1|Page
WGU D830 OA - Introduction to Cryptography | {LATEST
2026/ 2027 UPDATE} COMPLETE ACTUAL AND
AUTHENTIC EXAM | BRAND NEW!
Greece. ......ANSWER......In 500s BC, Greeks developed cylinder messages. The
cipher made use of a baton (called scytale). Each baton round had the capacity
to write letters. represents the secret key. First, a parchment was wound up in
spiral around the baton. Second, the text to be encrypted was written on the
parchment in straight lines (in rows). Third, the parchment was unrolled; and it
represented the ciphertext. The recipient of the ciphertext was required to
possess a baton (with the same diameter than that of the sender) on which the
received parchment was wound up to recover the plaintext. Figure 2.3.2
illustrates an example of a message encryption using the Greek scytale. The
plaintext is TWO THOUSAND SOLDIERS ONSITE ON SATURDAY; spaces are
removed before writing the plaintext on a parchment wound up around a baton
with a diameter of six letters. When the parchment is unrolled, the yielded
ciphertext is TUOSEUWSLOOROADNNDTNISSAHDEIAYOSRTT.
Rome ......ANSWER......With no doubt, the most known cipher was developed in
Rome 2000 years ago, and referred to as Caesar's cipher. The code was mainly
used to secure communications between commanders and troops on
battlefields or between Rome and provincial governors. Caesar's cipher is a
substitution cipher, where a letter is shifted by some fixed position number; i.e.
letter at position in alphabet is replaced by letter at position . This number, , was
the secret key of the cipher. Caesar's cipher is presented in more detail in
Section 5.8.
Vigenere's cipher ......ANSWER......In the 1500s, Blaise de Vigenere made
improvements to Caesar's cipher. A 26-by-26 table is used to substitute letters,
so that the same letter is not always replaced by the same letter as Caesar's
cipher does. This is the base concept of polyalphabetic ciphers, which are harder
pg. 1
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to break than Caesar's cipher. Vigenere's cipher is presented in more detail in
another chapter.
Jefferson's wheel cipher. ......ANSWER......In the late 1700s, Thomas Jefferson
came up with a cipher system similar to that of Vigenere, but with higher
security, because it has much more combinations to select keys. As illustrated by
Figure 2.3.3, Jefferson's cylinder is composed of 36 disks (or wells), stacked on
top of each other around an axle. The 26 Latin letters are randomly inscribed on
each disk. The disks are numbered; they are removable and can be mounted on
the axle in any desired order. The order of disk mounting represents the secret
key. Both the sender and receiver of a message had to mount the disks in the
same way.
Enigma machine. ......ANSWER......The Enigma machine, invented during World
War I and improved during World War II, was mainly used by the army of the
third Reich. Enigma is one of the first electromechanical devices used for
encryption and decryption; and the most used in practice at that time. It is a
very sophisticated version of Jefferson's cylinder. Enigma was very hard to break;
but Alan Turing did it. Enigma machine is presented in more detail in another
chapter.
Vernam's cipher. ......ANSWER......In 1917, the one-time pad (OTP) cipher was
(re)invented by Gilbert Vernam and patented in the US. OTP cannot be cracked,
because a single-use random key is required for each message, and the key must
be at least of the same bit-length than that of the message to encrypt. Vernam's
cipher was a cipher that combined a message with a key read from a paper tape
or pad; hence, the word pad in the name of the cipher. OTP was used in limited
number of applications, including military applications, because a new key is
required for each message. OTP is discussed in more detail in another chapter.
pg. 2
,3|Page
Modern cryptography. ......ANSWER......Much progress has been achieved since
the early 1970s and many cryptographic algorithms were proposed; they are
discussed in this book. From our point of view, the following six steps are the
most prominent in the modern cryptography:
In the early 1970s, IBM developed Lucifer cipher, which became the Data
Encryption Standard (DES) in 1976.
In 1976, Whitfield Diffie and Martin Hellman introduced the principle of public-
key cryptography.
In 1977, Ronald Rivest, Adi Shamir, and Leonard Adleman came up with the RSA
algorithm.
In 1987, Neal Koblitz proposed elliptic curves to design public-key ciphers.
In 1995, the first Secure Hash Algorithm (SHA1) was approved by the NIST.
