Launching a Gasless Transaction Experience with Session Keys

Gasless transactions allow users to interact with dApps without paying blockchain gas fees directly. Instead, a relayer or paymaster sponsored by the dApp or a third party covers the cost. This removes a major barrier to entry, especially for new users unfamiliar with holding native tokens for gas. However, each transaction still requires a user signature, which can be disruptive. This is where session keys come into play. They are temporary, limited-authority cryptographic keys that can sign transactions on a user's behalf for a predefined session, eliminating the need for repeated wallet pop-ups.

A session key is a smart contract wallet feature, often implemented via EIP-4337 Account Abstraction or specific SDKs like ZeroDev or Biconomy. The user signs a single, high-level "session" message that grants a temporary key (the session key) permission to execute specific actions, such as interacting with a particular contract or spending up to a set token amount. This delegation is time-bound and revocable. The session key, managed by the dApp's frontend, can then sign subsequent transactions without further user intervention, while the paymaster handles gas fees, creating a completely seamless experience.

Implementing this requires a few key components. First, you need an ERC-4337 smart account for the user, which can be created using providers like Stackup, Alchemy, or Pimlico. Second, you integrate a paymaster to sponsor gas; many providers offer testnet credits to start. Third, you generate a session key pair on the client side and have the user's main account sign a message granting it permissions via a specific session key module in their smart account. Finally, your dApp uses this session key to sign and send UserOperation objects through a bundler.

Here is a conceptual code snippet for setting up a session with the ZeroDev SDK:

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import { createSessionKey } from '@zerodev/sdk';

// 1. Create a session key module with specific permissions
const sessionModule = await createSessionKey({
  sessionKeyAddress: sessionPublicKey,
  permissions: [
    {
      target: gameContractAddress,
      valueLimit: ethers.parseEther('0.1'),
      functionSelector: 'playTurn(uint256)',
    }
  ],
  expiry: Math.floor(Date.now() / 1000) + 3600, // 1 hour session
});

// 2. Have the user's smart account enable the module
const op = await kernelClient.sendUserOperation({
  calls: [{
    to: kernelClient.address,
    value: 0n,
    data: encodeEnableSessionKey(sessionModule),
  }],
});

Use cases for this pattern are extensive. Gaming dApps can let players make multiple in-game moves in a single session. Social platforms can enable seamless posting or liking. Trading interfaces can allow for rapid, batch orders. The key is to balance convenience with security by strictly limiting the session key's permissions—defining allowed contracts, spend limits, and a short expiry time. Always inform users clearly about what powers they are delegating. For production use, audit your session key logic and consider using multi-chain paymasters if your dApp operates across several networks.

To get started, experiment on a testnet. Use the account-kit from Alchemy or Biconomy's dashboard to quickly prototype a gasless flow with session keys. The goal is to make blockchain interactions feel as smooth as web2, without compromising on user sovereignty or security. This combination of gas sponsorship and temporary delegation is becoming a standard for competitive dApp UX.

To implement session keys, you need a development environment capable of deploying and interacting with smart accounts. The most common setup involves using ERC-4337 (Account Abstraction) via a bundler service. You will need Node.js (v18+), a package manager like npm or yarn, and a code editor such as VS Code. For testing, you should have access to a blockchain node or RPC endpoint; services like Alchemy, Infura, or a local Anvil instance from Foundry are suitable. Basic familiarity with TypeScript/JavaScript and smart contract development (Solidity) is assumed.

The core concept enabling session keys is account abstraction, which separates transaction execution from fee payment. Instead of an Externally Owned Account (EOA), a smart contract account acts as the user's primary wallet. This account can be programmed with custom logic, such as delegating signing authority to a session key for a limited time or set of actions. You should understand the roles of the EntryPoint, Bundler, Paymaster, and Smart Account in the ERC-4337 architecture, as these components are necessary for gas sponsorship and transaction relaying.

