SYMMETRIC CIPHER MODEL
KEY TERMS IN CRYPTOGRAPHY
● Plain text - An original message
● Cipher Text - Encoded message
● Enciphering or encryption - The process of converting from plaintext
to ciphertext
● Deciphering or decryption - Restoring the plaintext from the
ciphertext is.
● Cryptography - The many schemes used for encryption . Such a scheme
is known as a cryptographic system or a cipher.
● Cryptanalysis - Techniques used for deciphering a message without
any knowledge of the enciphering details. Cryptanalysis is what the
layperson calls “breaking the code.”
● The areas of cryptography and cryptanalysis together are called
cryptology.
Main Requirements of a Encryption
1. The opponent should be unable to decrypt ciphertext or discover the key even if
he or she is in possession of a number of ciphertexts together with the plaintext
that produced each ciphertext.
2. Sender and receiver must have obtained copies of the secret key in a secure
fashion and must keep the key secure. If someone can discover the key and
knows the algorithm, all communication using this key is readable.
How Cryptographic systems are characterized?
1. Depending on transformation
2. Number of keys
3. Block/Stream Cipher
Key Points
Polyalphabetic Cipher
A poly-alphabetic cipher is any cipher based on substitution, using several substitution
alphabets. In polyalphabetic substitution ciphers, the plaintext letters are enciphered
differently based upon their installation in the text.
Rather than being a one-to-one correspondence, there is a one-to-many relationship
between each letter and its substitutes.
1. Vigenere Cipher
Encryption
Key
Key Index
Plain Text H E L L O
Plain Text Index
Cipher Index
Cipher Text
1. Vigenere Cipher
Decryption
Key T H E T H
Key Index 19 7 4 19 7
Cipher Text A L P E V
Cipher Index
Plain Index
Plain Text
KEY POINTS
The best known, and one of the simplest, polyalphabetic ciphers is the Vigenère cipher. In this scheme,
the set of related monoalphabetic substitution rules consists of the 26 Caesar ciphers with shifts of 0
through 25
The strength of this cipher is that there are multiple ciphertext letters for each plaintext letter, one for
each unique letter of the keyword. Thus, the letter frequency information is obscured. However, not all
knowledge of the plaintext structure is lost.
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VERNAM CIPHER (Encryption)
Plain Text O A K A B A
EX-OR
B
Plain Index 14 0 10
0 0
Plain Binary 01110 00000 01010
0 1
Key Text S O N
1 0
Key Index 18 14 13
1 1
Key Binary 10010 01110 01101
Cipher Binary
Cipher Index
VERNAM CIPHER (Decryption)
Cipher Text C O H
Cipher Index 2 14 7 A B A
EX-OR
B
Cipher Binary 00010 01110 00111 0 0
Key Text S O N 0 1
Key Index 18 14 13 1 0
Key Binary 10010 01110 01101 1 1
Plain Binary
Plain Index
Plain Text
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ONE TIME PAD
KEY POINTS
● Each new message requires a new key of the same length as the new message. Such a scheme,
known as a one-time pad, is unbreakable.
● Suppose that a cryptanalyst had managed to find these two keys. Two plausible plaintexts are
produced. How is the cryptanalyst to decide which is the correct decryption (i.e., which is the
correct key)? If the actual key were produced in a truly random fashion, then the cryptanalyst
cannot say that one of these two keys is more likely than the other.
● Thus, there is no way to decide which key is correct and therefore which plaintext is correct.
Therefore, the code is unbreakable.
● Two fundamental difficulties:
○ 1. There is the practical problem of making large quantities of random keys. Any heavily used system might
require millions of random characters on a regular basis. Supplying truly random characters in this volume is a
significant task.
○ 2. Even more daunting is the problem of key distribution and protection. For every message to be sent, a key
of equal length is needed by both sender and receiver. Thus, a mammoth key distribution problem exists.
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TRANSPOSITION -1) RAIL FENCE
ATTACK POSTPONED UNTIL TWO AM XYZ
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STEGANOGRAPHY
KEY POINTS
● A steganography technique involves hiding sensitive information within an ordinary, non-secret file or
message, so that it will not be detected. The sensitive information will then be extracted from the
ordinary file or message at its destination, thus avoiding detection. Steganography is an additional step
that can be used in conjunction with encryption in order to conceal or protect data.
● Steganography is a means of concealing secret information within (or even on top of) an otherwise
mundane, non-secret document or other media to avoid detection. It comes from the Greek words
steganos, which means “covered” or “hidden,” and graph, which means “to write.” Hence, “hidden
writing.”
● You can use steganography to hide text, video, images, or even audio data.
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