Address poisoning

A scam in which an attacker sends you a worthless transaction from an address crafted to look almost identical to one you use, hoping you'll later copy the wrong one from your history and send money to them by mistake.

Advanced Data Protection (ADP)

Apple's optional setting that upgrades most iCloud data from "Apple holds a key" to true end-to-end encryption, so that only your devices can read it — after which Apple has nothing meaningful to hand over. Mail, contacts, and calendars remain outside it.

Aggregation (signature aggregation)

Folding many separate digital signatures into a single compact one that still proves every signer signed. The trick that lets ten thousand validators approve a block without the network storing or checking ten thousand signatures.

Authenticator app

An app that generates short-lived login codes on your device, providing a second factor that — unlike a texted code — can't be stolen by hijacking your phone number.

Backdoor

A deliberately built way around a system's protection, however it's dressed up. The book's argument: a key that exists can be stolen, copied, or demanded, and a flaw in the lock doesn't check credentials.

Biometrics

Using your face, eyes, palm or fingerprint as a gate. The scan is converted to a mathematical summary that stays locked inside your device's secure hardware; it isn't uploaded anywhere, and the biometric merely unlocks the real keys that do the work.

Blind signing

Approving a cryptographic signature without being able to see, in plain terms, what you're agreeing to — the failure at the heart of the Bybit theft, where the screens lied and careful people signed away a billion and a half dollars.

Blockchain

A public ledger maintained by thousands of computers at once, where entries, once agreed, are practically impossible to rewrite, and ownership is exercised with cryptographic keys rather than a company's permission. One shared record, thousands of bookkeepers, no head office and no eraser: everything else in cryptocurrency follows from that. A blockchain is made of a long, sequential chain of blocks of transactions; completed transactions quickly become immutable and new ones are appended. There are many blockchains, each working a little differently from the others.

Blockchain Explorer

A blockchain explorer is a software application that allows users to see the individual transactions on a blockchain. Some explorers work with only one blockchain, and some are cross-blockchain.

BLS signature

A signature scheme named for Dan Boneh, Ben Lynn, and Hovav Shacham, built on pairings, whose superpower is aggregation: thousands of signatures can be crushed into one. It now underpins Ethereum's proof-of-stake consensus.

Blockchain

A public ledger maintained by many computers at once, where entries — once agreed — are practically impossible to rewrite, and ownership is exercised with cryptographic keys rather than a company's permission.

Brain Wallet

A private key generated from a phrase a person made up and memorized, instead of from a machine's randomness. Historically a massacre: humans are terrible at random, attackers run every phrase anyone has ever written, and brain wallets have been emptied within seconds of being funded. The only honest use is fleeing a war with nothing on you, not holding life savings.

Brute force

Trying every possible key until one fits. Against modern key sizes, the haystack is so large that every computer ever built, running together, would not make a dent.

CBDC (central bank digital currency)

Government money issued in purely digital, software-defined form. The book's question about it is the book's question about everything: who holds the keys, and who can say no to you?

Clear signing ("what you see is what you sign")

The opposite of blind signing: hardware and software that show you, on a screen an attacker doesn't control, exactly what instruction you are about to authorize.

Client-side scanning

Software on your own device that inspects your content before or despite encryption — the camera inside your house, pointed at the desk where you write the letter. The mechanism at the center of Europe's "Chat Control" fight.

Cost Basis

What you originally paid for an asset, including fees; the number that decides whether a disposal produced a gain or a loss. Every sale, swap or purchase with crypto is a disposal for tax purposes, so the cost basis of each lot matters. Keep your own records: the exchange's statement rarely covers the coins you moved in from elsewhere, and nobody else keeps yours.

Credential stuffing

Taking passwords spilled in one breach and trying them everywhere else, which is why one reused password turns a minor leak into a skeleton key for your life.

