1 / 25

EEC-484/584 Computer Networks

EEC-484/584 Computer Networks. Lecture 16 Wenbing Zhao Outline. Reminder Quiz#5 4/30 4-6pm Final Revised Wiki Page due 5/5 midnight Presentation: request for waiver will be handled FCFS Waiver request must have a revision summary and a url to your revised wiki page

Download Presentation

EEC-484/584 Computer Networks

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.


Presentation Transcript

  1. EEC-484/584Computer Networks Lecture 16 Wenbing Zhao

  2. Outline • Reminder • Quiz#5 4/30 4-6pm • Final Revised Wiki Page due 5/5 midnight • Presentation: request for waiver will be handled FCFS • Waiver request must have a revision summary and a url to your revised wiki page • Cipher modes • Public key algorithm • Digital signature • Message digest and secure hash functions • Public key infrastructure EEC-484/584: Computer Networks

  3. Stream Cipher Mode • To be insensitive to transmission error, an arbitrarily large sequence of output blocks, called the keystream, is treated like a one-time pad and XORed with the plaintext to get the ciphertext • It works by encrypting an IV, using a key to get an output block • The output block is then encrypted, using the key to get a second output block • This block is then encrypted to get a third block, and so on EEC-484/584: Computer Networks

  4. Stream Cipher Mode • The keystream is independent of the data • It can be computed in advance • It is completely insensitive to transmission errors Decryption Encryption EEC-484/584: Computer Networks

  5. Stream Cipher Mode • It is essential never to use the same (key, IV) pair twice with a stream cipher because doing so will generate the same keystream each time • Using the same keystream twice exposes the ciphertext to a keystream reuse attack • Stream cipher mode is also called output feedback mode EEC-484/584: Computer Networks

  6. Keystream Reuse Attack • Plaintext block, P0, is encrypted with the keystream to get P0 XOR K0 • Later, a second plaintext block, Q0, is encrypted with the same keystream to get Q0 XOR K0 • An intruder who captures both ciphertext blocks can simply XOR them together to get P0 XOR Q0, which eliminates the key • The intruder now has the XOR of the two plaintext blocks • If one of them is known or can be guessed, the other can also be found • In any event, the XOR of two plaintext streams can be attacked by using statistical properties of the message EEC-484/584: Computer Networks

  7. Counter Mode • To allow random access to encrypted data • The IV plus a constant is encrypted, and the resulting ciphertext XORed with the plaintext • By stepping the IV by 1 for each new block, it is easy to decrypt a block anywhere in the file without first having to decrypt all of its predecessors EEC-484/584: Computer Networks

  8. Public-Key Algorithms • Distributing keys => the weakest link in most cryptosystems • No matter how strong a cryptosystem was, if an intruder could steal the key, the system was worthless • Cryptologists always took for granted that the encryption key and decryption key were the same • Diffie and Hellman (1976) proposed a radically new kind of cryptosystem: encryption and decryption keys were different • D(E(P)) = P • It is exceedingly difficult to deduce D from E • E cannot be broken by a chosen plaintext attack EEC-484/584: Computer Networks

  9. Public-Key Algorithms • Public-key cryptography: • Encryption algorithm and the encryption key can be made public • How to establish a secure channel • Alice and Bob have never had previous contact • Alice sends Bob EB(P) (message P encrypted using Bob’s public encryption key EB) • Bob receives the encrypted message and retrieves the plaintext by using his private key P = DB(EB(P)) • Bobs then sends a reply EA(R) to Alice EEC-484/584: Computer Networks

  10. RSA • Rivest, Shamir, Adleman, 1978: a good method for public-key cryptography • RSA method: • Choose two large primes, p and q (typically 1024 bits) • Compute n = pq and z = (p-1)  (q-1) • Choose a number relatively prime to z and call it d • Find e such that ed = 1 mod z • To encrypt a message, P, Compute C = Pe (mod n) • To decrypt C, compute P = Cd (mod n) • The public key consists of the pair (e, n) • The private key consists of the pair (d, n) EEC-484/584: Computer Networks

