OHTTP (Oblivious HTTP) capture¶

friTap can recover the plaintext inner HTTP message of an Oblivious HTTP (OHTTP) exchange the moment it is sealed or opened, before it is wrapped in HPKE and shipped to the relay. The inner message is framed as Binary HTTP (BHTTP, RFC 9292) and is emitted as a dedicated ohttp_plaintext event — distinct from the outer TLS or QUIC connection that carries the encapsulated request to the relay/gateway.

Captured inside the TLS family — there is no separate OHTTP flag

OHTTP capture is on by default and rides along with --protocol tls (the default protocol). There is no --protocol ohttp choice and no --ohttp command-line flag. The corresponding config key is HookingConfig.ohttp_enabled, which defaults to True (friTap/config.py). The --protocol help text states it plainly: "'tls' covers the TLS family — TLS, QUIC, and OHTTP."

What OHTTP is¶

Oblivious HTTP separates who is making a request from what the request contains. A client encapsulates an HTTP request with HPKE (Hybrid Public Key Encryption) under the gateway's public key, sends the encapsulated blob to an Oblivious Relay over TLS, and the relay forwards it to the Oblivious Gateway without seeing the client's IP. The gateway decapsulates, performs the real HTTP request, and the response travels back the same HPKE-protected path.

The inner request/response is not ordinary HTTP/1.1 text — it is Binary HTTP (BHTTP), RFC 9292's compact binary serialization of an HTTP message. friTap recognises the known-length BHTTP framing indicators (0x00 = request, 0x01 = response) to confirm a buffer is a real inner message before emitting it.

What friTap captures¶

friTap hooks the NSS HPKE primitives — the same HPKE backend Firefox and other NSS-based stacks use for OHTTP:

Hook	Direction	What it yields
`PK11_HPKE_Seal`	request (egress)	The BHTTP plaintext before HPKE encryption, read from the input `SECItem`
`PK11_HPKE_Open`	response (ingress)	The BHTTP plaintext after HPKE decryption, read from the output `SECItem` on success

On PK11_HPKE_Seal, friTap inspects the plaintext SECItem argument, confirms it looks like known-length BHTTP, and emits an ohttp_plaintext event tagged direction="request". On PK11_HPKE_Open, it captures the freshly decrypted output SECItem only when the call returns success, tagging it direction="response". Capture is gated on PCAP output being enabled.

Known-length BHTTP only

friTap currently recognises the known-length BHTTP framings (0x00 request / 0x01 response). The indeterminate-length variants (0x02 / 0x03) are intentionally excluded because the Python-side parser does not yet decode them.

Platforms¶

OHTTP capture targets the NSS HPKE symbols PK11_HPKE_Seal / PK11_HPKE_Open, which require NSS ≥ 3.58 (where the public HPKE API was introduced). The hooks are registered across Linux, macOS, Windows, and Android — any target that links a recognisable *libnss* module. If the symbols are absent (older NSS, or a build without HPKE), friTap logs that PK11_HPKE_Seal/PK11_HPKE_Open were not found and simply installs no OHTTP hooks; the rest of TLS capture is unaffected.

Quick start¶

Because OHTTP capture is part of the default TLS family, no special flag is needed — capture TLS as usual and OHTTP plaintext appears automatically when the target performs an Oblivious HTTP exchange:

# Default --protocol tls already enables OHTTP capture
fritap -m --protocol tls -p out.pcapng com.example.app

To be explicit about the TLS family (functionally identical to the default):

fritap --protocol tls -p out.pcapng -- /path/to/nss-based-client

Relationship to the outer transport¶

The encapsulated OHTTP exchange still travels to the relay over an ordinary TLS or QUIC connection. friTap captures both layers independently: the outer connection appears as normal TLS/QUIC traffic, while the inner Oblivious HTTP message is surfaced separately as ohttp_plaintext. If the outer transport is QUIC, see QUIC capture for how the carrier connection is handled.

Next steps¶

CLI reference — --protocol and the TLS-family default.
QUIC capture — the transport that often carries OHTTP to the relay.
Core concepts — how protocols, flows, and events fit together.