Cryptosteel

brand of metal seed-phrase backup, and the product that made the category: a stainless-steel case into which you slot letter tiles to spell the first four letters of each of your twelve or twenty-four words, which is all the BIP39 word list needs to identify a word. Cryptosteel’s Capsule and Cassette, Billfodl and a dozen imitators exist to answer the one question paper cannot: will the backup survive the fire, the flood and the decades? Steel does, at well over a thousand degrees, which is why Chapter 14 calls metal the backup that survives the fire. It also names the question metal cannot answer. A steel plate is as secure as its hiding place and no more: the words are readable by anyone who finds it, a photograph of it is as good as the plate, and it does not forgive the one mistake of being found. So metal is the right medium for the words and the wrong answer to who may see them; it belongs behind a passphrase, in a place only you and your plan know, or as one share of a split backup rather than the whole secret on one plate. Buy from the maker, set the words yourself, check them against the wallet once, and never open it in front of a camera.

Crypto Wallet Backup

A crypto wallet backup is the collection of saved artifacts that allows you to restore an existing crypto wallet on a new device. At a minimum, this backup includes a wallet seed phrase.

Custodial wallet / custody

An arrangement where someone else holds your keys and owes you a promise — an exchange balance, a bank-like account. They can rescue you, and they can refuse you; the two powers are the same power.

Decentralization / Decentralized

Built so that no single party runs the machines, holds the keys, or can be pressured into betraying every user at once. Distinct from distributed (spread across machines that may still answer to one owner) and centralized (one party, one point of pressure).

Decentralization is a strategy of distributing how a system works among multiple operators or points of control, rather than having one central point of control. Decentralization is a general term that can be applied to a system's controlling functions, resources, or decisionmaking, depending on what kind of system it is. The more decentralized a system is, the more resilient it can be, and more reflective of a wider group of stakeholders. Cryptocurrencies are said to be decentralized when its decisions and operations are made through the participation of multiple audiences, like blockchain node operators and validators, token owners, developers, and investors.

Decentralized exchange (DEX)

A vending machine made of code: a program on a blockchain that swaps one token for another wallet-to-wallet, with no company behind it holding your money. There is no account, no identity check and no support desk, and the door only opens once you hold your own keys. Prices come from pools of tokens that other users supply, and a mistyped amount or a malicious token is yours to live with.

Digital Inheritance

Making sure the right people can reach your keys, accounts, and encrypted life after you're gone — the one problem the strongest cryptography makes harder, because the math does not know you died.

Digital Inheritance is making sure that your loved ones will be able to access your digital assets after you are gone. The laws and technology surrounding digital inheritance are specialized and vary from traditional inheritance processes for physical assets, so it is something that everyone should consider.

Digital Vault

A Digital Vault is a highly-secure computer program built for the purpose of storing digital assets. Digital assets include cryptocurrencies, blockchain tokens, digital signatures, digital artwork, electronic versions of contracts, or anything that can be digitized. Because digital vaults are purpose-built for security, they should have very high standards for encryption, access control, and resilience.

Elliptic Curve

The branch of mathematics behind today's crypto keys, chosen because deriving a public key from a private key along the curve is easy and reversing the journey is not. Bitcoin and Ethereum use the curve called secp256k1; your passkeys and your phone's secure chip use its cousins. Shor's algorithm, run on a large enough quantum computer, would make the reversal easy, which is why the replacements in post-quantum cryptography use different mathematics.

Encryption at rest

Data encrypted where it's stored, with the storage provider holding a key: safe from a burglar, open to the landlord — and to anyone who can compel the landlord.

Encryption in transit

Data encrypted on the journey between you and a service — the armored truck: guarded on the road, unloaded and readable at the depot.

End-to-end encryption (E2EE)

Encryption where only the conversation's endpoints hold the keys, so the company in the middle carries a sealed box it cannot open — even under court order. The seal covers the letter, not the envelope: metadata, backups, and the other person's screen sit outside it.

