Cryptography & network security exercise 3

Cryptography & Network Security Exercise 3 1 BK TP.HCM Content  Authentication  Digital Signatures 2 BK TP.HCM Message Authentication • Authentication ▫ establishing or confirming something • Message Authentication ▫ a mechanism or service used to verify the integrity of a message 3 BK TP.HCM Authentication requirements • Attacks ▫ Masquerade ▫ Content modification ▫ Sequence modification ▫ Timing modification 4 BK TP.HCM Authentication Functions • Message encryption • Message Authentication Code (MAC) • Hash function 5 BK TP.HCM Message encryption 6 BK TP.HCM Message Authentication Code (MAC) MAC = C(K, M) 7 BK TP.HCM Hash function • h = H(M) 8 BK TP.HCM Hash function - Birthday attack • Birthday problem ▫ 23 randomly chosen people ▫ 50% probability that some pair of them will have the same birthday )!365(365 !3651)( n np n   9 BK TP.HCM Hash function - Birthday attack • Birthday problem • General problem: 10 BK TP.HCM Hash function - Birthday attack • Strategy ▫ Source A uses hash code to sign a message and encrypts hash code with A’s PRa ▫ The opponent generates 2m/2 variations of message (valid and fraudulent)  A pair of messages producing the same hash code ▫ The opponent offers the valid variation to A for signature ▫ Attach this signature to fraudulent message 11 BK TP.HCM Message Authentication Questions ▫ 11.1 – 11.7 ▫ 12.2 Problems ▫ 11.1 -11.2, 11.4 – 11.6 ▫ 12.2 - 12.3 12 BK TP.HCM Digital Signatures Practical Signature Schemes 13 BK TP.HCM Digital Signatures Distribution of Public Keys ▫ public announcement ▫ publicly available directory ▫ public-key authority ▫ public-key certificates 14 BK TP.HCM Digital Signatures  PKI - Public Key Infrastructure ▫ X.509 Authentication service ▫ Based on asymmetric cryptography ▫ Basic function - authentication of public keys ▫ Achieved by signing public keys ▫ Public key certificates issued by certifying authorities (CA) ▫ Permits different public key algorithms ▫ Revocation of certificates 15 BK TP.HCM Digital Signatures  PKI - Public Key Infrastructure ▫ X.509 Authentication service ▫ Based on asymmetric cryptography ▫ Basic function - authentication of public keys ▫ Achieved by signing public keys ▫ Public key certificates issued by certifying authorities (CA) ▫ Permits different public key algorithms ▫ Revocation of certificates 16 BK TP.HCM Digital Signatures Questions ▫ 10.1 – 10.5 ▫ 13.7 – 13.9  Problems ▫ 13.3 17