Fingerprint, handshake and nodes suggest BitTorrent handshake evidence peer ID information exchange.

In many BitTorrent affidavits, the government treats the handshake as if it is a confession: “the target peer identified itself, so we know what happened.” That framing is convenient, but it skips important technical steps.

This post explains, in lawyer terms, what the BitTorrent handshake actually establishes, what Peer ID can show (and what it cannot), and how to ask cross-exam questions that are anchored to logs rather than narrative.

If you want the broader investigation context first, start with:

The handshake in plain English

The BitTorrent “handshake” is the initial exchange that lets two peers confirm they’re talking about the same torrent and establishes basic identifiers for the session.

At a high level, the handshake communicates:

  • A protocol identifier (“BitTorrent protocol”)
  • Reserved flags (features/extensions)
  • The infohash (identifier for the torrent’s “info” dictionary)
  • The peer’s peer_id value (a 20-byte client identifier) [1]

This matters because it is often the earliest recorded artifact tying a specific IP:port to a specific torrent identifier.

If the case briefing is leaning heavily on “bitfield proves possession” language, see this companion post: BitTorrent bitfield evidence.

What the handshake proves (usually)

When logged correctly, a handshake tends to support claims like:

  • This IP:port responded as a BitTorrent peer for a given infohash at a specific time.
  • A connection existed long enough to exchange the handshake.
  • The peer presented a peer_id value during that connection.

That is meaningful. It is also narrower than the way affidavits sometimes describe it.

What the handshake does NOT prove (by itself)

Here are the common gaps between protocol reality and affidavit language:

  • Handshake ≠ complete file download: A handshake can occur without any pieces being transferred.
  • Handshake ≠ file on disk: It does not show that the target device stored the file at the relevant time.
  • Handshake ≠ “knowing” conduct: It does not prove intent, knowledge, or who was at the keyboard.
  • Handshake ≠ identity of a person: It identifies a peer connection, not a human being.

This is why you should treat handshake evidence as a building block that must be combined with additional logs (piece requests, piece responses, hash verification, and timekeeping) before broad conclusions are justified.

Peer ID: what it can show (and what it cannot)

Peer ID is a 20-byte value that BitTorrent clients present in the handshake [1]. In practice, Peer ID often provides client-family hints (e.g., a prefix suggesting a particular client implementation/version style).

What Peer ID can help with

  • Client-family clues: Some Peer ID formats include recognizable prefixes (depending on the client).
  • Session correlation: In some contexts, Peer ID can help correlate logs across events or runs (carefully).
  • Sanity checks: If the government claims “client X,” Peer ID may be one supporting data point.

What Peer ID cannot prove

  • The identity of the person using the computer.
  • Intent or knowledge regarding the content.
  • Complete possession of a file.
  • Exclusive control over the network or device.

Even technically, Peer IDs can be generated in ways that limit their stability. Some clients randomize identifiers; some environments reset them; NAT and shared networks can introduce attribution ambiguity.

A practical (often missed) attribution step: look for Peer ID traces on the seized device

Even if Peer ID changes across sessions, traces of Peer ID values can sometimes be found on the seized computer’s file system, depending on the BitTorrent client and how it logs or persists session state. When present, that can strengthen the linkage between:

  • The Peer ID observed in network/tool logs, and
  • The specific device that was imaged and examined.

Where to look (client-dependent):

  • Client log files (session/network logs that may record peer_id values)
  • Resume/state files (artifacts that track active torrents, progress, and sometimes connection/session metadata)
  • Configuration directories (especially when a client logs debug-level network events)

In practice, many law-enforcement workflows appear to stop at tool outputs and do not pursue this additional correlation step. If attribution is contested, it is worth asking explicitly whether the examiner searched for Peer ID traces (and if not, why not).

Common defense misconceptions (and the clean correction)

These misconceptions show up on both sides. Fixing them early helps you stay credible.

  • Misconception: “A handshake proves the complete file was downloaded.”
    • Correction: A handshake proves a BitTorrent connection and shared identifiers; you still need piece-transfer and verification evidence.
  • Misconception: “Peer ID identifies the user.”
    • Correction: Peer ID identifies a client session identifier, not a person.
  • Misconception: “Infohash equals the file hash.”
    • Correction: Infohash is a torrent identifier (metadata). A file hash is computed over file bytes. For that distinction and why it matters to probable cause language, see Hashes and known-file databases.

Cross-exam roadmap: questions that map to the logs

Your goal is to get the witness to commit to testable propositions: what was recorded, how, and what is missing.

A) Ports, NAT, and attribution

  • “Is the port in your report the same port used in the handshake record?”
  • “What evidence shows the target device—not a router, NAT mapping, or another host—was the BitTorrent endpoint?”
  • “Did you verify whether the network was shared (Wi‑Fi guests, roommates, business/public access)?”

B) Time sync and timestamp handling

  • “What time source was used (NTP, domain time, manual set)?”
  • “What time zone are your logs in?”
  • “Are the timestamps consistent across your tool output, ISP returns, and any packet captures?”

C) Capture method and raw data availability

  • “Do you have packet capture, or only tool-generated logs?”
  • “If there is no packet capture, what exactly does the tool log, and how is it generated?”
  • “Can you point to the log line showing the infohash and peer_id values you relied on?”

D) Peer ID recording and interpretation

  • “How did you interpret the Peer ID—did you use a published mapping, training material, or an assumption?”
  • “Does your report treat Peer ID as a client hint or as a unique device identifier?”
  • “Did you observe changes in Peer ID across sessions?”

E) Handshake vs. data transfer

  • “Show the log line that demonstrates any piece request.”
  • “Show the log line that demonstrates any piece response.”
  • “Show the log line that demonstrates hash verification of downloaded data.”

If those exhibits are not present, that does not automatically defeat the case, but it helps you narrow what the evidence truly supports.

What to request in discovery (exhibits that matter)

Ask for materials that let you independently evaluate handshake claims:

  • Run logs / exports showing: infohash, peer_id, IP:port, timestamps, and session identifiers.
  • Connection details: handshake success/failure, retries, timeouts, and any peer selection logic.
  • Raw capture (if available): packet capture files, hashes of capture files, and chain-of-custody documentation.
  • Interpretation artifacts: any training material or documentation used to interpret Peer ID formats.

If you want a one-stop foundational primer on digital hashing and why chain-of-custody details matter when evidence is moved between systems, see Digital Hashing for Lawyers: Understanding the Essentials for Criminal Defense [2].

Bottom line

Handshake evidence can be strong for what it actually shows: a BitTorrent connection and identifiers tied to a torrent. It becomes overstated when it is used as a shortcut to prove full download, possession on disk, or identity of the person.

If you want help turning tool output into targeted discovery requests or cross-exam questions that map cleanly to what the logs actually show, contact Lucid Truth Technologies using the LTT contact form: Contact.

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This article is for informational purposes and does not provide legal advice. Every case turns on specific facts and controlling law in your jurisdiction. Work with qualified counsel and, where appropriate, a qualified expert.

References

[1] B. Cohen, “The BitTorrent Protocol Specification (BEP 3),” BitTorrent.org, 2017. [Online]. Available: https://www.bittorrent.org/beps/bep_0003.html

[2] Lucid Truth Technologies, “Digital Hashing for Lawyers: Understanding the Essentials for Criminal Defense,” Lucid Truth Technologies, 2025. [Online]. Available: https://lucidtruthtechnologies.com/digital-hashing-for-lawyers/