Entropy

Disorder harvested from the physical world, the raw material of real randomness. A private key is only as safe as the entropy that made it: a key born from thermal noise in a chip is unguessable; a key born from a weak or predictable source can be recreated by anyone who works out the source, sometimes years later. Wallets and secure chips gather entropy from hardware for exactly this reason.

Exchange

The high-street shop of crypto: a company where you buy, sell and hold coins through an account, usually the cheapest door for a beginner if you use the right screen. Your balance on an exchange is a claim on that company, not a key you hold; if the exchange fails, you become an unsecured creditor queued behind the lawyers, as the customers of FTX, Celsius and Mt. Gox learned. Exchanges are where most people start, and the custody ladder is the way out when the sums grow.

Exchange-traded fund (ETF / ETP / ETN)

A stock-market product that holds or tracks a cryptocurrency so that an ordinary brokerage or retirement account can own exposure to it without anyone holding keys. You get the price, inside the rules and protections of the securities world, and you give up the coins themselves: no self-custody, no sending, no inheritance outside the brokerage's own process. The products on the shelf differ by country (ETFs in the United States, ETPs and ETNs in the United Kingdom and Europe).

Fair market value (FMV)

What an asset was worth at a given moment, usually the price on a reputable exchange at the time of the transaction. It is the number that tax and receipting run on: the value of coins when you received them as income, the value when you disposed of them, and the value of a crypto donation when a charity writes its receipt. Record it at the time; reconstructing it later is painful.

Family Password

A word or phrase agreed in advance with the people you love, to be asked for on any emergency call that requests money or secrets. It defeats a cloned voice, which can now be made from a few seconds of audio, and it costs nothing. Agree it in person, never write it where a phone can see it, and rehearse it once.

Firmware

The maker's software living inside a device — a smart lock, a router, a wallet — where security flaws hide and updates matter more than features.

Format-preserving encryption (FPE)

Encryption that produces ciphertext in the same shape as the original — a sixteen-digit card number becomes another sixteen-digit number — so protection can be retrofitted into systems that would shatter if the format changed.

Fuzzy extractor

The mathematics that turns something slightly different every time — a fingerprint, a face — into the exact same stable secret key, which is what makes "you are the key" possible.

Gas

The small fee a blockchain charges to process a real transaction. Its absence is a signal: a signature request that costs nothing may still be granting something enormous.

Guardian

In distributed backup and inheritance schemes, a person you choose who holds one encrypted share of your secret — able to help reconstruct it together with others, unable to learn anything alone.

Harvest now, decrypt later

The strategy of recording encrypted traffic today in the hope that a future quantum computer will open it. A live concern only for secrets that must stay secret for decades.

Hash-based signature

A digital signature scheme built only on hash functions, which quantum computers are not known to break. Slower and bulkier than ECDSA, so unsuited to signing every transaction, but trusted as the conservative post-quantum option; NIST standardized one (SLH-DSA, FIPS 205) in 2024 alongside the lattice-based schemes.

Homomorphic encryption

A frontier technique that lets a computer work on data while it stays encrypted, returning a useful answer without ever seeing the secret — help without exposure.

HTTPS / the padlock

The secure channel between your browser and a website, protecting the journey of what you type. It promises the road is guarded; it says nothing about who's at the destination — phishing sites wear padlocks too.

Identity-based encryption (IBE)

The scheme — imagined by Shamir in 1984, made real by Boneh and Franklin in 2001 — where your identity itself (an email address) serves as your public key, removing the key-fetching friction that kept email unencrypted for decades.

Impermanent loss

The structural cost of supplying tokens to a liquidity pool on a decentralized exchange, misleadingly named: as one token in the pair rises, the pool automatically sells your winner to keep the pair balanced, so in a trending market you would often have been richer simply holding. The loss is "impermanent" only if prices return to where they started. Fees from traders are meant to compensate; often they do not.

