ISO/IEC 18033-2:20 06规定了用于数据机密性的加密系统（密码）。加密（或加密）技术的主要目的是保护存储或传输的数据的机密性。将加密算法应用于数据（通常称为明文或明文）以产生加密数据（或密文）；这个过程称为加密。加密算法应该被设计成使得密文不会产生关于明文的任何信息，除了它的长度。与每个加密算法相关联的是相应的解密算法，它将密文转换回其原始明文。 非对称(即公钥)加密方案允许发送者使用接收者的公钥将消息的加密传输给接收者，接收者可以使用他的私钥解密给定的密文，从而获得原始消息。这样的方案应该是安全的，即关于消息的信息不应该泄露给（资源受限的）攻击者，即使该攻击者发起所谓的“选择密文”攻击，在该攻击中他可以获得其他密文的解密。这是针对公钥加密方案提出的最强攻击类型。 ISO/IEC 18033-2:20 06规定了这种方案的功能接口，并且另外规定了许多看起来对所选密文攻击是安全的特定方案。不同的方案在安全属性和效率之间提供了不同的折衷。

ISO/IEC 18033-2:2006 specifies encryption systems (ciphers) for the purpose of data confidentiality. The primary purpose of encryption (or encipherment) techniques is to protect the confidentiality of stored or transmitted data. An encryption algorithm is applied to data (often called plaintext or cleartext) to yield encrypted data (or ciphertext); this process is known as encryption. The encryption algorithm should be designed so that the ciphertext yields no information about the plaintext except, perhaps, its length. Associated with every encryption algorithm is a corresponding decryption algorithm, which transforms ciphertext back into its original plaintext. An asymmetric, i.e. public-key, encryption scheme allows a sender to use a recipient's public key to transmit an encryption of a message to the receiver, who can use his secret key to decrypt the given ciphertext, thereby obtaining the original message. Such a scheme should be secure in the sense that no information about the message should be leaked to a (resource-bounded) attacker, even if that attacker mounts a so-called 'chosen ciphertext' attack, in which he may obtain decryptions of other ciphertexts. This is the strongest type of attack that has been proposed for a public-key encryption scheme. ISO/IEC 18033-2:2006 specifies the functional interface of such a scheme, and in addition specifies a number of particular schemes that appear to be secure against chosen ciphertext attack. The different schemes offer different trade-offs between security properties and efficiency.