<strong>Cryptography</strong> is about how to communicate securely — and how to do so even when there is a <em>malicious</em> person, people, organization(s), or other entities who are trying to stop you from doing so.
</p>
<p>
<strong>Cryptography</strong> is an important part of <strong>privacy</strong>.
</p>
<p>
<strong>Cryptography</strong> is also sometimes called <strong>cryptology</strong>.
</p>
<section>
<h2>Goals</h2>
<p>
<strong>Cryptography</strong> is generally used to try provide three different but related things:
</p>
<ul>
<li><strong>confidentiality</strong>,</li>
<li><strong>integrity</strong>, and</li>
<li><strong>authenticity</strong>.</li>
</ul>
<p>
These three goals form the foundation of the security that <strong>cryptography</strong> tries to provide.
</p>
</section>
<dl>
<dt>Confidentiality</dt>
<dd>
<p>
<strong>Confidentiality</strong> is the goal that only the person, people, organization(s), or other entities you addressed the message to can read the message.
<strong>Confidentiality</strong> is about how to hide information so that only the whomever or whatever you want to read, listen to, or watch, etc a message can do so.
(In <strong>cryptography</strong>, things such as text, audio, and video all get called a "<strong>message</strong>".)
</p>
<p>
For example, if I record a video on my mobile phone and send it to my wife, <strong>confidentiality</strong> has it so that only my wife can watch the video, and no one else.
Not even the people and organizations that provide the communication infrastructure that I use to send the video to my wife.
</p>
<p>
<strong>Confidentiality</strong> is important for <strong>privacy</strong>.
</p>
</dd>
<dt>Integrity</dt>
<dd>
<p>
<strong>Integrity</strong> is the goal that a message’s contents cannot be tampered with.
</p>
<p>
For example, if I write this message to my wife —
</p>
<blockquote>
<p>
Meet me at park by the kids' school at 3pm.
</p>
</blockquote>
<p>
— <strong>integrity</strong>, for example, has it so someone else cannot change my message to:
</p>
<blockquote>
<p>
Meet me at in the dark scary alley behind the grocery store at 2pm.
</p>
</blockquote>
<p>
With <strong>integrity</strong>, the message my wife receives will be exactly what I sent her.
What <strong>plain-text</strong> and <strong>cipher-text</strong> as well as <strong>encryption</strong> and <strong>decryption</strong> are is probably most easily understood with some examples.
</p>
<section>
<h4>Example №1</h4>
<p>
If I write this message to my wife —
</p>
<blockquote>
<p>
COULD YOU PICK UP THE KIDS FROM SCHOOL TODAY AT 2PM, PLEASE?
</p>
</blockquote>
<p>
— that is <strong>plain-text</strong>.
</p>
<p>
I might <em>not</em> want anyone else to be able to read that message (other than my wife).
So I can hide that message from others by turning that <strong>plain-text</strong> message into a <strong>cipher-text</strong> message to try to accomplish that.
</p>
<p>
The process of turning a <strong>plain-text</strong> message into a <strong>cipher-text</strong> message is called <strong>encryption</strong>.
</p>
<p>
What would a <strong>cipher-text</strong> message for that <strong>plain-text</strong> message look like‽
—
well, it depends on what <strong>encryption algorithm</strong> we use.
</p>
<p>
For a very very simple example, if we were using the <strong>ROT-13</strong><em>encryption algorithm</em>, then the <strong>cipher-text</strong> would be:
</p>
<blockquote>
<p>
PBHYQ LBH CVPX HC GUR XVQF SEBZ FPUBBY GBQNL NG 2CZ, CYRNFR?
</p>
</blockquote>
<p>
(We will look closer at <strong>ROT-13</strong> later.)
</p>
<p>
For my wife to read my message, she would have to turn that <strong>cipher-text</strong> back into <strong>plain-text</strong>.
</p>
<p>
The process of turning a <strong>cipher-text</strong> message into a <strong>plain-text</strong> message is called <strong>decryption</strong>.
</p>
</section>
<section>
<h4>Example №2</h4>
<p>
Let's look at another example.
We will use the same <strong>plain-text</strong> —
</p>
<p>
I.e. again I will write this message to my wife —
</p>
<blockquote>
<p>
COULD YOU PICK UP THE KIDS FROM SCHOOL TODAY AT 2PM, PLEASE?