In 1998, Joan Daemen and Vincent Rijmen came up with the symmetric cipher
Rijndael, which became AES in 2001.
Ciphertext ......ANSWER......it is data after encryption; it is not readable for
human beings or usable by any application, without possession of the
decryption key.
Cryptographic key ......ANSWER......(also called secret key or simply key): it is a
parameter used by encryption and decryption algorithms. It is the most critical
material.
Static keys ......ANSWER......are intended to be used for a long period of time
(e.g. the PIN code to access a bank account), while ephemeral keys are used for
a very limited time (e.g. to encrypt one message).
Encryption algorithm ......ANSWER......(also called enciphering): it is the process
of creating a ciphertext. It makes use of a key to scramble the input (i.e. the
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plaintext), so that the result (i.e. the ciphertext) looks like a noise for any
observer who does not know the decryption key.
Decryption algorithm: ......ANSWER......it is the process of transforming a
ciphertext into a plaintext; i.e. it is the reverse process of encryption.
Encryption key: ......ANSWER......it is a secret value that is known to the sender.
Decryption key: ......ANSWER......it is a value that is known to the receiver. The
decryption key is related to the encryption key, but it is not always identical to
it.
Cipher ......ANSWER......(also called encryption system): encryption and
decryption algorithms form together a cipher.
Symmetric cryptosystems ......ANSWER......Till the late 1970s, all cryptosystems
were symmetric. Even today, symmetric cryptosystems are used extensively in
many cryptosystems, in particular to encrypt and decrypt data. Collectively, the
algorithms used in symmetric cryptosystems are referred to as symmetric
cryptography. The well-known symmetric algorithms include AES (Advanced
Encryption Standard) and DES (Digital Encryption Standard).
Key establishment ......ANSWER......agreement on a secret symmetric key
requires to make use of secure channel (e.g. a phone call, a bag, post mail, etc.)
or a specific key-agreement protocol.
Trust ......ANSWER......since both parties (i.e. sender and recipient) use the same
key, there is an implicit requirement that they trust each other. If an attacker
exploits the weaknesses of one party to recover the key, the other party is not
pg. 4
WGU D830 OA - Introduction to Cryptography | {LATEST
2026/ 2027 UPDATE} COMPLETE ACTUAL AND
AUTHENTIC EXAM | BRAND NEW!
Greece. ......ANSWER......In 500s BC, Greeks developed cylinder messages. The
cipher made use of a baton (called scytale). Each baton round had the capacity
to write letters. represents the secret key. First, a parchment was wound up in
spiral around the baton. Second, the text to be encrypted was written on the
parchment in straight lines (in rows). Third, the parchment was unrolled; and it
represented the ciphertext. The recipient of the ciphertext was required to
possess a baton (with the same diameter than that of the sender) on which the
received parchment was wound up to recover the plaintext. Figure 2.3.2
illustrates an example of a message encryption using the Greek scytale. The
plaintext is TWO THOUSAND SOLDIERS ONSITE ON SATURDAY; spaces are
removed before writing the plaintext on a parchment wound up around a baton
with a diameter of six letters. When the parchment is unrolled, the yielded
ciphertext is TUOSEUWSLOOROADNNDTNISSAHDEIAYOSRTT.
Rome ......ANSWER......With no doubt, the most known cipher was developed in
Rome 2000 years ago, and referred to as Caesar's cipher. The code was mainly
used to secure communications between commanders and troops on
battlefields or between Rome and provincial governors. Caesar's cipher is a
substitution cipher, where a letter is shifted by some fixed position number; i.e.
letter at position in alphabet is replaced by letter at position . This number, , was
the secret key of the cipher. Caesar's cipher is presented in more detail in
Section 5.8.
Vigenere's cipher ......ANSWER......In the 1500s, Blaise de Vigenere made
improvements to Caesar's cipher. A 26-by-26 table is used to substitute letters,
so that the same letter is not always replaced by the same letter as Caesar's
cipher does. This is the base concept of polyalphabetic ciphers, which are harder
pg. 1
,2|Page
to break than Caesar's cipher. Vigenere's cipher is presented in more detail in
another chapter.
