handle Bytes Index [0] selector and fix case input

capability/policy/selector/selector.go

@@ -222,6 +222,24 @@ func resolve(sel Selector, subject ipld.Node, at []string) (ipld.Node, []ipld.No
 				} else {
 					cur = basicnode.NewString(string(str[idx]))
 				}
+			} else if cur != nil && cur.Kind() == datamodel.Kind_Bytes {
+				b, err := cur.AsBytes()
+				if err != nil {
+					return nil, nil, err
+				}
+				idx := seg.Index()
+				if idx < 0 {
+					idx = len(b) + idx
+				}
+				if idx < 0 || idx >= len(b) {
+					if seg.Optional() {
+						cur = nil
+					} else {
+						return nil, nil, newResolutionError(fmt.Sprintf("index out of bounds: %d", seg.Index()), at)
+					}
+				} else {
+					cur = basicnode.NewInt(int64(b[idx]))
+				}
 			} else if seg.Optional() {
 				cur = nil
 			} else {

capability/policy/selector/supported_test.go

@@ -37,7 +37,7 @@ func TestSupportedForms(t *testing.T) {
 		{Name: "Index", Selector: `.[0]`, Input: `[1, 2]`, Output: `1`},
 		{Name: "Negative Index", Selector: `.[-1]`, Input: `[1, 2]`, Output: `2`},
 		{Name: "String Index", Selector: `.[0]`, Input: `"Hi"`, Output: `"H"`},
-		{Name: "Bytes Index", Selector: `.[0]`, Input: `{"/":{"bytes":"AAE"}`, Output: `0`},
+		{Name: "Bytes Index", Selector: `.[0]`, Input: `{"/":{"bytes":"AAE"}}`, Output: `0`},
 		{Name: "Array Slice", Selector: `.[0:2]`, Input: `[0, 1, 2]`, Output: `[0, 1]`},
 		{Name: "Array Slice", Selector: `.[1:]`, Input: `[0, 1, 2]`, Output: `[1, 2]`},
 		{Name: "Array Slice", Selector: `.[:2]`, Input: `[0, 1, 2]`, Output: `[0, 1]`},