Review of basic cryptographically algorithm Asymmetric encoding (Private and Public Keys), Hash Function, Digital Signatures and Certification

 Cryptography  Encryption and decryption  History of cryptography  Conventional cryptography  Public key cryptography  Key  Digital signature  Hash function  Digital certificates  Summary  Resources

 cryptography is the science of using mathematics to encrypt and decrypt data  strong/week  cryptographically algorithm – cipher  cryptanalysis - attackers

plaintext encryptiondecryption ciphertextplaintext

 steganography  substitute encryption  shift of letters  tables  Vigneres crypt THISISTHEMESSAGE KEYKEYKEYKEYKEYK ATYEYEATHPHEEQKH.- /..../---/.---

 secret-key or symmetric-key encryption  Data Encryption Standard DAS plaintext encryptiondecryption ciphertextplaintext

Key management:  fast  expensive key  useful for not moving date  secure the way of the key

 Asymmetric coding plaintext encryptiondecryption chiphertext plaintext Public key Private key

Key management:  Solves the problem of key distribution Systems:  Elgamal (Tahela Elgamala)  RSA(Rivest, Shamir, Adleman)  DSA (Digital Signature Algorithm)

 PGP - private and public key cryptography  compresses the plaintext  program used for sending s

plaintext is encrypted with a session key session key is encrypted with public key cipher text and encrypted session key

encrypted messageencrypted session key chipertext recipient’s private key used to decrypt session key session key used to decrypt ciphertext original plaintext

 big numbers measured in bits  public and private keys are mathematically related  larger keys - cryptographically secure for a longer period of time  very difficult to derive the private key from the public key  stored in encrypted form

 verify that the information is intact  authentication and data integrity  non-repudiation  purpose as a handwritten signature

Signing Verifying Public key Private key

 A one-way hash function takes variable- length message of any length  Produces a fixed-length output (160-bits)  Ensures that, if the information is changed an entirely different output value is produced

plaintext Plaintext + signature Hash function

 certs  changing information about the correct key  is a form of credential  used to thwart attempts to substitute one person’s key for another

A digital certificate consists of three things:  public key  certificate information (“Identity” information about the user, such as name, user ID, and so on.)  one or more digital signatures  certificate server

certification user ID cerrtificate user ID signature

 Now you know what is Cryptography  Cryptography uses nowadays private or public keys or both  Digital signature is like your personal signature  Hash function creates something like a fingerprint  Digital certificates is like your ID card in the society

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