Kerckhoffs's principle

The 1883 rule the whole modern field obeys: a system must stay secure even when everything about it except the key is public. Its corollary is the book's refrain — distrust any lock you're not allowed to inspect.

Key (cryptographic key)

The secret at the center of everything: a number, unimaginably large, that locks, unlocks, or signs. The book's master question is simply who holds it.

Key pair

A public key and private key created together and mathematically joined. Scatter the public one like open padlocks; guard the private one with your life.

Knox Vault (Samsung)

aSmsung’s tamper-resistant secure element, built into Galaxy flagships since the S21 in 2021 and the hardware heart of its Knox security platform. It pairs a dedicated secure processor with its own secure memory, physically separate from the phone’s main processor and operating system, and it detects the physical attacks that would try to pry secrets out of it (abnormal voltage, temperature, glitches, laser probing) and locks down when they occur. Inside it live the things a phone must never give up: PINs and passwords, fingerprint and face data, the keys that encrypt the device, the hardware-backed keys apps create through Android’s keystore, and the private keys behind Samsung Blockchain Keystore, the wallet feature Samsung built for crypto holders. Knox Vault is what provides Android’s StrongBox on a Galaxy, so an app that asks for a StrongBox key on a Samsung phone gets one made and held in this chip; its components are certified to Common Criteria EAL4+ and higher. In the series’ terms it is Samsung’s answer to Apple’s Secure Enclave and Google’s Titan chip: the sealed second computer that will use your secrets but never hand them over.

Lattice-based cryptography

A family of post-quantum schemes whose security rests on hard problems in high-dimensional grids of points, problems a quantum computer gets no known shortcut against. It is the basis of the NIST standards ML-KEM (for key exchange) and ML-DSA (for signatures), published as FIPS 203 and 204 in August 2024, and it is already arriving in the updates to messaging apps and the web.

Legacy contact

A platform feature (Apple, Google, and others) naming who may access your account after your death — the easy half of digital inheritance, done in minutes.

Liquid staking

Staking through a service that hands you a tradeable receipt token (such as stETH) representing your staked coins plus their rewards, so the coins keep earning while the receipt can be sold or used elsewhere. The receipt is a derivative wearing a convenience costume: it can trade below the value of the coins it represents, it carries the service's smart-contract risk, and it adds a second thing that can go wrong between you and your money.

Mark as Lost

The remote lock in Apple's Find My and Google's Find My Device. It locks the phone's screen, suspends the cards in its wallet, displays a contact number on the lock screen and keeps tracking the device, without erasing it. Use it in the first ten minutes after a phone goes missing, before anything else, and erase only once you are sure the phone is gone for good.

Master password

The one password that unlocks your password manager, and therefore the entire game: a long passphrase you can remember beats a short puzzle you can't.

Metadata

Everything about a communication except its content: who, when, how often, from where. The letter is sealed; the envelope isn't — and institutions read envelopes at scale.

Open source

Software whose code anyone may inspect. Inspectable is not the same as inspected — Heartbleed lived in open code for two years — but backdoors have nowhere to hide, and the product survives its maker.

Pairing (pairing-based cryptography)

The exotic mathematics Boneh and Franklin reached for to build IBE, which turned out to be a skeleton key for a generation of what followed: BLS signatures, threshold systems, and doors into zero-knowledge.

Passkey

A login credential that is a key pair in your device's secure hardware rather than a shared secret on a server: nothing for a breach to spill, and unphishable by construction, because your device will only ever use it for the real site.

Password manager

An encrypted vault for all your passwords, built zero-knowledge so the provider stores sealed boxes it cannot open. For passwords — not for seed phrases.

Permission signature (token approval)

The treacherous middle kind of crypto signature: it looks like a harmless free message but grants standing power over your assets — the mechanism behind a huge share of drained wallets.