You will need to choose a Smart Account implementation and a Session Key library. Popular options include Safe{Core} Account Abstraction Kit, ZeroDev Kernel, Biconomy Smart Account, or building a custom account using the Solidity reference implementations. For session keys, libraries like ERC-7579 (Modular Account Standard) or specific SDKs from Alchemy (Account Kit), ZeroDev, or Biconomy provide abstractions. Install the necessary SDKs, such as @account-abstraction/sdk or provider-specific packages, and ensure you have test ETH on a supported network like Sepolia or Polygon Amoy for deployment.

ERC-4337 is an Ethereum standard that enables account abstraction without requiring consensus-layer changes. It introduces a new transaction type called a UserOperation, which is bundled and executed by a network of bundlers. This allows smart contract wallets, known as account contracts, to define their own logic for transaction validation and execution. Users can now enjoy features like gas sponsorship, batch transactions, and session keys, moving beyond the limitations of traditional Externally Owned Accounts (EOAs). The standard is deployed at 0x5FF137D4b0FDCD49DcA30c7CF57E578a026d2789 on mainnet.

A paymaster is a smart contract that can sponsor gas fees for users. It is a core component of the ERC-4337 architecture, interacting with the EntryPoint contract. When a user submits a UserOperation, they can specify a paymaster. The paymaster can then pay for the transaction's gas costs, either as a promotional service, in exchange for a fee paid in a different token, or based on custom business logic. This enables true gasless transactions for end-users. Popular implementations include the VerifyingPaymaster for whitelisted operations and TokenPaymaster for fee payment in ERC-20 tokens.

Session keys are a powerful pattern for improving UX. They are temporary private keys, authorized by the user's main account contract, to perform a limited set of actions for a specific period or number of transactions. For example, a gaming dApp could request a session key that allows only approve and transferFrom functions on a specific NFT contract for the next 8 hours. This eliminates the need for users to sign a wallet pop-up for every in-game action, creating a console-like experience while maintaining security boundaries. Session key logic is implemented within the account contract's validation function.

To launch a gasless experience, you integrate these components. First, users need an ERC-4337 compatible smart account, like those from Safe{Wallet}, ZeroDev, or Biconomy. Your dApp's backend then generates a session key pair and requests a signature from the user's main account to authorize it with specific permissions. When the user acts, the dApp signs a UserOperation with the session key, routes it to a bundler, and attaches a paymaster to cover gas. The entire flow is invisible to the user.

Implementing this requires careful security consideration. Session key permissions must be narrowly scoped—limiting allowed contracts, function selectors, spending limits, and time windows. Use established libraries like @account-abstraction/sdk or permit2-inspired schemes for authorization. Always conduct audits on custom account and paymaster logic. For developers, the Account Kit from Alchemy and Stackup's Bundler APIs provide robust infrastructure to build and test these flows on networks like Ethereum, Polygon, and Base.

An ERC-4337 Paymaster is a smart contract that can pay for the gas fees of UserOperations on behalf of your users. It is a critical component for enabling gasless transactions or allowing users to pay with ERC-20 tokens. When you submit a UserOperation to the mempool, you specify a paymasterAndData field, which instructs the network's EntryPoint contract to validate and execute the transaction using the designated Paymaster's logic and funds. This decouples the fee payment from the user's wallet, abstracting away the need for native ETH.

You have several options for your Paymaster strategy. You can deploy and manage your own custom Paymaster contract, which gives you full control over sponsorship logic and whitelisting. Alternatively, you can integrate with a third-party Paymaster-as-a-Service provider like Biconomy, Stackup, or Alchemy, which handle infrastructure, gas price estimation, and fund management. For a gasless experience, a verifying paymaster that signs off on sponsored transactions is common. Your choice depends on your need for control versus development speed.

Integrating a Paymaster begins with updating your bundler or client configuration. For example, using the userop.js library, you would construct your UserOperation with the appropriate paymasterAndData. If using a service like Biconomy, you would first get a paymasterAndData response from their API. The code snippet below shows a basic integration pattern:

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const userOp = await smartAccount.buildUserOp([tx], {
  paymasterServiceData: {
    mode: 'SPONSORED', // Gasless mode
  },
});

This tells the bundler to fetch sponsorship data from the configured Paymaster service before submitting the operation.

