Quantum resistance
Also covering Postquantum cryptography
Quantum resistance is the ability for cryptographic protocols to remain secure in the presence of fast quantum computers.
Bitcoin uses a variety of cryptographic protocols with varying degrees of vulnerability to fast quantum computers:

● SHA256, SHA256d, and RIPEMD160 used variously in Bitcoin’s proof of work and to uniquely identify blocks, scripts, individual transactions, and collections of transactions, plus used for hash locks in HTLCs, would have their security strength reduced to its square root by Grover’s algorithm running on an idealized quantum computer. That means an algorithm like SHA256 that currently has an estimated second preimage resistance of 256 bits (2^{256}) would be reduced to 128 bits (2^{128} is the square root of 2^{256}). That loss can be overcome by using a roughly equivalent algorithm with twice as many security bits, e.g. going from SHA256 to SHA512.

● ECDSA public keys used in Bitcoin are vulnerable to a factorization attack using Shor’s algorithm. This would completely eliminate the security of ECDSA, assuming an idealized quantum computer. Since public keys used for proposed schnorr signatures are essentially identical to those used for ECDSA, the same attack applies. Quantumresistant alternatives to ECDSA are known, but they involve much larger key and signature sizes, so most developers seem to prefer to delay upgrading until it’s necessary.

● Noise is the protocol framework used for encrypted communication in LN. Optech has not seen discussion of its quantum resistance, but we believe the way LN currently uses it depends on the security of ECDSA, so if fast quantum computers are developed, they may be able to decrypt old communication between LN nodes. Alternative key exchange mechanisms such as NewHope are known and are proposed for implementation in other parts of Bitcoin, such as the version 2 Bitcoin transport protocol.
The worst case for attacks assumes an idealized quantum computer with sufficient capacity and reliability to perform the attack. It’s likely that the capacity and reliability of quantum computers will increase gradually over time, meaning the security of the cryptography used in Bitcoin will similarly decrease gradually over time, with attacks progressing from computationally infeasible, to theoretically possible but implausible, to extraordinarily expensive, to very expensive, to practical. As long as this progression is followed and is possible to publicly track, it’s likely Bitcoin can continue using its currently highly space efficient cryptography while it remains safe, and then upgrade to postquantum cryptography when it looks like it’ll soon become necessary.
Optech newsletter and website mentions
2021
2020
 Question about paying public keys directly versus hash indirection
 Question about whether taproot create security risk from quantum threats?
2019
 Question about whether hashing pubkeys really provides quantum resistance?
 Bitcoin Core contributor meeting transcripts: taproot quantum discussion
2018
 BIP151 discussion, including about NewHope quantumresistant key exchange
 PR opened for initial BIP151 support, NewHope quantum resistance to follow
See also
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