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Q-Day: what happens when the mathematics behind digital trust becomes vulnerable? Q-Day is the point when a sufficiently powerful, fault-tolerant quantum computer can break widely deployed public-key cryptography. There is no confirmed date. Today’s quantum computers cannot perform these attacks at the required scale. But the exposure begins before that capability arrives. Technically, the threat unfolds in three stages: 1. Before Q-Day: harvest now, decrypt later. Adversaries can collect encrypted communications today and retain them for future decryption. Diplomatic exchanges, intellectual property, medical records and strategic infrastructure data may remain valuable for decades. Their protection must outlast the arrival of quantum cryptanalysis. 2. At the capability threshold: key recovery becomes possible. Shor’s algorithm, running on a sufficiently capable quantum computer, could factor RSA keys and solve the discrete logarithm problems underlying Diffie–Hellman and elliptic-curve cryptography. Attackers could then recover vulnerable private keys or reconstruct shared secrets from recorded exchanges. Classical forward secrecy alone does not protect recorded elliptic-curve key exchanges against this attack. 3. As attacks become operational: digital trust can be exploited. Recovered signing keys could enable forged certificates, impersonated services and malicious software updates that pass vulnerable signature checks. Devices relying on RSA or ECDSA for secure boot—including industrial equipment and satellites—could face serious risks where attackers can deliver forged firmware. These outcomes would depend on computing resources, attack time, access and each system’s remaining controls. Q-Day will not automatically switch off the Internet. Nor does it mean all encryption breaks. AES-256 remains a strong option against known quantum attacks. The immediate structural weakness is public-key key establishment and digital signatures. The practical response is to inventory cryptography, deploy standardized post-quantum mechanisms such as ML-KEM for key establishment and ML-DSA or SLH-DSA for signatures, and ensure devices can securely update their algorithms and trust anchors. Migration dates are preparation deadlines, not predictions of Q-Day. The UK roadmap calls for discovery and planning by 2028, high-priority migration by 2031, and completion by 2035. These threats, their timing and the practical steps organizations can take will be discussed with experts during SEALSQ Quantum Day. Join the discussion on protecting digital identities, secure semiconductors and critical infrastructure in the quantum era. Request attendance at @SEALSQcorp Quantum Days 2026⁠ sealsq.com/quantum-days-2026… The decisive question is whether our systems will be ready before an adversary has the capability to attack them. #SEALSQ #QuantumDays2026 #QuantumSecurity #PostQuantumCryptography #Cybersecurity #DigitalTrust

$LAES - SEALSQ and TechAgency Partner to Guide Organizations Through the Post-Quantum Transition