Public Key Encryption, Secure WWW Transactions & Digital Signatures

1. Public Key Encryption, Secure WWW Transactions & Digital Signatures

2. Securing Data Transmission • Most Internet communications are sent "in the clear," without the benefit of encryption • The primary reasons for that practice are speed and simplicity • encrypting and decrypting data are computationally intensive operations • Encrypting all Internet traffic would burden connected computers to the point where data processing would be unacceptably slow

3. Securing Data Transmission • Internet communications are inherently plain text, a byproduct of the Internet's design • To acquire sensitive data, attackers may employ • packet sniffing (reading all data on a particular data line) • gaining access to target systems by guessing user passwords • Commercial transactions, however must be secured • Transmitting sensitive data in an unencrypted state magnifies the possibility that any successful attack would lead to significant consequences

4. Securing Data Transmission • The goal of secure servers is to protect Internet communications in four ways: • Ensuring the communications are private • Verifying the communications have not been altered by transmission errors or a third party • Ensuring server and client are who they claim to be • Ensuring the information to be transferred was written by the signed author

5. Securing Data Transmission • These four types of protection are implemented through cryptographic techniques used to • secure World Wide Web (WWW) transactions • verify the server's identity • send encrypted requests and responses between browsers and servers • provide for verification of document authorship and browser user identity

6. Encryption Techniques • Simple Encryption • Cryptography ("secret writing") obscures a message's meaning by replacing characters in the original message (plaintext) with other characters (ciphertext) • One early encryption scheme was Caesar's Cipher, where each letter was replaced with the letter three places behind it in the alphabet (wrapping around as necessary) • Using this method, the plaintext "cat" would be encrypted as the ciphertext "fdw" • "buzz" becomes "excc"

7. Encryption Techniques • Computers make such simple algorithms worthless • Attackers can use freely available programs running on home systems to break these simple ciphers quite easily • On the plus side, home computers also have the capacity to run powerful encryption schemes which may only be broken by trying every possible key (a brute force attack) • It is possible to encrypt messages in such a way that • decryption with the key takes a few seconds • a brute force attack would have little chance of succeeding in one million years

8. Modern Encryption • Contemporary cryptographic techniques use such complex transposition schemes that humans cannot perform the operations within a reasonable time • Computers are required • These methods may be divided into two general groups: • symmetric • asymmetric

9. Symmetric Algorithms • A cryptographic method is symmetric, or single-key, when the same key is used to encrypt and decrypt a message. Though symmetric algorithms are simple to use, they have two major drawbacks: • The key must be communicated by another secure channel, such as a trusted courier • Should the key be compromised, an attacker could read every message between the original parties and produce fake messages (provided the encryption algorithm is known) • Examples of symmetric encryption algorithms include the US developed Digital Encryption Standard (DES) and the Swiss International Data Encryption Algorithm (IDEA)

10. Asymmetric Algorithms • In 1974, Whitfield Diffie and Martin Hellman developed a cryptographic scheme known as public key cryptography • Their method has a tremendous advantage over symmetric cryptography: the key need not be passed in an unencrypted state • However, it is considerably slower than private-key encryption

11. What is public key encryption? • Public key encryption (PKE) uses a system of two keys: • a private key, which only you use • a public key, which other people use • Public keys are often stored on public key servers • A document that is encrypted with one of these keys can be decrypted only with the other key in the pair Here is a sample PUBLIC KEY: -----BEGIN PGP PUBLIC KEY BLOCK----- Version: 5.0 mQCNAi44C30AAAEEAL1r6ByIvuSAvOKIk9ze9yCK+ZPPbRZrpXIRFBbe+U8dGPMb 9XdJS4L/cy1fXr9R9j4EfFsK/rgHV6i2rE83LjOrmsDPRPSaizz+EQTIZi4AN99j iBomfLLZyUzmHMoUoE4shrYgOnkc0u101ikhieAFje77j/F3596pT6nCx/9/AAUR tCRBbmRyZSBCYWNhcmQgPGFiYWNhcmRAd2VsbC5zZi5jYS51cz6JAFUCBRAuOA6O 7zYZz1mqos8BAXr9AgCxCu8CwGZRdpfSs65r6mb4MccXvvfxO4TmPi1DKQj2FYHY jwYONk8vzA7XnE5aJmk5J/dChdvfIU7NvVifV6AF =GQv9 -----END PGP PUBLIC KEY BLOCK-----

