Cryptographic Concepts

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1 Outline Identify the different types of cryptography Learn about current cryptographic methods Chapter #23: Cryptography Understand how cryptography is applied for security Given a scenario, utilize general cryptography concepts Cryptographic Concepts 2 Cryptography Cryptanalysis Cryptography is the science of encrypting information Transposition cipher where the same letters are used but the order is changed Shift cipher where one letter of the alphabet is shifted a set number of places in the alphabet for another letter Cryptanalysis process of analyzing available information to attempt to return the encrypted message to its original form Differential cryptanalysis comparing the input plaintext to the output ciphertext to try and determine the key Linear cryptanalysis uses both plaintext and ciphertext, but it puts the plaintext through a simplified cipher to try and deduce what the key is likely to be in the full version of the cipher 3 4 1

2 Algorithms Symmetric Encryption Encryption schemes are based upon an algorithm step-by-step problem-solving procedure made up of mathematical steps for encrypting and decrypting information They are designed to use a key a special piece of data used in both the encryption and decryption processes The algorithms are the same, but every implementation uses a different key Usually use same key for encryption and decryption Encryption key can be calculated from decryption key and vice versa Require sender and receiver to agree on a key before they communicate securely Security lies with the key Also called secret key algorithms, single-key algorithms, or onekey algorithms 5 6 Asymmetric Encryption Symmetric vs. Asymmetric Use different keys for encryption and decryption Decryption key cannot be calculated from the encryption key Anyone can use the key to encrypt data and send it to the Type of Algorithm Advantages Disadvantages Symmetric Single key Requires sender and receiver to agree on a key before transmission of data Security lies only with the key High costs host; only the host can decrypt the data Asymmetric Encryption and decryption keys Security of keys can be compromised when Also known as public key algorithms are different malicious users post phony keys Decryption key cannot be calculated from encryption key 7 8 2

3 Key Exchange Algorithms Central foundational element of a secure symmetric encryption system. out-of-band exchange curriers Diffie-Hellman key exchange in-band exchange Stream algorithms Operate on the plaintext one bit at a time Block algorithms Encrypt and decrypt data in groups of bits, typically 64/128/512 bits in size mod function XOR function (114 XOR 220) XOR = Algorithms ECC (Elliptic Curve Cryptography) ECC works on the basis of elliptic curves y 2 = x 3 + ax 2 + b They have a special property you can add two points on the curve together and get a third point on the curve Users agree on a curve and a fix point With a secret random number calculate a public key That can be used to encrypt messages

4 Hashing Hashing A hash is a special mathematical function that performs one-way encryption Hashing can produce a unique value corresponding to the data entered Message authentication code (MAC) and the computational Can be attacked with what is called a collision attack two different messages that hash to the same value Two of the popular hash algorithms are Secure Hash Algorithm (SHA) and Message Digest of varying versions (MD2, MD4, MD5) number shows that no intermediary has modified the message SHA Message Digest (MD) 2 64 = bits ( chars) Encoding creates 32 bit long words in hexa format 16 of them make up a 512 bit block They will be processed into a 160 bit string Hash output of 128 bits Optimization MD2 -> 8 bit, MD4 & MD5 -> 32 bit CPU MD2 128 bit blocks; MD4 & MD5 512 bit blocks MD4 is only 2x MD2 collision can harm security MD5 improves MD4 by adding a 64 bit version of the original length. It uses different non-linear functions applied 16 times on three of the four 32 bit segments

5 Hashing Summary Usage Less data needs to be signed by the more complex asymmetric encryption Maintains assurances about message integrity Security typically defined as a product of five components: confidentiality, integrity, availability, authentication, and nonrepudiation. Encryption can address four of these five components: confidentiality, integrity, authentication, and nonrepudiation Confidentiality Integrity Symmetric encryption is favored both on stored data and on transmitted data For stored items there is typically no need for a public key For transmitted data, the typical case is to use public key cryptography to exchange the secret key, and then to use symmetric cryptography to ensure the confidentiality of the data does protect confidentiality, but its Size and speed make asymmetric cryptography more efficient at protecting small units It is provided with one-way hash functions and digital signatures

6 Authentication Nonrepudiation Proof you are who you say you are Generally used before communication Password, token (digital certification), or biometric Asymmetric encryption is better suited It means that the senders cannot later deny that they sent the message Nonrepudiation is tied to asymmetric cryptography and cannot be implemented with symmetric algorithms Digital Signatures Key Escrow Digital signatures are based upon both hashing functions and asymmetric cryptography User can decrypt the hash with the public key of the Key escrow and key recovery System by which your private key is kept both by you and by a third party originator, that user knows that the hash was encrypted by the corresponding private key nonrepudiation

7 There is no 100 percent secure system, and there is nothing that is foolproof! Stay Alert! 7

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