feat(calfeed): RFC-5545 iCal renderer with injection-safe escaping

2 files changed, 144 insertions, 0 deletions

diff --git a/internal/calfeed/ical.go b/internal/calfeed/ical.go
new file mode 100644
index 0000000..bce3710
--- /dev/null
+++ b/internal/calfeed/ical.go
@@ -0,0 +1,92 @@
+package calfeed
+
+import (
+	"strconv"
+	"strings"
+	"time"
+)
+
+// icalEscape neutralises RFC-5545 TEXT specials AND all CR/LF, so untrusted
+// celebration names / citations cannot inject iCal lines or properties.
+func icalEscape(s string) string {
+	s = strings.ReplaceAll(s, "\\", "\\\\")
+	s = strings.ReplaceAll(s, ";", "\\;")
+	s = strings.ReplaceAll(s, ",", "\\,")
+	s = strings.ReplaceAll(s, "\r\n", "\\n")
+	s = strings.ReplaceAll(s, "\r", "\\n")
+	s = strings.ReplaceAll(s, "\n", "\\n")
+	return s
+}
+
+// foldLine folds a content line at 75 octets with a leading space on
+// continuations (RFC 5545 §3.1). Counts bytes; folding runs after escaping.
+func foldLine(line string) string {
+	if len(line) <= 75 {
+		return line
+	}
+	var b strings.Builder
+	for i := 0; i < len(line); {
+		end := i + 75
+		if i > 0 {
+			end = i + 74 // account for the leading space
+		}
+		if end > len(line) {
+			end = len(line)
+		}
+		if i > 0 {
+			b.WriteString("\r\n ")
+		}
+		b.WriteString(line[i:end])
+		i = end
+	}
+	return b.String()
+}
+
+func calName(form string) string {
+	if form == "old" {
+		return "Lectio — Extraordinary Form"
+	}
+	return "Lectio — Ordinary Form"
+}
+
+// ICal renders days as an RFC-5545 VCALENDAR, one all-day VEVENT per day.
+func ICal(form string, days []DayView, stamp time.Time) []byte {
+	var lines []string
+	add := func(s string) { lines = append(lines, foldLine(s)) }
+	add("BEGIN:VCALENDAR")
+	add("VERSION:2.0")
+	add("PRODID:-//lectio//calendar//EN")
+	add("CALSCALE:GREGORIAN")
+	add("METHOD:PUBLISH")
+	add("X-WR-CALNAME:" + icalEscape(calName(form)))
+	ds := stamp.UTC().Format("20060102T150405Z")
+	for _, d := range days {
+		date := strings.ReplaceAll(d.Date, "-", "") // YYYYMMDD
+		next, _ := time.Parse("2006-01-02", d.Date)
+		end := next.AddDate(0, 0, 1).Format("20060102")
+		var desc []string
+		desc = append(desc, "Season: "+d.Season+" (week "+strconv.Itoa(d.Week)+")")
+		if d.Observed.Rank != "" {
+			desc = append(desc, "Rank: "+d.Observed.Rank)
+		}
+		desc = append(desc, "Colour: "+d.Colour)
+		for _, r := range d.Readings {
+			desc = append(desc, r.Part+": "+r.Citation)
+		}
+		add("BEGIN:VEVENT")
+		add("UID:" + d.Date + "-" + form + "@lectio") // input-free, stable
+		add("DTSTAMP:" + ds)
+		add("DTSTART;VALUE=DATE:" + date)
+		add("DTEND;VALUE=DATE:" + end)
+		add("SUMMARY:" + icalEscape(d.Observed.Name))
+		// join with real \n, then escape the whole string so \n -> \\n and every
+		// TEXT special is neutralised in one pass.
+		add("DESCRIPTION:" + icalEscape(strings.Join(desc, "\n")))
+		if d.Colour != "" {
+			add("CATEGORIES:" + icalEscape(strings.ToUpper(d.Colour)))
+		}
+		add("END:VEVENT")
+	}
+	add("END:VCALENDAR")
+	return []byte(strings.Join(lines, "\r\n") + "\r\n")
+}
diff --git a/internal/calfeed/ical_test.go b/internal/calfeed/ical_test.go
new file mode 100644
index 0000000..fb7ee9a
--- /dev/null
+++ b/internal/calfeed/ical_test.go
@@ -0,0 +1,52 @@
+package calfeed
+
+import (
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestICalEscapeInjection(t *testing.T) {
+	// A malicious custom-calendar name must not be able to inject lines/props.
+	got := icalEscape("Evil\r\nBEGIN:VEVENT\nSUMMARY:hijack; a,b\\c")
+	if strings.ContainsAny(got, "\r\n") {
+		t.Fatalf("unescaped newline survived: %q", got)
+	}
+	for _, sub := range []string{`\n`, `\;`, `\,`, `\\`} {
+		if !strings.Contains(got, sub) {
+			t.Fatalf("missing escape %q in %q", sub, got)
+		}
+	}
+}
+
+func TestICalStructure(t *testing.T) {
+	days := []DayView{{
+		Date: "2026-01-06", Season: "time-after-epiphany", Week: 1, Colour: "white",
+		Observed: CelView{Name: "The Epiphany of the Lord", Rank: "class-1"},
+		Readings: []ReadingView{{Part: "gospel", Citation: "Matt 2:1-12"}},
+	}}
+	out := string(ICal("old", days, time.Date(2026, 7, 27, 12, 0, 0, 0, time.UTC)))
+	for _, want := range []string{
+		"BEGIN:VCALENDAR", "VERSION:2.0", "PRODID:-//lectio//calendar//EN",
+		"BEGIN:VEVENT", "UID:2026-01-06-old@lectio", "DTSTART;VALUE=DATE:20260106",
+		"DTEND;VALUE=DATE:20260107", "SUMMARY:The Epiphany of the Lord",
+		"CATEGORIES:WHITE", "DTSTAMP:20260727T120000Z", "END:VEVENT", "END:VCALENDAR",
+	} {
+		if !strings.Contains(out, want) {
+			t.Fatalf("missing %q in:\n%s", want, out)
+		}
+	}
+	// injection attempt via day count: exactly one VEVENT
+	if strings.Count(out, "BEGIN:VEVENT") != 1 {
+		t.Fatalf("expected 1 VEVENT")
+	}
+}
+
+func TestFoldLine(t *testing.T) {
+	long := "SUMMARY:" + strings.Repeat("x", 200)
+	for _, line := range strings.Split(foldLine(long), "\r\n") {
+		if len(line) > 75 {
+			t.Fatalf("line exceeds 75 octets: %d", len(line))
+		}
+	}
+}