Bitcoin: How to calculate lambda using the ecdsa library?

I can provide you with an article on calculating lambda using the ECDSA library and explain how it works.

Understanding Bitcoin’s Lambda Function

In Bitcoin, the lambda function (λ) plays a crucial role in the ECDSA (Elliptic Curve Digital Signature Algorithm) implementation. The λ function is used for both signing and verifying digital signatures.

The formula for calculating λ is:

λ = (3 x2^2) (2 * y2)^-1 mod p

λ = (y₂ – y₁) * (x₂ – x₁)^-1 mod p

However, in practice, the values ​​of λ are corrected by the ECDSA library. To understand how this works, let’s dive into the details.

The Corrected Formula

To calculate the corrected formula for λ, we need to modify the original equation:

λ = (3 x2^2) (2 * y2)^-1 mod p

λ = (y₂ – y₁) * (x2 – x₁)^-1 mod p

The correction comes from the property of modular arithmetic and the fact that λ is not always equal to the original formula. This means that when you plug in values ​​for x, y, and p, the result will be different than what it would be if you used the original formula.

How ​​ECDSA Calculates Corrected Lambda

The ECDSA library uses a combination of mathematical operations to calculate corrected λ values. Here are the steps involved:

• Generate elliptic curve points

  : The ECDSA library generates two elliptic curve points, (x1, y1) and (x2, y2), using a specific elliptic curve (e.g., secp256k1).

• Calculate the multiplicative inverse: The library calculates the multiplicative inverse of (y2 – y1) modulo p using the extended Euclidean algorithm or other methods.

• Apply the corrected formula: The corrected λ value is calculated by multiplying x2^2 and 2*y2^-1 modulo p, where y2^-1 is the multiplicative inverse found in step 2.

ECDSA Implementation

The ECDSA library typically provides a function to calculate corrected lambda values ​​using the following steps:

• ecdsa SigningHash::VerifySignature(const std::vector& signature): This function takes a signature as input and returns a boolean indicating whether it’s valid.

• ecdsa SigningHash::CalculateCorrectedLambda(): This function calculates the corrected λ value for the given elliptic curve points.

Example Code

Here’s an example implementation of calculating corrected lambda values ​​using the secp256k1 elliptic curve:

#include 


// Generate a new private key

const secp256k1_keypair sk = secp256k1::generatePrivate();


// Get the public key and private value

const secp256k1_point pubKey = secp256k1::publicKey(sk);


// Calculate corrected lambda values ​​for the elliptic curve points (x, y)

std::vector x2_y2_values ​​= {{pubKey.x.pow(2) }, {pubKey.y.pow(2)}};

uint64_t corrected_lambda_values[2] = {};

for (size_t i = 0; i < 2; ++i) {

    for (int j = 0; j < 4; ++j) {

        // Calculate the multiplicative inverse of (y2 - y1)

        uint8_t inv_y2_minus_y1 = secp256k1::inverseModulo((pubKey.y.pow(2) - pubKey.y).x, 65537);

        

        // Apply the corrected formula

        corrected_lambda_values[i] = ((3  x2_y2_values[i][0].xpow(2))  (2 * x2_y2_values[i][1]).yinv) % sk.p;

    }

}


// Print the corrected lambda values

for (size_t i = 0; i < 2; ++i) {

    std::cout << "Corrected λ value for x²²: " << corrected_lambda_values[i].to16() << "n";

}

Note that this is a simplified example and may not cover all the edge cases. In real-world applications, you should consult the ECDSA library documentation and consider factors like performance, security, and scalability when implementing lambda calculations.

I hope this article helps! Let me know if you have any questions or need further clarification on any of the topics covered.