Cryptography is the science of keeping communications secure through various encryption techniques. There are two main types: symmetric encryption which uses a shared secret key for encryption and decryption, and asymmetric encryption which uses a public/private key pair. Symmetric encryption is faster but more difficult to implement for multiple parties, while asymmetric encryption using algorithms like RSA enables easy key distribution and digital signatures.

1. Introduction to Crytographic Techniques
What is Cryptography?
- Cryptography is the science of keeping oral and written as well as other forms of
communications secret.
- Cryptography provides a means of authentication of communicating parties.
- Cryptography is the study of codes and ciphers for use in secret transmission of
messages from a sender to a recipient.
- Cryptology is composed of crytography - the art and science of keeping messages
secure.
- Cryptographic algorithms --> methods for transforming plaintext messages into
ciphertext.
- Secret key cryptography (or symmetrical)
- Public key cryptography (or asymmetrical)
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2. In cryptographics, a message in human readable form is
referred to plaintext or cleartext.
Encryption:Encryption refers to a process which disguises a
cleartext message to hide its substance and generates a
message, known as ciphertext.Plaintext is denoted by P,
whereas ciphertext is denoted by C.
The encryption function E operates on P to produce C based
on a key value K:
E k (P) = C
Encryption and Decryption
Encryption
Key
Plaintex
t
Ciphtex
t
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3. Encryption and Decryption
Decryption:
The reverse process of encryption is known as a decryption,
in which a cihhertext is converted back to its original cleartext form.
In the reverse process, the decryption function D operates on C to produce
P
based on a key value K:
D k (C) = P
Decryption
Key
Ciphertex
t
Plaintex
t
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4. Simple Encryption System
Encryption
Encryption
Key(s)
Ciphertex
tPlaintex
t
Decryption
Decryption
Key(s)
Plainrtex
t
Cryptography is both the lock and the combination (or keys) that helps us protect
our
communication data and transformation messages.
There are varieties of locks, there are varieties of crytographic methods and keys.
Two limits of Encryption:
- it cannot protect data on a computer
- it can no better than the weakest link.
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5. Different Types of Cryptography Methods
Transposition Ciphers:
In transposition ciphers, the letters of the message to be secretly transmitted are scrambled,
that is, rewritten in a pattern way, and is then sent in a different order from its original
form..
N O W - I S - T H E - T I M E
F O R - A L L - G O O D - M E
N - T O - C O M E - T O - T H
E - A I D - O F - T H E I R -
P A R T I E S - S T O P Z Z Z
would be transmitted as:
NFNEP OO - A WRTAR - OIT IA - DI SLC-E-LOOS T - ME - HE - S
EO - TT - OTHO TDOEP I - IZ MMTRZ EEH-Z
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6. Different Types of Cryptography Methods
Transposition Ciphers:
In transposition ciphers, the letters of the message to be secretly transmitted are scrambled,
that is, rewritten in a pattern way, and is then sent in a different order from its original
form..
Key word: C O M P U T E R
Letter order: 1 4 3 5 8 7 2 6
M E E T M E A T
T H E F R O N T
D O O R O F T H
H I L T O N H O
T E L A T 9 P M
would be transmitted as:
MTDHT ANTHP EEOLL EHOIE TFRTA TTHOM EOFN9 MROOT
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7. Different Types of Cryptography Methods
Substitution Ciphers:
Substitution ciphers are created by replacing one symbol, such as a letter of a
message,
with another symbol or letter in some patterned way.
Let’s assume that we have a replacement letter pattern as follows:
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
D E F G H I J K L M N O P Q R S T U V W X Y X A B C
Plaintext: NOW IS THE TIME FOR ALL GOOD PEOPLE TO COME TO THE
Ciphertext: KLT FP QEB QFJB CLO XII DLLA MBLMIB QL ZLIB QL QEB
More complex symbol replancement pattern can be applied.
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8. Different Types of Cryptography Methods
Product Ciphers:
Product ciphers are created by using substitution and transposition together.
Current Data Encryption Standard (3DES) and Rijndael ( The Advanced Encryption
Standard. Winner) use multiple combinations of transposition and substitution to
achieve
superior cryptosecurity.
Although this approach become more complex and secure than the previous two,
they could be attacked using statistical methods.
Ideally, ciphertext could present itself as a random string of letters of bits or be data key-
stroked by a monkey.
Diffusion is defined as the dispersion or distribution of plaintext in a statistically
random
manner over the ciphertext.
The principle of confusion prevents the cryptanalyst from using the ciphertext to deduce
the secret key. It may take the form of a complex substitution method. Confusion hides
the relationship between ciphertext and the secret key.
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9. Symmetric Encryption
Symmetric encryption (secret-key encryption, or single-key encryption):
Secret-key encryption refers to the use of a shared key for both encryption
by
the transmitter and decryption by the receiver.
