Implement support for AEAD tag at the end

Using the AEAD tag at the end is the standard way of doing AEAD. Several
APIs even only support the tag at the end (e.g. mbed TLS). Having the tag at
the front or end makes no difference for security but allows streaming HW
implementations like NICs to be much more efficient as they do not need to
buffer a whole packet content and encrypt it to finally write the tag but
instead just add the calculated tag at the end of processing.

Change-Id: I00821d75342daf3f813b829812d648fe298bea81
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <20240214132719.3031492-1-frank@lichtenheld.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg28239.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>

src/openvpn/crypto.c

@@ -104,14 +104,10 @@ openvpn_encrypt_aead(struct buffer *buf, struct buffer work,
         ASSERT(cipher_ctx_reset(ctx->cipher, iv));
     }
 
-    /* Reserve space for authentication tag */
-    mac_out = buf_write_alloc(&work, mac_len);
-    ASSERT(mac_out);
-
     dmsg(D_PACKET_CONTENT, "ENCRYPT FROM: %s", format_hex(BPTR(buf), BLEN(buf), 80, &gc));
 
     /* Buffer overflow check */
-    if (!buf_safe(&work, buf->len + cipher_ctx_block_size(ctx->cipher)))
+    if (!buf_safe(&work, buf->len + mac_len + cipher_ctx_block_size(ctx->cipher)))
     {
         msg(D_CRYPT_ERRORS,
             "ENCRYPT: buffer size error, bc=%d bo=%d bl=%d wc=%d wo=%d wl=%d",
@@ -121,9 +117,16 @@ openvpn_encrypt_aead(struct buffer *buf, struct buffer work,
     }
 
     /* For AEAD ciphers, authenticate Additional Data, including opcode */
-    ASSERT(cipher_ctx_update_ad(ctx->cipher, BPTR(&work), BLEN(&work) - mac_len));
+    ASSERT(cipher_ctx_update_ad(ctx->cipher, BPTR(&work), BLEN(&work)));
     dmsg(D_PACKET_CONTENT, "ENCRYPT AD: %s",
-         format_hex(BPTR(&work), BLEN(&work) - mac_len, 0, &gc));
+         format_hex(BPTR(&work), BLEN(&work), 0, &gc));
+
+    if (!(opt->flags & CO_AEAD_TAG_AT_THE_END))
+    {
+        /* Reserve space for authentication tag */
+        mac_out = buf_write_alloc(&work, mac_len);
+        ASSERT(mac_out);
+    }
 
     /* Encrypt packet ID, payload */
     ASSERT(cipher_ctx_update(ctx->cipher, BEND(&work), &outlen, BPTR(buf), BLEN(buf)));
@@ -133,6 +136,14 @@ openvpn_encrypt_aead(struct buffer *buf, struct buffer work,
     ASSERT(cipher_ctx_final(ctx->cipher, BEND(&work), &outlen));
     ASSERT(buf_inc_len(&work, outlen));
 
+    /* if the tag is at end the end, allocate it now */
+    if (opt->flags & CO_AEAD_TAG_AT_THE_END)
+    {
+        /* Reserve space for authentication tag */
+        mac_out = buf_write_alloc(&work, mac_len);
+        ASSERT(mac_out);
+    }
+
     /* Write authentication tag */
     ASSERT(cipher_ctx_get_tag(ctx->cipher, mac_out, mac_len));
 
@@ -353,7 +364,6 @@ openvpn_decrypt_aead(struct buffer *buf, struct buffer work,
     static const char error_prefix[] = "AEAD Decrypt error";
     struct packet_id_net pin = { 0 };
     const struct key_ctx *ctx = &opt->key_ctx_bi.decrypt;
-    uint8_t *tag_ptr = NULL;
     int outlen;
     struct gc_arena gc;
 
@@ -406,19 +416,29 @@ openvpn_decrypt_aead(struct buffer *buf, struct buffer work,
 
     /* keep the tag value to feed in later */
     const int tag_size = OPENVPN_AEAD_TAG_LENGTH;
-    if (buf->len < tag_size)
+    if (buf->len < tag_size + 1)
     {
-        CRYPT_ERROR("missing tag");
+        CRYPT_ERROR("missing tag or no payload");
     }
-    tag_ptr = BPTR(buf);
-    ASSERT(buf_advance(buf, tag_size));
-    dmsg(D_PACKET_CONTENT, "DECRYPT MAC: %s", format_hex(tag_ptr, tag_size, 0, &gc));
 
-    if (buf->len < 1)
+    const int ad_size = BPTR(buf) - ad_start;
+
+    uint8_t *tag_ptr = NULL;
+    int data_len = 0;
+
+    if (opt->flags & CO_AEAD_TAG_AT_THE_END)
+    {
+        data_len = BLEN(buf) - tag_size;
+        tag_ptr = BPTR(buf) + data_len;
+    }
+    else
     {
-        CRYPT_ERROR("missing payload");
+        tag_ptr = BPTR(buf);
+        ASSERT(buf_advance(buf, tag_size));
+        data_len = BLEN(buf);
     }
 
+    dmsg(D_PACKET_CONTENT, "DECRYPT MAC: %s", format_hex(tag_ptr, tag_size, 0, &gc));
     dmsg(D_PACKET_CONTENT, "DECRYPT FROM: %s", format_hex(BPTR(buf), BLEN(buf), 0, &gc));
 
