Quantum computing, is an exciting and rather unusual field of informatics. Recently I had privilege to participate on The Quantum Panel, as part of the Payments Canada conference, where I shared some of my view with wider audience.

1. Why Should You Pay Attention to Quantum Computing?
For over a year now, I have been uncovering the secrets of quantum computing, which is an
exciting and rather unusual field of informatics. It is somewhat weird and yet extremely
fascinating discipline, by being very, very different than anything I have learned or encountered
so far in my career, as a software developer and high-tech executive. Last week I had privilege
to participate on The Quantum Panel, as part of the Payments Canada conference.
But how could this mainly personal interest, be so relevant to payments innovation, which was
and still is my main focus, for almost a decade now? As scientific field that is still very much in
its infancy, quantum computing is also rapidly developing and maturing. It will likely impact
every security we have built and relied on so far. It is not anymore question of IF, but WHEN is
that going to happen. That’s why I felt that right now is the appropriate time for me to invest
some time and try to understand its potential.
The business of electronic payments is based on trust, basically on being able to protect the
communication channels and storage of sensitive information, like user authentication
credentials, account numbers, personally identifiable information, etc. This is not an easy task,
as there are many dishonest actors, who would be more than happy to take advantage of any
security holes left behind, and benefit from stealing someone’s identity, money, etc.
The Current State of The Cryptographic Art
In order to secure the financial system, including payments, we rely on the special area of
mathematics and number theory, known as cryptography. Clearly, solid cryptography is not
guarantee alone. There can always be insiders, who might be bribed (or simply disgruntled) and
leak the secrets to an adversary. However, without help of rock solid cryptography, we would
not stand a slightest chance.
Today’s cryptography is segmented into 2 main fields: symmetric and asymmetric (also known
as Public Key Infrastructure or PKI).
Symmetric cryptography is based on a principle that the same secret key is shared by all parties
(sides) of the communication. The same key is also used for both encryption and decryption.
Example of symmetric cryptographic algorithms would be DES (today there is stronger version
known as triple-DES or 3DES) and AES. Various key sizes are mandated to make it unbreakable
by brute force, i.e. by trying of every possible key value of the same length. As long as all
participants can secretly agree on a secret key value and keep it secret, all is good. But that’s
not easy to achieve, especially if those parties are remote from each other and / or if they can’t
necessarily trust each other.
Asymmetric cryptography is based on principle that every participant has their own key pair,
which is comprised of PUBLIC and PRIVATE key components. Participant’s public key can be
shared with anyone else, while private key must stay protected. Public key is used for

2. encrypting information and the corresponding private key is used for decrypting. On the other
hand, private key can be used to digitally sign the information, as a proof of non-repudiation
and public key can be used to verify that digital signature. Public and private keys are tightly
mathematically related, in a way, that it is practically impossible to reverse engineer private key
from the public key, by using classic computers, in any usable timeframe. By some estimates,
even by using classic supercomputers, one would need several billion years to reverse engineer
private key from RSA public key that is at least long 2048 bits (or 256 digits). This is in fact
fantastic news, as it means that we can freely share our public key with anyone we want to
communicate, without fear that them ever being able to figure out our private key value. Two
most popular PKI cryptography algorithms are RSA (acronym for Rivest, Shamir, and Adelman,
the inventors) and ECC (acronym for Elliptic Curve Cryptography).
For secure internet communication used in e-commerce, mobile / online banking and B2B / P2P
payments communicating parties use the combination of symmetric and asymmetric
cryptography. In simple terms, they first exchange with each other their public keys. Then each
side combines their private key and the other party’s public key, and uses well-known algorithm
(Diffie-Hellman), to generate the secret ‘session’ symmetric key that is used to encrypt and
decrypt the messages, during the lifetime of that particular communication session.
Why can’t today’s classical supercomputers reverse engineer private key from the public key,
even with all of the computing power they have, when those two keys are tightly related? The
answer is - because classic computers are highly iterative in nature. If you imagine a single
memory register that is 2048 bits long, it can hold only one of the possible values from the
range [0 … 22048
-1], at each moment in time. That means that classic computer can basically
only manipulate a single value at a time. You also intuitively feel that 22048
is an beautifully large
number (about as large as 10682
). Now, in order to process that large value range, only single
value at the time, even with today’s ultra-fast CPU clock speeds, it will take an awfully large
amount of time. Certainly, a lot longer than the usefulness of the information we were trying to
protect in the first place. That inability of mainstream modern classic computers (even
supercomputers) to process and examine very large ranges of values and discover any secret
patterns or rules buried in there, is basically the strong foundation for today electronic security.
The Quantum Computing Threat
However, quantum computers are completely different type of machines. They differ from
classic computers, first and foremost, because they are governed by and can take advantage of
the laws of the sub-atomic world, known as quantum mechanics. In quantum world bits are
called Qbits and they can be implemented in variety of ways, for example as photons polarized
in different ways, or electrons spinning in different directions, etc.
According to the laws of quantum mechanics, each Qbit can be in both states ‘0’ and ‘1’ at the
same time, with equal probability amplitude, equal to
!
√#
. Following the same logic, a
hypothetical quantum register, long 2048 Qbits, can be at the same time, in all of the possible
states from the [0 … 22048
-1] range (known as ‘linear superposition’). This also means that on

