Having covered stream and block symmetric ciphers and asymmetric ciphers, this week Leo and I describe and discuss "cryptographic hashes", the final component to comprise a complete fundamental cryptographic function suite. We discuss the roles of, and attacks against, many common and familiar cryptographic hashes including MD5 and SHA1.

Having discussed symmetric (private) key ciphers during the last two weeks, this week Leo and I examine asymmetric key cryptography, commonly known as "Public Key Cryptography". We begin by examining the first public key cryptosystem, known as the Diffie-Hellman Key Exchange, invented in 1976. Then we describe the operation of general purpose public key cryptosystems such as the one invented by RSA.

Leo and I answer last week's Puzzler/BrainTeaser which explored the idea of using two private one-time pad "keys," like two padlocks, to securely convey a message between two parties, neither of whom would have the other's key. Then we continue our ongoing tour of fundamental crypto technology by describing the operation of Symmetric Block Ciphers.

Leo and I briefly review last week's topic of symmetric stream ciphers, then we pose the first Security Now! Puzzler/BrainTeaser which proposes a secure means for sending encrypted messages where neither party knows the other's key. The Puzzler/BrainTeaser will be answered and resolved at the start of next week's episode. Then, as always in our Q&A episodes, we answer questions and discuss issues raised by listeners.

Leo and I continue our multi-episode tour of cryptographic technology. This week we analyze the cryptographic operation of secret decoder rings which we use to develop a solid foundation of cryptographic terminology. We then examine the first of two forms of symmetric, private key cryptography known as symmetric stream ciphers. Two weeks from now, after next week's Q&A episode, we'll discuss the operation of symmetric block ciphers.

Leo and I open our multi-week discussion of the operation and technology of cryptography. This first week we start by examining the social consequences and ethical implications of common citizens being empowered with freely available cryptographic technology that no force on Earth - no government agency, no corporation, no private individual - can crack within their lifetimes.

Leo and I discuss the design, operation, and complete lack of security of Ethernet - the LAN technology that virtually all of the world uses. We explain how this lack of security enables a wide range of serious attacks to be perpetrated by any other machine sharing the same Ethernet - such as in a wireless hotspot, within a corporate network, or even in a wired hotel where the entire hotel is one big exploitable Ethernet LAN. GRC's ARP Cache Poisoning page contains a detailed explanation of these problems with diagrams and links to readily available Ethernet ARP exploitation malware.ARP Cache Poisoning: /nat/arp.htm

Leo and I discuss questions asked by listeners of our previous episodes. We tie up loose ends, explore a wide range of topics that are too small to fill their own episode, clarify any confusion from previous installments, and present real world "application notes" for any of the security technologies we have previously discussed.

Having covered the operation of the Internet's WAN (Wide Area Network) technology in the past two weeks, this week Leo and I turn to discussing the way Local Area Networks (LANs) operate and how they interface with the Internet WAN. We address the configuration of subnet masks, default gateways, and DHCP to explain how packets are routed among machines and gateways within a LAN.

During this 38-minute, part 2 episode of "How the Internet Works," Leo and I briefly review last week's discussion of the ICMP protocol, then discuss the operational details of the Internet's two main data-carrying protocols: UDP and TCP.