The Paymaster must undergo a validation step during the UserOperation lifecycle. The EntryPoint calls validatePaymasterUserOp on your Paymaster contract. This function should verify the request (e.g., check a signature for a whitelisted user) and deposit funds for gas. If validation passes, the Paymaster's balance is used for the transaction. Post-execution, the EntryPoint deducts the actual gas cost. It's crucial that your Paymaster holds a sufficient ETH balance or has a strategy to replenish it, or transactions will start to fail.

For developers launching a session key system, the Paymaster is what makes the experience seamless. A user can approve a session with a one-time gas payment, and all subsequent actions signed by that session key can be sponsored. This requires the Paymaster to validate that the UserOperation originates from a pre-approved session key. Implementing this involves storing a mapping of valid sessions in the Paymaster or having it verify a cryptographic signature from a trusted session key manager contract. This setup is the backbone of a smooth, gasless user experience.

A session key is a temporary, limited-authority cryptographic key that a user delegates to an application. Instead of signing every transaction with their primary wallet (which requires gas fees and constant approval), the user signs a single session authorization. This creates a session key that can automatically sign predefined types of transactions on their behalf for a set duration or until a spending limit is reached. This is the core mechanism that enables a seamless, gasless user experience.

Designing a session involves defining its authorization rules. These are the constraints encoded into the signed authorization message and enforced by a smart contract. Critical parameters include: the validUntil timestamp, a paymaster address to sponsor gas, a sessionKey address, and permissions. Permissions are granular, such as allowing transactions only to a specific target contract, only calling certain selector functions, and setting a maximum value limit for native token transfers.

Implementation typically uses EIP-4337 Account Abstraction smart accounts or a dedicated session key manager contract. The flow is: 1) The dApp frontend generates a session key pair. 2) The user's primary wallet signs a structured message (EIP-712) granting permissions to that key. 3) This signature is passed to a smart contract (like a SessionKeyManager) that validates and stores the permissions. 4) For subsequent actions, the dApp uses the session key to sign UserOperations, which are submitted via a Bundler and have gas paid by a Paymaster.

Security is paramount. Best practices include: setting short session durations (e.g., 24 hours), implementing low value limits, restricting contract calls to a whitelist, and enabling users to revoke sessions instantly. Tools like the ZeroDev Kernel smart account or Biconomy's SessionKeyManager provide audited modules for this. Always emit events for session creation and revocation so users can monitor activity via block explorers.

Here is a simplified example of a session authorization data structure for EIP-712 signing:

solidityCopy
struct SessionData {
    address sessionKey;
    address paymaster;
    uint48 validUntil;
    Permission[] permissions;
}
struct Permission {
    address targetContract;
    bytes4 functionSelector;
    uint256 valueLimit;
}

The user signs the hash of this structured data. The verifying contract will check this signature and enforce the rules before allowing a session-signed operation to proceed.

By implementing session keys, you shift the transaction signing burden from the user's main wallet to a temporary, restricted agent. This allows your dApp to batch actions, sponsor gas fees seamlessly, and create user experiences comparable to Web2 applications, all while maintaining clear security boundaries and user control over permissions.

The core of the gasless flow is the session key signature. When a user initiates a transaction, your dApp's frontend must generate a signature from the user's session key wallet, not their primary wallet. This signature authorizes the specific action, such as a swap or a mint, within the pre-defined rules of the session. Libraries like Viem or Ethers.js are used to sign the transaction data (or a user operation hash in an Account Abstraction context) with the session key's private key, which is securely managed in the user's browser session.

This signed payload is then sent to your backend relayer. The relayer's primary responsibilities are to validate the session signature against the on-chain session rules and, if valid, to sponsor the transaction. It checks the session's validUntil timestamp, the allowedContracts, and any spendingLimit. Crucially, the relayer uses its own funded wallet to pay the gas fees, submitting the final transaction to the blockchain. For AA wallets, this involves constructing and sending a UserOperation to a bundler service like those from Stackup or Pimlico.

A robust implementation requires careful error handling and state management. Your frontend should handle cases where the session has expired, the action is not permitted, or the relayer is temporarily unavailable. It's also essential to provide clear user feedback, such as confirming the session key is active and showing that gas fees are being covered. The transaction hash returned by the relayer should be displayed so users can track their gasless transaction on a block explorer.

