@hunhani.Thank you for your interest and support in the previous posts. I will do my best to meet your expectations.
Last time, We have learned about the current status of technology development for the quantum computer.
Chapter 7. Can Quantum Computers Kill Blockchain? This time, we will deal with how valid the following sentence is. “quantum computer will eventually kill the block chain system.”
This is what many people already know, but I will keep it up. Blockchain is a chain-like structure of blocks, which are the books on Peer to Peer (P2P) transaction data are recorded, and connected in sequence according to time. Once every 10 minutes, the transaction specifications were bundled into a single block and then make each block into a public record, like a chain. The entire block of the transaction book are exposed and distributed to all network participants using the blockchain to be managed. 51% of blockchain network must agree that each block is a valid transaction through consensus so that new blocks can be linked to the existing blockchain and the transaction records which are once connected blockchain is permanently stored in an unmodifiable state. If someone wants to manipulate the transaction details, it is necessary to unrealistically operate the whole blockchain by hacking all the network participants' computers within 10 minutes of forming one block. Therefore, in principle, this is not possible. Transparency, fairness and safety are guaranteed at the same time. In addition, without the need for a third party intervention, the blockchain system itself authorizes direct inter-personal transactions in real time through autonomous delegation, thus realizing transaction authorization and decentralization of information.
As you all know, the cryptocurrency uses a block chain system. Let's take bitcoin for example. The bitcoin receiver shares the public key with the sender. The public key corresponds to a wallet address, which is generally not a problem when it is exposed to others. To use bitcoin, the bitcoin holder must use the private key. If someone else finds out their private key, they can use that bitcoin. Private keys are protected from algorithms that use the difficulty of a large number of prime factorization. Existing super computer can not even dream to solve them, but quantum computer can calculate a private key from a public key in just a few minutes. If someone find out the private key of the address, it is instantaneous that the balance will be stolen. Theoretically, if the quantum computer finds out all the private keys, it can access all the bitcoins. For this reason, when the quantum computer is commercialized, it is considered that the current blockchain system can collapse by destroying both the public key and private key.
Bitcoin uses the Secure Hash Algorithm (SHA) designed by the USA National Security Agency (NSA). A hash is a value obtained by a function that converts data of arbitrary length into data of fixed length (which finds the actual physical address from the virtual address). Since the hash function is deterministic, the original input data is different if the two hash values are different. At the same time, the hash function is not a one-to-one function, so even if it has the same hash value, it does not guarantee that the original input data will match 100%. Therefore, even if the input data changes slightly, the hash value greatly changes. Hashing makes it easy to find duplicated records in a large repository, which not only accelerates database searches, but also makes it easier to structure the ste of data. Because of this, hash is widely applied to computer software capable of very fast data retrieval. Furthermore, the cryptographic hash function has been applied to the cryptocurrency as well as the bitcoin, based on the fact that the integrity of the transmitted data can be verified and the hashing value alone makes it difficult to determine the original input data.
Returning to the SHA again, the SHA function represents a set of cryptographic hash functions related to each other. The first function in the SHA function family is officially called SHA, but it is also called SHA-0 to distinguish it from later designed functions. Two years later, SHA-1, a variant of SHA-0, was announced, and four variants of SHA-224, SHA-256, SHA-384 and SHA-512 were announced in succession. SHA-1 is the most used of SHA functions and is used in many security protocols and programs such as TLS, SSL, PGP, SSH, and IPSec. Current bitcoin also depends on SHA-256. Although SHA-256 is now very safe, one day, if a quantum computer is commercialized, we must use a much stronger encryption algorithm than the SHA series.
Bitcoin key generation is managed by ECDSA (elliptic curve digital signature algorithm). The public key is created by multiplying the randomly generated private key by the points of the predetermined elliptic curve. With elliptic curve multiplication, the private and public keys are paired with each other and protected by a set of non-reversible hashing functions. Miners must find a solution to a specially designed hash problem to authenticate transactions in the block chain and be rewarded with the newly generated bit coin. The quantum computer can use Grover's algorithm to find a unique input value with a high probability in the black box function that generates the measured output value, thereby improving the second speed in the hash function and reducing the length of the generated key by half. Therefore, a miner using a quantum computer can significantly improve the speed at which a hash problem solution can be found. On the other hand, the length of the generated key may be reduced and the security effect may also be reduced. This can be solved by artificially doubling the length of the key. Of course, this approach can cause controversy regarding block size limitations, and above all, the fork implementation can be much more difficult.
There are some cryptocurrencies developed for the purpose of countering quantum computers, such as QRL (Quantum Resistant Ledger) and Byteball (GBYTE). They use a totally new block chaining system designed to be particularly secure for quantum computers. In particular, unlike blockchain systems used by other cryptography, QRL can apply a pre-quantum secure signature for transactions using a hash-based signature scheme called the eXtended Merkle Signature Scheme (XMSS). In addition, QRL uses a proof-of-stake algorithm with low power consumption through iterative hash-chains and provably secure hash-based pseudo-random number functions. It is possible to operate a signature that is 25% smaller than the cryptocurrencies using a conventional signature scheme vulnerable to a quantum computer, so that QRL can run the node even in a low-power device such as a laptop, and is based on the most secure and verifiable practical blockchain system.
At a time when a quantum computer is not commercially available, if some forces own a quantum computer and exploit it, it will pose a huge threat to all industries of modern technology based on IT as well as cryptocurrency. Furthermore, it will have unimaginable adverse effects throughout the military, financial and administrative security sectors. The computing power of a quantum computer can be used to disrupt the cryptocurrency market by hacking the blockchain system, but it can also be used to enhance the security of a blockchain network and mine the cryptocurrency. Ultimately, to protect cryptocurrency from quantum computers, we must incorporate a new cryptographic standard into existing cryptographic protocols that can stand up to quantum computers. There are already a number of cryptographic developers developing technologies against quantum computers, and the basic technology already exists. But it would take time for such technology to be introduced and used throughout the cryptocurrency community. We still have enough time to prepare. My personal opinion is that, even if the threat of a quantum computer is realistic, we expect to adopt a new, appropriate standard that can handle quantum computers in a block-chain network.