12. PKE Example • Alice wants to send a message to Bob using PGP (a popular public key encryption system) • She encrypts the message with Bob's public key and sends it using her favorite email program • Once the message is encrypted with Bob's public key, only Bob can decrypt the message using his private key • Even major governments using supercomputers would have to work for a very long time to decrypt this message without the private key

13. What are digital signatures? • Consider Alice and Bob again: • how can Bob be sure that it was really Alice who sent the message, and not the criminally-minded Eve pretending to be Alice? • This is where digital signatures come in • Before encrypting the message to Bob, Alice can sign the message using her private key • when Bob decrypts the message, he can verify the signature using her public key

14. Steps in using digital signatures • Alice creates a digest of the message — a sort of digital fingerprint • A message can be easily confirmed as the source of a digest • Alice then encrypts the digest with her private key • The encrypted digest is the digital signature • The encrypted digest is sent to Bob along with the message • When Bob receives the message, he decrypts the digest using Alice's public key • Bob then creates a digest of the message • Bob compares the digest that he created with the one that Alice encrypted • If the digests match, then Bob can be confident that the signed message is indeed from Alice • If they don't match, then the message has been tampered with — or isn't from Alice at all

15. Digital Signatures & Encrypted Message • Alice creates a digest of the message • Alice then encrypts the digest with her private key • Alice creates an augmented message • Message + encrypted digest • The augmented message is encrypted using Bob's public key • The message is sent to Bob • When Bob receives the message, he decrypts the augmented message using his private key • Bob decrypts the digest using Alice's public key • Bob then creates a digest of the message • Bob compares the digest that he created with the one that Alice encrypted • If the digests match, then Bob can be confident that the signed message is indeed from Alice • If they don't match, then the message has been tampered with — or isn't from Alice at all

16. A weakness in PKE • What if I need to verify a signature from someone I don't know, or be sure that the key is really theirs? • This scheme has a weakness in that it is possible for attackers to • intercept a public key in transit • substitute another • compromise any subsequent communications until the change is discovered • This is called a man-in-the-middle attack

17. Certificate Authorities • An attacker must monitor the communications channel and alter every message sent between two parties to avoid discovery, so completing this type of attack is far from trivial • However, a determined party could make such an effort and either gain valuable information or substitute false messages. • Fortunately, this weakness may be overcome by having a trusted authority external to the communication verify the legitimacy of the keys in use • These parties, known as Certification Authorities (CA's), issue certificates which attest to varying degrees of confidence in the certificate holder's identity and affiliation

18. Secure Socket Layer Protocol • Computer communications may be thought of as a series of layers, starting from the hardware (machine) level and moving up to the data being transmitted • The network layer refers to the link between machines, whether it be the Internet, a corporate Local Area Network (LAN) or a satellite link • A socket is an abstraction of the point where the computer and network intersect; following this analogy, the SSL Protocol ensures that the communications channel itself is encrypted "from socket to socket"

19. Establishing a SSL Connection • When a SSL client and server begin communicating, they need to establish how messages will be handled during the session • Specifically, the machines must select cryptographic algorithms, optionally authenticate each other, and use public-key encryption techniques to generate random values to be used as keys for this session only • These processes are performed in the connection initialization sequence, or handshake • To initiate a session, the client sends a message to the server requesting a connection • The server will accept the connection and respond with • a list of its cryptographic capabilities • its public key • (optionally) a copy of its certificate with a request for the client's certificate

20. Establishing a SSL Connection • The client matches its capabilities to those of the server and responds with similar information, encoded with the server's public key and any symmetrical encryption or hashing requested by the server • Certificates are optional as SSL supports three authentication modes: • authentication of both parties • server authentication with an unauthenticated client • total anonymity • completely anonymous sessions are vulnerable to man-in-the-middle attacks • Once the connection has been established, SSL • takes messages to be transmitted • fragments the data into manageable blocks • optionally compresses the data • generates a fingerprint (signature) • encrypts • and transmits the result