For developers using ERC-7579-compliant modular smart accounts, the process integrates with the account's module system. The session key is installed as a validation module. When the relayer submits the UserOperation, the account's validateUserOp function will call into this module to verify the session signature and rules, ensuring only authorized, gasless transactions are executed. This standardizes the security model across different smart account implementations.

Finally, consider the user experience flow from start to finish: 1) User connects primary wallet and approves a session. 2) User interacts with your dApp, triggering a transaction. 3) Frontend signs with the session key. 4) Backend relayer validates and sponsors the tx. 5) User sees a confirmation—all without ever needing native gas tokens or signing multiple times for the same session. This seamless flow is what defines a premium Web3 onboarding and interaction experience.

A session key without constraints is a significant security risk. The primary defense is implementing a spending limit policy. This defines the maximum value a session can transact, often denominated in the native token (e.g., 0.1 ETH) or a specific ERC-20. This limit should be enforced on-chain within the session validation logic, checking that the cumulative value of all actions in a session does not exceed the user-authorized cap. Tools like ERC-7579 standards or smart account modules from Safe, ZeroDev, or Biconomy provide frameworks for building these rules.

Rate limiting is equally critical to prevent spam or automated draining attempts. Implement constraints such as: - Maximum number of transactions per session - Minimum time between transactions (throttling) - A total gas usage budget for the session. These limits protect the dApp from being exploited to spam the network or exhaust a relayer's gas subsidies. Logic for rate limiting can be handled off-chain by your backend service before signing or on-chain within a validation module.

All sessions must have a finite lifetime. Implement an expiration timestamp, after which the session key's signature is no longer valid. Common durations range from hours for active trading sessions to days for less sensitive interactions. The expiration should be a signed parameter within the session payload itself, verified on-chain. Combine this with a mechanism for users to manually revoke sessions at any time via their primary wallet, which updates a revocation registry or invalidates the session nonce.

For sustainability, consider the economic model. Who pays the gas? If the dApp subsidizes gas via a paymaster, implement a usage-based budget per user or session to prevent unsustainable costs. Alternatively, use a gas tank model where users pre-deposit funds, or a hybrid model where the first N transactions are free. Monitor metrics like average gas cost per session and user lifetime value to calibrate these parameters. Open-source tools like Pimlico's Gas Manager offer configurable policies for this.

Finally, integrate monitoring and alerts. Track events like: - Session creation and revocation - Limit breaches (failed transactions) - Abnormal gas consumption patterns. Services like Tenderly Alerts or OpenZeppelin Defender can notify your team of suspicious activity. This proactive monitoring completes the loop, turning your gasless system from a convenient feature into a secure, production-ready infrastructure.

The system you've built demonstrates the core value proposition of session keys: user experience and scalability. By pre-authorizing a set of rules—like spending limits, contract addresses, and a time window—you enable users to perform multiple actions without constant wallet pop-ups and gas fee approvals. This is critical for applications like gaming, social networks, or frequent trading interfaces where frictionless interaction is paramount. The security model shifts from per-transaction signatures to the careful definition of the session's permissions.

For production use, several critical enhancements are necessary. First, implement robust key management: session private keys should be securely stored and rotated frequently, never exposed client-side. Second, add comprehensive validation on your backend or in a dedicated relayer to enforce the session's rules (e.g., checking the maxAmount before submitting to the network). Third, integrate a gas sponsorship mechanism using a paymaster like those from Stackup, Biconomy, or Pimlico. These services can abstract away the complexity of funding and submitting UserOperation bundles to the ERC-4337 mempool.

To explore further, consider these advanced patterns. Delegatable Sessions allow a session key to grant sub-keys with more restricted permissions. Time-locked or conditional sessions can be invalidated based on on-chain events. For broader inspiration, review how major dApps implement sessions: check the ERC-4337 account abstraction examples or the ZeroDev Kernel for a production-ready smart account framework that natively supports session keys. The goal is to create a seamless, secure, and sustainable gasless experience that aligns with your application's specific needs.