Change payload type to byte[]

sigstore-java/src/main/java/dev/sigstore/bundle/Bundle.java

@@ -138,7 +138,7 @@ public interface MessageDigest {
   public interface DsseEnvelope {
 
     /** An arbitrary payload that does not need to be parsed to be validated */
-    String getPayload();
+    byte[] getPayload();
 
     /** Information on how to interpret the payload */
     String getPayloadType();
@@ -158,12 +158,18 @@ default byte[] getPAE() {
               + " "
               + getPayloadType()
               + " "
-              + getPayload().length()
+              + getPayloadAsString().length()
               + " "
-              + getPayload())
+              + getPayloadAsString())
           .getBytes(StandardCharsets.UTF_8);
     }
 
+    @Lazy
+    @Gson.Ignore
+    default String getPayloadAsString() {
+      return new String(getPayload(), StandardCharsets.UTF_8);
+    }
+
     @Lazy
     @Gson.Ignore
     default byte[] getSignature() {

sigstore-java/src/main/java/dev/sigstore/bundle/BundleReader.java

@@ -103,7 +103,7 @@ static Bundle readBundle(Reader jsonReader) throws BundleParseException {
       var dsseEnvelopeProto = protoBundle.getDsseEnvelope();
       var dsseEnvelopeBuilder =
           ImmutableDsseEnvelope.builder()
-              .payload(dsseEnvelopeProto.getPayload().toStringUtf8())
+              .payload(dsseEnvelopeProto.getPayload().toByteArray())
               .payloadType(dsseEnvelopeProto.getPayloadType());
       for (int sigIndex = 0; sigIndex < dsseEnvelopeProto.getSignaturesCount(); sigIndex++) {
         dsseEnvelopeBuilder.addSignatures(

sigstore-java/src/main/java/dev/sigstore/dsse/InTotoPayload.java

@@ -52,6 +52,6 @@ interface Subject {
   }
 
   static InTotoPayload from(DsseEnvelope dsseEnvelope) {
-    return GSON.get().fromJson(dsseEnvelope.getPayload(), InTotoPayload.class);
+    return GSON.get().fromJson(dsseEnvelope.getPayloadAsString(), InTotoPayload.class);
   }
 }