Deep Dive into TON Smart Contracts: Implementing Proof-of-Work in the TestGiver Contract

In the rapidly evolving landscape of blockchain technology, smart contracts have emerged as a pivotal force driving the development of decentralized applications (DApps). TON (The Open Network) is an emerging blockchain platform that has garnered attention from numerous developers and enterprises due to its efficiency and scalability. This article will provide an in-depth analysis of a TON smart contract — TestGiver — which utilizes a Proof-of-Work (PoW) mechanism to safeguard its functions, allowing users to obtain test tokens by solving PoW challenges. We will examine the contract’s structure, key functions, security, and scalability in detail.

Overview of the TestGiver Contract

The TestGiver contract is a smart contract specifically designed for the TON blockchain, primarily aimed at distributing tokens in a test environment. By incorporating a PoW mechanism, the contract ensures that only users who invest computational resources and successfully solve a specific challenge can acquire tokens. This mechanism not only increases the difficulty of obtaining tokens but also enhances the security of the entire system.

Contract Structure

TON smart contracts are written in a low-level programming language similar to assembly, which provides high execution efficiency but increases the complexity of development and comprehension. The structure of the TestGiver contract can be broken down into several parts:

• Data Storage: The contract stores critical data such as sequence numbers, public keys, seeds, and PoW complexity.
• Interface Functions: The contract provides interfaces for external calls, such as PoW verification and parameter updates.
• Internal Functions: Auxiliary functions used internally by the contract, such as data parsing and hash calculation.

Key Functions and Logic

1. Inline Assembly Function: ufits

The first highlight of the contract is the use of the inline assembly function ufits. This function is used to convert an integer into an unsigned integer with a fixed bit width, which is useful for bit manipulation and ensuring the correctness of data size.

int ufits(int x, int bits) impure asm "UFITSX";

The impure keyword indicates that the function may have side effects, while asm signifies that it is implemented in assembly language. This low-level operation offers the contract great flexibility and performance advantages.

2. Handling Internal Messages: recv_internal

The contract defines a recv_internal function to handle internal messages. In this specific contract, the function does not perform any operations, but it leaves room for handling internal messages that may be required in the future.

() recv_internal(slice in_msg) impure {
  ;; do nothing for internal messages
}

Internal messages are typically triggered by the contract itself or by other contracts, differing from external user interactions. Leaving such a handling function is good programming practice.

3. PoW Verification: check_proof_of_work

The check_proof_of_work function is the core of the contract, responsible for verifying the PoW solutions submitted by users.

() check_proof_of_work(slice cs) impure inline_ref {
  // ... PoW verification logic ...
}

The detailed steps of this function are as follows:

• Calculate Hash: First, the function calculates the hash of the input message. This is the first step of PoW verification, ensuring the integrity of the message.
• Load State Data: Next, the function loads key information such as sequence numbers, public keys, seeds, and PoW complexity from the contract’s data storage.
• Verify Hash: The function checks whether the calculated hash is lower than the stored PoW complexity. If it is, the user has successfully solved the PoW challenge.
• Verify Additional Data: To prevent replay attacks, the function also verifies that the seed and additional data match.
• Update State: If PoW verification passes, the function accepts the message, recomputes the PoW complexity, and updates the contract’s state, including a new seed and the timestamp of the last successful attempt.

4. Adjusting PoW Complexity: rescale_complexity

The rescale_complexity function readjusts the PoW complexity if there have been no successful PoW solutions for an extended period.

() rescale_complexity(slice cs) impure inline_ref {
  // ... logic for readjusting PoW complexity ...
}

This function ensures that the difficulty of the PoW problem remains appropriate over time, preventing the problem from becoming too difficult and hindering users from obtaining tokens. The main logic of the function includes:

• Check Expiry Time: The function first checks if the expiry time has been exceeded.
• Calculate Time Interval: It then calculates the time interval since the last successful PoW solution.
• Adjust Complexity: Based on the time interval and the target interval, the function computes the new PoW complexity.
• Update State: Finally, the function updates the contract’s state with the new PoW complexity and the timestamp of the last successful attempt.

