After completing this video, the learner will understand the different between codes and ciphers as well as the difference between block and stream ciphers.
- [Voiceover] There's a little more terminology that you'll need to know before we start diving into the details of Cryptography. Let's talk about codes and ciphers. Codes and ciphers are different concepts. People often use these terms interchangeably but they are actually two very different things. You need to be sure that you understand the difference between codes and ciphers when you take the exam. A code is a system that substitutes one meaningful word or phrase for another.
This might be done for secrecy purposes, or it might be done for efficiency of communications allowing a short message to convey a detailed meaning. One example of a code that you've probably already heard of is the 10 code system used by police and other organizations who communicate by radio. They have a long list of codes that allow the person sending a message to simply say two numbers and convey a long meaning. For example 10-four means I acknowledge your message.
While 10-nine means please repeat your last transmission. This is an example of using codes for the efficiency of communication. You'll also see codes pop up often in spy movies. When a secret operative calls in from the field and says the blue cow jumped over the moon but really means that the subject of a surveillance operation disappeared. That's an example of using codes for secrecy. Ciphers on the other hand are systems that use mathematical algorithms to encrypt and decrypt messages.
All of the cryptographic algorithms we talk about in this course are examples of ciphers not codes. Ciphers have two different ways of processing a message. Stream ciphers work on one character of the message at a time. They perform their action on an single character, or a single bit, and then move on to the next character or bit. Block ciphers work on junks of the message known as blocks at the same time.
They might take 100 characters of a message, for example, and encrypt them all at the same time. Ciphers perform their encryption and decryption operations using two basic building blocks. Substitution ciphers actually change the characters in a message. A simple substitution cipher might, for example, shift all of the letters in a message by two positions changing the A's to C's, the B's to D's, and so on.
When someone wants to decrypt the message, they simply shift the characters back by two positions. Transposition ciphers don't change the characters in a message but instead they rearrange them. They're basically scrambling up the message in a way that only someone who knows the decryption key can unscramble them. We just talked about two very basic examples of substitution and transposition ciphers. You wouldn't use these today because they're very easy to crack, but these two operations do form the basis of the modern encryption algorithms that we'll talk about later in this course.
This course is part of a six-course series on the CompTIA Security+ exam, and is useful for IT professionals who wish to learn more about information security as well as students preparing to take the Security+ exam.
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- Choosing encryption algorithms
- Applying symmetric and asymmetric cryptography standards
- Implementing key management, including key exchange and key stretching
- Working with public keys, trust models, and digital certificates
- Using transport encryption protocols
- Securing wireless networks