Understanding Policy Scripts

Comprehensive guide to Cardano policy scripts, their structure, components, and best practices when minting tokens with the Anvil API.

Introduction

Policy scripts are a fundamental component of the Cardano blockchain that define who has permission to mint or burn assets under a specific policy ID. These scripts create the cryptographic foundation of your NFT or token collection and establish the rules for minting authority. This guide explains how policy scripts work, their structure, and best practices for implementation.

What is a Policy Script?

A policy script is a logical expression that defines the conditions under which tokens can be minted or burned under a specific policy ID. The policy ID is derived by hashing the policy script, creating a unique identifier for your asset collection on the blockchain.

When included in a minting or burning transaction, policy scripts are evaluated by the Cardano network to verify that the transaction satisfies all conditions before allowing new assets to be created or burned.

For a detailed, step-by-step guide on creating policy scripts using the Anvil API, see our Creating Policy Scripts guide.

Policy Script Structure

Policy scripts in the Anvil API follow a specific JSON structure that maps to Cardano's native script format. Each script uses a discriminated union with a type field that determines the script's behavior:

// Base structure for all policy scripts
{
  "type": "sig" | "before" | "after" | "all" | "any" | "atLeast",
  // Additional properties based on the type
}

The underlying zod schema validates the following structures:

// Main Policy Script Schema
export const policyScriptSchema = z.discriminatedUnion("type", [
  // Signature requirement
  z.object({ type: z.literal("sig"), keyHash: z.string() }),
  
  // Time constraints
  z.object({ type: z.enum(["before", "after"]), slot: z.number() }),
  
  // Logical AND/OR
  z.object({ 
    type: z.enum(["any", "all"]), 
    scripts: z.array(/*recursive policy scripts*/) 
  }),
  
  // At least N of M
  z.object({ 
    type: z.literal("atLeast"), 
    required: z.number(),
    scripts: z.array(/*recursive policy scripts*/) 
  }),
]);

Script Types

Signature Scripts

Signature scripts (sig) require a specific key to sign the transaction. This is the most common constraint used to ensure only authorized parties can mint tokens:

{
  "type": "sig",
  "keyHash": "KEY_HASH_HERE"
}

The keyHash is the hash of the verification key that must sign the transaction.

Time Constraints

Time constraints limit minting to specific time windows using slot numbers:

{
  "type": "before",
  "slot": 100000000
}

or

{
  "type": "after",
  "slot": 50000000
}

  • before: The transaction must be submitted before the specified slot number

  • after: The transaction must be submitted after the specified slot number

Logical Operators

In Simple Terms: all means "AND" (everything must be true), any means "OR" (at least one thing must be true). So if you want to allow either Alice OR Bob to sign, use any. If you need both Alice AND Bob to sign, use all.

Logical operators combine multiple constraints:

{
  "type": "all",
  "scripts": [
    // Array of other scripts that must ALL evaluate to true
  ]
}

or

{
  "type": "any",
  "scripts": [
    // Array of other scripts where at least ONE must evaluate to true
  ]
}

Threshold Requirements

The atLeast operator allows you to specify a minimum number of conditions that must be met:

{
  "type": "atLeast",
  "required": 2,
  "scripts": [
    // Array of scripts where at least 'required' number must evaluate to true
  ]
}

This is particularly useful for multi-signature scenarios where you might want to require, for example, at least 2 out of 3 possible signers to authorize minting.

Best Practices

Security Considerations

  1. Always Include a Signature Requirement: While technically optional, you should always include at least one signature constraint in your policy scripts for security. A policy without signature requirements allows anyone to mint tokens with your policy ID.

  2. Time-Limited Policies: For NFT collections, consider implementing time-limited minting windows using both signature and time constraints:

    {
  "type": "all",
  "scripts": [
    {
      "type": "sig",
      "keyHash": "KEY_HASH_HERE"
    },
    {
      "type": "before",
      "slot": 12345678
    }
  ]
}

    This pattern ensures your collection becomes immutable after a certain date, which is important for NFT value and trust.

  3. Multi-Signature Requirements: For projects with multiple stakeholders, consider using nested logical operators:

    {
  "type": "all",
  "scripts": [
    {
      "type": "any",
      "scripts": [
        { "type": "sig", "keyHash": "OWNER1_KEY_HASH_HERE" },
        { "type": "sig", "keyHash": "OWNER2_KEY_HASH_HERE" }
      ]
    },
    { "type": "before", "slot": 12345678 }
  ]
}

Implications for Token Operations

Time-Limited Policies and Token Operations

When using time-limited policies (with a before constraint), understand the implications for various operations:

  1. Minting New Tokens:

    • Only possible before the specified slot time

    • Attempting to mint after this time will fail validation

    • Keep track of your policy's deadline to avoid transaction failures

  2. Burning Tokens:

    • Requires the same policy script verification as minting

    • Cannot burn tokens after the time limit expires

    • To burn tokens, use a negative quantity in your mint request

  3. Metadata Updates:

    • On-chain metadata (label 721) cannot be modified after being written

    • For updatable NFTs, consider using CIP-68 which stores data in datums

    • Time-limitations affect your ability to issue updated versions of tokens

Important: If your use case requires future updates to tokens, avoid time-limited policies or ensure the time window is sufficient for your project's lifecycle.

Using Policy Scripts with the Anvil API

When to Include Policy Scripts

The policyScripts object in your Anvil API request is:

  • Required: When minting tokens with a new policy ID that hasn't been used before

  • Optional: When minting additional tokens with an existing policy ID

Performance Note: Including the policy script even for existing policies improves performance. Without it, the Anvil API will automatically fetch the script from the blockchain in the background, which adds processing time to your request.

Script Verification

When you provide a policy script, the Anvil API performs several validations:

  1. The policy script is parsed and its hash is computed

  2. The computed hash is compared to the policy ID specified in the policyId field

  3. If the hashes don't match, the transaction is rejected with a Policy script and policy Id do not match error

Example API Usage

{
  "changeAddress": "CHANGE_ADDRESS_HERE",
  "utxos": "UTXOS_HERE",
  "mint": {
    "assets": [
      {
        "version": "cip25",
        "assetName": { "name": "mynft", "format": "utf8" },
        "metadata": { /* Use https://metadraft.io to generate accurate metadata easily. */ }, 
        "policyId": "POLICY_ID_HERE",
        "quantity": 1
      }
    ],
    "policyScripts": {
      "POLICY_ID_HERE": {
        "type": "all",
        "scripts": [
          {
            "type": "sig",
            "keyHash": "KEY_HASH_HERE"
          },
          {
            "type": "before",
            "slot": 100000000
          }
        ]
      }
    }
  }
}

Common Errors and Troubleshooting

  • Unable to retrieve script for policy: The policy doesn't exist on-chain and wasn't provided in the request

  • Policy script and policy ID do not match: The provided script's hash doesn't match the specified policy ID

  • Missing policy scripts: Required for first-time minting but wasn't included in the request

References

PreviousNFT & FTNextCreating Policy Scripts

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