Encryption No. 1  Seattle Pacific University Encryption: Protecting Your Data While in Transit Kevin Bolding Electrical Engineering Seattle Pacific University Based on Chapter 18 of William Stallings, Data and Computer Communication, 6 th Ed.

Encryption No. 2  Seattle Pacific University Living in a Glass House Public networks are public... Chances for eavesdropping or worse LANs – may broadcast all packets to all stations within your collision domain WANs - shared public resources Service providers Hackers at every point... Network model is inherently insecure Passing data to an unreliable third party many times over

Encryption No. 3  Seattle Pacific University The Need for Encryption Transmission of sensitive data Classical use of encryption Sender encodes data in such a way that only the intended recipient(s) can decipher it Identity verification Sender provides a digital signature that cannot be easily produced by any other entity Receiver can easily verify signature Data encoding and Digital Signatures can be provided using encryption

Encryption No. 4  Seattle Pacific University Goals for an Encryption Scheme Difficult to break Probability of breaking the code within any reasonable amount of time should be very small Fast encoding and decoding If encoding/decoding is too complex or too slow, users will bypass it Easy distribution of keys Secret key distribution can be an issue Frequent key changes needed for security

Encryption No. 5  Seattle Pacific University Code Breaking Assume that the attacker has: Samples of matched plaintext and ciphertext Lots of computing power Knowledge of the plaintext language Cryptanalysis Gather information that you can Size of documents, plaintext/ciphertext pairs, frequency of documents, language Think really hard Brute force Randomly try keys until something works Bits in keyTime to crack (10 6 encryptions/s) min years x years x years

Encryption No. 6  Seattle Pacific University Symmetric Encryption Sender and all recipients use the same key Sender encrypts plaintext using the common secret key Encrypted ciphertext is sent over a public channel Receiver decrypts ciphertext back into plaintext using the common secret key PlaintextCiphertextPlaintextCiphertext Public Network Symmetric Encryption requires that sender and receiver both know the same secret key

Encryption No. 7  Seattle Pacific University DES - Data Encryption Standard DES – US government standard from Symmetric encryption 56-bit key Basic idea: Repeatedly XOR and shift XORing - Great for encryption plaintext xor key ciphertext xor key plaintext A special DES-cracking machine built by the Electronic Frontier Foundation in 1999 can crack DES in under three days. --> DES is dead

Encryption No. 8  Seattle Pacific University Replacing DES Triple DES (TDEA) Run DES three times, yielding a key length of 168 bits Short-term replacement for DES Advanced Encryption Standard (AES) / Rijndael US Approved in 2002 Block algorithm, 128, 192, or 256-bit key 128-bit approved through US “Secret” 192+ bits approved for US “Top Secret”

Encryption No. 9  Seattle Pacific University Key Distribution With symmetric codes, both sender and receiver need to have the secret key How does the sender safely send the key? 1. Send it manually via secure courier Expensive, time-consuming 2. Send a new key using the previous key Fine, but what if the old key was compromised? 3. Use a key distribution center Sender requests that a session key be sent to both the sender and receiver Key is used for this session only

Encryption No. 10  Seattle Pacific University Public-Key Encryption Receiver has two matched keys - Public and Private Public key can only encode – private key needed to decode Sender encrypts plaintext using the Public Key Encrypted ciphertext is sent over a public channel Receiver decrypts ciphertext back into plaintext using the Receiver’s Private Key Receiver sends Receiver’s Public Key to the Sender Public Network Pub Priv Pub Priv Sender Receiver PlaintextCiphertextPlaintextCiphertext Pub

Encryption No. 11  Seattle Pacific University RSA Public Key Encryption Rivest, Shamir, Adleman (1977) Involves the use of the product of two (large) prime numbers Public and Private Keys include the above product and some mathematical functions of the prime factors Cracking the code Factoring the product will crack the code... RSA authors (in 1977) predicted a 428-bit code would take 40,000,000,000,000 years to crack Internet consortium cracked it in bit or more keys are used today...

Encryption No. 12  Seattle Pacific University Public Key Issues Man-in-the-Middle Imposters An imposter might send you an encrypted message Requires sender authentication RSA is slow Complicated math, 1024-bit (or more) keys Symmetric codes are faster Use a combined public key/symmetric method Establish a session using public key methods Send the receiver a session key for a symmetric method Use the symmetric key for the rest of the session

Encryption No. 13  Seattle Pacific University Sender Authentication PlaintextCiphertext of Hash PlaintextCiphertext of Hash Public Network Pub Priv Pub Priv Sender Receiver Pub Sender “encrypts” document using sender’s private key This document is sent to the receiver Receiver decrypts signature using sender’s public key If document is readable, it must have been encrypted using the sender’s private key, which only the sender has access to

Encryption No. 14  Seattle Pacific University Public-Key Encryption + Authentication Sender sends document using public key encryption Digital Signature Ciphertext of DS Digital Signature Ciphertext of DS Public Network Pub Priv Pub Priv Sender Receiver Plaintext CiphertextPlaintextCiphertext Pub Sender computes a digital signature of the document using a one-way hash function Sender sends digital signature of document using authentication methods =? Receiver authenticates encrypted message message by confirming that digital signature matches received plaintext