diff --git a/lib/web_push.rb b/lib/web_push.rb
index 6f7992f..80ccaf6 100644
--- a/lib/web_push.rb
+++ b/lib/web_push.rb
@@ -2,7 +2,6 @@
 
 require 'openssl'
 require 'base64'
-require 'hkdf'
 require 'jwt'
 require 'uri'
 require 'net/http'
diff --git a/lib/web_push/encryption.rb b/lib/web_push/encryption.rb
index 4fe1dea..b6b7b45 100644
--- a/lib/web_push/encryption.rb
+++ b/lib/web_push/encryption.rb
@@ -8,6 +8,7 @@ def encrypt(message, p256dh, auth)
       assert_arguments(message, p256dh, auth)
 
       group_name = 'prime256v1'
+      hash = 'SHA256'
       salt = Random.new.bytes(16)
 
       server = OpenSSL::PKey::EC.generate(group_name)
@@ -25,11 +26,11 @@ def encrypt(message, p256dh, auth)
       content_encryption_key_info = "Content-Encoding: aes128gcm\0"
       nonce_info = "Content-Encoding: nonce\0"
 
-      prk = HKDF.new(shared_secret, salt: client_auth_token, algorithm: 'SHA256', info: info).read(32)
+      prk = OpenSSL::KDF.hkdf(shared_secret, salt: client_auth_token, info: info, hash: hash, length: 32)
 
-      content_encryption_key = HKDF.new(prk, salt: salt, info: content_encryption_key_info).read(16)
+      content_encryption_key = OpenSSL::KDF.hkdf(prk, salt: salt, info: content_encryption_key_info, hash: hash, length: 16)
 
-      nonce = HKDF.new(prk, salt: salt, info: nonce_info).read(12)
+      nonce = OpenSSL::KDF.hkdf(prk, salt: salt, info: nonce_info, hash: hash, length: 12)
 
       ciphertext = encrypt_payload(message, content_encryption_key, nonce)
 
diff --git a/spec/web_push/encryption_spec.rb b/spec/web_push/encryption_spec.rb
index 3aed2ac..8f0c76d 100644
--- a/spec/web_push/encryption_spec.rb
+++ b/spec/web_push/encryption_spec.rb
@@ -65,10 +65,10 @@ def decrypt payload
       content_encryption_key_info = "Content-Encoding: aes128gcm\0"
       nonce_info = "Content-Encoding: nonce\0"
 
-      prk = HKDF.new(shared_secret, salt: client_auth_token, algorithm: 'SHA256', info: info).read(32)
+      prk = OpenSSL::KDF.hkdf(shared_secret, salt: client_auth_token, info: info, hash: 'SHA256', length: 32)
 
-      content_encryption_key = HKDF.new(prk, salt: salt, info: content_encryption_key_info).read(16)
-      nonce = HKDF.new(prk, salt: salt, info: nonce_info).read(12)
+      content_encryption_key = OpenSSL::KDF.hkdf(prk, salt: salt, info: content_encryption_key_info, hash: 'SHA256', length: 16)
+      nonce = OpenSSL::KDF.hkdf(prk, salt: salt, info: nonce_info, hash: 'SHA256', length: 12)
 
       decrypt_ciphertext(ciphertext, content_encryption_key, nonce)
     end
diff --git a/web-push.gemspec b/web-push.gemspec
index 4f8e21d..678cf51 100644
--- a/web-push.gemspec
+++ b/web-push.gemspec
@@ -14,7 +14,6 @@ Gem::Specification.new do |spec|
 
   spec.required_ruby_version = '>= 3.0'
 
-  spec.add_dependency 'hkdf', '~> 1.0'
   spec.add_dependency 'jwt', '~> 2.0'
   spec.add_dependency 'openssl', '~> 3.0'