Hack@CHES 2026 - Phase 1 Bug Submissions

Team KattangalSec

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Bug 11 - Key Manager: EDN Entropy Error Path Unconnected, Replay Protection Disabled

Security feature bypassed

Key Manager entropy reseed integrity validation - the EDN error/replay-protection path that must gate reseed acceptance

Finding

The prim_edn_req instance in the Key Manager reseed controller has RepCheck=0 and leaves err_o() physically unconnected - any faulted entropy is accepted silently:

// keymgr_reseed_ctrl.sv:63-73
prim_edn_req #(.RepCheck(0)) u_edn_req (..., .err_o(), ...);

.err_o() is unconnected; cnt_err_o (line 109) is a dead-end local signal.

Location or code reference

The attack flow for this bug is linked in the Attachments section below.

New Tools

Yes - custom VCS fuzzing pipeline (standalone model) + module instantiation (Verilator 4.210): keymgr_reseed_ctrl_tb.sv instantiates the real keymgr_reseed_ctrl.sv with its genuine dependency chain; drives a reseed request, simulates the EDN responding with a FIPS error (edn_fips=1), and observes cnt_err_o stays 0 - the error is silently swallowed (see testbench/logs/rtl-test-simulation.log: *** BUG #18 CONFIRMED on actual OpenTitan RTL ***).

AI Tools

Yes

LLM

Yes

LLM Details

GPT-5.6-Sol from https://agentrouter.org/v1

Online LLM Details

Orchestrator session total: 38,176,115 in / 26,998 out tokens (the agentic session that ran the Lightsaber pipeline, session ses_031497488).

Estimated tokens for this bug’s run (from result.json turn log: 20260805_082405 keymgr agentic run (15 turns, 168,087 bytes) - shared with bugs 03 and 12): ~81,021 in / ~2,015 out. Estimate = file bytes / 4 + ~2,600 system-prompt tokens per turn; the run was part of a larger multi-IP campaign, so the per-bug figure is an approximation.

LLM Prompts

Step-by-step discovery and verification (source: Lightsaber agentic audit):

  1. Lightsaber’s keymgr agent read 12 keymgr files including keymgr_reseed_ctrl.sv and prim_edn_req.sv (run 20260805_082405).
  2. It flagged that prim_edn_req is instantiated with default RepCheck=0 and fips_o()/err_o() unconnected, and that the all-zero/all-one/previous checks are assertion-only (INC_ASSERT) - so a faulted/replayed EDN response is acknowledged as a successful reseed.
  3. Verified on the RTL: keymgr_reseed_ctrl_tb.sv drives an EDN FIPS error and observes the error is silently dropped (cnt_err_o=0).

Evidence:

Agent/session transcript: session transcript line 2455 (finding #4 (keymgr EDN)).

Detection method

Module instantiation (Verilator 4.210, full dependency closure) + AFL model. Security property: an EDN error during reseed must set the error output / block acceptance; the testbench shows the error is accepted silently.

Security impact

Fault-injected EDN entropy poisons the reseed LFSR; all subsequent key derivations use attacker-influenced entropy - predictable session keys. Replay protection (RepCheck) is also disabled, enabling entropy replay.

Adversary profile

Type 2 - physical attacker with the ability to fault the EDN entropy bus (voltage/EM glitch).

Proposed mitigation

prim_edn_req #(.RepCheck(1)) u_edn_req (..., .err_o(err), ...);  // connect + alert

CVSSv3.1 score and severity

6.8 - MEDIUM

CVSSv3.1 Details

CVSS:3.1/AV:P/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:N