5. Updating Contract Parameters: update_params

The update_params function is used to update the contract’s parameters, such as the seed and PoW complexity.

(slice, ()) ~update_params(slice ds, cell pref) inline_ref {
  // ... logic for updating contract parameters ...
}

This function is typically called when the contract administrator needs to adjust the PoW parameters. The main steps of the function are:

• Parse Input: The function parses the input parameters, including a flag indicating whether to reset the PoW complexity.
• Update Parameters: If the reset flag is set, the function generates a new seed randomly and resets the PoW complexity based on the input complexity level.
• Build New State: The function constructs a new contract state with the updated seed and PoW complexity, and the current time as the timestamp of the last successful attempt.
• Return Result: The function returns the updated contract state.

6. Handling External Messages: recv_external

The recv_external function is the main entry point for handling external messages and executes different functions based on the operation code, such as PoW verification or complexity adjustment.

() recv_external(slice in_msg) impure {
  // ... logic for handling external messages ...
}

The main logic of this function includes:

• Parse Operation Code: The function first parses the operation code of the incoming message to determine the operation to be executed.
• PoW Verification: If the operation code indicates PoW verification, the function calls check_proof_of_work.
• Complexity Adjustment: If the operation code indicates complexity adjustment, the function calls rescale_complexity.
• Signature Verification: The function verifies the signature of the message to ensure it comes from a legitimate sender.
• Update Sequence Number: The function updates the contract’s sequence number to prevent replay attacks.
• Handle References: The function processes references within the message, which may include sending new messages or updating contract parameters.

Getter Methods

To provide transparency of the contract’s state, the TestGiver contract implements several getter methods, allowing external queries of the contract’s current state:

seqno Method

int seqno() method_id {
  return get_data().begin_parse().preload_uint(32);
}

This method returns the current message sequence number, which is crucial for ensuring the uniqueness of messages and preventing replay attacks.

get_pow_params Method

(int, int, int, int) get_pow_params() method_id {
  var ds = get_data().begin_parse().skip_bits(32 + 32 + 256);
  var (seed, pow_complexity, xdata) = (ds~load_uint(128), ds~load_uint(256), ds.preload_ref());
  ds = xdata.begin_parse();
  return (seed, pow_complexity, ds~load_grams(), ds.preload_uint(32));
}

This method returns the PoW parameters, including the seed, PoW complexity, amount, and interval. These parameters are vital for users wishing to solve the PoW problem.

get_public_key Method

int get_public_key() method_id {
  var ds = get_data().begin_parse();
  ds~load_uint(32 + 32);
  return ds.preload_uint(256);
}

This method returns the contract’s public key, which is essential for verifying the signatures of messages and ensuring the security of the contract.

Security and Scalability

The TestGiver contract was designed with security and scalability in mind. Here are some key points:

Security

• PoW Mechanism: By requiring users to solve a PoW problem, the contract reduces the opportunity for malicious attackers, as they would need to invest significant computational resources.
• Signature Verification: The contract verifies the signatures of external messages to ensure that only users with the corresponding private key can execute specific operations.
• Sequence Numbers: By maintaining message sequence numbers, the contract prevents replay attacks, where attackers might resend the same message.

Scalability

• Parameter Adjustment: The contract allows the administrator to adjust the PoW complexity, enabling the contract to adapt to varying network conditions and user demands.
• Getter Methods: By providing getter methods, the contract allows external queries of its state, which is crucial for building applications that interact with the contract.

Conclusion

The TestGiver contract is a prime example of the powerful capabilities of TON smart contracts. By implementing a PoW mechanism, it not only provides a secure environment for distributing test tokens but also opens up new possibilities for decentralized applications on the TON blockchain. This article’s analysis aims to serve as a starting point for developers looking to gain a deeper understanding of TON smart contracts and to inspire the design and implementation of more innovative applications. As TON continues to evolve, we can anticipate seeing more efficient and secure smart contract applications that drive