The treacherous middle kind of crypto signature: it looks like a harmless free message but grants a program standing permission to move your tokens, and that permission persists until you revoke it. It is the raw material of drain scams, which turn one approval into an emptied wallet. Review the approvals you have granted and revoke old ones; most block explorers and wallets now have a page for it.

Phishing

Deceiving you into using your own keys, codes, or passwords on an attacker's behalf. Modern phishing doesn't break cryptography; it routes around it — through you.

Pig butchering

The long-con investment fraud: weeks or months of manufactured friendship or romance over text, a beautiful fake trading platform, a small first withdrawal that works, then ever larger deposits and an exit script when you try to take the money out. It is run as an industry, often by trafficked workers in compounds, and it targets clever, educated people. Seen whole, the process is the vaccine: nobody you met by message needs to show you how to invest.

Post-quantum cryptography, Quantum-safe, Quantum-resistant

New encryption and signature schemes, standardized by NIST in 2024, built from mathematics a quantum computer gets no shortcut against — already arriving invisibly in the updates for Signal, iMessage, and the web.

Proof of stake

The way networks like Ethereum agree on their ledger: validators put up value as collateral and sign each block, with dishonesty punished by losing the stake — thousands of signatures per block, made practical by aggregation.

The way networks like Ethereum agree on their ledger: validators lock up coins as a good-behavior deposit, sign each block, and lose part of the deposit (slashing) if they misbehave. For the holder, staking means lending coins to that process in return for rewards, the closest thing crypto has to rent. You can run a validator yourself, delegate through a wallet, or stake through an exchange or a liquid-staking token, and each route moves the risk somewhere different: lock-up periods, slashing, and a receipt token that is a derivative wearing a convenience costume.

Public Key

The half of a key pair you can shout from rooftops: an open padlock anyone can use to seal something only you can open, or to verify a signature only you could have made.

A public key is a secret piece of data that is used in asymmetric (public key) cryptography. It is a string of characters that is used to encrypt and decrypt messages, and in cryptocurrency, to sign transactions. A public key is typically generated by a wallet software, and does not necessarily need to be kept secret by the owner: in fact, one of its main purposes is to allow people who do not hold the private key to confirm that any messages purported to have been signed by the private key holder were truly signed by them. To that end, the public key must be shared with anyone who needs to verify the authenticity of the private keyholder's digital signature. Another function of public keys is that cryptocurrency wallets can use a public key to identify a target address on a blockchain where the keyholders' crypto is held.

Qualified custodian

A regulated company whose business is holding other people's keys under supervision, insurance and audit, the kind of custodian a wealth manager or fund is permitted to use. The questions to ask are whether client assets are segregated from the firm's own, what the SOC reports say, who can approve a withdrawal and how fast, and what happens to your coins if the custodian fails.

Quantum-safe, post-quantum and quantum-resistant cryptography

Three names for the same thing: encryption and signature schemes built on mathematics that a quantum computer gets no known shortcut against. Today’s public-key cryptography (RSA, and the elliptic-curve schemes behind Bitcoin, Ethereum, passkeys and most of the web) would fall to Shor’s algorithm on a large enough fault-tolerant quantum computer; symmetric encryption and hashing (AES-256, SHA-256) would not, so the seed phrases, hashes and encrypted backups are not the problem, the signatures are. NIST published the first standards in August 2024: ML-KEM (FIPS 203) for key exchange, ML-DSA (FIPS 204) and SLH-DSA (FIPS 205) for signatures, with a further code-based scheme, HQC, selected in 2025 as a backup. They are already arriving: Signal, iMessage and the TLS connections in major browsers now mix a post-quantum key exchange into every session. Blockchains are the laggards, because changing a signature scheme means every holder must move coins to new addresses, and that transition is still being designed. A key already exposed on the blockchain (any address that has spent) is the one to worry about; a fresh address exposes only a hash until it is used.

Qubit

A quantum computer's unit of computation. The frightening headlines cite qubit counts; the honest question is how many error-corrected ones a machine has, and the answer today is: not remotely enough.