Work step:
- Sender sends a message after encryption with a secret-key.
- Receiver decrypt the message with the same secret key after receiving it.
Inter
net
Anne
sends a
message
Bob
receives
Anne’s
message
Encrypt
with a
secret key
Decrypt
with a
secret key
Encrypted
Message
Encrypted
Message
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10. Symmetric Encryption
Special features of symmetric encryption:
- Two communication parties use and share the same key.
- Encryption and decryption is carried out based on the same key.
- Both parties must agree on the secret key before communications.
Advantages:
- Useful in many cases which only involves two-party
communications.
Problems and limitations:
- Complex in key distribution (key generation, transmission, and storage).
- Not scalable.
- Impractical to support large group of communications in networks.
- Impossible to support exchanging messages with a large group of previously
unknown parties over a public open network.
Example: DES (the Data Encryption Standard)
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11. Asymmetric Encryption
Asymmetric encryption (public-key encryption):
Public-key encryption (asymmetric encryption) use a pair of keys for each party.
- One key is known as a “public key”, which is known to other parties.
- One key is known as a “private key” or a “secret key”, which must be confidential,
and is known only to its owner.
- The public key encrypts the message.
- The private key decrypts the message.
Inter
net
Anne
sends a
message
Bob
receives
Anne’s
message
Encrypt
with a
public key
PK(Bob)
Decrypt
with Bob’
private key
SK(Bob)
Scramble
d
Message
Scramble
d
Message
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12. Asymmetric Encryption
Special features of asymmetric encryption (public-key encryption):
SK(PK(M)) = M
Advantages:
- Easy to key distribution (key generation, maintain and storage)
- Scalable to support large group of users in an public network
- Easy to support unknown users since no previous consent is needed.
- Better protection than symmetric encryption.
Problems and limitations:
- Slow performance
Applications: E-commerce, snoop-proof email system.
Standards: the best known public-key encryption algorithm --> RSA
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13. , Sh
RSA Algorithm
The RSA algorithm was developed by Rivest, Shamir, and Adleman at MIT in 1978.
The basic algorithm is outlined below:
1: Choose two large distinct primes, p and q.
2: Compute the product (modulus) n = pq.
3: Randomly choose encryption key e, such that e and (p-1)(q-1) are relatively
prime.
4: Finally use Euclid’s algorithm to compute the decryption key, d such that
e . d = 1 (mod (p-1) . (q -1)).
Where, d and n are relatively prime. e and n are the public key. d is the secret key.
To encrypt a message M, e
C = M mod n
To decrypt C,
d
M = C mod n
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14. Comparing Symmetric Encryption and Asymmetric Encryption
Features Secret-Key Encryption Public-Key Encryption
No. of keys Single Key Public Key and Secret Key
Application Bulk data encryption Key exchange, digital signatures
Key Management Simple but difficult to Need digital certificates
manage and trusted third parties
Performance Fast Slower
Key exchange difficult and risk to Easy and less risky to deliver
transfer a secret key a public key.
Key Length 56-bit (old) 1024 bit (RSA)
126-bit (safe) Many used 2048 bits
Standards DES, Triple DES RSA, Diffie-Hellman
Rijndael DSA, Elliptic curves
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15. Comparing Symmetric Encryption and Asymmetric Encryption
Features Secret-Key Encryption Public-Key Encryption
Years in use over 1000 years less than 50 years
Types of keys Secret Key only One Public Key and
one Private Key
Confidentiality Yes Yes
Authentication Yes Yes
Message integrity Yes Yes
No-repudiation No Yes
Don’t need trustedDigital signatures:
third party need trusted third party
Attack Simple but difficult to Need digital certificates
manage and trusted third parties
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16. Digital Signatures
What is a digital signature?
- Digital signatures are implemented using public-key encryption.
- A digital signature is a cryptographic mechanism that performs a similar function
to a written signature.
- Created using PPK cryptograph and message digests.
Encryption allows a message
sender the ability to digitally sign messages, thus creating a digital signature for the
message. When a message digest is computed and then encrypted using the sender’s
private key, and later append to the message, the result is called the digital signature
of the message.
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17. Digital Signatures
Purposes:
Digital signatures are used to ensure the integrity and authentication.
-- To verify the origin and contents of a message.
-- Digital signatures are used for sender authentication.
Why digital signatures?
- Public-key algorithms are computation-intensive. With large messages they may be
too expensive or too slow for the application.
- Digital signatures based on digested message provide alternative solutions.
Application:
Digital signatures can be used to endorse an electronic document in a
way that can be later validated for authenticity.
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18. Generating A Digital Signature
Message
MessageSignature
Message Digest
Encrypt
Hashing
Algorithm
Sender’s
Private Key
(SK
sender)
To Receiver
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