Security & trust — how verifiable an Akkubrief is

Trust isn't claimed, it's made verifiable. This page sums up what secures an Akkubrief technically — traceable rather than on faith.

Signed and verifiable in the browser

Every certificate is cryptographically signed with Ed25519. Verification runs entirely in the buyer's browser (WebCrypto): verify the signature, recompute the certificate ID against the content, optionally match the vehicle identification number. Any later change is immediately visible — including reusing someone else's certificate. The VIN appears only masked and as a SHA-256 checksum, never in clear text. You don't have to trust us — you can check it yourself.

The public key — verifiable for good

A signature is only worth as much as the key it is checked against. So our public Ed25519 key is written out here in plain text — and machine-readable at https://akku-brief.de/.well-known/akkubrief-signing-keys.json. Anyone checking a certificate can hold the two against each other. The private counterpart lives only in a sealed key vault and never leaves it.

Active signing key prod-2026-08, valid from 23 August 2026:

KqM/9Bi4hqPHLDbdXK2YMjoxhUoVLg8d5+Sw+aqNaTQ=

The sample keys stand for the demo and nothing else, and each private counterpart was discarded after signing once. Today's sample Akkubrief carries demo-2026-09:

aDY1AJZtshwqbcdcq5sStGyot8L9gNbkC9eguPcbgfI=

demo-2026-08b and demo-2026-08 signed the two earlier versions of the sample. They stay here so that a saved or printed copy of those remains verifiable too:

NMeMPEOU0wi0u2DrlGT5s6+vxNoOZmdYnTV/1y30M4c=
hPTCcdXExsyowcPNe+MCeuwS6ygw/5Yg+sJuWOheKTk=

What counts when checking is not only that the signature verifies, but which key it verifies against: only a key with purpose production stands for a real Akkubrief. A demo key signs the sample certificate and nothing else. The machine-readable list carries the purpose as a purpose field, and our own check in the browser rejects a document whose key does not match.

The fingerprint is the SHA-256 checksum over the 32 raw bytes of the key, not over the base64 text. One line recomputes it:

echo -n 'KqM/9Bi4hqPHLDbdXK2YMjoxhUoVLg8d5+Sw+aqNaTQ=' | base64 -d | openssl dgst -sha256

Every certificate names the key ID it was signed with inside its signed wording. If the key ever changes, the new one joins this list — the old one stays. Otherwise Akkubriefe already issued would stop being verifiable, and that is what we rule out: an issued certificate should stay verifiable even if Akkubrief one day no longer exists.

So that this stays more than a promise, the same key list is also deposited as a citable archive record with Zenodo, run by CERN: DOI 10.5281/zenodo.22072519. Zenodo records cannot be deleted — withdrawing one leaves a tombstone and the DOI stays resolvable. The same record holds an OpenTimestamps proof anchoring exactly these bytes in the Bitcoin blockchain, showing that the key already existed on that day. On rotation the new list joins as a new version under the same DOI.

Read-only access — your car stays yours

Sign-in happens directly at Tesla, not with us. We only ever receive read permissions on driving and charging data: no password, no vehicle control, no unlocking, no waking the vehicle. You can revoke access anytime in your Tesla account.

Data-minimal and protected

We store vehicle data encrypted and never sell it. We use no cookie trackers. Akkubrief runs on ISO 27001 and SOC 2 certified infrastructure (Supabase, Vercel). You can request deletion of your data at any time.

Calculated in the open

No provider on the market discloses how its battery score is produced — we do. How we compute battery health from the charging history, where the limits are, and what an Akkubrief is not, is documented in full in our methodology.

No result, no charge

If the data isn't enough for a reliable certificate, we don't issue one — and charge nothing. An Akkubrief is a data-based estimate of battery health, not an official document and not a guarantee.