3. quantum computer, the same mathematical transformation can be applied to all of the linear
superposition states from the [0 … 22048
-1] range, all AT THE SAME TIME, without need for
iterating.
Next, according to the same laws of quantum mechanics, each Qbit behaves as a ‘quantum
wave’, and those Qbits can be creatively manipulated to constructively interfere (i.e. ‘add up’
their quantum wave amplitudes) or destructively interfere (i.e. ‘cancel out’) for certain states of
the linear superposition.
Last but not the least, pair of Qbits (or even whole Qbit registers) can be made to be entangled,
so that changes made in one register immediately set the state of its ‘entangled brother’ in
certain way, governed by the laws of quantum mechanics.
All of these characteristics, combined together, give quantum computers huge and unfair
advantage over classic computer ‘cousins’ – especially for certain types of mathematical
problems, which are ultra-hard to be solved using classical computers alone.
Breaking the RSA Cryptography with Quantum Computers
Unfortunately, breaking the PKI cryptography (both RSA and ECC type), are examples of
mathematical problems, which may be ultra-hard for iterative classical computers, but are very
natural and easy for quantum computer, given large enough number of Qbits.
The RSA algorithm’s security is based on inability of classic computers to discover which two
distinct prime numbers p and q were multiplied to produce very large value n (called modulus).
The currently used length of n is 2048 bits. The values of prime numbers p and q, are also used
to calculate the private exponent es and public exponent ep. The public key is then pair of values
[n, ep], while private key is pair of values [n, es].
Peter Shor, a mathematician and cryptographic scientist, back in 1994, had formulated an
algorithm, which can combine steps executed on classical computer, together with steps
executed on quantum computer, for efficiently finding the prime factors p and q of a given
large number n, thus effectively breaking RSA’s security.
First thing that Peter Shor realized, by being a master of the number theory, was that in order
to factor n into p and q, all one needs is the ability to find the period of a special function
f(x) = (ax
mod n), for x = 0, 1, 2, 3, ...etc
where integer a < n, such that gcd (a, n) = 1, and ‘gcd’ stands for ‘greatest common divisor’.
The period is defined as the smallest integer r, such that
f(x) = (ax
mod n) = f(x+r) = (ax+r
mod n)
i.e. the ‘number of steps’ after which the results of applying f(x), start repeating.
Once the period r is found out, the target q and p values can easily be calculated, using the
classic computer, as:

4. p = gcd (n, ar/2
– 1)
q = gcd (n, ar/2
+ 1)
Finding the period of f(x) = (ax
mod n) function is extremely difficult computational problem to
be solved iteratively, when n is very large number (i.e. 2048 bits long), because this is not
smooth periodic function and the results of f(x) look rather like random noise to classic
computers.
However, for the quantum computer, having couple of large enough of Qbit registers, and due
to all of unique quantum mechanics characteristics mentioned above, this is a very natural
problem to solve. That’s is exactly what Peter Shor proposed.
On a very high level, without going into real details of his exact algorithm, Shor basically
proposed following:
• applying f(x) = (ax
mod n) function to a linear superposition of Qbit register states in one
of the Qbit registers
• creatively producing (and detecting using Quantum Fourier Transform) constructive
interference between Qbit waves of that register and another register, entangled with it
o Constructive interference is produced for states equal to the period r (or
multiples of the period r), while destructive interference happens for all other
states.
Pure genius. Done. RSA security broken easily, in a matter of hours, as long as there is readily
available access to a stable quantum computer, with two registers and enough number of Qbits
in each.
Shor then went on further and specified an algorithm for cracking the ECC security as well, by
using quantum computations for similarly critical steps (in this case it was ‘discrete logarithm
calculation’), ultra-hard problem for classic computer to solve.
Conclusion
Should we be worried now? Comforting news is that currently, quantum computers with such
large Qbit registers do not exist yet. However somewhat discomforting is that quantum
computing field is rapidly developing, with major world governments and all high technology
giants (Apple, Google, Microsoft, IBM, etc) actively funding their own quantum computing
research and are collaborating with advanced university institutes focused on quantum
computing. We now already have experimental quantum computers with close to 100 Qbits
available. That’s how Shor’s algorithms have been proven to work in practice. Microsoft is even
developing high level programming language called Q# with its own IDE. Educated forecasts
speculate that within a decade, we can expect to have quantum computers with large enough
Qbit registers, available for cracking 2048-bit long RSA keys. Especially if bad actors get access
to them.

5. There is lot at stake here, basically the electronic security of the whole financial system may
potentially be invalidated in about 10 years from now, if quantum computing progresses
without alternative quantum-safe PKI algorithms emerging in parallel.
Even with quantum-safe cryptographic algorithms becoming available, the industry must plan
for orderly transition and replacement of the legacy RSA and ECC algorithms with quantum safe
equivalents.
This upgrade is not going to be an easy and trivial task, as RSA and ECC algorithms are
embedded in all today’s protocols like VPN, SSL/TLS and payment protocols like EMV, etc. The
right time to start planning and acting is NOW. Financial industry needs to be proactive and
engage with quantum computing experts from universities, private research facilities and high-
tech giants, in order to stay well ahead of the curve and avoid being caught by surprise.
Ten years will fly very quickly. Before we realize, it will be gone. Let’s not hope someone else
will solve these issues, because none of us can stay isolated and hope they will not be affected,
because we will. Also let’s not hope that we can protect access to future quantum computing in
the cloud, by using legacy PKI J