diff --git a/SignalMetadataKitTests/src/SMKSenderCertificateTest.swift b/SignalMetadataKitTests/src/SMKSenderCertificateTest.swift
index fad9ad0..2e1dc15 100644
--- a/SignalMetadataKitTests/src/SMKSenderCertificateTest.swift
+++ b/SignalMetadataKitTests/src/SMKSenderCertificateTest.swift
@@ -159,6 +159,68 @@ class SMKSenderCertificateTest: XCTestCase {
         }
     }
 
+    func test_losslessRoundTrip() {
+        // To test a hypothetical addition of a new field:
+        //
+        // Step 1: tempororarily add a new field to the .proto.
+        //
+        //     index 537f8df..82e9263 100644
+        //     --- a/protobuf/OWSUnidentifiedDelivery.proto
+        //     +++ b/protobuf/OWSUnidentifiedDelivery.proto
+        //     @@ -40,6 +40,7 @@ message SenderCertificate {
+        //     optional bytes identityKey = 4;
+        //     // @required
+        //     optional ServerCertificate signer = 5;
+        //     +        optional string someFakeField = 999;
+        //     }
+        //
+        // Step 2: Serialize and print out the new fixture data (uncomment the following)
+        //
+        //     let serverKey = Curve25519.generateKeyPair()
+        //     let key = Curve25519.generateKeyPair()
+        //     let signer = try! getServerCertificate(serverKey: serverKey)
+        //     let builder = try! SMKProtoSenderCertificateCertificate.builder(sender: "+14152222222",
+        //                                                                     senderDevice: 1,
+        //                                                                     expires: 31337,
+        //                                                                     identityKey: key.ecPublicKey().serialized,
+        //                                                                     signer: signer)
+        //     builder.setSomeFakeField("crashing right down")
+        //
+        //     print("<SNIP>")
+        //     let serializedCertificateData = try! builder.buildSerializedData()
+        //     let certificateDataEncoded = serializedCertificateData.base64EncodedString()
+        //     print("let certificateDataEncoded = \"\(certificateDataEncoded)\"")
+        //
+        //     let certificateSignatureEncoded = try! Ed25519.sign(serializedCertificateData, with: serverKey).base64EncodedString()
+        //     print("let certificateSignatureEncoded = \"\(certificateSignatureEncoded)\"")
+        //
+        //     let trustRootPublicKeyDataEncoded = try! trustRoot.ecPublicKey().serialized.base64EncodedString()
+        //     print("let trustRootPublicKeyDataEncoded = \"\(trustRootPublicKeyDataEncoded)\"")
+        //     print("</SNIP>")
+
+        // Step 3: update the following *Encoded fixture data with the new values from above.
+        let certificateDataEncoded = "CgwrMTQxNTIyMjIyMjIQARlpegAAAAAAACIhBdyYGjVpE02g7CUlCvGNElHZNZmGy3Xhh5y+TuPh6dQIKmkKJQgBEiEFeszl2BGIxS95K+anx30GX6+Tgoqp70/aWKNEkH/5TGkSQPbz1mzKfidiWTuT8pRdnYYchEnL+ln5i/mVq5JP1MzzmqVnx8bzkFhfT4EGYSDY5rQoVfb5JnV0Kf3Aavdkd426PhNjcmFzaGluZyByaWdodCBkb3du"
+        let certificateSignatureEncoded = "Ii8DBO6yapzQwc0kJ6M5EhuFsgHccjlzFSJow408O1tceRVZiYGpR5MZO1SBgKHH2GEayiBNpvayFIL2i4POig=="
+        let trustRootPublicKeyDataEncoded = "BanGdQtiGO0KYbSu/rBz3MZvO+LGkjGVceXfmQV8eNwM"
+
+        let certificateData = Data(base64Encoded: certificateDataEncoded)!
+        let certificateSignature = Data(base64Encoded: certificateSignatureEncoded)!
+        let trustRootPublicKeyData = Data(base64Encoded: trustRootPublicKeyDataEncoded)!
+
+        // The rest of the test should be stable.
+        let senderCertificateData = try! SMKProtoSenderCertificate.builder(certificate: certificateData,
+                                                                           signature: certificateSignature)
+            .buildSerializedData()
+
+        let senderCertificate = try! SMKSenderCertificate(serializedData: senderCertificateData)
+
+        let stableTrustRoot = try! ECPublicKey(serializedKeyData: trustRootPublicKeyData)
+
+        let certificateValidator = SMKCertificateDefaultValidator(trustRoot: stableTrustRoot)
+        XCTAssertNoThrow(try certificateValidator.throwswrapped_validate(senderCertificate: senderCertificate,
+                                                                         validationTime: 31336))
+    }
+
     // MARK: - Utils
 
     // private SignalProtos.ServerCertificate getServerCertificate(ECKeyPair serverKey) throws InvalidKeyException, InvalidCertificateException {