Add test for non-standard use-case of KMS Envelope Keys.

In tink-crypto/tink-go#10 it
was mentioned that KMS Envelope Keys can and are used with a TINK prefix.

Add this test to make sure that we don't accidentally break this.

PiperOrigin-RevId: 614604104

go/aead/kms_envelope_aead_key_manager_test.go

@@ -17,11 +17,13 @@
 package aead_test
 
 import (
+	"bytes"
 	"testing"
 
 	"google.golang.org/protobuf/proto"
 	"github.com/google/tink/go/aead"
 	"github.com/google/tink/go/core/registry"
+	"github.com/google/tink/go/insecurecleartextkeyset"
 	"github.com/google/tink/go/keyset"
 	"github.com/google/tink/go/mac"
 	"github.com/google/tink/go/testing/fakekms"
@@ -127,3 +129,54 @@ func TestNewKMSEnvelopeAEADKeyWithInvalidSerializedKeyFormat(t *testing.T) {
 		t.Errorf("a.Encrypt() err = nil, want error")
 	}
 }
+
+func TestKMSEnvelopeAEADWithTinkPrefix(t *testing.T) {
+	keyURI := "fake-kms://CM2b3_MDElQKSAowdHlwZS5nb29nbGVhcGlzLmNvbS9nb29nbGUuY3J5cHRvLnRpbmsuQWVzR2NtS2V5EhIaEIK75t5L-adlUwVhWvRuWUwYARABGM2b3_MDIAE"
+	client, err := fakekms.NewClient(keyURI)
+	if err != nil {
+		t.Fatal(err)
+	}
+	registry.RegisterKMSClient(client)
+	defer registry.ClearKMSClients()
+
+	// Keyset that was created with
+	// aead.CreateKMSEnvelopeAEADKeyTemplate(keyURI, aead.AES256GCMKeyTemplate()), and then serialized
+	// with Tink's JSON keyset witer. Then, the output prefix type was changed from "RAW" to "TINK".
+	jsonKeysetWithTinkPrefix := `{"primaryKeyId":3980895889, "key":[{"keyData":{"typeUrl":"type.googleapis.com/google.crypto.tink.KmsEnvelopeAeadKey", "value":"EsMBCoYBZmFrZS1rbXM6Ly9DTTJiM19NREVsUUtTQW93ZEhsd1pTNW5iMjluYkdWaGNHbHpMbU52YlM5bmIyOW5iR1V1WTNKNWNIUnZMblJwYm1zdVFXVnpSMk50UzJWNUVoSWFFSUs3NXQ1TC1hZGxVd1ZoV3ZSdVdVd1lBUkFCR00yYjNfTURJQUUSOAowdHlwZS5nb29nbGVhcGlzLmNvbS9nb29nbGUuY3J5cHRvLnRpbmsuQWVzR2NtS2V5EgIQIBgB", "keyMaterialType":"REMOTE"}, "status":"ENABLED", "keyId":3980895889, "outputPrefixType":"TINK"}]}`
+
+	parsedHandle, err := insecurecleartextkeyset.Read(
+		keyset.NewJSONReader(bytes.NewBuffer([]byte(jsonKeysetWithTinkPrefix))))
+	if err != nil {
+		t.Fatalf("insecurecleartextkeyset.Read() err = %v, want nil", err)
+	}
+
+	primitive, err := aead.New(parsedHandle)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	plaintext := []byte("message")
+	associatedData := []byte("example KMS envelope AEAD encryption")
+
+	ciphertext, err := primitive.Encrypt(plaintext, associatedData)
+	if err != nil {
+		t.Fatal(err)
+	}
+	gotPlaintext, err := primitive.Decrypt(ciphertext, associatedData)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Equal(gotPlaintext, plaintext) {
+		t.Fatalf("got plaintext %q, want %q", gotPlaintext, plaintext)
+	}
+
+	// Also verify that the ciphertext has a TINK prefix
+	gotPrefix := ciphertext[:5]
+	// The Tink prefix is 0x01 followed by the 4 bytes key ID. The key ID is 3980895889, which is
+	// equal to 0xed47a691.
+	wantPrefix := []byte{0x01, 0xed, 0x47, 0xa6, 0x91}
+	if !bytes.Equal(gotPrefix, wantPrefix) {
+		t.Fatalf("ciphertext[:5] = %q, want %q", gotPrefix, wantPrefix)
+	}
+
+}