> This page is from **SQOUT** by QuDef — https://sqout.qudef.com/ > > SQOUT, by QuDef, is a security intelligence and knowledge framework for quantum technologies. Its catalogue describes attacks, vulnerabilities, countermeasures and security proofs for quantum communication and quantum computing systems. > > How to use and cite this content: https://sqout.qudef.com/llms.txt # Dictionary Source: https://sqout.qudef.com/dictionary Cite as: SQOUT (QuDef), https://sqout.qudef.com/dictionary - **Afterpulsing**: Spurious click caused by carriers trapped during a previous avalanche. - **Asymmetric Mach-Zehnder interferometer** (AMZI): Unbalanced interferometer converting phase differences into distinguishable arrival times. - **Attack potential**: Rating of the expertise, equipment and time an attack requires to succeed. - **Attack surface**: The set of points where an attacker can try to interact with a system. - **Authentication**: Confirming a message came from the claimed party, required on the classical channel. - **Avalanche photodiode** (APD): Semiconductor detector where one absorbed photon triggers a measurable current avalanche. - **Active quenching**: Stopping an avalanche with driven electronics that reset the detector quickly. - **Free-running detector**: Detector sensitive continuously rather than only inside timed gates. - **Gated detector**: Detector made sensitive only during short windows synchronised to expected arrivals. - **Geiger mode**: Detector regime above breakdown where a single photon triggers a full avalanche. - **Linear mode**: Detector regime below breakdown where output scales with incident optical power. - **Negative-feedback avalanche diode** (NFAD): Avalanche diode with integrated feedback resistance for fast self-quenching. - **Passive quenching**: Stopping an avalanche with a resistor, leaving a slow recharge the attacker can exploit. - **Single-photon avalanche diode** (SPAD): Avalanche photodiode biased above breakdown so it responds to individual photons. - **Beam splitter**: Optic dividing a beam between two paths, the basis of passive basis choice. - **Blinding pulse**: The bright light that holds a detector insensitive to single photons during an attack. - **Common Criteria** (CC): International standard for specifying and evaluating IT security requirements. - **Common Vulnerabilities and Exposures** (CVE): Public catalogue assigning each disclosed vulnerability a stable identifier. - **Continuous Wave** (CW) - **Countermeasure**: Control that prevents, detects or reduces the impact of an attack. - **CV-QKD**: Continuous-variable QKD — quantum information encoded in field quadratures (Gaussian states). - **Defence in depth**: Layering independent controls so no single failure exposes the whole system. - **Denial of service** (DoS): Attack that prevents legitimate key exchange rather than trying to learn the key. - **Detection efficiency**: Probability that a photon reaching a detector produces a recorded click. - **Detector dead time**: Interval after a click during which a detector cannot register another photon. - **Device-Independent QKD** (DI-QKD): Security argued from observed statistics alone, trusting neither source nor detectors. - **Evaluation Assurance Level** (EAL): Common Criteria rating describing how rigorously a product was evaluated. - **Exploit**: Concrete technique or code that turns a vulnerability into an actual compromise. - **Faraday mirror**: Mirror that rotates polarisation on reflection, stabilising two-way plug-and-play systems. - **Forward secrecy**: Property where compromising a long-term secret does not expose past session keys. - **Intensity modulator**: Component setting pulse brightness, used for decoy states and signal preparation. - **Kill chain**: Ordered stages an attacker moves through, used to plan detection and disruption. - **Man-in-the-middle attack** (MITM): Attacker sits between two parties, impersonating each to the other. - **MDI-QKD**: Measurement-device-independent QKD — detector is treated as untrusted (Lo-Curty-Qi 2012). - **Mean photon number** (MPN): Average photons per pulse, the parameter decoy states vary to expose splitting. - **One-time pad**: Cipher that is unbreakable when the key is random, secret and used exactly once. - **Optical isolator**: Component passing light one way only, used to block injected probing light. - **Optical power monitor**: Component measuring incoming light level to reveal bright-light attacks. - **Patch**: Change issued to remove a vulnerability from a deployed system. - **Penetration testing**: Authorised simulated attack used to find weaknesses before a real attacker does. - **Phase encoding**: Carrying bits in the relative optical phase between two pulses. - **Phase modulator**: Component imposing a chosen optical phase, used to encode or decode bits. - **Phase randomisation**: Source assumption that pulse-to-pulse phases are uniformly random, enforcing Poisson statistics. - **Plug-and-play system**: Two-way architecture where pulses originate at the receiver and return encoded. - **Polarisation