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[Openvpn-devel,S] Change in openvpn[master]: Do not attempt to decrypt packets anymore after 2**36 failed decryptions

Message ID a159b2d02a7bc4ec90fd87fbd1a0755dfd692f85-HTML@gerrit.openvpn.net
State New
Headers show
Series [Openvpn-devel,S] Change in openvpn[master]: Do not attempt to decrypt packets anymore after 2**36 failed decryptions | expand

Commit Message

ralf_lici (Code Review) Dec. 20, 2024, 2:49 p.m. UTC 
Attention is currently required from: flichtenheld.

Hello flichtenheld,

I'd like you to do a code review.
Please visit

    http://gerrit.openvpn.net/c/openvpn/+/843?usp=email

to review the following change.


Change subject: Do not attempt to decrypt packets anymore after 2**36 failed decryptions
......................................................................

Do not attempt to decrypt packets anymore after 2**36 failed decryptions

To avoid attacks (especially on Chacha20-Poly1305) we do not allow
decryption anymore after 2**36 failed verifications.

Change-Id: I81440ac28a1ad553652e201234e5ddfe03a8c190
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
---
M src/openvpn/crypto.c
M src/openvpn/crypto.h
M src/openvpn/ssl.c
3 files changed, 40 insertions(+), 1 deletion(-)



  git pull ssh://gerrit.openvpn.net:29418/openvpn refs/changes/43/843/1
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Patch

diff --git a/src/openvpn/crypto.c b/src/openvpn/crypto.c
index faf69fc..88a9f24 100644
--- a/src/openvpn/crypto.c
+++ b/src/openvpn/crypto.c
@@ -405,7 +405,13 @@ 
 {
     static const char error_prefix[] = "AEAD Decrypt error";
     struct packet_id_net pin = { 0 };
-    const struct key_ctx *ctx = &opt->key_ctx_bi.decrypt;
+    struct key_ctx *ctx = &opt->key_ctx_bi.decrypt;
+
+    if (cipher_decrypt_verify_fail_exceeded(ctx))
+    {
+        CRYPT_DROP("Decryption failed verification limit reached.");
+    }
+
     int outlen;
     struct gc_arena gc;
 
@@ -416,6 +422,8 @@ 
     ASSERT(buf->len > 0);
     ASSERT(ctx->cipher);
 
+
+
     dmsg(D_PACKET_CONTENT, "DECRYPT FROM: %s",
          format_hex(BPTR(buf), BLEN(buf), 80, &gc));
 
@@ -511,6 +519,7 @@ 
     if (!cipher_ctx_final_check_tag(ctx->cipher, BPTR(&work) + outlen,
                                     &outlen, tag_ptr, tag_size))
     {
+        ctx->failed_verifications++;
         CRYPT_DROP("packet tag authentication failed");
     }
     ASSERT(buf_inc_len(&work, outlen));
diff --git a/src/openvpn/crypto.h b/src/openvpn/crypto.h
index 4579b65..bb417e1 100644
--- a/src/openvpn/crypto.h
+++ b/src/openvpn/crypto.h
@@ -181,6 +181,8 @@ 
      * with the current key in number of 128 bit blocks (only used for
      * AEAD ciphers) */
     uint64_t plaintext_blocks;
+    /** number of failed verification using this cipher */
+    uint64_t failed_verifications;
 };
 
 #define KEY_DIRECTION_BIDIRECTIONAL 0 /* same keys for both directions */
@@ -623,6 +625,29 @@ 
 cipher_get_aead_limits(const char *ciphername);
 
 /**
+ * Check if the number of failed decryption is over the acceptable limit.
+ * We
+ */
+static inline bool
+cipher_decrypt_verify_fail_exceeded(const struct key_ctx *ctx)
+{
+    /* Use 2**36, same as TLS 1.3 */
+    return ctx->failed_verifications >  (1ull << 36);
+}
+
+/**
+ * Check if the number of failed decryption is approaching the limit and we
+ * should try to move to a new key
+ */
+static inline bool
+cipher_decrypt_verify_fail_warn(const struct key_ctx *ctx)
+{
+    /* Use 2**35, half the amount after which we refuse to decrypt */
+    return ctx->failed_verifications >  (1ull << 35);
+}
+
+
+/**
  * Blocksize used for the AEAD limit caluclation
  *
  * Since cipher_ctx_block_size() is not reliable and will return 1 in many
diff --git a/src/openvpn/ssl.c b/src/openvpn/ssl.c
index c77c4ed..73201ef 100644
--- a/src/openvpn/ssl.c
+++ b/src/openvpn/ssl.c
@@ -3042,6 +3042,11 @@ 
         return true;
     }
 
+    if (cipher_decrypt_verify_fail_warn(&key_ctx_bi->decrypt))
+    {
+        return true;
+    }
+
     return false;
 }
 /*