Glossary
The cryptography and privacy vocabulary Tessera is built on, in plain language. Each term links to the deeper explainer in theLearn hub.
- (ε,δ)-differential privacy
- The parameterised form of DP: ε bounds how much one delivery can shift the published counts, and δ allows a small failure probability. At ε=0.1, an adversary classifying Tessera bucket counts reaches AUC 0.526 — near the 0.5 random baseline.
- AES-256-GCM
- An authenticated encryption cipher providing confidentiality and integrity. Tessera seals message payloads with AES-256-GCM; encrypting a 1 KB payload takes ~0.02 ms (experiment E1).
- Blinded pseudonym
- A per-delivery public identifier Y' = Y + t·G, where t = H(seed ‖ session_id). Only a contact holding the shared seed can recompute t and recognise the sender; to everyone else Y' looks uniformly random — giving cross-recipient and cross-delivery unlinkability.
- Bloom filter
- A compact probabilistic set-membership structure. Recipients use a Bloom fingerprint to test whether a bucket entry is plausibly theirs, allowing fast matching with a tunable, sub-1% false-positive rate.
- Bucketed broadcast
- A routing scheme where each delivery’s commitment maps to one of a fixed number of buckets (64 by default). Recipients scan their bucket and match with a Bloom fingerprint, providing k-anonymity without a central mix.
- Cover traffic
- Dummy or noise messages that obscure whether a real delivery occurred. Tessera’s cover traffic is load-independent shifted-Laplace noise on bucket counts, making the published count (ε,δ)-differentially private.
- Differential privacy (DP)
- A formal privacy guarantee: adding calibrated noise so that the presence or absence of any single record barely changes the output distribution. Tessera applies (ε,δ)-DP to per-bucket delivery counts.
- EUF-CMA
- Existential Unforgeability under Chosen-Message Attack — the standard security notion for signatures and proofs. Tessera’s Schnorr proof is EUF-CMA secure under the discrete-log assumption, confirmed empirically at a 0/5000 forgery rate (E6).
- Fiat–Shamir transform
- A technique that turns an interactive proof into a non-interactive one by deriving the verifier’s challenge from a hash of the transcript. Tessera uses it so a proof can be produced offline and verified later without a live exchange.
- k-anonymity
- A privacy property meaning each delivery is indistinguishable from at least k−1 others. Tessera’s bucketing provides k ≥ 75 at default parameters (experiment E2), so an observer cannot single out a recipient within a bucket.
- Key blinding
- Adding a deterministic, seed-derived offset to a public key so the same identity appears as a different, unlinkable key to each recipient and on each delivery. The offset t = H(seed ‖ session_id) is the blinding factor.
- Metadata
- The data about a message rather than its content — who sent it, to whom, when, how large, and how often. Content encryption (like Signal) hides the message body but leaves metadata, which reveals the social graph.
- Metadata privacy
- Hiding the who/when/how-often of communication from network observers, routing infrastructure, and colluding parties — not just the message content. Tessera achieves this with blinded pseudonyms and DP cover traffic.
- Mixnet
- A network of relays that shuffle and delay messages to break the link between sender and receiver. Tessera achieves comparable metadata privacy with a lighter, decentralized bucketed-broadcast overlay — and adds authentication a mixnet lacks.
- Pairwise local enrolment
- Establishing a contact by exchanging public keys and a shared seed directly between two parties, with no directory or registration server. It removes the central authority that signed messengers rely on.
- Schnorr proof
- A zero-knowledge proof of knowledge of a discrete logarithm. In Tessera it lets a sender prove they hold the secret key x behind a public point Y = x·G, revealing nothing else. Its unforgeability reduces to the discrete-logarithm assumption.
- Zero-knowledge proof (ZKP)
- A proof that a statement is true while revealing nothing beyond its truth. Tessera’s ZKP proves sender identity without exposing the secret key or linking deliveries across recipients.