IEEE Consumer Electronics Magazine - July/August 2021 - 83

Figure 1. Operations within the PUF-Cash protocol responsible for exchanging issued e-Cash tokens for
blinded e-Cash tokens.
and automatically adjusts with dynamically
changing environmental conditions.
Figure 1 illustrates the entities and message
exchange operations associated with the blinding
process, which is a subset of the overall protocol.
Protocol steps are ordered by the
circled numbers and the arrows indicate the
direction of the information exchange. The following
section elaborates on this component
of the protocol, namely the issuance and
blinding steps.
Issuance and Blinding Steps
1) Alice withdraws funds from her account at
the Bank. The Bank generates a unique token
(128-bit random number) for each 1 cent of
the withdrawal amount, then encrypts them
using a PUF-generated session key and transmits
them to Alice. The Bank records the
issued tokens and Alice's session key in a
database of open transactions.
2) Alice randomly selects a Master TTP, TTPM,
XOR encrypts her issued tokens with her session
key and transmits them to TTPM.
3) TTPM transmits Alice's encrypted issued
tokens to the Bank. The Bank validates the
issued tokens by XOR decrypting them with
each of the session keys stored in the database
of open transactions, searching for a match to
those stored in the database. All matching
tokens are removed from the database to prevent
double spending. If validated, the Bank
sends Alice's session key and an acknowledgment
toTTPM.
July/August 2021
4) TTPM initiates the blinding operation: It runs
an AI algorithm to select a subset of Slave
TTPs from the set TTP1 to TTPn. A second
instance of an AI algorithm runs to determine
a set of fractions, which dictate howthe issued
tokens are partitioned among the set of Slave
TTPs for blinding. The AI algorithms utilized
for these operations are presented in the
" Artificial Intelligent Transactions " section.
Since all TTPs are enrolled in a trusted network,
all messages between the Slave TTPs
are transferred over encrypted channels.
5) The Slave TTPs generate a new blinded token
(128-bit random number) for each 1 cent in
the conversion request from TTPM. The
Slave TTPs encrypt the blinded tokens using
their session keys and transmit them to the
Bank. The Bank acknowledges receipt and
adds them to a second blinded token database
of open transactions.
6) Once acknowledged, each Slave TTP transmits
the blinded tokens to TTPM.
7) TTPM encrypts the aggregate set of blinded
tokens with Alice's session key and transmits
them to Alice.
The PUF-Cash protocol addresses the core
components of e-Cash. The issued token database
and validation process carried out by the
Bank (step 3) protects against double spending
and provides a mechanism to recover lost funds.
The random number generation and encrypted
communication between all entities in the blinding
operation guards against man in the middle
and replay attacks. Use of AI algorithms for tasks
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IEEE Consumer Electronics Magazine - July/August 2021

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