Jefferson's wheel cipher. ......ANSWER......In the late 1700s, Thomas Jefferson
came up with a cipher system similar to that of Vigenere, but with higher
security, because it has much more combinations to select keys. As illustrated by
Figure 2.3.3, Jefferson's cylinder is composed of 36 disks (or wells), stacked on
top of each other around an axle. The 26 Latin letters are randomly inscribed on
each disk. The disks are numbered; they are removable and can be mounted on
the axle in any desired order. The order of disk mounting represents the secret
key. Both the sender and receiver of a message had to mount the disks in the
same way.
Enigma machine. ......ANSWER......The Enigma machine, invented during World
War I and improved during World War II, was mainly used by the army of the
third Reich. Enigma is one of the first electromechanical devices used for
encryption and decryption; and the most used in practice at that time. It is a
very sophisticated version of Jefferson's cylinder. Enigma was very hard to break;
but Alan Turing did it. Enigma machine is presented in more detail in another
chapter.
Vernam's cipher. ......ANSWER......In 1917, the one-time pad (OTP) cipher was
(re)invented by Gilbert Vernam and patented in the US. OTP cannot be cracked,
because a single-use random key is required for each message, and the key must
be at least of the same bit-length than that of the message to encrypt. Vernam's
cipher was a cipher that combined a message with a key read from a paper tape
or pad; hence, the word pad in the name of the cipher. OTP was used in limited
number of applications, including military applications, because a new key is
required for each message. OTP is discussed in more detail in another chapter.
pg. 2
,3|Page
Modern cryptography. ......ANSWER......Much progress has been achieved since
the early 1970s and many cryptographic algorithms were proposed; they are
discussed in this book. From our point of view, the following six steps are the
most prominent in the modern cryptography:
In the early 1970s, IBM developed Lucifer cipher, which became the Data
Encryption Standard (DES) in 1976.
In 1976, Whitfield Diffie and Martin Hellman introduced the principle of public-
key cryptography.
In 1977, Ronald Rivest, Adi Shamir, and Leonard Adleman came up with the RSA
algorithm.
In 1987, Neal Koblitz proposed elliptic curves to design public-key ciphers.
In 1995, the first Secure Hash Algorithm (SHA1) was approved by the NIST.
In 1998, Joan Daemen and Vincent Rijmen came up with the symmetric cipher
Rijndael, which became AES in 2001.
Ciphertext ......ANSWER......it is data after encryption; it is not readable for
human beings or usable by any application, without possession of the
decryption key.
Cryptographic key ......ANSWER......(also called secret key or simply key): it is a
parameter used by encryption and decryption algorithms. It is the most critical
material.
Static keys ......ANSWER......are intended to be used for a long period of time
(e.g. the PIN code to access a bank account), while ephemeral keys are used for
a very limited time (e.g. to encrypt one message).
Encryption algorithm ......ANSWER......(also called enciphering): it is the process
of creating a ciphertext. It makes use of a key to scramble the input (i.e. the
pg. 3
, 4|Page
plaintext), so that the result (i.e. the ciphertext) looks like a noise for any
observer who does not know the decryption key.
Decryption algorithm: ......ANSWER......it is the process of transforming a
ciphertext into a plaintext; i.e. it is the reverse process of encryption.
Encryption key: ......ANSWER......it is a secret value that is known to the sender.
Decryption key: ......ANSWER......it is a value that is known to the receiver. The
decryption key is related to the encryption key, but it is not always identical to
it.
Cipher ......ANSWER......(also called encryption system): encryption and
decryption algorithms form together a cipher.
Symmetric cryptosystems ......ANSWER......Till the late 1970s, all cryptosystems
were symmetric. Even today, symmetric cryptosystems are used extensively in
many cryptosystems, in particular to encrypt and decrypt data. Collectively, the
algorithms used in symmetric cryptosystems are referred to as symmetric
cryptography. The well-known symmetric algorithms include AES (Advanced
Encryption Standard) and DES (Digital Encryption Standard).
Key establishment ......ANSWER......agreement on a secret symmetric key
requires to make use of secure channel (e.g. a phone call, a bag, post mail, etc.)
or a specific key-agreement protocol.
Trust ......ANSWER......since both parties (i.e. sender and recipient) use the same
key, there is an implicit requirement that they trust each other. If an attacker
exploits the weaknesses of one party to recover the key, the other party is not
pg. 4
