conditions: verify that AST condition result is bool

lib/challenge/register.go

@@ -11,6 +11,7 @@ import (
 	"github.com/go-jose/go-jose/v4/jwt"
 	"github.com/goccy/go-yaml/ast"
 	"github.com/google/cel-go/cel"
+	"github.com/google/cel-go/common/types"
 	"io"
 	"math/rand/v2"
 	"net/http"
@@ -71,6 +72,13 @@ func (r Register) Create(state StateInterface, name string, pol policy.Challenge
 		if err != nil {
 			return nil, 0, fmt.Errorf("error compiling conditions: %v", err)
 		}
+
+		if out := ast.OutputType(); out == nil {
+			return nil, 0, fmt.Errorf("error compiling conditions: no output")
+		} else if out != types.BoolType {
+			return nil, 0, fmt.Errorf("error compiling conditions: output type is not bool")
+		}
+
 		reg.Condition, err = http_cel.ProgramAst(state.ProgramEnv(), ast)
 		if err != nil {
 			return nil, 0, fmt.Errorf("error compiling program: %v", err)