Message Encryption Source a Destination B M M K E(K, M K (a) Symmetric encryption: confidentiality and authentication It may be difficult to determine automatically if incoming ciphertext decrypts to intelligible plaintext message should have suitable structure redundancy or a checksum to detect any changes 復大软件学院
11 Message Encryption • It may be difficult to determine automatically if incoming ciphertext decrypts to intelligible plaintext • message should have suitable structure, redundancy or a checksum to detect any changes
Message Encryption Source a Destination B M M K E(K, M K (a) Symmetric encryption: confidentiality and authentication A→B:E(K,M Provides confidentiality Only A and B share K Provides a degree of authentication Could come only from A Has not been altered in transit -Requires some formatting/redundancy Does not provide signature Receiver could forge message Sender could deny message (a) Symmetric encryption 復大软件学院
12 Message Encryption
Message Encryption if public-key encryption is used encryption provides no confidence of sender, if the sender uses private key to encrypt the message since anyone potentially knows public-key however if sender signs message using their private-key then encrypts with recipients public key have both confidentiality and authentication again need to recognize corrupted messages but at cost of two public-key uses on message 13 復大软件学院
13 Message Encryption • if public-key encryption is used: – encryption provides no confidence of sender, if the sender uses private key to encrypt the message – since anyone potentially knows public-key • however if • sender signs message using their private-key • then encrypts with recipients public key • have both confidentiality and authentication – again need to recognize corrupted messages – but at cost of two public-key uses on message
Message Encryption M PUb E(PUb, M) (b) Public-key encryption: confidentiality E(PRa, M (e) Public-key encryption: authentication and signature M E M PR, E(PRa, M) PUb E(PUb, E(PRa, M)) PRb E(PRa, M) PU (d) Public-key encryption: confidentiality, authentication, and signature 復大软件学院
14 Message Encryption
eto Message Encryption A→B:E(PUb,M Provides confidentialit Only B has PR, to decrypt . Provides no authentication Any party could use PU, to encrypt message and claim to be A (b) Public-key(asymmetric)encryption: confidentiality A→B:E(PR,M .Provides authentication and signature Only A has PR, to encrypt Has not been altered in transit Requires some formatting/redundancy Any party can use PU, to verify signature (c) Public-key encryption: authentication and signature A→B:E(PUb,E(PR,M) .Provides confidentiality because of PUb .Provides authentication and signature because of pr 15 大 (d) Public-key encryption: confidentiality, authentication, and signature
15 Message Encryption