2003 lecture crypto1

Many of following slides are adapted from:Introduction to Web Security By Adam Cain 2nd WWW Workshop, South Korea , 11/1/95and from a tutorialat the website of PrivacyExchangehttp://www.privacyexchange.org/: 

Many of following slides are adapted from: Introduction to Web Security By Adam Cain 2nd WWW Workshop, South Korea , 11/1/95 and from a tutorial at the website of PrivacyExchange http://www.privacyexchange.org/

Symmetric-key Cryptography: 

Symmetric-key Cryptography Data encrypted and decrypted with same key Classical examples: Caesar cipher, one-time pad, Enigma Machine

Caesar Cipher (n=3) (Review): 

Caesar Cipher (n=3) (Review) If we use the algorithm of simply moving each letter n places down the alphabet (here n=3) then the original alphabet we were using, or the Plain Text becomes the following Cipher Text, as follows:

Encrypting a Message (Review): 

Encrypting a Message (Review) Bob wants to send a secret message to his friend Alice. He encrypts his message with the key of n=3: "This is a secret message" becomes "Wklv lv d vhfuhw phvvdjh"

Decrypting a Message (Review): 

Decrypting a Message (Review) Alice receives Bob's encrypted message. If she is knows the key (n=3) she decrypts the message by reversing the encryption process. She takes the ciphertext: "Wklv lv d vhfuhw phvvdjh" and applies the Caesar Cipher using her key to render it: "This is a secret message"

Symmetric-key Cryptography: key properties: 

Symmetric-key Cryptography: key properties Decrypting a ciphertext with the same key the plaintext was encoded with yields the plaintext. Decrypting a ciphertext with a key the plaintext was not encoded with yields junk. From a ciphertext alone, it should be very hard to recover the plaintext.

The Enigma Machine: Beyond Caesar Ciphers: 

The Enigma Machine: Beyond Caesar Ciphers (From Alan Turing: The Enigma; Simon and Schuster; 1983, by Andrew Hodges )

The Enigma Machine: Beyond Caesar Ciphers: 

The Enigma Machine: Beyond Caesar Ciphers (From Alan Turing: The Enigma; Simon and Schuster; 1983, by Andrew Hodges )

Symmetric-key Cryptography: Drawbacks: 

Symmetric-key Cryptography: Drawbacks Strength of scheme largely depends on size of key   How do the two parties get the shared, secret key?   For a community of n people, each person needs a key for each other person, so each needs n keys, for a total of n2 keys. For 10,000 people, up to 5 million keys must be kept secret.

Public Key Cryptography : 

Public Key Cryptography Each user has a keypair, consisting of a public and private key Anything encrypted with one key may only be decrypted by the other. To make message readable only by B, encrypt message using B's public key

Public-key Cryptography: Key Properties: 

Public-key Cryptography: Key Properties Given a keypair (Kpublic, Kprivate), Decrypting a ciphertext which was encrypted with Kpublic using Kprivate yields the plaintext. Decrypting a ciphertext which was encrypted with Kprivate using Kpublic yields the plaintext. (Why? We’ll see...) Decrypting a ciphertext encrypted with one key of a keypair with any other key but the other key of that keypair yields junk. From a ciphertext alone, it should be very hard to recover the plaintext.

An Encrypted Message (using PGP) : 

An Encrypted Message (using PGP) From president@whitehouse.gov Wed May 18 15:33:50 1994 Date: Thu, 6 July 95 04:49:54 EDT X-Ph: V4.3@argus.cso.uiuc.edu From: "William J. Clinton" Subject: Re: your secret assignment To: a-cain@uiuc.edu -----BEGIN PGP MESSAGE----- Version: 2.6.2 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lnFq -----END PGP MESSAGE-----

Digital Signatures : 

Digital Signatures Using Public-Key Crypto for Strong Authentication Switch the roles of the keys Encrypt with Private key ("signing") Decrypt with Public key ("verifying" ) Anyone (B) can read the message, But only A could have generated it

A Digitally Signed Message (PGP) : 

A Digitally Signed Message (PGP) -----BEGIN PGP SIGNED MESSAGE----- Dear Alice: I'm getting very tired of cryptographers talking about us behind our back. Why can't they keep their nosesin their own affairs?! Really, it's enough to make me paranoid. Sincerely, Bob -----BEGIN PGP SIGNATURE----- Version: 2.6.2 iQB1AwUBL4XFS2F2HFbSU7RpAQEqsQMAvo3mETurtUnLBLzCj9/U8oOQg/T7iQcJvzMedbCfdR6ah8sErMV+3VRid64o2h2XwlKAWpfVcC+2v5pba+BPvd86KIP1xRFIe3ipmDnMaYP+iVbxxBPVELundZZw7IRE=Xvrc -----END PGP SIGNATURE-----

Key length and security in real use: 

Key length and security in real use One can … attack encryption by trying to break the symmetric key….  Adding one bit to the length of a symmetric key doubles the number of possible keys and the amount of time that is needed to find the right one.  For example, the number of possible keys in a 56-bit encrypted message is 2 to the 56th power (256).  That would be 72 quadrillion keys, or 72,057,594,037,927,936.  The number of possible keys in a 57-bit encrypted message is twice that, or about 144 quadrillion. (from RSA Security)