Threat model

What arca protects against, honestly — and what it explicitly does not.

Protects against

• Plaintext in the repo — code and config carry arca:// refs, never values. There is nothing to leak in a commit, a diff, a paste, or a prompt context.
• Plaintext in shell history / process listings — values never enter via CLI arguments (args land in ~/.bash_history and /proc/*/cmdline). put/rotate read stdin or a hidden /dev/tty prompt.
• Plaintext in agent context — arca run injects values into the child's environment; the agent that spawned it sees the child's output, never the variable. [secrets] bounds the injectable surface so a run can't dump the whole vault.
• Accidental printing — Secret has no Display/Debug of the value (.expose() only), list is names + metadata only, and the TUI's posture is browse-never-spill (reveal is a deliberate two-step, audited).
• Ciphertext at rest, everywhere — every vault impl stores age ciphertext. The local file is unreadable without the identity; the cf remote store holds ciphertext over the wire and in D1 — a compromised worker yields blobs, not values. With [vaults.*] private_refs = true it can't even see which secrets exist: storage keys are HMAC-derived hex and the real ref travels in an age-encrypted name blob.
• Token leakage on the cf wire — scoped tokens (ARCA_VAULT_TOKENS) bound a leaked credential to its {env, read, write} scope instead of every env forever, and ArcaHmac request signing means the token never travels at all (a ±60s window bounds replay). All auth compares are constant-time.
• Silent misuse — every materialization, denial, and error emits an ndjson audit event on stderr, so capture is a wrapper away. check fails closed on a vault that won't decrypt, and on a mixed-mode cf vault (private config + plaintext rows).

Does NOT protect against

• A determined local actor — ARCA_ACTOR is self-claimed (attribution, not identity) and [guardrails] are advisory: anyone who can edit arca.toml or run the binary can bypass them. They prevent accidents, not adversaries. Real multi-actor policy and enforcement belong to peregrin.
• The identity's host protection — the master key lives in the session keyring when one exists (ADR-0002), which protects it about as well as your login session. On headless/CI it is still identity.txt (0600), raw key material — protect it like a private SSH key. Either way, UID-level compromise reaches it: the keyring raises the bar, it is not a TPM.
• The child process — once run injects env vars, the child owns them (/proc/<pid>/environ, core dumps, the child logging its own env). arca's guarantee ends at exec.
• Deploy sinks — arca deploy pushes plaintext into the sink by design (CF Secrets Store is write-only); a sink compromise is outside arca's boundary — it crosses exactly once, at push.
• Access patterns — private_refs hides which secrets exist, not when they're touched: request timing and row counts are still observable to the store. Oblivious access is out of scope.
• Host compromise in general — an attacker who can read your memory, ptrace the broker, or impersonate your UID already won; arca raises the bar for accidental and casual leaks, not root.

In one line

arca makes the default posture "values never appear in repos, prompts, args, logs, or agent context" — and records every time one still crosses.