feat(bskyweb): add HMAC download token for invite passes

bskyweb/cmd/bskyweb/passtoken.go

@@ -0,0 +1,61 @@
+// bskyweb/cmd/bskyweb/passtoken.go
+package main
+
+import (
+	"crypto/hmac"
+	"crypto/sha256"
+	"encoding/base64"
+	"encoding/json"
+	"errors"
+	"strings"
+	"time"
+)
+
+var (
+	ErrTokenExpired = errors.New("invite pass token expired")
+	ErrTokenInvalid = errors.New("invite pass token invalid")
+)
+
+type passTokenPayload struct {
+	Did   string `json:"d"`
+	Theme string `json:"t"`
+	Exp   int64  `json:"e"`
+}
+
+func MintPassToken(secret []byte, did, theme string, now time.Time, ttl time.Duration) (string, error) {
+	p := passTokenPayload{Did: did, Theme: theme, Exp: now.Add(ttl).Unix()}
+	body, err := json.Marshal(p)
+	if err != nil {
+		return "", err
+	}
+	bodyB64 := base64.RawURLEncoding.EncodeToString(body)
+	mac := hmac.New(sha256.New, secret)
+	mac.Write([]byte(bodyB64))
+	sig := base64.RawURLEncoding.EncodeToString(mac.Sum(nil))
+	return bodyB64 + "." + sig, nil
+}
+
+func VerifyPassToken(secret []byte, token string, now time.Time) (string, string, error) {
+	parts := strings.SplitN(token, ".", 2)
+	if len(parts) != 2 {
+		return "", "", ErrTokenInvalid
+	}
+	mac := hmac.New(sha256.New, secret)
+	mac.Write([]byte(parts[0]))
+	want := base64.RawURLEncoding.EncodeToString(mac.Sum(nil))
+	if !hmac.Equal([]byte(want), []byte(parts[1])) {
+		return "", "", ErrTokenInvalid
+	}
+	body, err := base64.RawURLEncoding.DecodeString(parts[0])
+	if err != nil {
+		return "", "", ErrTokenInvalid
+	}
+	var p passTokenPayload
+	if err := json.Unmarshal(body, &p); err != nil {
+		return "", "", ErrTokenInvalid
+	}
+	if now.Unix() > p.Exp {
+		return "", "", ErrTokenExpired
+	}
+	return p.Did, p.Theme, nil
+}

bskyweb/cmd/bskyweb/passtoken_test.go

@@ -0,0 +1,58 @@
+// bskyweb/cmd/bskyweb/passtoken_test.go
+package main
+
+import (
+	"errors"
+	"testing"
+	"time"
+)
+
+func TestPassToken_RoundTrip(t *testing.T) {
+	secret := []byte("test-secret-32-bytes-of-random!!")
+	now := time.Date(2026, 6, 25, 12, 0, 0, 0, time.UTC)
+	tok, err := MintPassToken(secret, "did:plc:abc", "dusk", now, 60*time.Second)
+	if err != nil {
+		t.Fatalf("mint: %v", err)
+	}
+	gotDid, gotTheme, err := VerifyPassToken(secret, tok, now.Add(10*time.Second))
+	if err != nil {
+		t.Fatalf("verify: %v", err)
+	}
+	if gotDid != "did:plc:abc" || gotTheme != "dusk" {
+		t.Fatalf("mismatch: %q %q", gotDid, gotTheme)
+	}
+}
+
+func TestPassToken_Expired(t *testing.T) {
+	secret := []byte("test-secret-32-bytes-of-random!!")
+	now := time.Date(2026, 6, 25, 12, 0, 0, 0, time.UTC)
+	tok, _ := MintPassToken(secret, "did:plc:abc", "day", now, 60*time.Second)
+	_, _, err := VerifyPassToken(secret, tok, now.Add(61*time.Second))
+	if !errors.Is(err, ErrTokenExpired) {
+		t.Fatalf("want ErrTokenExpired, got %v", err)
+	}
+}
+
+func TestPassToken_BadSignature(t *testing.T) {
+	secretA := []byte("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa")
+	secretB := []byte("bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb")
+	now := time.Date(2026, 6, 25, 12, 0, 0, 0, time.UTC)
+	tok, _ := MintPassToken(secretA, "did:plc:abc", "day", now, 60*time.Second)
+	_, _, err := VerifyPassToken(secretB, tok, now)
+	if !errors.Is(err, ErrTokenInvalid) {
+		t.Fatalf("want ErrTokenInvalid, got %v", err)
+	}
+}
+
+func TestPassToken_Mutated(t *testing.T) {
+	secret := []byte("test-secret-32-bytes-of-random!!")
+	now := time.Date(2026, 6, 25, 12, 0, 0, 0, time.UTC)
+	tok, _ := MintPassToken(secret, "did:plc:abc", "day", now, 60*time.Second)
+	// flip a byte in the middle of the payload portion
+	mutated := []byte(tok)
+	mutated[10] ^= 0x01
+	_, _, err := VerifyPassToken(secret, string(mutated), now)
+	if !errors.Is(err, ErrTokenInvalid) {
+		t.Fatalf("want ErrTokenInvalid, got %v", err)
+	}
+}