TapDance: End-to-Middle Anticensorship without Flow Blocking

Asymmetric IP routing is a fundamental constraint on prior E2M designs: tier-2 ISPs typically see around 25% of packets on asymmetric paths, while tier-1 ISPs can have up to 90% of packets on asymmetric flows. Because Telex requires observing both directions of a connection to derive the client-server TLS master secret, this asymmetry severely constrains where it can be deployed. TapDance resolves this by using chosen-ciphertext steganography to leak the master secret from client to station in a single upstream packet, making it functional under fully asymmetric routing.

§1, §6 evaluation dpitraffic-shape generic

TapDance introduces chosen-ciphertext steganography, which allows the client to embed an arbitrary-length hidden message inside a valid TLS ciphertext without invalidating the TLS MAC or session. By exploiting ciphertext malleability in both stream-cipher (counter) mode and CBC mode, the client can choose specific byte values to appear in the ciphertext while constraining plaintext to a safe ASCII range (0x40–0x7F), encoding 6 bits of tag data per ciphertext byte. This provides unbounded covert-channel bandwidth, compared to the fixed 224-bit TLS nonce used by Telex and Decoy Routing or the 24-bit TCP ISN used by Cirripede.

§3, §6 defense dpirandom-payload-detect generic

All three prior end-to-middle (E2M) schemes — Telex, Cirripede, and Decoy Routing — require an inline flow-blocking component at the participating ISP, which adds latency, introduces a single point of failure, and may violate carrier SLAs. In private discussions with ISPs, the authors found that despite willingness to assist Internet freedom technically and financially, none were willing to deploy existing E2M technologies due to these operational impacts. TapDance removes the inline blocking requirement entirely, requiring only a passive tap and packet-injection capability.

§1, §6 deployment middlebox-interferenceip-blocking generic

Scanning a 1% sample of the IPv4 address space and the Alexa top-1-million domains, the authors found that over half of all TLS hosts will leave an incomplete HTTP request connection open for at least 60 seconds before sending data or closing the connection; many had timeouts exceeding 5 minutes. The 16-core TapDance station prototype processes over 12,000 tag verifications per second per core, with approximately 90% of CPU time consumed by a single ECC point multiplication on Curve25519. The station adds a median latency of 270 milliseconds to page downloads versus direct connections, and a single station instance can be overwhelmed by approximately 1.2 Gbps of TLS application-layer traffic.

§7.3, §8, Figure 5 evaluation dpitraffic-shape generic

Because TapDance does not block client-to-server packets, a censor can inject a TCP packet with a stale acknowledgment number directly to the true decoy server; the server will reply with its actual TCP sequence state, which will differ from the sequence numbers the TapDance station has been using — confirming the flow is proxied. This active packet-injection attack is qualitatively easier to execute against TapDance than against Telex or Cirripede, which used inline blocking to prevent such probes from reaching the server. Table 1 in the paper confirms that TapDance, unlike Telex, lacks replay/preplay attack resistance and has no traffic-analysis defense.

§5.2, Table 1 detection active-probingpacket-injectionrst-injection generic