Source A Destination B- EKR[H(M)] KUa KRa M DP EP) M Compare ((a)Authentication only Ekv[K;] KUb KRb DP -po M (b)Confidentiality only KUb EkU,[Ks] KRb EKR [H(M] KUa KRa K;-EP) DP M Compare (c)Confidentiality and authentication Figure 15.1 PGP Cryptographic Functions 2022/10/9 现代密码学理论与实践-15：电子邮件的安全 9/612022/10/9 现代密码学理论与实践-15：电子邮件的安全 9/61