网络安全 L2 Introduction to Cryptography 密码学

Definitions
1. crypto - hidden/secret + grafia - writing
2. “the science and study of secret writing”
3. Cryptography is the science of protecting data, which provides means of
converting data into unreadable form, so that
        1. the data cannot be accessed for unauthorised use
        2. the content of the data frames is hidden
        3. the authenticity of the data can be established
        4. the undetected modification of the data is avoided
        5. the data cannot be disowned by the originator of the message

Four goals of cryptography

1. Confidentiality - prevent unauthorised access;
2. Integrity - no modification of existing information;
3. Authentication - no identifying either entities or data origins;
4. Non-repudiation - preventing denials of messages sent

Goal 1: Confidentiality
1. This comprises two separate requirements:
        1. no observer can access the contents of the message.确保只有授权的接收者能够阅读或访问消息，防止未授权的第三方获取敏感信息。
        2. no observer can identify the sender and receiver.保护通信双方的身份信息，防止第三方知道谁在发送消息以及消息是发送给谁的。
2. The terms privacy or secrecy are also used to mean confidentiality

• 隐私（Privacy）：这个术语通常用于描述个人信息的保护，确保个人信息不被未授权的第三方获取或滥用。在信息安全的上下文中，隐私与保密性密切相关，因为它涉及到保护个人数据不被泄露。

• 秘密（Secrecy）：这个术语通常用于描述信息保持未知或不公开的状态。在信息安全中，秘密性是指确保信息不被未授权的实体所知晓，这与保密性的要求是一致的。

Goal 2: Integrity
1. This requires that the recipient can be sure that:
        1. the message has not been changed or lost during transmission.
        2. the message has not been prevented from reaching the recipient.
        3. the message has not reached the recipient twice. 

Goal 3: Authentication
1. This requires that:
        1. the sender can be sure that the message reaches the intended recipient, and only
the intended recipient, and
        2. the recipient can be sure that the message came from the sender and not an
imposter. The act by an imposter of sending such a message is referred to as “spoofing”

Goal 4: Non-repudiation
1. This requires that:
        1. the sender cannot deny that the message was sent by him.
        2. the recipient cannot deny that the message was received by him.

Terminology
1. Plain text (or clear text) - text that can be read by a human
2. Encryption - process of transforming plaintext into ciphertext
3. Cipher text (or encrypted text) - text that ne