoding of an ISC request to be included in a request output Metadata feature.
+
+## Motivation
+
+In order to interact with ISC chains using L1 output requests, the metadata for a request must be specified in the Metadata feature of the created output.
+
+## Specification
+
+(LE stands for Little Endian, BE stands for Big Endian.)
+
+### Request Metadata
+
+Encoding of the request metadata follows the following structure, in strict order:
+
+| Description      | Type             | Encoding                                                                           |
+| ---------------- | ---------------- | ---------------------------------------------------------------------------------- |
+| SenderContract   | ContractIdentity | [custom](./###ContractIdentity)                                                    |
+| TargetContract   | HName (uint32)   | LE encoding uint32 (4 bytes)                                                       |
+| TargetEntryPoint | HName (uint32)   | LE encoding uint32 (4 bytes)                                                       |
+| GasBudget        | uint64           | LE encoding uint64 (8 bytes) - Value +1 <br/>(1 must be encoded as 2, 3 as 4 ,etc) |
+| Params           | Dictionary       | [custom](./###Dictionary)                                                          |
+| Allowance        | Assets           | [custom](./###Assets)                                                              |
+
+
+### Hname
+
+The hnames can be calculated from: Blake2B Hash of an UTF-encoded string, then taking the initial 4 bytes as a LE encoded uint32:
+
+```go
+Blake2B("accounts") // yields 0x3c4b5e02.....
+```
+
+### ContractIdentity
+
+Is used to identify a contract on a given chain (for the time being, contracts can only be of type `EVM` or `ISC`).
+
+When sending requests from non-chain identities (regular user wallet), the `ContractIdentity` should always be `null`.
+
+There are 3 types of `ContractIdentity`:
+
+- `null` - encoded as a single Kind `0` byte
+- `EVM` - encoded as a Kind byte `1`, followed by the EVM address (20 bytes)
+- `ISC` - encoded as a kind byte `2`, followed by an uint32 LE encoding (4 bytes)
+
+### Dictionary
+
+A dictionary is a set of Key/Value pairs.
+Must be encoded in the following way:
+
+- Length Prefix (number of kv pairs) using [Optimized Size Encode](https://github.com/iotaledger/wasp/blob/c362291b053f70c9b14a16961dd74b3b4176bba5/packages/util/rwutil/convert.go#L108-L137)
+- Each kv pair: [
+  - `Key` bytes, prefixed with size in [Optimized Size Encode](https://github.com/iotaledger/wasp/blob/c362291b053f70c9b14a16961dd74b3b4176bba5/packages/util/rwutil/convert.go#L108-L137)
+  - \+ `Value` bytes prefixed with size in [Optimized Size Encode](https://github.com/iotaledger/wasp/blob/c362291b053f70c9b14a16961dd74b3b4176bba5/packages/util/rwutil/convert.go#L108-L137)]
+
+### Assets
+
+The Assets encoding starts with a bitmask:
+
+```go
+hasBaseTokens   = 0x80
+hasNativeTokens = 0x40
+hasNFTs         = 0x20
+```
+
+followed by the 3 optional parts, in strict order:
+
+- Base Tokens - uint64 - LE encoding
+- Native Tokens:
+  - length prefix (number of different tokens - uint16 LE encoding)
+  - \+ each [TokenID (38 bytes) + amount ([uint256 special encoding](./###Uint256))]
+- NFTs
+  - length prefix (number of NFTs - uint16 LE encoding)
+  - \+ the bytes off all the NFTIDs (32 bytes each)
+
+### Uint256
+
+Uint256 values are encoded using BE and unused bytes removed. Must prefixed by the length of the encoded bytes using the [Optimized Size Encode](https://github.com/iotaledger/wasp/blob/c362291b053f70c9b14a16961dd74b3b4176bba5/packages/util/rwutil/convert.go#L108-L137).
+
+## Example
+
+Follows an example of a deposit from L1 to an EVM address on a target chain:
+
+```go
+{
+	SenderContract: nil,
+	TargetContract: 1011572226 ( = 0x3c4b5e02),
+	EntryPoint: 603251617 ( = 0x23f4e3a1),
+	GasBudget: 10000,
+	Params: [
+		{ k: "a", v: EthereumAgentID( 
+                    address: 0xE913CAc59E0bA840039aDD645D5df83C294CC230, 
+                    chainID: 0xe14c3499349cb8d2fd771e09829883e4ecfae02e6b09c9b6a0fb3c7504b4e2f4)
+                },
+	],
+	Allowance: {
+		BaseTokens:0,
+		NativeTokens:[
+                    {
+                        ID: 0x08e14c3499349cb8d2fd771e09829883e4ecfae02e6b09c9b6a0fb3c7504b4e2f40100000000, 
+                        Amount: 50
+                    }
+                ],
+		NFTs: [],
+	},
+}
+```
+
+The `TargetContract` is `"accounts"` and `TargetEntrypoint` is `"transferAllowanceTo"`.
+
+The `EthereumAgentID` is encoded as: kind byte of value 3 + chainID bytes + ethereumAddress bytes.
+
+results in the following metadata:
+
+```hex
+0x00025e4b3ca1e3f423914e0101613503e14c3499349cb8d2fd771e09829883e4ecfae02e6b09c9b6a0fb3c7504b4e2f4e913cac59e0ba840039add645d5df83c294cc230400108e14c3499349cb8d2fd771e09829883e4ecfae02e6b09c9b6a0fb3c7504b4e2f401000000000132
+```
+
+## Libraries
+
+The Wasp repo already provides function to Encode/Decode request metadata (in Go).
+
+A javascript library should be produced to be re-used across client applications.
+
+## Copyright
+
+Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).