From 10560371ab9f866c904fa507af856df1316075aa Mon Sep 17 00:00:00 2001
From: Camden Dixie O'Brien <cdo@wip.sh>
Date: Tue, 25 Feb 2025 21:58:39 +0000
Subject: [PATCH] Fix CHOICE DER support

---
 client/src/der.vala         | 7 ++++---
 client/tests/der_tests.vala | 4 ++--
 2 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/client/src/der.vala b/client/src/der.vala
index 968a0e7..82332a8 100644
--- a/client/src/der.vala
+++ b/client/src/der.vala
@@ -48,7 +48,7 @@ namespace StudySystemClient.Der {
 
 	private static Datum decode_datum(uint8 type, uint8[] content)
 			throws DecodeError {
-		if ((type & 0xc0) == Choice.BASE_TYPE)
+		if ((type & ~Choice.ID_MASK) == Choice.BASE_TYPE)
 			return new Choice.from_content(type, content);
 		switch (type) {
 		case Boolean.TYPE:
@@ -266,7 +266,8 @@ namespace StudySystemClient.Der {
 	}
 
 	public class Choice : Datum {
-		internal const uint8 BASE_TYPE = 0x80;
+		internal const uint8 BASE_TYPE = 0xa0;
+		internal const uint8 ID_MASK = 0x1f;
 
 		public int id { get; private set; }
 		public Datum value { get; private set; }
@@ -282,7 +283,7 @@ namespace StudySystemClient.Der {
 			throws DecodeError {
 			this.type = type;
 			content = bytes;
-			id = type & 0x3f;
+			id = type & ID_MASK;
 			value = decode(bytes);
 		}
 	}
diff --git a/client/tests/der_tests.vala b/client/tests/der_tests.vala
index da564a0..22ea1ae 100644
--- a/client/tests/der_tests.vala
+++ b/client/tests/der_tests.vala
@@ -175,7 +175,7 @@ void main(string[] args) {
 	Test.add_func("/encode/choice/1:foo", () => {
 			var choice = new Der.Choice(1, new Utf8String("foo"));
 			var expected = new uint8[] {
-				0x81, 0x05, 0x0c, 0x03, 0x66, 0x6f, 0x6f
+				0xa1, 0x05, 0x0c, 0x03, 0x66, 0x6f, 0x6f
 			};
 			test_encode(choice, expected);
 		});
@@ -266,7 +266,7 @@ void main(string[] args) {
 	Test.add_func("/decode/choice/1:foo", () => {
 			var expected_id = 1;
 			var bytes = new uint8[] {
-				0x81, 0x05, 0x0c, 0x03, 0x66, 0x6f, 0x6f
+				0xa1, 0x05, 0x0c, 0x03, 0x66, 0x6f, 0x6f
 			};
 			Der.Choice choice;
 			try {