- 1.1 Opinions, Products
- 1.2 Roadmap to the Book
- 1.3 Terminology
- 1.4 Notation
- 1.5 Cryptographically Protected Sessions
- 1.6 Active and Passive Attacks
- 1.7 Legal Issues
- 1.8 Some Network Basics
- 1.9 Names for Humans
- 1.10 Authentication and Authorization
- 1.11 Malware: Viruses, Worms, Trojan Horses
- 1.12 Security Gateway
- 1.13 Denial-of-Service (DoS) Attacks
- 1.14 NAT (Network Address Translation)
This chapter is from the book
This chapter is from the book
This chapter is from the book
1.6 Active and Passive Attacks
A passive attack is one in which the intruder eavesdrops but does not modify the message stream in any way. An active attack is one in which the intruder may transmit messages, replay old messages, modify messages in transit, or delete or delay selected messages in transit. Passive attacks are less risky for the attacker, because it is tricky to detect or prove someone has eavesdropped. If the attacker is not on the path between Alice and Bob, the passive attack can be done by having an accomplice router make copies of the traffic and send them to the attacker, who can then analyze the data later, in private.
A typical active attack is one in which an intruder impersonates one end of the conversation, or acts as a meddler-in-the-middle (MITM). (Note that the acronym MITM used to be expanded to be man-in-the-middle, but the industry is trying to move to more inclusive language. In this case, it is acknowledging that being annoying is not gender-specific.) A MITM attack is where an active attacker, say, Trudy, acts as a relay between two parties (Alice and Bob), and rather than simply forwarding messages between Alice and Bob, modifies, deletes, or inserts messages. If Trudy were faithfully forwarding messages, she could be acting as a passive eavesdropper, or she could be a correctly functioning router.
If Alice communicates with Bob using a secure session protocol with strong cryptographic protection, Trudy would gain no information by eavesdropping and would not be able to modify messages without being detected.
However, Trudy might be able to impersonate Bob’s IP address to Alice, tricking Alice into establishing a secure session between Alice and Trudy. Then Trudy can simultaneously impersonate Alice to Bob, and establish a secure session between Trudy and Bob. (See Figure 1-1.)
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Figure 1-1. MITM Attack
Alice and Bob will think they are talking to each other, but in fact they are each talking to Trudy. Data sent by Alice to Bob will be decrypted by Trudy using the session secret for the Alice-Trudy secure session, and encrypted for Bob with the session secret for the Trudy-Bob secure session. It is difficult for Alice and Bob to know that they have a MITM. Alice could attempt to make sure she’s really talking to Bob by asking questions such as “What did I order when we first met for dinner?”, but Trudy can forward the questions and answers. We will explain in §11.6 Detecting MITM how Alice and Bob can detect a MITM. And as we will explain in later chapters, if Alice has credentials for Bob that Trudy cannot impersonate, Alice and Bob can prevent a MITM.