Private messaging and encryption glossary

A reference. Last updated 1 September 2026.

Private messaging leans on a specific vocabulary, and a lot of it gets used loosely in marketing. This glossary defines the core terms of encryption and metadata in plain English, accurately and without spin. Each entry stands on its own, so you can link to a single definition or read straight through.

Ciphertext: The scrambled, unreadable output of encryption. Ciphertext is what a relay server or a network observer sees in transit: without the correct key it reveals nothing about the original message, although its length can still hint at how large that message was.
Cryptographic primitive: A low-level, well-studied building block of cryptography, such as a cipher, a hash function, or a key-agreement algorithm. Secure apps combine audited primitives rather than inventing their own. Nyra builds on the independently audited @noble primitives, though Nyra itself has not yet had an independent audit. More on this is on the security page.
Deniability: A property of some messaging protocols where, after a conversation, neither party can cryptographically prove to an outsider who wrote a given message. Both people can still read the messages as they arrive; the protocol simply does not produce a signature that would stand up later as proof of authorship.
Disappearing messages: Messages set to delete themselves from both devices after a chosen time. This limits how much history survives to be seized, subpoenaed, or read by someone who later picks up a device. In Nyra they are on by default with a 7 day timer, explained on the disappearing messages page.
Double Ratchet: The algorithm that gives Signal-style messengers a fresh key for almost every message. It turns two ratchets, one driven by new Diffie-Hellman keys exchanged as people reply and one driven by each message sent, so that compromising a single key does not expose the rest of the conversation.
Duress PIN: A second PIN or passphrase that triggers a protective action instead of a normal unlock, such as hiding or erasing sensitive data. It is designed for moments when someone is forced to open an app. Not every messenger offers one.
End-to-end encryption: Encryption where a message is sealed on the sender's device and can be opened only on the recipient's device, so the servers and networks that relay it cannot read the content. It protects what a message says, but not the metadata around it, such as who is talking to whom and when.
Forward secrecy: A property where past messages stay protected even if your long-term key is later stolen, because each message uses a short-lived key that is thrown away after use. It is sometimes called perfect forward secrecy. The Double Ratchet provides it, and a few protocols drop it as a deliberate trade-off.
Identity key: The long-lived key pair that represents an account over time. The assurance that you are still talking to the same person rests on this key staying constant, which is exactly what a safety number check compares.
Man-in-the-middle attack: An attack where someone secretly sits between two parties, relaying and possibly altering their messages while each believes they are talking directly to the other. In encrypted messaging the defense is verifying keys out of band, for example by comparing a safety number, so a substituted key is caught.
Metadata: Everything about a message except its content: who sent it, who received it, when, how often, how large it was, and from what network address. End-to-end encryption protects content; metadata is what stays visible around it. We cover it in depth on the metadata page.
Military-grade encryption: A marketing phrase with no fixed technical meaning. It usually gestures at AES-256, a strong and widely used cipher, but a messenger's real security comes from its whole design rather than one label. Treat the phrase as a slogan, not a specification.
Onion routing: A technique that wraps a message in several layers of encryption and passes it through a chain of relays, each of which removes one layer and learns only the next hop. No single relay sees both the origin and the destination. Tor is the best-known example, and the Session messenger runs on an onion-routed network.
Open source: Software whose source code is published so that anyone can inspect, audit, and build it. Open source makes security claims checkable rather than taken on trust, though publishing code is not the same as a formal audit, and users only truly run the published code when builds are reproducible.
Padding: Adding filler to messages so that many different messages share the same size on the wire. Padding weakens the size signal in metadata, making it harder to tell a short note from a long one by length alone. Nyra plans to pad envelopes into fixed size buckets, but this is not built yet.
Plaintext: The original, readable message, either before it is encrypted or after it is decrypted. In a sound end-to-end encrypted system, plaintext exists only on the sender's and recipient's devices and never on the relay server.
Prekey: A public key you upload to a server in advance so that someone can begin an encrypted conversation with you while you are offline. Prekeys let the first message be encrypted without both people being online at the same moment, and they are a core part of the X3DH handshake.
Private key: The secret half of a key pair, kept only on your device and never shared. It decrypts messages sent to you and proves your identity. Anyone who obtains your private key can read those messages and impersonate you.
Proof of work: A requirement that a sender's device solve a small computational puzzle before an action is accepted. Because each puzzle costs real time and energy, proof of work makes mass abuse such as spam or flooding expensive while barely inconveniencing honest users.
Public key: The shareable half of a key pair. Other people use your public key to encrypt messages that only your matching private key can open, and to check signatures you create. Public keys can be handed out openly, which is why safety numbers exist to confirm you hold the right one.
Recovery phrase (seed phrase): A list of words that encodes the secret from which your account keys are derived. Whoever holds the phrase controls the account, and if you lose it, nobody can restore the account for you. In Nyra, losing your recovery phrase means the account is gone forever.
Safety number (key verification): A short code, shown as digits or a QR code, that two people compare to confirm they hold each other's real keys rather than an attacker's. Matching codes rule out a man-in-the-middle. Nyra presents this as a shared constellation: same sky on both screens means the connection is secure.
Sealed sender: A technique introduced by Signal that hides the sender's identity from the relay server. The server can still deliver a message to the recipient, but it does not learn which account sent it, removing one piece of metadata.
Self-custody: An arrangement where you alone hold the keys to your account and no company can access or reset them. It gives you full control and removes a central point that could be compelled or breached, at the cost that recovery is entirely your responsibility.
Sender anonymity: The property of the sender's identity being hidden from some party, whether the relay server, the network, or in certain designs the recipient. It is separate from content secrecy: a message can be unreadable yet obviously from a known account, or readable yet not traceable to any sender. Using no phone number is one way to keep the sender from being identified.
Sender Keys: An efficiency mechanism for group messaging in which a sender encrypts a message once under a shared group key instead of separately for every member. It keeps large groups practical while preserving end-to-end encryption, at the cost of extra steps whenever the membership changes.
Sybil attack: An attack in which one actor spins up many fake identities to gain outsized influence over a system, for instance to swamp a peer-to-peer network or a reputation scheme. Defenses work by making each identity costly to create, for example through proof of work.
X3DH: Short for Extended Triple Diffie-Hellman, the key-agreement handshake that lets two people establish a shared secret, even when one of them is offline, using published prekeys. It sets up the initial keys that the Double Ratchet then keeps advancing. Nyra uses X3DH together with the Double Ratchet, as described on the security page.
Zero-knowledge: In cryptography, a precise term for a proof that convinces someone a statement is true while revealing nothing beyond that fact. In marketing it is often stretched to mean little more than "we cannot see your data", which is a weaker and vaguer claim. Read it carefully and ask what is actually being proven, and to whom.