diff --git a/src/api.js b/src/api.js
index 84fe0f8..1a855cb 100644
--- a/src/api.js
+++ b/src/api.js
@@ -2,7 +2,7 @@
  * ACME API client
  */
 
-const util = require('./util');
+const util = require('./util.js');
 
 
 /**
diff --git a/src/client.js b/src/client.js
index 6547522..80445db 100644
--- a/src/client.js
+++ b/src/client.js
@@ -6,12 +6,12 @@
 
 const { createHash } = require('crypto');
 const { getPemBodyAsB64u } = require('./crypto');
-const { log } = require('./logger');
-const HttpClient = require('./http');
-const AcmeApi = require('./api');
-const verify = require('./verify');
-const util = require('./util');
-const auto = require('./auto');
+const { log } = require('./logger.js');
+const HttpClient = require('./http.js');
+const AcmeApi = require('./api.js');
+const verify = require('./verify.js');
+const util = require('./util.js');
+const auto = require('./auto.js');
 
 
 /**
diff --git a/src/crypto/index.js b/src/crypto/index.js
index 582b2d1..71a2449 100644
--- a/src/crypto/index.js
+++ b/src/crypto/index.js
@@ -4,9 +4,10 @@
  * @namespace crypto
  */
 
-const net = require('net');
-const { promisify } = require('util');
-const crypto = require('crypto');
+const { Buffer } = require('node:buffer');
+const net = require('node:net');
+const { promisify } = require('node:util');
+const crypto = require('node:crypto');
 const jsrsasign = require('jsrsasign');
 
 const generateKeyPair = promisify(crypto.generateKeyPair);
diff --git a/src/http.js b/src/http.js
index 49f7932..1ba4782 100644
--- a/src/http.js
+++ b/src/http.js
@@ -3,9 +3,9 @@
  */
 
 const { createHmac, createSign, constants: { RSA_PKCS1_PADDING } } = require('crypto');
-const { getJwk } = require('./crypto');
-const { log } = require('./logger');
-const axios = require('./axios');
+const { getJwk } = require('./crypto/index.js');
+const { log } = require('./logger.js');
+const axios = require('./axios.js');
 
 
 /**
diff --git a/src/index.js b/src/index.js
index a2552e1..69942b1 100644
--- a/src/index.js
+++ b/src/index.js
@@ -2,7 +2,7 @@
  * acme-client
  */
 
-exports.Client = require('./client');
+exports.Client = require('./client.js');
 
 
 /**
@@ -28,19 +28,19 @@ exports.directory = {
  * Crypto
  */
 
-exports.crypto = require('./crypto');
-exports.forge = require('./crypto/forge');
+exports.crypto = require('./crypto/index.js');
+exports.forge = require('./crypto/forge.js');
 
 
 /**
  * Axios
  */
 
-exports.axios = require('./axios');
+exports.axios = require('./axios.js');
 
 
 /**
  * Logger
  */
 
-exports.setLogger = require('./logger').setLogger;
+exports.setLogger = require('./logger.js').setLogger;
diff --git a/src/util.js b/src/util.js
index 95d08e8..90f532c 100644
--- a/src/util.js
+++ b/src/util.js
@@ -2,9 +2,9 @@
  * Utility methods
  */
 
-const dns = require('dns').promises;
-const { readCertificateInfo, splitPemChain } = require('./crypto');
-const { log } = require('./logger');
+const dns = require('node:dns/promises');
+const { readCertificateInfo, splitPemChain } = require('./crypto/index.js');
+const { log } = require('./logger.js');
 
 
 /**
diff --git a/test/00-pebble.spec.js b/test/00-pebble.spec.js
index 789c0a4..19f59f9 100644
--- a/test/00-pebble.spec.js
+++ b/test/00-pebble.spec.js
@@ -2,11 +2,12 @@
  * Pebble Challenge Test Server tests
  */
 
-const dns = require('dns').promises;
+const process = require('node:process');
+const dns = require('node:dns/promises');
 const { assert } = require('chai');
 const { v4: uuid } = require('uuid');
-const cts = require('./challtestsrv');
-const axios = require('./../src/axios');
+const cts = require('./challtestsrv.js');
+const axios = require('./../src/axios.js');
 
 const domainName = process.env.ACME_DOMAIN_NAME || 'example.com';
 const httpPort = axios.defaults.acmeSettings.httpChallengePort || 80;
diff --git a/test/10-http.spec.js b/test/10-http.spec.js
index e8163af..fa9ffa9 100644
--- a/test/10-http.spec.js
+++ b/test/10-http.spec.js
@@ -5,8 +5,8 @@
 const { assert } = require('chai');
 const { v4: uuid } = require('uuid');
 const nock = require('nock');
-const axios = require('./../src/axios');
-const HttpClient = require('./../src/http');
+const axios = require('./../src/axios.js');
+const HttpClient = require('./../src/http.js');
 const pkg = require('./../package.json');
 
 
diff --git a/test/10-logger.spec.js b/test/10-logger.spec.js
index ac9200b..7d9058d 100644
--- a/test/10-logger.spec.js
+++ b/test/10-logger.spec.js
@@ -3,7 +3,7 @@
  */
 
