DEQUICO, BEVERLY JOYCE P.
Cryptography
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RA: Cryptography
S18C
A means of encrypting information and communications with codes so that only those who are supposed
to read and process the information may do so.
It involves inventing cipher systems which basically are secret ways of writing.
Goals of Encryption
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Confidentiality - no third party can read the message
Integrity - no third party may change the message
Authenticity - the receiver is assured that the message was sent by the sender
Nonrepudiation - the sender cannot deny sending the message
Processes of Encryption and Decryption
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Encryption - process of translating plaintext (plain text data) into ciphertext (random and meaningless)
Decryption - process of converting ciphertext back to plaintext.
Uses of Encryption and Decryption in Real-life Situations
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The use of cryptography makes it easier to provide a secure service.
Cash withdrawal through an ATM or other paying options,
Email and file storage utilizing privacy softwares
Secure online browsing
Storing passwords and information
The use of a GSM mobile phone
Examples of Cryptosystems
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A cryptosystem is a suite of cryptographic algorithms needed to implement a particular security
service.
Shift Cipher - it entails replacing each letter in the message with a letter that is some specified
number of positions farther along in the alphabet. Julius Caesar also used this when communicating
with his officers.
Transposition Cipher - the letters are simply moved around (each letter retains its identity but
changes its position) and the rule is to reverse the sequence of the characters; rail fence encoding is
an example of this.
Symmetric And Asymmetric Cryptosystems
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Symmetric Cryptosystem:
- Private Key Cryptography
- single key for both encryption and decryption
- the size of cipher text is same or smaller than the original plain text
- encryption process is very fast
- used when a large amount of data is required to transfer
- provides confidentiality
- resource utilization is low
Asymmetric Cryptosystem:
- Public Key Cryptography
- requires two key one to encrypt and the other one to decrypt
- the size of cipher text is same or larger than the original plain text
- encryption process is slow
- used to transfer small amount of data
- provides confidentiality, authenticity, and non-repudiation
- resource utilization is high
Procedure on how the public key and private key of an RSA cryptosystem are generated.
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RSA Cryptosystem is based on a trapdoor function. It is named after its inventors Rivest, Shamir, and
Adelman (RSA).
- easy going one way: multiplication
- hard going back: factoring
The Public key is used for encryption, and the Private Key is used for decryption.
Performing KEY GENERATION
1. Choose two prime numbers p and a
2. Compute N = pq and R = (p-1)(q-1)
3. Choose a number e such that it is less than N and relatively prime to R
4. Find d such that it is relatively prime to R and ed = 1 (mod R)
5. Public key: (N, e); Private key: (N, d)
Performing ENCRYPTION OR DECRYPTION
1. Message → Numeric form
- ASCII code chart, for example
2. Encryption (using N,e)
- Cipher = Messagee (Mod N)
3. Decryption using (N, d)
- Message = Cipherd (Mod N)
How public key is used to generate the ciphertext from the plaintext
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Perform N = pq to get the public key (N,e)
Mapped the given plain text into the number of the letter to the alphabet
Use cipher = Messagee (Mod N), evaluate each letter to get the full cipher key
How private key is used to retrieve the plaintext to ciphertext
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Perform ed = 1 (mod R), to get R use the formula: R = (p-1)(q-1)
Use message = Cipherd (Mod N), evaluate each letter to retrieve the plain text