From 900693b830c34ef54b73bf3c1e88175d864719e9 Mon Sep 17 00:00:00 2001
From: Drew DeVault <sir@cmpwn.com>
Date: Thu, 7 Jan 2021 09:38:30 -0500
Subject: [PATCH] crypto: harden API surface

Make it explicit that expiration is not being tested with Decrypt, and
test that the expiration is meaningful with DecryptWithExpiration.
---
 auth/middleware.go    | 2 +-
 crypto/crypto.go      | 5 ++++-
 crypto/crypto_test.go | 2 +-
 model/cursor.go       | 2 +-
 4 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/auth/middleware.go b/auth/middleware.go
index 15e8d0e6a1dc84a235cbb5f2908ffadc227dd7b1..14f418e38b77357f882c1dae4fd244a271b7b731 100644
--- a/auth/middleware.go
+++ b/auth/middleware.go
@@ -215,7 +215,7 @@ type AuthCookie struct {
 
 func cookieAuth(cookie *http.Cookie, w http.ResponseWriter,
 	r *http.Request, next http.Handler) {
-	payload := crypto.Decrypt([]byte(cookie.Value))
+	payload := crypto.DecryptWithoutExpiration([]byte(cookie.Value))
 	if payload == nil {
 		authError(w, "Invalid authentication cookie", http.StatusForbidden)
 		return
diff --git a/crypto/crypto.go b/crypto/crypto.go
index 025d7453ae5cbbf1bb183ddac277c9d00e5ff874..babfabd68eb55ca09543e27f96b1c120b5ef8b65 100644
--- a/crypto/crypto.go
+++ b/crypto/crypto.go
@@ -63,12 +63,15 @@ func Encrypt(payload []byte) []byte {
 	return msg
 }
 
-func Decrypt(payload []byte) []byte {
+func DecryptWithoutExpiration(payload []byte) []byte {
 	return fernet.VerifyAndDecrypt(payload,
 		time.Duration(0), []*fernet.Key{fernetKey})
 }
 
 func DecryptWithExpiration(payload []byte, expiry time.Duration) []byte {
+	if expiry == 0 {
+		panic(fmt.Errorf("DecryptWithExpiration given expiration of zero. Use DecryptWithoutExpiration if you really meant it."))
+	}
 	return fernet.VerifyAndDecrypt(payload, expiry, []*fernet.Key{fernetKey})
 }
 
diff --git a/crypto/crypto_test.go b/crypto/crypto_test.go
index eb7e7cad4519715e79e7310690f7259881668b57..7cc93b5a970ff5c81a9d478dd66f948abd23c00a 100644
--- a/crypto/crypto_test.go
+++ b/crypto/crypto_test.go
@@ -57,7 +57,7 @@ func TestEncrypt(t *testing.T) {
 	assert.NotNil(t, enc)
 	assert.NotEqual(t, enc, []byte("Hello, world!"))
 
-	dec := Decrypt(enc)
+	dec := DecryptWithoutExpiration(enc)
 	assert.NotNil(t, dec)
 	assert.Equal(t, dec, []byte("Hello, world!"))
 }
diff --git a/model/cursor.go b/model/cursor.go
index 6f87b229b3e1cf24393d349936efa98036998052..3dffa2029ef27222f26198debfee151afbfd806c 100644
--- a/model/cursor.go
+++ b/model/cursor.go
@@ -20,7 +20,7 @@ func (cur *Cursor) UnmarshalGQL(v interface{}) error {
 	if !ok {
 		return fmt.Errorf("cursor must be strings")
 	}
-	plain := crypto.Decrypt([]byte(enc))
+	plain := crypto.DecryptWithoutExpiration([]byte(enc))
 	if plain == nil {
 		return fmt.Errorf("Invalid cursor")
 	}