encoding**: Carrying bits in the polarisation state of each photon. - **Post-quantum cryptography** (PQC): Classical algorithms designed to resist attack by a large quantum computer. - **Protection Profile** (PP): Common Criteria document stating the security requirements for a class of product. - **QBER**: Quantum bit-error rate — fraction of sifted bits that disagree between Alice and Bob. - **Quality of Service** (QoS) - **Quantum Bit Error Rate** (QBER) - **Quantum communication**: Transmitting information encoded in quantum states, of which key distribution is one use. - **Alice** - **Bell inequality**: Test whose violation proves correlations cannot be explained by any local hidden variable. - **Blind quantum computing**: Delegating a computation to a server that learns neither input nor algorithm. - **Bob** - **Channel loss**: Fibre or free-space attenuation, scaling as 10^(-alpha·L/10) per km. - **CHSH inequality** (CHSH): The standard four-correlator Bell test used to certify device-independent security. - **Classical channel**: The authenticated public link carrying sifting, error correction and privacy amplification. - **Conference key agreement** (CKA): Establishing one shared key among three or more parties at once. - **Eve** - **Monogamy of entanglement**: Limit on how far entanglement can be shared, bounding what an eavesdropper can hold. - **No-cloning theorem**: Result that an unknown quantum state cannot be copied, the basis of quantum security. - **Position verification**: Proving a party is physically where it claims, using timing and quantum states. - **Quantum channel**: The path carrying quantum signals, assumed fully under an adversary's control. - **Quantum coin flipping**: Two mistrustful parties agreeing a random bit neither can bias alone. - **Quantum digital signature** (QDS): Signing scheme whose unforgeability rests on quantum rather than computational assumptions. - **Quantum fingerprinting**: Comparing large inputs by exchanging exponentially smaller quantum states. - **Quantum Key Distribution** (QKD) - **B92**: Two-state protocol encoding bits in a pair of non-orthogonal states rather than four. - **BBM92**: Entanglement-based counterpart of BB84 in which both parties measure photons of a shared pair. - **Coherent One-Way** (COW): Protocol encoding bits in the time position of coherent pulses, with a monitoring line. - **Continuous-Variable QKD** (CV-QKD) - **Channel transmittance**: Fraction of signal power surviving the channel, estimated during parameter estimation. - **Coherent state**: Minimum-uncertainty state of light produced by a laser, the usual CV-QKD carrier. - **Covariance matrix**: Second-moment description of a Gaussian state, sufficient on its own for security analysis. - **Direct reconciliation**: Correcting errors toward the sender's data, which fails beyond 3 dB channel loss. - **Electronic noise**: Receiver noise of electronic rather than optical origin, added after the signal arrives. - **EPR state**: Strongly correlated two-mode state used in the entanglement-based view of CV protocols. - **Excess noise**: Channel noise above the vacuum level, the quantity that limits continuous-variable key rate. - **Gaussian channel**: Channel that maps every Gaussian state to another Gaussian state. - **Gaussian state**: State fully described by the first and second moments of its quadratures. - **Heterodyne detection**: Measuring both quadratures at once, at the cost of extra vacuum noise. - **Homodyne detection**: Measuring one quadrature of a light field by mixing it with a reference beam. - **Local oscillator** (LO): Bright reference beam a continuous-variable receiver mixes with the signal. - **Low-density parity-check code** (LDPC): Sparse-graph error-correcting code that performs near the limit at low signal-to-noise. - **Modulation variance**: Spread of the sender's Gaussian modulation, a parameter optimised per channel. - **Multidimensional reconciliation**: Mapping Gaussian data onto a sphere so binary codes can correct it at low signal-to-noise. - **Noise budget**: Accounting of every noise contribution, used to decide which parts are trusted. - **Phase noise**: Random drift between signal and reference phase, which appears in the measurement as excess noise. - **Phase recovery**: Estimating and correcting the signal-to-reference phase from pilot measurements. - **Pilot tone**: Reference signal sent alongside the quantum signal so the receiver can recover phase. - **Post-selection**: Discarding data where the legitimate parties hold no advantage over the attacker. - **Quadrature**: One of two conjugate components of a light field, the variables CV-QKD encodes on. - **Reverse reconciliation**: Correcting errors toward the receiver's data, which removes the 3 dB loss limit. - **Shot noise**: Irreducible fluctuation of a coherent light field, the reference all CV noise is measured against. - **Shot-noise unit** (SNU): Normalisation in which the vacuum fluctuation variance equals exactly one. - **Slice reconciliation**: Quantising continuous values