Node.js crypto.sign() Function

Last Updated : 23 Jul, 2025

Cryptography is the process of converting plain text into unreadable which is hashed from text and vice-versa and the crypto.sign() is used to create signature of data.

Syntax:

crypto.sign(algorithm, data, key)

Parameters: This function accepts the following parameters:

algorithm: It is a string-type value. A signature can be created by applying the name of signature algorithms, like ‘SHA256’, in place of a digest algorithm.
data: It should be an object of buffer, TypedArray, or DataView. Read Buffer.from() method to convert string to Buffer.
key: It should be the privateKey of keyObject. If you have not any private keys then you can create private and public key using crypto.generateKeyPairSync() method.

Module Installation: Install the required module using the following command:

npm install crypto

Return Value: It returns the signature value base on the specified algorithm, data & key. The returned value is Buffer by default but possible to convert on other format using buffer.toString() method.

Example 1: Sign a string.

signature.js

const crypto = require('crypto');
const buffer = require('buffer');

// Create a private key
const { privateKey, publicKey } = crypto.generateKeyPairSync('rsa', {
  modulusLength: 2048,
});

// Convert string to buffer 
const data = Buffer.from("I Love GeeksForGeeks");

// Sign the data and returned signature in buffer 
const sign = crypto.sign("SHA256", data , privateKey);

// Convert returned buffer to base64
const signature = sign.toString('base64');

// Printing the signature 
console.log(`Signature:\n\n ${signature}`);

Run the signature.js file using the following command:

node signature.js

Output:

Example 2: Sign a file.

signature.js

const crypto = require('crypto');
const buffer = require('buffer');
const fs = require('fs');

// Create a private key
const { privateKey } = crypto.generateKeyPairSync('rsa', {
  modulusLength: 2048,
});

console.log("Reading File...\n");
// Reading file
const text = fs.readFileSync('./doc.txt');
console.log(`File content: ${text}`);

// Convert string to buffer 
const data = Buffer.from(text);

// Sign the data and returned signature in buffer 
const sign = crypto.sign("SHA256", data , privateKey);

// Convert returned buffer to base64
const signature = sign.toString('base64');

// Printing the signature 
console.log(`Signature:\n\n ${signature}`);

Run the signature.js file using the following command:

node signature.js

Output:

Example 3: Sign JSON data.

signature.js

const crypto = require('crypto');
const buffer = require('buffer');

// Create a private key
const { privateKey } = crypto.generateKeyPairSync('rsa', {
  modulusLength: 2048,
});

// JSON object
const person = {
    name: "Raktim Banerjee",
    email:"[email protected]",
    address: "4 main street"
}

// Convert Stringified json data to buffer  
const data = Buffer.from( JSON.stringify(person) );

// Sign the data and returned signature in buffer 
const sign = crypto.sign("SHA256", data , privateKey);

// Convert returned buffer to base64
const signature = sign.toString('base64');

// Printing the signature 
console.log(`Signature:\n\n ${signature}`);

Run the signature.js file using the following command:

node signature.js

Output:

Reference:https://nodejs.org/api/crypto.html#crypto_crypto_sign_algorithm_data_key_callback

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