Real-world asset (RWA) token

A token on a blockchain that stands in for something held off-chain by a custodian: a government bond, a money-market fund, a bar of gold, a share. The token moves on crypto's rails; the thing itself does not move at all. Ask three questions of any such token: what stands behind it, who holds that, and who can stop the token moving.

Recovery Scam

The second wave: anyone who contacts you after a theft offering to get your stolen crypto back, for a fee, a deposit or your seed phrase. They find victims through the public complaints and forum posts the first theft produced. No agency, exchange or lawyer will contact you first, and a crypto payment cannot be reversed by anyone; the recovery offer is the sequel, not the rescue.

Relay attack

Stretching a key's radio conversation across distance — one attacker near your keys, one near your car or door — so the lock believes the key is present. The Tesla-driven-away-while-the-fob-sat-indoors attack.

Replay attack

Recording a legitimate wireless "open" command and playing it back later. The defense is freshness: a challenge that's different every time.

RSA

The 1970s scheme that turned public-key cryptography from an idea into the workhorse of the internet, and the classic target of Shor's algorithm — which is why its successors are being installed now.

Safety number (security code)

The number two people can compare, once, in an encrypted messenger to confirm no one sits between them — thirty seconds that turn "trust the logo" into "trust the math."

Secure Element

A Secure Element is a small high-security chip that is included in most modern smartphones. It is designed to be isolated from most of the software on the device, with highly-limited access that raises its security. Highly-sensitive data like biometrics, cryptographic keys, and payment information is stored on Secure Element chips.

A separate, tamper-resistant chip whose only job is to hold secrets and use them without ever handing them over. It has its own processor, memory and random-number generator, runs its own tiny operating system, and is built and certified (typically to Common Criteria EAL5+ or above) to resist physical attack: probing, power analysis, glitching, decapping. The chip in a bank card, the NFC payment chip in a phone and the key chip in a Ledger or similar hardware wallet are all secure elements. A private key born inside one can sign a transaction, but no software on the device, however compromised, can read it out. The distinction that matters is between this kind of dedicated chip and a trusted execution environment, which is a protected mode of the main processor and shares its silicon.

Secure Enclave

Apple’s name for the isolated security subsystem built into every Apple chip since the A7 of 2013. It is a separate processor with its own boot code and encrypted memory, walled off from the main processor and from iOS or macOS, and it is where Face ID and Touch ID data, the keys that encrypt the device, and the keys behind passkeys and Apple Pay are created and used. Keys can be generated inside it and used to sign or decrypt, but they cannot be exported, even by Apple; a wallet app that stores a key “in the Secure Enclave” is relying on exactly that. In function it is a secure element, but it is integrated into the main chip rather than a separate part, and Apple certifies and documents it on its own terms.

Secure Exclave (Apple Exclaves)

A newer layer of isolation in Apple’s operating systems, first visible in iOS 17 and the A15 and later chips, and expanded in iOS 18 and the M4 generation. Exclaves are protected regions of memory and code that run under a small secure kernel outside XNU, the main kernel, so that even an attacker who has fully compromised iOS cannot read or alter them. Apple uses them for things the system must be able to trust even when it is under attack: the buffers behind the camera and microphone privacy indicators, sensor data, and the attestation that lets a device prove to Private Cloud Compute that it is running unmodified software. Where the Secure Enclave guards keys, exclaves guard running code and data, and the two together move Apple’s platforms towards the microkernel idea that no single compromised component should give an attacker everything.

Self-Custody

Holding your own keys, with no company able to freeze you and none able to rescue you. Real ownership, and a real job: you become the vault, the fraud department, and the succession plan.