into slices that are error-corrected separately. - **Squeezed state**: State whose noise in one quadrature falls below the vacuum level, at the other's expense. - **Three-decibel loss limit**: Range bound on direct reconciliation, lifted by reconciling toward the receiver instead. - **Trusted noise**: Detector noise assumed outside the attacker's reach, which raises the computed key rate. - **Two-mode squeezed vacuum** (TMSV): Entangled Gaussian state, the entanglement-based equivalent of Gaussian modulation. - **Untrusted noise**: Noise attributed entirely to the attacker, the conservative assumption for a security proof. - **Vacuum fluctuations**: Quantum noise present even with no light, setting the floor for any measurement. - **Weak coherent pulse**: Laser pulse approximating a low-photon-number state, used in real QKD systems. - **Counterfactual QKD** (CFQKD): Key exchange in which, ideally, no signal photon ever travels the quantum channel. - **CV B92**: Continuous-variable analogue of the two-state B92 protocol. - **CV MDI-QKD**: Continuous-variable protocol measured at an untrusted central relay. - **Decoy-state BB84**: BB84 run with several pulse intensities so photon-number-splitting is detectable. - **Differential Phase Shift QKD** (DPS-QKD) - **Differential phase time shifting** (DPTS): Protocol combining differential phase encoding with time-shifted pulse positions. - **Discrete-modulation CV-QKD** (DM CV-QKD): Continuous-variable scheme sending a small finite set of coherent states. - **Discrete-Variable QKD** (DV-QKD) - **Afterpulse probability**: Chance that an avalanche leaves trapped carriers that fire a spurious later click. - **Basis bias**: Choosing one basis more often than the other to raise the sifted-key yield. - **Basis reconciliation**: Publicly comparing measurement bases so incompatible results can be discarded. - **BB84**: The original quantum key distribution protocol (Bennett & Brassard 1984), using non-orthogonal bases. - **Coincidence window**: Time interval within which two detections count as one correlated pair. - **Dark count**: Detector click in absence of an incoming photon — sets a noise floor. - **Decoy intensity**: Lower intensity setting used to certify the single-photon yield in decoy-state QKD. - **Decoy state**: Protocol variant using multiple intensities to certify the single-photon contribution. - **Decoy-state estimation**: Using several intensities to bound the single-photon yield and its error rate. - **Detector efficiency**: Probability that a photon arriving at the detector produces a click. - **Detector gate width**: Duration of the window in which a gated detector is sensitive. - **Gain**: Probability that a sent pulse produces a detection, per intensity setting. - **Key sifting ratio**: Fraction of detections surviving basis reconciliation. - **Multi-photon pulse**: Pulse containing more than one photon, the opening that photon-number splitting exploits. - **Photon-number distribution**: Statistical spread of photons per pulse, Poissonian for an attenuated laser. - **Photon-number-splitting attack**: Multi-photon-pulse attack that lets Eve siphon photons without disturbance. - **Poissonian source**: Attenuated laser whose photon number per pulse follows a Poisson distribution. - **Sifted key**: Bits remaining after Alice and Bob discard rounds in which their bases disagreed. - **Sifting factor**: Probability that Alice and Bob choose the same basis (q ≈ 1/2 for symmetric BB84). - **Single-photon source**: Source emitting exactly one photon per pulse; required for the Shor-Preskill proof. - **Single-photon yield**: Detection probability for the single-photon component, the quantity decoy states bound. - **Unambiguous State Discrimination** (USD) - **Visibility**: Interference contrast, which sets the achievable error rate in phase-encoded systems. - **Yield**: Probability that an n-photon pulse produces a detection at the receiver. - **E91**: Ekert's entanglement-based protocol that certifies secrecy through violation of a Bell inequality. - **Entanglement-Based QKD** (EB-QKD) - **Gaussian-Modulated Coherent State** (GMCS): Continuous-variable scheme modulating coherent-state amplitude and phase from a Gaussian distribution. - **Grosshans-Grangier 2002** (GG02): Continuous-variable protocol using Gaussian-modulated coherent states and homodyne detection. - **LM05**: Deterministic two-way protocol where the receiver encodes on a returning qubit. - **Local local oscillator CV-QKD** (LLO CV-QKD): Continuous-variable scheme generating the reference beam at the receiver, not the sender. - **MDI three-state protocol**: Measurement-device-independent scheme using three rather than four prepared states. - **Measurement-Device-Independent QKD** (MDI-QKD) - **PE-BB84**: BB84 variant with an explicit parameter-estimation stage built into sifting. - **Phase-Matching QKD** (PM-QKD): Twin-field variant matching optical phase at a central node to extend range. - **Prepare-and-Measure QKD** (P&M QKD) - **Round-Robin Differential Phase Shift** (RRDPS): Protocol bounding