initial fee service spec

CHIPs/chip-0039.md

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+| CHIP Number  | 0039                                                                  |
+| :----------- | :-------------------------------------------------------------------- |
+| Title        | Fee Service Standard                                                  |
+| Description  | A standard for fee services that attach fees when XCH isn't available |
+| Author       | [Freddie Coleman](https://github.com/freddiecoleman)                  |
+| Editor       | [Dan Perry](https://github.com/danieljperry)                          |
+| Comments-URI | [CHIPs repo, PR #138](https://github.com/Chia-Network/chips/pull/138) |
+| Status       | Draft                                                                 |
+| Category     | Standards Track                                                       |
+| Sub-Category | Primitive                                                             |
+| Created      | 2024-12-17                                                            |
+
+## Abstract
+
+This proposal describes a standard for fee services that attach fees when the spender doesn't have access to XCH. This service allows any spend bundle to have fees attached through a standardized protocol.
+
+## Motivation
+
+There are scenarios where a valid transaction cannot be submitted or will take a long time to be included in a block because there is no XCH available to pay for fees. Common scenarios include:
+
+- Coins controlled by wallets that don't have access to XCH
+- Multi-signature wallets that want to separate fee management from asset management
+- Wallets that hold CATs and NFTs but no XCH
+- Vaults that want to perform recovery or rekey operations without holding XCH
+
+Currently, these systems either need to:
+
+1. Hold XCH specifically for fees
+2. Rely on manual fee attachment
+3. Use custom, non-standardized fee services
+
+A standardized fee service interface would allow any application to request fee attachment from any compatible service, improving interoperability and reducing complexity.
+
+## Backwards Compatibility
+
+This standard introduces new service definitions but does not modify existing blockchain behavior. It is fully backwards compatible with existing spend bundles and transaction mechanisms.
+
+## Rationale
+
+The design focuses on the core functionality needed to attach fees to transactions:
+
+- Simple request/response protocol
+- Minimal required fields
+- Clear separation between fee attachment and transaction submission
+
+## Specification
+
+### Core Concepts
+
+A fee service must provide the capability to attach fees to a spend bundle. This involves:
+
+1. Receiving a spend bundle that requires fees
+2. Creating additional coin spends to provide the necessary fees
+3. Returning a new spend bundle that includes both the original spends and the fee spends
+
+### Protocol
+
+The fee attachment protocol consists of two main operations:
+
+1. Fee Attachment Request
+2. Fee Attachment Response
+
+#### Fee Attachment Request
+
+A request to attach fees must include:
+
+- Spend Bundle:
+- Network identifier (e.g. mainnet, testnet11)
+- Submit flag - whether to let the service submit the spend bundle or just return it
+
+#### Fee Attachment Response
+
+The response must include:
+
+- Modified spend bundle with fees attached
+- Amount of fees added
+- Error information if the request failed
+
+### Error Conditions
+
+Services must handle and clearly indicate the following error conditions:
+
+1. Invalid spend bundle format
+2. Unsupported network
+3. Fee calculation failures
+
+### Implementation Requirements
+
+Fee services should:
+
+1. Never modify the intention or effect of the original spend bundle
+2. Only add coin spends necessary for fee payment
+3. Provide clear documentation of:
+   - Supported networks
+   - Any rate limits or restrictions
+   - Error conditions and responses
+
+### Security Requirements
+
+Fee services should:
+
+1. Validate all input spend bundles
+2. Use secure transport (e.g. TLS)
+3. Implement appropriate rate limiting
+
+## Examples
+
+### Basic Fee Attachment Flow
+
+1. Client has a valid spend bundle without fees
+2. Client sends spend bundle to fee service
+3. Service calculates required fees
+4. Service creates new coin spends for fees
+5. Service combines original spend bundle with fee spends
+6. Service returns complete spend bundle to client
+7. Client can verify and submit the transaction
+
+### Example Spend Bundle Modification
+
+```
+# Original spend bundle:
+{
+    "coin_spends": [
+        {
+            "coin": {
+                "parent_coin_info": "0x...",
+                "puzzle_hash": "0x...",
+                "amount": 1000
+            },
+            "puzzle_reveal": "0x...",
+            "solution": "0x..."
+        }
+    ],
+    "aggregated_signature": "0x..."
+}
+
+# After fee service:
+{
+    "coin_spends": [
+        # Original spend
+        {
+            "coin": {
+                "parent_coin_info": "0x...",
+                "puzzle_hash": "0x...",
+                "amount": 1000
+            },
+            "puzzle_reveal": "0x...",
+            "solution": "0x..."
+        },
+        # Added fee spend
+        {
+            "coin": {
+                "parent_coin_info": "0x...",
+                "puzzle_hash": "0x...",
+                "amount": 100
+            },
+            "puzzle_reveal": "0x...",
+            "solution": "0x..."
+        }
+    ],
+    "aggregated_signature": "0x..."  # Updated to include fee spend
+}
+```
+
+## Test Cases
+
+1. Basic fee attachment to a simple spend bundle
+2. Fee attachment to a multiple-coin-spend transaction
+3. Handling of invalid spend bundles
+4. Verification that original transaction intent is preserved
+5. Rate limit handling
+6. Network selection validation
+
+## Security Considerations
+
+Fee services introduce a trust relationship - the service must be trusted to:
+
+1. Not front-run transactions
+2. Not leak transaction details
+3. Provide reliable fee attachment
+4. Maintain availability
+5. Not modify transaction intent
+
+Implementations should clearly document their trust model and security properties.
+
+## Copyright
+
+Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).