  11. RSA • An example of the RSA algorithm • P = 3, q = 11 => n = 33 and z = 20 • A suitable value for d = 7 • e can be found by solving the eq. 7e = 1 (mod 20) => e = 3 • C = P3 (mod 33), P = C7 (mod 33) EEC-484/584: Computer Networks

  12. Digital Signatures • Requirement on digital signatures: one party can send a signed message to another party in such a way that the following conditions hold: • The receiver can verify the claimed identity of the sender • The sender cannot later repudiate the contents of the message • The receiver cannot possibly have concocted the message himself EEC-484/584: Computer Networks

  13. Symmetric-Key Signatures • Big Brother (BB): a central authority that knows everything and whom everyone trusts • Each user chooses a secret key and shares it with BB • Digital signatures with Big Brother EEC-484/584: Computer Networks

  14. Public-Key Signatures • Digital signatures using public-key cryptography • Requires E(D(P)) = P (in addition to D(E(P)) = P) EEC-484/584: Computer Networks

  15. Message Digests • Message digest (MD):using a one-way hash function that takes an arbitrarily long piece of plaintext and from it computes a fixed-length bit string • Given P, it is easy to compute MD(P) • Given MD(P), it is effectively impossible to find P • Given P no one can find P’ such that MD(P’) = MD(P) • A change to the input of even 1 bit produces a very different output EEC-484/584: Computer Networks

  16. Hash Functions: MD5 and SHA-1 • Hash function: mangling bits in a sufficiently complicated way that every output bit is affected by every input bit • MD5is the fifth in a series of message digests designed by Ronald Rivest (1992) • MD5 generates a 128-bit fixed value • SHA-1: Secure Hash Algorithm 1, developed by National Security Agency (NSA) and blessed by NIST • SHA-1 generates 160-bit message digest EEC-484/584: Computer Networks

  17. Digital Signatures Using Message Digests EEC-484/584: Computer Networks

  18. Message Authentication Code • MACs are used between two parties that share a secret key in order to validate information transmitted between these parties • The MAC mechanism that is based on cryptographic hash functions is called HMAC. Basic idea: • Append the key to the plaintext and generate a digest using a hash function • Ship the plaintext together with the digest EEC-484/584: Computer Networks

  19. Management of Public Keys • Problem statement • Certificates • X.509 • Public key infrastructure EEC-484/584: Computer Networks

  20. Problems with Public-Key Management • If Alice and Bob do not know each other, how do they get each other’s public keys to start the communication process ? • It is essential Alice gets Bob’s public key, not someone else’s • A way for Trudy to subvert public-key encryption EEC-484/584: Computer Networks

  21. Certificates • Certification Authority (CA): an organization that certifies public keys • It certifies the public keys belonging to people, companies, or even attributes • CA does not need to be on-line all the time (in ideal scenarios) • A possible certificate and its signed hash EEC-484/584: Computer Networks

  22. X.509 • Devised and approved by ITU • The basic fields of an X.509 certificate EEC-484/584: Computer Networks

  23. Public-Key Infrastructures • A Public-Key Infrastructure (PKI) is needed for reasons of • Availability, Scalability, Ease of management • A PKI has multiple components • Users, CAs, Certificates, Directories • A PKI provides a way of structuring these components and define standards for the various documents and protocols • A simple form of PKI is hierarchical CAs EEC-484/584: Computer Networks

  24. Public-Key Infrastructures • Hierarchical PKI • A chain of trust/certification path: A chain of certificates going back to the root EEC-484/584: Computer Networks

  25. Public-Key Infrastructures • Revocation: sometimes certificates can be revoked, due to a number of reasons • Reinstatement: a revoked certificate could conceivably be reinstated • Each CA periodically issues a CRL (Certificate Revocation List) giving the serial numbers of all certificates that it has revoked • A user who is about to use a certificate must now acquire the CRL to see if the certificate has been revoked • Having to deal with revocation (and possibly reinstatement) eliminates one of the best properties of certificates, namely, that they can be used without having to contact a CA EEC-484/584: Computer Networks

More Related