an eavesdropper's information without monitoring the error rate. - **SARG04**: BB84 variant whose sifting step resists photon-number-splitting on weak coherent pulses. - **Sending-or-Not-Sending** (SNS): Twin-field variant tolerant of phase misalignment because bits ride on sending choices. - **Six-state protocol** (SSP): BB84 extended to three mutually unbiased bases, tolerating a higher error rate. - **Squeezed-state CV-QKD**: Continuous-variable protocol using squeezed light rather than coherent states. - **Toshiba T12**: Decoy-state BB84 variant with biased basis choice used in commercial systems. - **Two-way CV-QKD**: Continuous-variable scheme in which signals travel to the far party and back. - **Quantum money**: Unforgeable tokens whose security rests on the impossibility of copying quantum states. - **Quantum Non-Demolition** (QND) - **Quantum oblivious transfer**: Transfer where the sender does not learn which message the receiver obtained. - **Quantum secret sharing** (QSS): Splitting a secret among parties so only an authorised subset can reconstruct it. - **Quantum secure direct communication** (QSDC): Sending a message itself over the quantum channel rather than first sharing a key. - **Security proof**: Terminology specific to QKD security-proof construction and analysis. - **Asymptotic regime**: Infinite-data analysis ignoring finite-statistics corrections. - **Coherent attack**: Most general attack: Eve coherently entangles a probe with every pulse. - **Collective attack**: Eve uses an identical probe for every pulse, but measurements may be joint. - **Composable security**: Security definition under which the protocol remains secure when composed with others. - **Detection loophole**: Gap in a Bell test caused by low detector efficiency, which faked violations exploit. - **Devetak-Winter bound**: Key rate as the parties' mutual information minus the attacker's Holevo information. - **Error correction**: Public-channel reconciliation that fixes the bit errors counted by the QBER. - **Fair sampling assumption**: Assumption that detected events represent all events, broken by efficiency mismatch. - **Finite-key analysis**: Security accounting for statistical fluctuations in a finite-size run. - **GLLP analysis** (GLLP): Security treatment separating single-photon from multi-photon contributions in an imperfect source. - **Hashing inequality**: Bound used to relate von-Neumann entropy to the smooth min-entropy. - **Holevo bound**: Upper limit on the classical information obtainable from a quantum ensemble. - **Individual attack**: Eve attacks each pulse independently with independent measurements. - **Information-theoretic security**: Security holding against any adversary regardless of computing power or time. - **Leftover hash lemma**: Result stating how much uniform key a hash can extract from a partly known string. - **Linear program**: Optimisation method used in Curty 2014 to tightly bound the single-photon contribution. - **Locality loophole**: Gap in a Bell test where the measurement events are not space-like separated. - **Min-entropy**: Measure of the guessing probability of a variable, setting how much key can be extracted. - **Mutual information**: Amount of information two correlated data sets share. - **Optimality of Gaussian attacks**: Theorem that the worst case against a Gaussian-modulated protocol is itself Gaussian. - **Parameter estimation**: Using a sampled subset of results to bound the eavesdropper's information. - **Phase error**: Error rate inferred for the complementary (test) basis; bounds Eve information. - **Privacy amplification**: Final step that compresses a partially-leaked key into a shorter secret one. - **Purification**: Describing a prepare-and-measure protocol by an equivalent entangled source, simplifying its proof. - **Quantum bit error rate threshold**: Error level above which no secret key can be distilled and the session aborts. - **Reconciliation efficiency**: How close error correction comes to the theoretical minimum leakage. - **Secret key fraction**: Portion of raw measurement data that survives distillation to become key. - **Secret-key rate**: Asymptotic length of secret key extracted per channel use (or per sifted signal). - **Security assumption**: Condition a proof relies on, which an implementation must actually satisfy. - **Security parameter**: Explicit bound on the probability that the delivered key is not perfectly secret. - **Sifting**: Discarding results where the parties used incompatible measurement settings. - **Smooth min-entropy**: Min-entropy allowing a small failure probability, the quantity finite-key proofs bound. - **Squashing model**: Reduction that maps a multi-mode detector to an equivalent single-photon detector. - **Symplectic eigenvalue**: Eigenvalue of a covariance matrix under symplectic transformation, used to compute entropies. - **Trusted device assumption**: Assumption that named hardware behaves as modelled and is outside the attacker's reach. - **Universal hashing**: Family of hash functions whose collision