Page 104 - Handbook of Deep Learning in Biomedical Engineering Techniques and Applications

92 Chapter 4 A critical review on using blockchain technology in education domain

denial-of-service (DDoS) protection. Hacking the network
becomes costly. The major issue is that generation of billions of
hashes every second is energy intensive. There is also a possibility
of moving toward centralization when miners group together to
increase their computation power. Hence, miners with more
computation power may be able to control the network. This
might lead to 51% attack when a group has control over more
than 51% nodes in the network. PoW in Bitcoins is able to
perform only 10e20 transactions per second over the entire
global network.
(b) Proof of capacity (PoC): PoC is similar to PoW, but it uses hard
disk capacity of the miners instead of the computational
capacity. The miners have to store a huge number of hash
solutions on the mining device’s hard disk. The larger the ca-
pacity of the hard drive, the more the solutions can be stored.
Thus, during the plotting phase, list of all possible nonce
values is created through repeated hashing of data, including
a miner’s account. Each such nonce contains 8192 hashes
paired into scoops. Adjacent hashes are paired to a scoop.
Hence, there are 4095 scoops. Hash values in these scoops
identify blockID, previous hash, and data values.
In PoW, nonce with suitable value is identified. This is done by
hashing blockID, previous hash, data value, and nonce (random)
to generate a hash below a target value. During mining, once the
blockID, previous hash, and data values are known, a hash can be
computed. This can be mapped into scoopID of various nonce. A
deadline is stored in each scoop. These data are extracted from
the matching scoops of all nonces. The nonce with least deadline
is chosen. The miner who does his first then fills in the nonce in
the block and attaches it to the BC.
PoC works by storing a list of possible solutions on the mining
device’s hard drive even before the mining activity commences.
The larger the hard drive, the more the possible solution values
one can store on the hard drive, and the more the chances a
miner has to match the required hash value from his list, result-
ing in more chances to win the mining reward. The Permacoin
and Spacemint are the two cryptocurrencies employing the
PoC. It is faster than PoW. It is more energy efficient than PoW.
But PoC creates redundant disk spaces and favors miners holding
large storage units. There is also a possibility of hacking the solu-
tions stored in the hard disk.
(c) Proof of existence: When a user submits a document/data, the
hash calculated is linked with the time of data submission.
This ensures that data had existed at a particular time instant.
The data are not exposed as it is maintained separately