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Cryptography and Network Security . Basics of computer and network S ecurity. Computer security : involves implementing measures to secure a single computer ( protecting the ressources stored on that computer).
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Basics of computer and network Security • Computer security: involvesimplementingmeasures to secure a single computer (protecting the ressources stored on that computer). • Network security: involvesprotecting all the ressources on a network. We must consider not only the computers on the network but other network devices and data transmittedacross the network.
Basics of computer and network Security • Security attack: any action that comprises the security of information owned by an organization.
Basics of computer and network Security • Network security: canprovide one of the five services: confidentiality, integrity, authentification, nonrepudiation. • Confidentiality(Privacy): the transmitted message must makesense to only the intendedreceiver. To the others, the message must begarbage. When a costumer communicateswithherbank, sheexpectsthather communication istotalyconfidential.
Basics of computer and network Security 2. Integrity: the data must arrive to the receiverexactly as theywere sent. A request for transferring 100$ should not bechanged to a request for 1000$. 3. Authentification: the receiverneeds to be sure of the sender’sidentity. 4.Nonrepudiation: a sender must not be able to denysending a message thathe or she , in fact, didsend.
Cryptography • Definition • Cryptography, a word with Greek origins, means “secret writing”. • It refers to the science of transferring messages to make them secure and immune to attacks. • Components • Plaintext : is the original message before being transformed. • Ciphertext: is the message after transformation. • Cipher : is the encryption and decryption algorithm. • Key: is an number (or set of numbers) that the cipher, as an algorithm, operates on.
Cryptography • In symmetric-key cryptography, the same key is used by the sender (for encryption) and the receiver (for decryption). • The key is shared.
Cryptography • In asymmetric-key cryptography, one key is used by the sender (for encryption) and other key is used by receiver (for decryption). • The keys are different.
Simple Example of Symmetric Key Encryption • Shift Cipher ( or Caesar cipher) • in this cipher, the encryption algorithm is “shift key characters down”, key is N number • The decryption algorithm is “shift key characters up”, key is N number • Example: • Use the shift cipher with key=15 to encrypt the message “HELLO”? Show the ciphertext.
Hash Algorithm • As the Cryptography is used to assure the privacy (confidentiality), Hash algorithm is used to ensure the integrity of the message. • At sender site, hash code is generated. • At receiver site, a new hash code for message is generated and compares with the hash code that sent, if match, message hasn’t been altered.
Simple Example of Hash Function • The hash algorithm: take every third letter of the message (ignore punctuation), and convert the letter to a number (a=1, b=2…z=26). Add the numbers together. Hello, This is a sample message to demonstrate signatures. 12 +20 +19 +1 +13 +5 +19 +7 +15 +13 +19 +1 +19 +14 +21 +19 = 217 (therefore the hash value is 217)
Digital Signautre • Digital signature is a data string which associates a message (in digital form) with some originating entity. • Digital signature verification is a method for verifying that a digital signature is authentic (was indeed created by the specific entity). • The main difference from a handwritten signature is that digital signature of a message is intimately connected with the message, and for different messages is different, whereas the handwritten signature is adjoined to the message and always looks the same.