probability is bounded for any input pair. - **Wegman-Carter authentication**: Information-theoretically secure message authentication using a short shared key. - **Superdense coding**: Sending two classical bits in one qubit using pre-shared entanglement. - **Quantum hacking**: Attacking a quantum cryptosystem through its physical implementation rather than its protocol. - **Implementation attack**: Attack exploiting the gap between an idealised protocol and the hardware realising it. - **After-gate attack**: Sending bright pulses just after a gate closes, when the detector responds classically. - **Backflash**: Light emitted by an avalanching detector that can leak which detector fired. - **Beam-splitting attack**: Tapping a fraction of every pulse from the channel and storing it for later measurement. - **Bright illumination attack**: Detector control achieved by shining bright light into the receiver's optical input. - **Calibration attack**: Manipulating a system's calibration routine so it adopts attacker-favourable settings. - **Dead time attack**: Exploiting the interval after a click when a detector cannot register another photon. - **Detector blinding**: Driving a single-photon detector out of Geiger mode so it stops seeing single photons. - **Detector control attack**: Steering a receiver's detectors so the attacker decides which detector clicks. - **Detector efficiency mismatch** (DEM): Exploitable difference in detector sensitivity across time, wavelength or spatial mode. - **Entangling cloner attack**: Canonical collective attack where the attacker taps with a beamsplitter and stores one mode. - **Faked state**: Classical light pulse crafted to force a chosen detection outcome in a blinded detector. - **Induced-photorefraction attack**: Changing a lithium niobate modulator's response with light to bias its encoding. - **Injection locking attack**: Forcing a sender's laser to follow injected light, controlling the emitted state. - **Laser damage attack**: Permanently altering a component with high-power light to create an exploitable weakness. - **Local oscillator calibration attack**: Exploiting a receiver that derives its noise reference from a beam the attacker can alter. - **Local oscillator intensity attack**: Manipulating reference-beam power so the receiver mis-calibrates its shot-noise scale. - **Memory attack**: Exploiting a device that retains state between sessions to leak earlier key material. - **Phase-remapping attack**: Shifting the modulator's timing so the sender's phase states are remapped exploitably. - **Pulse illumination attack**: Blinding with pulsed rather than continuous light, evading average-power monitors. - **Saturation attack**: Overwhelming a detector or monitor so genuine signals stop being registered correctly. - **Seed light attack**: Injecting light into a sender's laser cavity to read or steer its output. - **Spatial mode mismatch**: Efficiency that varies with the incoming beam's spatial mode, opening a side channel. - **Time-shift attack**: Shifting signal arrival time to exploit detectors whose efficiency peaks at different moments. - **Trojan horse attack**: Probing a party's optics with injected light and reading the reflection for settings. - **Wavelength-dependent attack**: Using light outside the design band, where optics and monitors behave differently. - **Zero-error attack**: Detector control precise enough that the legitimate parties see no raised error rate. - **Intercept-resend attack**: Measuring every signal and resending a fresh state, the baseline eavesdropping strategy. - **Side-channel attack**: Attack using unintended physical leakage rather than a flaw in the protocol itself. - **Quantum network**: Connected set of quantum links sharing entanglement or key across many parties. - **Availability**: Fraction of time a link or service actually delivers key to its consumers. - **Backbone link**: Long-haul connection carrying aggregated traffic between metropolitan networks. - **Dense wavelength division multiplexing** (DWDM): Carrying many channels down one fibre, letting quantum and classical traffic share it. - **End-to-end security**: Property where only the two endpoints hold the key, with no trusted intermediary. - **Entanglement-based network**: Network distributing entanglement rather than relaying key through trusted nodes. - **Entanglement distribution**: Delivering entangled pairs to distant parties as the resource for key or teleportation. - **Entanglement swapping**: Joining two entangled pairs so distant particles become entangled without meeting. - **ETSI GS QKD 004**: Standard defining an application interface for key delivery with explicit key streams. - **ETSI GS QKD 014**: Standard defining a REST interface for delivering keys to applications. - **ETSI ISG QKD**: The ETSI industry group that publishes the QKD interface and security standards. - **Failover**: Switching to an alternative path when a link or node stops delivering key. - **Free-space QKD**: Key distribution through air or vacuum rather than optical fibre. - **ISO/IEC 