Security through Encryption James P. Skon

Encryption

Overview History What is encryption/decryption? Symmetrical encryption Public-key encryption Current encryption standards Encryption

Caesar cipher Encryption

German Enigma Encryption

Cryptography Is Is not A tremendous tool The basis for many security mechanisms Is not The solution to all security problems Reliable unless implemented properly Reliable unless used properly Something you should try to invent yourself unless you spend a lot of time becoming an expert you subject your design to outside review Encryption

Basic Cryptographic Concepts Encryption scheme: functions to encrypt, decrypt data Symmetric encryption Block, stream ciphers, same key for both sides Hash function, MAC Map any input to short hash; ideally, no collisions MAC (keyed hash) used for message integrity Public-key cryptography PK encryption: public key does not reveal key-1 Signatures: sign data, verify signature Encryption

Unsecure message Plain Text ah-ha!! Encryption Dear Sally, The secret ... ah-ha!! Encryption

network transactions Assume attackers can control the network We will talk about how they do this later Attackers can intercept packets, tamper with or suppress them, and inject arbitrary packets Encryption

Encryption Ciphertext Plain Text Encryption Algorithm Ad44fgt6&55tgt Ciphertext Ee8e0w8e*WEdlGlr98& d(D9ED- …. Encryption Algorithm Plain Text Dear Sally, The secret ... Encryption

Decryption Plain Text Ciphertext Decryption Algorithm Dear Sally, Ad44fgt6&55tgt Plain Text Decryption Algorithm Dear Sally, The secret ... Ciphertext Ee8e0w8e*WEdlGlr98& d(D9ED- …. Encryption

Secure Message Ciphertext Plain Text Plain Text ????? Encryption Ee8e0w8e*WEdlGlr98&d(D9ED fssdfsdf sfd sd- Dear Sally, The secret ... Dear Sally, The secret ... Plain Text Plain Text ????? Encryption

Encryption Destination Source insecure key Plaintext Plaintext key Algorithm Encrypted message Algorithm Plaintext key Encryption

Cryptanalysis breaking the code with brute force computation attack Types Ciphertext-only attack Known-plaintext attack Chosen-plaintext attack Adaptive-chosen-plaintext attack Others... Encryption

Symmetric Key Encryption Both parties must have the same key Often encryption and decryption done by the exact same algorithm Typically the algorithm is publicly known (only the key is secret) Called the “secret key” method, since secrecy of key is essential Encryption

One-time pad Method Each character from the plaintext is encrypted by a modular addition with a character from a secret random key (or pad) of the same length as the plaintex Key must be length of message Must be random, never reused Encryption

One Time Pad Method Message: "THE BRITISH ARE COMING” Key: DKJFOISJOGIJPAPDIGN A B C D E F G H I J K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Encryption (T(19)+D(03)=22) MOD 26 = 22 = W (H(07)+K(10)=17) MOD 26 = 17 = R (E(04)+J(09)=13) MOD 26 = 13 = N (B(01)+F(05)=06) MOD 26 = 06 = G (R(17)+O(14)=31) MOD 26 = 05 = F Encryption

One Time Pad THEBRITISHARECOMING DKJFOISJOGIJPAPDIGN Result: WRNGFQLRGNIATCDPQTT Decryption (W(22)-D(03)= 19 +26) MOD 26 = 19 = T (R(17)-K(10)= 07 +26) MOD 26 = 07 = H (N(13)-J(09)= 04 +26) MOD 26 = 04 = E (G(06)-F(05)= 01 +26) MOD 26 = 01 = B (F(05)-O(14)=-09 +26) MOD 26 = 17 = R Encryption

Stream ciphers Generate a long key with a pseudo random number generator. Encryption

Block Cipher DES, AES Build a key for each block from the start key, and the previous block Encryption

Strong Symmetric Key Encryption Weak encryption is vulnerable to cryptanalysis Strong encryption is an algorithm for which cryptanalysis is intractable (possible, but takes too long) Strength of an algorithm is related to key length (algorithm complexity) Encryption

Symmetric Key Encryption - Algorithms DES (Data Encryption Standard) 56-bit key Adopted in 1977 by the National Bureau of Standards. Originally proposed key length of 128-bits. Powerful computers can now break DES. Double and Triple DES - use two or three keys for greater safety Encryption

Major Systems Data Encryption Standard (DES) 1978 56-bit keys very hard to break a key requires 255 steps on average never been broken in genera, but can be broken for a specific key given enough CPU power. Usually implemented in hardware Encryption

Major Systems Advanced Encryption Standard adopted by the U.S. government and is now used worldwide Algorithm well known, generally trusted Attacks: Brute Force Cryptoanalysis In 2011 a key-recovery attack created which is about 4 times faster then brute force. Encryption

Symmetric Key Problems How do we safely exchange keys? How do we authenticate the source (I.e. who really sent this message) Encryption

Encryption secret-key Sender and receiver must have an identical key Key is used for both encryption and decryption Problems: Secret keys must be exchanged in private. Each side must trust each other with their secret key. Called the key management problem. Encryption

Encryption public-key Solution to key management problem. Each persons has a pair of keys, a public key and a private key Public key is published in trusted directory Private key known only to owner. Encryption

Encryption public-key Anyone can use public key to encrypt data to be sent to the owner of the public key. ONLY the owner of the private key can decrypt the message. Thus security is assured across unsecure media. Encryption

Public-key Encryption Destination Source insecure Private key Plaintext Algorithm Encrypted message Algorithm Plaintext Public key Encryption

Authentication process whereby the receiver of a digital message can be confident of the identity of the sender or the integrity of the message Encryption

Authentication Public-key Encryption provides concept of digital signature General operation: Bob sends to Mary Mary’s Public key Mary’s Private key Message decryption algorithm Message encryption algorithm encrypted message Bob’s Public key A mathematical relationship that will hold ONLY if Bob’s private key was used. function Digital signature Bob’s Private key function Encryption

Advantages of Public Key Systems Private keys need not be transmitted Digital signature possible Messages can be proven authentic to a third party (such as judge) allowing for legal agreements via network Encryption

Disadvantages of Public Key Systems Speed symmetrical (secret) key algorithms much, much faster Public key very computationally intensive Encryption

Disadvantages of Public Key Systems Solution to speed problem: For a transfer, generate a random session key Send the session key using a public key method Use the session key with a symmetrical key algorithm for actual data transfer. Encryption

Crypto Policy Perspectives Issues Law Enforcement National Security Right to Privacy Encryption