BTP's secret retention period for transport t is Rt + 2C + Lt, where Rt is the rotation period, C is the maximum clock-skew tolerance, and Lt is the maximum transport latency. With Rt = 2C + Lt only two temporary secrets need simultaneous storage. Concrete durations: TCP with automatic clocks (C=10s, Lt=60s) requires 2 minutes 40 seconds; TCP with manual clocks (C=1800s) requires 4 hours 2 minutes; mail with manual clocks (Lt=2 weeks) requires 4 weeks 4 hours.

The paper explicitly flags that BTP's fixed-size b-byte connection tag creates an active-probing oracle: a censor that sends b−1 bytes and observes no close, then sends one more byte and observes a close, can confirm the endpoint is running BTP. Preventing such active-probing attacks is identified as future work.

§8 detection

BTP's forward secrecy guarantee depends on reliably destroying old keys, but the paper notes that secure deletion from persistent storage—especially solid-state storage—is difficult with current operating systems and hardware. The recommended mitigation is passphrase-derived encryption of stored secrets, though this shifts the problem to passphrase protection.

§6.3 defense

BTP achieves forward secrecy over unidirectional transports—where ephemeral in-band key exchange is impossible—by using a one-way key derivation function (NIST SP 800-108) to produce sequential temporary secrets from an initial shared secret. Once both devices destroy a given temporary secret, no keys derived from it can be reconstructed even if devices are later compromised.

§2, §5.2–5.3 defense

BTP's wire protocol contains no handshakes, timeouts, or plaintext headers. Connections open with a pseudo-random b-byte tag that the recipient can compute in advance from its key state, making BTP frames indistinguishable from random data to a passive observer who does not know the shared secret.

§2, §3.2 defense