diff --git a/passkit/lib/src/write_signature.dart b/passkit/lib/src/write_signature.dart
new file mode 100644
index 0000000..428b6f6
--- /dev/null
+++ b/passkit/lib/src/write_signature.dart
@@ -0,0 +1,56 @@
+import 'dart:typed_data';
+
+import 'package:passkit/src/apple_wwdr_certificate.dart';
+import 'package:pem/pem.dart';
+import 'package:pkcs7/pkcs7.dart';
+import 'package:pointycastle/pointycastle.dart';
+
+Uint8List writeSignature(
+  String userCertificate,
+  Uint8List manifestBytes,
+) {
+  final privateKey = _createPrivateKey(userCertificate);
+  final issuer = X509.fromPem(userCertificate);
+
+  final pkcs7Builder = Pkcs7Builder();
+  // Add one or more certificates
+  pkcs7Builder.addCertificate(wwdrG4);
+
+  // Create a signature information object
+  final signerInfo =
+      Pkcs7SignerInfoBuilder.rsa(issuer: issuer, privateKey: privateKey);
+
+  // Add the digest to sign
+  signerInfo.addSMimeDigest(digest: manifestBytes, signingTime: DateTime.now());
+
+  // Add the signature information
+  pkcs7Builder.addSignerInfo(signerInfo);
+
+  // Add a certificate revocation list
+  pkcs7Builder.addCRL(
+    CertificateRevocationList.fromDer(
+      Uint8List.fromList(worldwide_Developer_Relations_G4),
+    ),
+  );
+
+  final pkcs7 = pkcs7Builder.build();
+  return pkcs7.der;
+}
+
+RSAPrivateKey _createPrivateKey(String certificate) {
+  final pem = decodePemBlocks(PemLabel.privateKey, certificate);
+
+  final modulus =
+      ASN1Object.fromBytes(Uint8List.fromList(pem[1])) as ASN1Integer;
+  final exponent =
+      ASN1Object.fromBytes(Uint8List.fromList(pem[3])) as ASN1Integer;
+  final p = ASN1Object.fromBytes(Uint8List.fromList(pem[4])) as ASN1Integer;
+  final q = ASN1Object.fromBytes(Uint8List.fromList(pem[5])) as ASN1Integer;
+
+  return RSAPrivateKey(
+    modulus.integer!,
+    exponent.integer!,
+    p.integer,
+    q.integer,
+  );
+}
diff --git a/passkit/pubspec.yaml b/passkit/pubspec.yaml
index a011afb..b9d7313 100644
--- a/passkit/pubspec.yaml
+++ b/passkit/pubspec.yaml
@@ -23,7 +23,9 @@ dependencies:
   intl: ^0.19.0
   json_annotation: ^4.9.0
   meta: ^1.0.0
-  pkcs7: ^1.0.3
+  pem: ^2.0.0
+  pkcs7: ^1.0.0
+  pointycastle: ^3.9.0
 
 dev_dependencies:
   build_runner: ^2.3.2