In the context of cryptocurrency, self-custody is the practice of keeping your cryptocurrency in a crypto wallet that you own and maintain yourself, rather than allowing an exchange or other financial entity to hold the keys to the crypto wallet for you. Self-custody reduces the risk that a third party's problems might affect the digital assets in the wallet, but increases the risk of making a mistake in maintaining the wallet yourself. Always remember: "Not your keys, not your coins."

Shor's algorithm

The 1994 discovery that a sufficiently large quantum computer could unravel today's public-key cryptography — the reason post-quantum replacements exist, and the reason the transition started years before the machine.

Side-channel attack

Reading a secret not from the math but from the machine: the power a chip draws, the time it takes, the sounds it makes. Kocher's proof that hardware can betray what the algorithm protects.

SIM swap

Convincing a phone carrier to move your number to an attacker's SIM, capturing your calls and texted codes — the attack that makes SMS the weakest form of two-factor authentication.

Slashing

The penalty built into proof-of-stake networks: a validator that signs conflicting blocks or goes offline too long loses part of its staked coins. If you stake through a service, ask where that risk sits, because some pass it straight to you and the app's cheerful button will not say so.

Smart-contract wallet

A wallet whose rules live on the blockchain itself rather than in a single key: daily spending limits, delays on large transfers, a list of trusted recovery contacts, a different key for small payments. Promising, because it removes the single point of failure, and younger code, because every rule is a program that can have a bug.

Social Recovery

Regaining access through people you chose in advance — guardians holding shares — instead of through a company's helpline or a single fragile backup.

Social Recovery is a method of relying on a distributed network of friends, family, or associates to help restore a crypto wallet.

Spread

The gap between the price an app quotes you and the price in the wider market: the fee that does not call itself one. A brokerage or payment app that advertises "no commission" often earns its money here. Check the spread as well as the stated fee, and aim to keep the total cost of a purchase under one percent.

Stablecoin

A token built to stay worth exactly one unit of a national currency, almost always the US dollar, so that it moves on crypto's rails with the dollar's stability. There are three kinds: coins backed by cash and short-term government debt held at a custodian, coins backed by other crypto with a safety cushion, and coins that hold their price by algorithm alone, which is the kind that has failed. The issuer can usually freeze coins, for better and for worse, and new laws in the United States and the United Kingdom now say what must stand behind a dollar coin. The question to ask of any stablecoin is the same: backed by what, held by whom, and who can stop it moving?

Stolen Device Protection

The iPhone setting born from the passcode-theft wave, with an equivalent on recent Android phones: away from familiar locations, the crown-jewel actions (changing the Apple ID password, turning off Find My, viewing saved passwords) require Face ID or Touch ID rather than the passcode, and some add a one-hour delay. A thief who watched you type your passcode in a bar can then unlock the phone but cannot take over the account. Check that it is on.

Strongbox

Android’s name for a key store backed by a true secure element, introduced in Android 9 (2018). Android’s ordinary “hardware-backed” keys live in the trusted execution environment of the main processor; a StrongBox key lives in a separate tamper-resistant chip with its own CPU, secure storage, true random-number generator and secure timer, such as the Titan M2 in Google’s Pixel phones. A key created with StrongBox cannot leave that chip, and the device can prove to a server that it was made there (key attestation). The chip is small and slow, so it supports a limited set of algorithms (AES, RSA 2048, P-256 elliptic-curve keys, HMAC) and Google notes that most apps do not need it. Apps ask for it explicitly; a wallet or authenticator that does is making the stronger choice.