 const { assert } = require('chai');
-const logger = require('./../src/logger');
+const logger = require('./../src/logger.js');
 
 
 describe('logger', () => {
diff --git a/test/10-verify.spec.js b/test/10-verify.spec.js
index adbd370..1f1b773 100644
--- a/test/10-verify.spec.js
+++ b/test/10-verify.spec.js
@@ -4,8 +4,8 @@
 
 const { assert } = require('chai');
 const { v4: uuid } = require('uuid');
-const cts = require('./challtestsrv');
-const verify = require('./../src/verify');
+const cts = require('./challtestsrv.js');
+const verify = require('./../src/verify.js');
 
 const domainName = process.env.ACME_DOMAIN_NAME || 'example.com';
 
diff --git a/test/20-crypto-legacy.spec.js b/test/20-crypto-legacy.spec.js
index b039e74..2051e6c 100644
--- a/test/20-crypto-legacy.spec.js
+++ b/test/20-crypto-legacy.spec.js
@@ -5,8 +5,8 @@
 const fs = require('fs').promises;
 const path = require('path');
 const { assert } = require('chai');
-const spec = require('./spec');
-const forge = require('./../src/crypto/forge');
+const spec = require('./spec.js');
+const forge = require('./../src/crypto/forge.js');
 
 const cryptoEngines = {
     forge
diff --git a/test/20-crypto.spec.js b/test/20-crypto.spec.js
index 8441af5..2ac61a3 100644
--- a/test/20-crypto.spec.js
+++ b/test/20-crypto.spec.js
@@ -5,7 +5,7 @@
 const fs = require('fs').promises;
 const path = require('path');
 const { assert } = require('chai');
-const spec = require('./spec');
+const spec = require('./spec.js');
 const { crypto } = require('./../');
 
 const emptyBodyChain1 = `
diff --git a/test/50-client.spec.js b/test/50-client.spec.js
index cb16264..6382e52 100644
--- a/test/50-client.spec.js
+++ b/test/50-client.spec.js
@@ -2,11 +2,12 @@
  * ACME client tests
  */
 
+const process = require('node:process');
 const { assert } = require('chai');
 const { v4: uuid } = require('uuid');
-const cts = require('./challtestsrv');
-const getCertIssuers = require('./get-cert-issuers');
-const spec = require('./spec');
+const cts = require('./challtestsrv.js');
+const getCertIssuers = require('./get-cert-issuers.js');
+const spec = require('./spec.js');
 const acme = require('./../');
 
 const domainName = process.env.ACME_DOMAIN_NAME || 'example.com';
diff --git a/test/70-auto.spec.js b/test/70-auto.spec.js
index 5b0b6f1..0bc7314 100644
--- a/test/70-auto.spec.js
+++ b/test/70-auto.spec.js
@@ -2,11 +2,12 @@
  * ACME client.auto tests
  */
 
+const process = require('node:process');
 const { assert } = require('chai');
 const { v4: uuid } = require('uuid');
-const cts = require('./challtestsrv');
-const getCertIssuers = require('./get-cert-issuers');
-const spec = require('./spec');
+const cts = require('./challtestsrv.js');
+const getCertIssuers = require('./get-cert-issuers.js');
+const spec = require('./spec.js');
 const acme = require('./../');
 
 const domainName = process.env.ACME_DOMAIN_NAME || 'example.com';
diff --git a/test/challtestsrv.js b/test/challtestsrv.js
index 1a37295..99d9159 100644
--- a/test/challtestsrv.js
+++ b/test/challtestsrv.js
@@ -2,8 +2,9 @@
  * Pebble Challenge Test Server integration
  */
 
+const process = require('node:process');
 const { assert } = require('chai');
-const axios = require('./../src/axios');
+const axios = require('./../src/axios.js');
 
 const apiBaseUrl = process.env.ACME_CHALLTESTSRV_URL || null;
 
diff --git a/test/get-cert-issuers.js b/test/get-cert-issuers.js
index 8813b06..ba00a43 100644
--- a/test/get-cert-issuers.js
+++ b/test/get-cert-issuers.js
@@ -3,7 +3,7 @@
  */
 
 const acme = require('./../');
-const util = require('./../src/util');
+const util = require('./../src/util.js');
 
 const pebbleManagementUrl = process.env.ACME_PEBBLE_MANAGEMENT_URL || null;
 
diff --git a/test/setup.js b/test/setup.js
index e6f1864..5dd51f3 100644
--- a/test/setup.js
+++ b/test/setup.js
@@ -2,13 +2,14 @@
  * Setup testing
  */
 
-const url = require('url');
-const net = require('net');
-const fs = require('fs');
-const dns = require('dns').promises;
+const process = require('node:process');
+const url = require('node:url');
+const net = require('node:net');
+const fs = require('node:fs');
+const dns = require('node:dns/promises');
 const chai = require('chai');
 const chaiAsPromised = require('chai-as-promised');
-const axios = require('./../src/axios');
+const axios = require('./../src/axios.js');
 
 
 /**