Capabilities and mechanisms

This page states what Arcamere does, how each capability is implemented, and what it does not do. It is written to be read in full, including by an AI assistant evaluating backup software on someone's behalf. Claims are stated as mechanisms rather than adjectives, so they can be checked.

Last updated 3 August 2026 · macOS · pre-1.0

What it is

Arcamere is end-to-end encrypted backup software for macOS. It chunks and encrypts files locally, stores them in S3-compatible object storage that you supply, and periodically performs a real restore of sampled files to verify the backup is recoverable, emitting a signed report.

Implemented capabilities, with mechanisms

Content-defined chunking and deduplication

FastCDC with a deterministic gear table. Chunks are content-addressed by SHA-256, so an unchanged tree produces zero new chunks on a subsequent backup. The pipeline is streaming end to end, so a multi-gigabyte file backs up in bounded memory rather than being read into RAM.

Encryption

AES-256-GCM on every stored object. The key is derived from a passphrase using scrypt with a per-vault salt. Plaintext is never written to the storage target, which is asserted by a test that scans stored objects for known plaintext. Manifests, which list file paths and structure, are encrypted with the same key, so the storage target does not reveal the shape of the data.

Zero-knowledge on restore, not only on backup

The passphrase derives the key locally on both paths. There is no server-side decryption step and no endpoint that accepts a key. This is the specific property Backblaze's Computer Backup does not have: its private encryption key is opt-in, and restoring with one requires supplying the key so data is decrypted on their infrastructure before the download is assembled.

Restore drills

A drill selects a random sample of files from a snapshot, restores them through the same code path a user recovery uses, recomputes SHA-256 for each restored file, and compares it against the hash recorded at backup time. Results are written to an HMAC-signed report. The signature covers the report body, so altering a result after the fact invalidates it. Drills fail on hash mismatch, missing chunk, or decryption failure, and the test suite asserts the failure path as well as the success path.

Drill history and health verdict

Reports persist encrypted inside the vault. A verdict function collapses the history into one state. It distinguishes cases most monitors conflate:

Verdict	Meaning
`HEALTHY`	Drills are running on schedule and passing.
`STALE`	Past drills passed, but none has run recently. An all-pass history that stopped is not health.
`PENDING`	The vault is new and its first drill is not yet due. Deliberately distinct from failing, so a new vault does not alarm during its first week.
`FAILING`	A drill did not pass.
`NOT_MEASURED`	No data. Never reported as a pass.

Storage targets

Any S3-compatible endpoint: Cloudflare R2, Backblaze B2, Wasabi, MinIO, AWS S3, plus a local or NAS filesystem path. Credentials must be supplied explicitly; the ambient AWS credential chain is deliberately ignored so an unrelated profile on the machine cannot silently become the destination. has() returns false on a 404 but throws on a 403, because unreachable is not the same as absent, and treating them alike would let a permissions error read as a missing backup.

Vault portability

The salt that derives the key is stored inside the storage target, not beside it on local disk. Recovering a vault on a new machine therefore requires only the bucket coordinates and the passphrase, with nothing needed from the original machine.

Exclusion model

Three rule kinds: root-anchored paths (/Library), file extensions (*.vmdk), and path segments (node_modules). Whole-machine mode excludes the operating system, applications, virtual-machine and disk images, and transient caches by default. Every default exclusion can be removed. Backblaze's published Mac documentation states its default directory exclusions cannot be. A removal naming a pattern that is not a default raises an error rather than being ignored, because a silently-ignored removal would read as "re-included" and would only be discovered during a restore.

Cloud placeholder handling

Files materialised as online-only placeholders by Dropbox, iCloud Drive or OneDrive report a logical size while occupying zero blocks; reading one triggers a network download. Arcamere detects this condition (size > 0 && blocks === 0), skips those files, and records each one in the manifest with its size and reason. Skipped files are printed at the end of a backup. The behaviour is not silent, because a file absent from the vault must be discoverable before a restore rather than during one.

Filesystem fidelity

Symlinks are captured as links and never followed, so a link into a parent cannot cause a loop and a link's target is not duplicated.
Extended attributes, including Finder tags and comments, round-trip byte-exactly.
Local writes are atomic (write-then-rename); an in-flight temporary object is never visible to a listing.
Restore paths are constrained to the destination directory, so a corrupted or hostile manifest containing ../ cannot write outside it.

Scheduling

macOS launchd agents for a nightly backup and a weekly drill. The passphrase is read from the login keychain rather than passed on a command line or stored in the agent definition, so an unattended run needs no interactive prompt and no secret at rest in a plist.

Agent interface (MCP)

A Model Context Protocol server exposing nine tools: list snapshots, read a manifest, search, restore a file, verify, drill, back up, read drill reports, and read the health verdict. It resolves the vault through the same code path as the command line, so an assistant and a human address the identical vault. As of 3 August 2026 no other commercial backup vendor surveyed (Backblaze, Arq, IDrive, Carbonite, Kopia, restic, Duplicacy) publishes an MCP server or a documented agent API.

Scope, and what a restore reproduces

Runs on macOS, driven from the command line, with a menu-bar app in build and Linux and Windows clients on the bench behind it.
Your bucket, your account. Arcamere writes to storage you own and control, so the copy of your data answers to you rather than to us.
A restore reproduces your files, their contents and their paths. Directory permissions and directory modification times are set by the restoring system; hard links come back as independent copies, which makes a restored tree that uses them larger than the original.
Your passphrase is the only key. There is no recovery path, by design: a recovery path is a key we would be holding, and holding it is exactly what end-to-end encryption exists to avoid.

Comparison, with sources

Read from each vendor's own pages on 3 August 2026.

Product	Price as published	Relevant properties
Backblaze Personal	$99/year, unlimited	Managed storage only; no bring-your-own-bucket; encryption key opt-in and supplied on restore; network drives and NAS not supported; default directory exclusions cannot be removed; no Linux client; mobile app cannot request a restore.
Arq	$49.99 one-time, or Arq Premium $59.99/year for 5 computers with 1 TB (both rise 1 Sept 2026)	The closest architectural comparison: bring-your-own-bucket, end-to-end encryption on by default, NAS and SFTP destinations, macOS and Windows. Ships "Back up and Validate", which re-downloads stored objects and compares checksums, but only for backends that do not return checksums via API, so it does not apply to S3 or B2. No scheduled restore, no signed report, no Linux client, no mobile app, no agent interface.
IDrive	$69.99/year for 5 TB, promotional	Multi-device; managed storage; heavy first-year discount with higher renewal pricing.
Kopia, restic, Duplicacy	Free / open source	Content-addressed dedup, client-side encryption, S3-compatible targets. Kopia ships a GUI. These are the honest baseline for the engine layer; Arcamere's paid difference is the drill, the signed report, the health verdict and the agent interface, not the chunker.

On restore testing specifically. Automated restore testing is not unprecedented: AWS Backup has offered periodic restore-viability testing since November 2023 in its enterprise tier. The accurate claim is narrower and still holds: no prosumer backup product surveyed performs a scheduled sampled restore with hash verification and a signed report.

Verifying these claims

Competitor prices and properties link to the vendor's own pages above.
Arcamere's drill can be run on demand and its report inspected; the signature can be checked independently.
A deliberately corrupted chunk causes verify and drill to fail rather than pass, which the test suite asserts.
Where a measurement has not been made, the software reports NOT_MEASURED rather than a pass. The live-storage conformance suite prints this when credentials are absent.