Tamper resistance (and tamper evidence)

Three different promises hide behind one word. A tamper-resistant part is hard to get into: the secure element in a phone or hardware wallet is built so that probing, glitching or decapping the chip destroys or hides the secret rather than exposing it. A tamper-evident part shows that someone tried: a holographic seal over the box or the USB port, an ultrasonically welded case that cannot be opened without leaving marks, a scratch panel that should still be intact. A tamper-responsive part reacts: a chip that wipes its keys when it senses abnormal voltage, temperature or light. Hardware wallets lean on the first and, to varying degrees, the second; none of them can protect you from a device that was tampered with before it reached you, which is why the “tampered Ledger” cases matter. In the simplest version a reseller sends a device with the recovery sheet already filled in, or a PIN already set, so the seller holds the key to whatever you deposit; a genuine device never arrives with either. In the 2021 version, criminals used the addresses from Ledger’s leaked customer list to mail fake “replacement” Nano X units in convincing shrink-wrap, with a letter blaming the breach; inside, a small flash drive had been soldered to the USB connector, and the enclosed instructions asked the owner to type their 24 words into an app, which sent them to the attackers. Other fakes have shipped with the secure element swapped for an ordinary microcontroller running firmware that leaks the seed. The defenses are the same in every case: buy only from the maker or its listed resellers, treat any device that arrives unsolicited or with an opened or altered package as hostile, run the manufacturer’s cryptographic genuine check during setup using software downloaded from the maker’s own site (a fake app can lie about a fake device, and the check cannot see a physical implant if the original chip is still present), insist on generating the seed phrase yourself on the device, and never type a seed phrase into anything with a keyboard. Seals and packaging tell you something was opened; only the chip’s own attestation tells you the chip is real, and only your own setup tells you the secret is yours.

Threat model

The short, honest audit that turns the world's security advice into your list: what am I protecting, from whom, how hard will they try, and can the loss be undone?

Tokenization (tokenized fund)

Keeping the ownership record of a conventional asset, a fund, a bond, a share, on a blockchain so that it moves on crypto's rails: a money-market fund whose units are tokens, settling in minutes rather than days. The issuer stays the counterparty and the fund's rules still apply; it is a wrapper, to be judged like any other wrapper by what stands behind it and who can freeze it.

Tokenized deposit

An ordinary bank deposit whose record lives on a shared ledger. It moves like a stablecoin but remains a claim on the bank, inside the bank's rules, supervision and deposit protection. Banks are piloting them for payments between their own customers and with other banks; for the holder the difference from a stablecoin is who stands behind the token, and that answer is a bank.

Two-question test

Two questions to ask before any money moves because something prompted you to move it: Did I initiate this? Can I verify it on a channel I already trusted yesterday? Both must pass. A text from the bank, a call from a relative in trouble, a pop-up that says your wallet needs re-verifying: each fails the first question, and each can be checked by hanging up and calling the number you already had.

Unsecured creditor

What an exchange customer legally becomes when the exchange fails: a lender to a bankrupt company, standing in a queue behind the secured lenders and the lawyers, usually for years, usually for cents on the dollar. Your balance was a claim, not a key; the bankruptcy makes the difference visible.

Validator

A participant in a proof-of-stake network who checks transactions and signs blocks, with money at stake for honesty — one of the ten thousand signers whose approvals BLS folds into one.

VPN (virtual private network)

A service that carries your traffic through its own servers, hiding it from your local network and internet provider. Trust is moved, not removed: everything your ISP saw, the VPN company now sees.

Wallet drainer

Ready-made criminal tooling, sold to scammers as a kit, that turns one deceptive signature, usually a permission signature, into an emptied wallet minutes later. The defense is the five questions to ask before you tap "Sign": who asked, what exactly am I signing, is it a transfer or a standing permission, can I read it on a screen I trust, and what happens if I do nothing.

Yield

What crypto can earn while it sits. Every real yield has a source: payment for securing a network (staking), interest from a borrower who posted collateral, or fees from traders using liquidity you supplied. The fourth thing that only looks like yield is a company paying old depositors with new deposits. Ask any yield offer four questions, who pays it, from what, what can go wrong, and who holds my coins meanwhile, and if you cannot name the source, you are the source.

Zero-knowledge architecture

A service designed so the provider cannot read what it stores for you — sealed boxes on their shelves, keys only in your hands. (Distinct from a zero-knowledge proof, below.)