     /* Buffer overflow check (should never fail) */
@@ -427,20 +447,19 @@ openvpn_decrypt_aead(struct buffer *buf, struct buffer work,
         CRYPT_ERROR("potential buffer overflow");
     }
 
-    {
-        /* feed in tag and the authenticated data */
-        const int ad_size = BPTR(buf) - ad_start - tag_size;
-        ASSERT(cipher_ctx_update_ad(ctx->cipher, ad_start, ad_size));
-        dmsg(D_PACKET_CONTENT, "DECRYPT AD: %s",
-             format_hex(BPTR(buf) - ad_size - tag_size, ad_size, 0, &gc));
-    }
+
+    /* feed in tag and the authenticated data */
+    ASSERT(cipher_ctx_update_ad(ctx->cipher, ad_start, ad_size));
+    dmsg(D_PACKET_CONTENT, "DECRYPT AD: %s",
+         format_hex(ad_start, ad_size, 0, &gc));
 
     /* Decrypt and authenticate packet */
     if (!cipher_ctx_update(ctx->cipher, BPTR(&work), &outlen, BPTR(buf),
-                           BLEN(buf)))
+                           data_len))
     {
         CRYPT_ERROR("cipher update failed");
     }
+
     ASSERT(buf_inc_len(&work, outlen));
     if (!cipher_ctx_final_check_tag(ctx->cipher, BPTR(&work) + outlen,
                                     &outlen, tag_ptr, tag_size))

src/openvpn/crypto.h

@@ -279,6 +279,10 @@ struct crypto_options
     /**< Bit-flag indicating that renegotiations are using tls-crypt
      *   with a TLS-EKM derived key.
      */
+#define CO_AEAD_TAG_AT_THE_END  (1<<8)
+    /**< Bit-flag indicating that the AEAD tag is at the end of the
+     *   packet.
+     */
 
     unsigned int flags;         /**< Bit-flags determining behavior of
                                  *   security operation functions. */

src/openvpn/init.c

@@ -2328,6 +2328,10 @@ tls_print_deferred_options_results(struct context *c)
         {
             buf_printf(&out, " dyn-tls-crypt");
         }
+        if (o->imported_protocol_flags & CO_AEAD_TAG_AT_THE_END)
+        {
+            buf_printf(&out, " aead-tag-end");
+        }
     }
 
     if (buf_len(&out) > strlen(header))

src/openvpn/options.c

@@ -8692,6 +8692,10 @@ add_option(struct options *options,
                 options->imported_protocol_flags |= CO_USE_DYNAMIC_TLS_CRYPT;
             }
 #endif
+            else if (streq(p[j], "aead-tag-end"))
+            {
+                options->imported_protocol_flags |= CO_AEAD_TAG_AT_THE_END;
+            }
             else
             {
                 msg(msglevel, "Unknown protocol-flags flag: %s", p[j]);

src/openvpn/push.c

@@ -29,6 +29,7 @@
 
 #include "push.h"
 #include "options.h"
+#include "crypto.h"
 #include "ssl.h"
 #include "ssl_verify.h"
 #include "ssl_ncp.h"
@@ -688,6 +689,11 @@ prepare_push_reply(struct context *c, struct gc_arena *gc,
         buf_printf(&proto_flags, " dyn-tls-crypt");
     }
 
+    if (o->imported_protocol_flags & CO_AEAD_TAG_AT_THE_END)
+    {
+        buf_printf(&proto_flags, " aead-tag-end");
+    }
+
     if (buf_len(&proto_flags) > 0)
     {
         push_option_fmt(gc, push_list, M_USAGE, "protocol-flags%s", buf_str(&proto_flags));

tests/unit_tests/openvpn/test_ssl.c

@@ -265,6 +265,17 @@ uninit_crypto_options(struct crypto_options *co)
 
 }
 
+/* This adds a few more methods than strictly necessary but this allows
+ * us to see which exact test was run from the backtrace of the test
+ * when it fails */
+static void
+run_data_channel_with_cipher_end(const char *cipher)
+{
+    struct crypto_options co = init_crypto_options(cipher, "none");
+    co.flags |= CO_AEAD_TAG_AT_THE_END;
+    do_data_channel_round_trip(&co);
+    uninit_crypto_options(&co);
+}
 
 static void
 run_data_channel_with_cipher(const char *cipher, const char *auth)
@@ -274,21 +285,25 @@ run_data_channel_with_cipher(const char *cipher, const char *auth)
     uninit_crypto_options(&co);
 }
 
+
 static void
 test_data_channel_roundtrip_aes_128_gcm(void **state)
 {
+    run_data_channel_with_cipher_end("AES-128-GCM");
     run_data_channel_with_cipher("AES-128-GCM", "none");
 }
 
 static void
 test_data_channel_roundtrip_aes_192_gcm(void **state)
 {
+    run_data_channel_with_cipher_end("AES-192-GCM");
     run_data_channel_with_cipher("AES-192-GCM", "none");
 }
 
 static void
 test_data_channel_roundtrip_aes_256_gcm(void **state)
 {
+    run_data_channel_with_cipher_end("AES-256-GCM");
     run_data_channel_with_cipher("AES-256-GCM", "none");
 }
 
@@ -318,6 +333,8 @@ test_data_channel_roundtrip_chacha20_poly1305(void **state)
         skip();
         return;
     }
+
+    run_data_channel_with_cipher_end("ChaCha20-Poly1305");
     run_data_channel_with_cipher("ChaCha20-Poly1305", "none");
 }