</p>
</blockquote>
<p>
— and again that will be the <strong>plain-text</strong>.
</p>
<p>
And again, I will <em>not</em> want anyone else to be able to read that message (other than my wife).
So I will hide that message from others by turning that <strong>plain-text</strong> message into a <strong>cipher-text</strong> message to try to accomplish that.
</p>
<p>
<strong>But this time we will use a different <em>encryption algorithm</em>.</strong>
</p>
<p>
With this example, we will use another very very simple <em>encryption algorithm</em> — we will use a <strong>substitution cipher</strong>.
To use a <strong>substitution cipher</strong> as your <em>encryption algorithm</em> you need to pick a <em>substitution-alphabet</em>.
The <em>substitution-alphabet</em> we will use is —
</p>
<table>
<tr>
<thscope="row">normal English alphabet</th>
<td>A</td>
<td>B</td>
<td>C</td>
<td>D</td>
<td>E</td>
<td>F</td>
<td>G</td>
<td>H</td>
<td>I</td>
<td>J</td>
<td>K</td>
<td>L</td>
<td>M</td>
<td>N</td>
<td>O</td>
<td>P</td>
<td>Q</td>
<td>R</td>
<td>S</td>
<td>T</td>
<td>U</td>
<td>V</td>
<td>W</td>
<td>X</td>
<td>Y</td>
<td>Z</td>
</tr>
<tr>
<thscope="row">cipher alphabet</th>
<td>Q</td>
<td>K</td>
<td>T</td>
<td>N</td>
<td>H</td>
<td>F</td>
<td>P</td>
<td>R</td>
<td>M</td>
<td>S</td>
<td>U</td>
<td>J</td>
<td>W</td>
<td>C</td>
<td>B</td>
<td>D</td>
<td>Z</td>
<td>G</td>
<td>X</td>
<td>I</td>
<td>V</td>
<td>Y</td>
<td>O</td>
<td>E</td>
<td>A</td>
<td>L</td>
</tr>
</table>
<p>
(We will look closer at <strong>substitution ciphers</strong> later.
Don't worry if, for example, what a <em>substitution-alphabet</em> is doesn't make sense yet.)
</p>
<p>
With this, if we <strong>encrypt</strong> our <strong>plain-text</strong>, our <strong>cipher-text</strong> would be:
And again, for my wife to be able to read my message, she would need to have to <strong>decrypt</strong> the <strong>cipher-text</strong> back into the <strong>plain-text</strong> to get:
</p>
<blockquote>
<p>
COULD YOU PICK UP THE KIDS FROM SCHOOL TODAY AT 2PM, PLEASE?
</p>
</blockquote>
</section>
</section>
<p>
So, now that you have seen those examples, let's define those terms.
</p>
<dl>
<dt>Plain-Text</dt>
<dd>
<p>
<strong>Plain-Text</strong> is the original message.
</p>
<p>
In <strong>cryptography</strong>, text, audio, and video all get called a "<strong>plain-text</strong>".
Which can be a bit confusing and even misleading as, in regular language, the word "<em>text</em>" tends not to include <em>audio</em> and <em>video</em>.
But in <strong>cryptography</strong> it does.
</p>
<p>
Usually when <strong>cryptography</strong> specialists talk about <strong>plain-text</strong>, they are doing so in relation to <strong>cipher-text</strong>.
</p>
</dd>
<dt>Cipher-Text</dt>
<dd>
<p>
To make a <strong>plain-text</strong> message <strong>confidential</strong>, we turn it into a <strong>cipher-text</strong>.
</p>
<p>
We turn a <strong>plain-text</strong> message into a <strong>cipher-text</strong> message to hide the message so that only the person, etc you want to read, listen to, or watch, etc the message can do so.
</p>
</dd>
<dt>Encryption</dt>
<dd>
<p>
<strong>Encryption</strong> is the process of turning <strong>plain-text</strong> into a <strong>cipher-text</strong>.
</p>
<p>
What the <strong>cipher-text</strong> ends up being depends on what <strong>encryption algorithm</strong> you end up using.
</p>
</dd>
<dt>Decryption</dt>
<dd>
<p>
<strong>Decryption</strong> is the process of turning <strong>cipher-text</strong> back into <strong>plain-text</strong>.
