How Spies Communicate – Mocomi.com

HOW SPIES
COMMUNICATE
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General Knowledge for Kids
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HOW SPIES
COMMUNICATE
(Encryption) (Decryption)
Cryptography also known as cryptology is the
practice and study of techniques used for secure
communication in the presence of third parties.

HOW SPIES
COMMUNICATE
Cryptography techniques prior to the modern
age was effectively synonymous with encryp-
tion - the conversion of information from a read-
able state to apparent visible nonsense. The
person or originator of an encrypted message
shared the decoding technique needed to re-
cover the original information only with in-
tended recipients, thereby preventing un-
wanted persons to do the same.

HOW SPIES
COMMUNICATE
Since World War I and the invention of the com-
puter, methods used to carry out cryptology
have become extremely complex and its appli-
cation more widespread. Cryptography today
uses the disciplines of mathematics, computer
science, and electrical engineering. The applica-
tion of cryptography is now used for ATM cards,
computer passwords, and electronic commerce.

HOW SPIES
COMMUNICATE
Cipher Machine
Until modern times cryptography referred
almost exclusively to encryption, which is the
process of converting plain text into cipher text.
Decryption is the reverse, in other words,
moving from the cipher text back to plain text.
Terminology

HOW SPIES
COMMUNICATE
Cipher Output

HOW SPIES
COMMUNICATE
History of cryptography and cryptanalysis
A cipher (or cypher) is a pair of algorithms that
create the encryption and the reversing decryp-
tion. The operation of a cipher is controlled both
by the algorithm and in each instance by a "key".
The "key" is a secret parameter known only to
the communicants for the specific exchange of
messages.
Cryptanalysis is the study of how to crack en-
cryption algorithms or their implementations.
Before the modern era, cryptology was con-
cerned solely with encryption. Encryption was
used to attempt to ensure secrecy in communi-
cations, such as those of spies, military leaders,
and diplomats.

HOW SPIES
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Cryptanalysis
In recent times, the field has expanded beyond
confidentiality concerns and now include tech-
niques for message integrity checking,
sender/receiver identity authentication, digital
signatures, interactive proofs and secure com-
putation, among others.

HOW SPIES
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Cryptography
Classic cryptography
The earliest forms of secret writing required
little more than pen and paper, as most people
could not read. Those who were more literate
and had literate opponents required actualy
cryptography.

HOW SPIES
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Caesar Cipher
The main classical cipher types are transposition
ciphers, which rearrange the order of letters in a
message (e.g., 'hello dolly' becomes 'ehlol yllod'
in a simple rearrangement scheme, and substi-
tution ciphers, which systematically replace let-
ters or groups of letters with other letters or
groups of letters (e.g., 'fly at once' becomes 'gmz
bu podf') by replacing each letter with the one
following it in the Latin alphabet). These simple
versions however have never offered much con-
fidentiality from enterprising opponents.
A P P L E
D S S O H

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Cipher machine in the shape of a book
An early substitution cipher was the Caesar
cipher, in which each letter in the plaintext was
replaced by a letter some fixed number of posi-
tions further down the alphabet. Suetonius, a
Roman historian reports that Julius Caesar used
it with a shift of three to communicate with his
generals.
Atbash is an example of an early Hebrew cipher.
The earliest known use of cryptography is some
carved ciphertext on stone in Egypt at around
1900 BCE.

HOW SPIES
COMMUNICATE
Computer era
Many mechanical encryption/decryption de-
vices were invented early in the 20th century,
and several patented, among them the rotor
machines—famously including the Enigma ma-
chine used by the German government and
military from the late '20s and during World War
II. The ciphers implemented by these machines
brought about a substantial increase in crypta-
nalytic difficulty after WWI.
Cryptanalysis of the new mechanical devices
proved to be both difficult and laborious. In
Great Britain, cryptanalytic efforts during WWII
spurred the development of more efficient
means for carrying out repetitious tasks.

HOW SPIES
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Smart Card
This culminated in the development of the Co-
lossus, the world's first fully electronic, digital,
programmable computer, which assisted in the
decryption of ciphers generated by the German
Army's Lorenz SZ40/42 machine.
Just as the development of digital computers
and electronics helped in cryptanalysis, it made
possible much more complex ciphers.

HOW SPIES
COMMUNICATE
Image references:
Cipher machine/Cipher output/Cryptanalysis/Cryptography/Cipher machine in the shape of a book/Smart
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Magnifying Glass/computer - source: www.openclipart.org. This image is public domain. You may use this
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Furthermore, computers allowed for the encryp-
tion of any kind of data unlike classical ciphers
which only encrypted written language texts.
Computer use has thus supplanted linguistic
cryptography, both for cipher design and
cryptanalysis.
Decode these substitution ciphers. (J bn dszqu-
bobmztu)
Start communicating with your friends in school
through code. Those chits you pass on hence-
forth will always be safe.
Project
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