From 00e2f5eea29994d19293ec4e8c8775ba73678598 Mon Sep 17 00:00:00 2001
From: Tomas Mraz <tomas@openssl.org>
Date: Tue, 4 Jul 2023 17:30:35 +0200
Subject: [PATCH] Do not ignore empty associated data with AES-SIV mode

The AES-SIV mode allows for multiple associated data items
authenticated separately with any of these being 0 length.

The provided implementation ignores such empty associated data
which is incorrect in regards to the RFC 5297 and is also
a security issue because such empty associated data then become
unauthenticated if an application expects to authenticate them.

Fixes CVE-2023-2975

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/21384)

(cherry picked from commit c426c281cfc23ab182f7d7d7a35229e7db1494d9)
---
 .../implementations/ciphers/cipher_aes_siv.c   | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/providers/implementations/ciphers/cipher_aes_siv.c b/providers/implementations/ciphers/cipher_aes_siv.c
index 45010b90db..b396c8651a 100644
--- a/providers/implementations/ciphers/cipher_aes_siv.c
+++ b/providers/implementations/ciphers/cipher_aes_siv.c
@@ -120,14 +120,18 @@ static int siv_cipher(void *vctx, unsigned char *out, size_t *outl,
     if (!ossl_prov_is_running())
         return 0;
 
-    if (inl == 0) {
-        *outl = 0;
-        return 1;
-    }
+    /* Ignore just empty encryption/decryption call and not AAD. */
+    if (out != NULL) {
+        if (inl == 0) {
+            if (outl != NULL)
+                *outl = 0;
+            return 1;
+        }
 
-    if (outsize < inl) {
-        ERR_raise(ERR_LIB_PROV, PROV_R_OUTPUT_BUFFER_TOO_SMALL);
-        return 0;
+        if (outsize < inl) {
+            ERR_raise(ERR_LIB_PROV, PROV_R_OUTPUT_BUFFER_TOO_SMALL);
+            return 0;
+        }
     }
 
     if (ctx->hw->cipher(ctx, out, in, inl) <= 0)
-- 
2.27.0