23837**: Standard on security requirements and test methods for QKD components. - **ITU-T Y.3800**: Recommendation describing the overall framework for networks supporting QKD. - **Key consumption rate**: Speed at which applications draw key, which must not exceed the distillation rate. - **Key management system** (KMS): System storing, distributing and retiring key material for the applications consuming it. - **Key buffering**: Holding produced key in reserve so demand can exceed instantaneous production. - **Key consumer**: Application that draws key from the management layer and uses it to protect data. - **Key delivery interface**: API through which an application asks the key manager for key material. - **Key destruction**: Irreversibly erasing key material once it is spent or expired. - **Key exhaustion**: Condition where consumers draw key faster than the links can produce it. - **Key identifier** (Key ID): Label letting both endpoints request the same key without transmitting it. - **Key lifecycle**: Stages a key passes through from generation to destruction. - **Key provisioning**: Making key material available to the endpoints that will consume it. - **Key store**: Protected storage holding key material until an application requests it. - **Key synchronisation**: Keeping both endpoints' key stores consistent in content and ordering. - **Key rate budget**: Planned allocation of a link's key production across its consuming applications. - **Key relay**: Passing key material hop by hop across trusted nodes to link distant parties. - **Metropolitan network**: Short-range deployment linking sites across a single city. - **Multi-hop path**: Route crossing several links, requiring key relay at each intermediate node. - **Network controller**: Component deciding how key and traffic are routed across a quantum network. - **Network topology**: Arrangement of nodes and links, typically point-to-point, ring, star or mesh. - **Optical switch**: Component routing a quantum signal between paths without measuring it. - **Path computation**: Choosing which route a key request should take across the network. - **QKD link**: A single point-to-point quantum connection between two nodes. - **QKD node**: Site housing QKD equipment that terminates one or more quantum links. - **Quantum backbone**: Core long-distance segment of a quantum network. - **Quantum key pool**: Buffer of already-distilled key material held ready for consuming applications. - **Quantum memory**: Device storing a quantum state long enough to synchronise operations across a network. - **Quantum repeater**: Node extending entanglement across long distances without measuring the transmitted key. - **Quantum teleportation**: Transferring a quantum state using shared entanglement plus classical communication. - **Satellite QKD**: Key distribution using a spacecraft as a relay or source to span continents. - **Software-defined networking** (SDN): Centralised control plane that configures links, increasingly used to manage QKD paths. - **Trusted node**: Relay that decrypts and re-encrypts key material, and so must itself be trusted. - **Quantum random number generator** (QRNG): Device deriving unpredictable numbers from a quantum process rather than an algorithm. - **Raman scattering**: Fibre noise from bright classical channels that can swamp a co-propagating quantum signal. - **Red team**: Group that attacks a system adversarially to test its defences. - **Repeaterless bound** (PLOB): Limit on secret key rate achievable over a lossy link without repeaters. - **Residual risk**: Risk that remains after a countermeasure has been applied. - **Security certification**: Independent confirmation that a product meets a stated security standard. - **Signal-to-Noise Ratio** (SNR) - **Superconducting nanowire single-photon detector** (SNSPD): Detector sensing single photons through loss of superconductivity in a cooled nanowire. - **Symmetric encryption**: Encryption where both parties hold the same secret key, the usual consumer of QKD output. - **Threat actor**: Person or group conducting attacks, described by capability, resources and intent. - **Threat model**: Explicit statement of what an attacker can do and what is assumed trustworthy. - **Time-bin encoding**: Carrying bits in which of several time slots a photon occupies. - **Timing jitter**: Variation in the delay between photon arrival and the detector's output pulse. - **Traffic Light Protocol** (TLP): Labelling scheme stating how widely a piece of shared information may be redistributed. - **Twin-Field QKD** (TF-QKD): Protocol pairing single photons at a middle node to beat the repeaterless rate bound. - **Variable optical attenuator** (VOA): Adjustable component setting the mean photon number of outgoing pulses. - **Vulnerability**: Weakness in a system that an attack can exploit to defeat a security goal. - **Watchdog detector**: Extra monitor at the receiver input that flags unexpected incoming optical power. - **Zero-day**: Vulnerability being exploited before the vendor has published a fix.