EEC 693/793 Special Topics in Electrical Engineering Secure and Dependable Computing

1. EEC 693/793Special Topics in Electrical EngineeringSecure and Dependable Computing Lecture 6 Wenbing Zhao Department of Electrical and Computer Engineering Cleveland State University wenbing@ieee.org

2. Outline • Public-key algorithms • Digital signatures • Message digest • Message authentication code • Public key management EEC693: Secure & Dependable Computing

3. 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 EEC693: Secure & Dependable Computing

4. 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 EEC693: Secure & Dependable Computing

5. 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) EEC693: Secure & Dependable Computing

6. 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) EEC693: Secure & Dependable Computing

7. 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 fabricated the message himself EEC693: Secure & Dependable Computing

8. 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 EEC693: Secure & Dependable Computing

9. Public-Key Signatures • Digital signatures using public-key cryptography • Requires E(D(P)) = P (in addition to D(E(P)) = P) EEC693: Secure & Dependable Computing

10. 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 • Requirement on the hash function: • 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 EEC693: Secure & Dependable Computing

11. Digital Signatures Using Message Digests EEC693: Secure & Dependable Computing

12. MD5 • One of the most widely used hash functions • MD5 is the fifth in a series of message digests designed by Ronald Rivest (1992) • It operates by mangling bits in a sufficiently complicated way that every output bit is affected by every input bit • MD5 generates a 128-bit fixed value EEC693: Secure & Dependable Computing

13. SHA-1 • SHA-1: Secure Hash Algorithm 1, developed by National Security Agency (NSA) and blessed by NIST. It generates 160-bit message digest • Use of SHA-1 and RSA for signing nonsecret messages EEC693: Secure & Dependable Computing

14. 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: • Append the key to the plaintext and generate a digest using a hash function • Ship the plaintext together with the digest EEC693: Secure & Dependable Computing

15. Management of Public Keys • Problem statement • Certificates • X.509 • Public key infrastructure EEC693: Secure & Dependable Computing

16. 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 EEC693: Secure & Dependable Computing

17. 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 EEC693: Secure & Dependable Computing

18. X.509 • Devised and approved by ITU • The basic fields of an X.509 certificate EEC693: Secure & Dependable Computing

19. 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 EEC693: Secure & Dependable Computing

20. Public-Key Infrastructures • Hierarchical PKI • A chain of trust/certification path:A chain of certificates going back to the root EEC693: Secure & Dependable Computing

21. 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 EEC693: Secure & Dependable Computing