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Encryption Vs. Decryption: What’s the Difference?

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We tend to associate encryption with the challenges of securing digital data in the modern world. In truth, encryption is rooted in practices that date back thousands of years. The technology and codes have changed, but some of the fundamental principles are the same.

Applying encryption adds a level of security to the data that can help prevent the file contents from being understood by any unauthorized person who gets hold of it. Even if the data is accessed, it requires decryption to extract its meaning.

When more than one key is involved in the process, it's also possible to use to authenticate the sender. (Read Expert Feedback: What Data Encryption Advancements Should Businesses Be Aware Of?)

What's the Difference Between Encryption Vs. Decryption?

Encryption is the process of using an algorithm to transform information to make it unreadable for unauthorized users. Once the information is encoded, it requires decryption to be understood. (Read Encryption Just Isn't Enough: 3 Critical Truths About Data Security.)

Decryption is the process of transforming data that has been rendered unreadable through encryption back to its unencrypted form.

The encoded data reverts back to its original form, whether it contains texts or images, so that it makes sense to the human reader and/or the computer system. This process may be automated or be conducted manually.

Typically, there is a form of key involved. (Read 10 Best Practices for Encryption Key Management and Data Security.)


A History Lesson on Encryption and Decryption

scytale used for transposition ciphers

CC BY-SA 3.0

A Scytale was what ancient Greeks used to make a simple transposition cipher. All it took was a strip of leather on which the letters were written and cylinder around which to wrap it. The sensitive data that was protected this way was likely centered around battle strategies. (Read Encryption Backdoors: The Achilles Heel to Cybersecurity?)

The encryption is the result of the letters being taken out of the order necessary to read and make sense of the message when they are unwrapped. In this case, the right cylinder functions as the key because it is what would get the letters properly aligned once the strip was wrapped once again.

The cylinder would be what is called a pre-shared key (PSK) in cryptography, that is a secret key that was shared ahead of the secret message being sent on it. It’s letting the other party know what code the hidden message will be in. (Read Cryptography: Understanding Its Not-So-Secret Importance to Your Business.)

The Scytale method of encryption is the first one mentioned in A Brief History of Cryptological Systems, an instructive and entertaining read about strategies to prevent unauthorized people from reading secret message.

A Key Etched in Stone

What may be the most famous stone in the world is housed in the British Museum. The museums’s blog on the historic Rosetta Stone explains that Napoleon’s army found it in the Nile delta town for which it is named in 1799. At that time, no one had the capability to read hieroglyphs. It was a code with no key.

That is until scholars studied the Rosetta Stone. It opened the way to meaning through two components. One was that the same message was carved into in three languages, including Ancient Greek, which scholars could read.

The other was an identifiable cartouche that indicated which symbols stood for the name of the king Ptolemy.That was the basis of finding which of the 53 lines of Ancient Greek corresponded to the 14 lines of hieroglyphics and figure out the meaning of individual symbols.

It then took a couple of scholars 20 years to work it all out.

Decryption Keys in Modern Times

While the Rosetta Stone did function effectively as a decryption key, we need something easier to work with than a 1,680 pound rock for our everyday needs. The keys used in computer encryption are based on algorithms which scramble the plaintext data to render it into apparently random gibberish.

Applying the decryption key will put it back into understandable plaintext. There are different possible setups with single or double sets of keys.

Symmetric key encryption

Symmetric key encryption is based on algorithms that apply the same keys for both encryption and decryption. It’s the same concept that worked for the Scytale in which the same size cylinder is used both to set the code and to rewrap the strips to make sense of the apparently random letters.

The same key that rendered the plaintext into ciphertext will turn the ciphertext back into plaintext In his blog, Panayotis Vryonis offers the analogy of locking something away in a box. The same key used to remove the contents from view is used to unlock the box and reveal them.

Asymmetric Key Encryption

This is also sometimes called public key encryption. The name is a bit misleading because the asymmetry actually depends on having both a public and a private key. The public key is used to encrypt the message that is decrypted with the private key.

You can also encrypt data with the private key and have the receiver decrypted with the public key. The point is just that different keys are used for two functions.

Vryonis once again offers an image of a locked box to understand the concept: “This lock has three states: A (locked), B (unlocked) and C (locked). And it has two separate (yes, two) keys. The first one can only turn clockwise (from A to B to C) and the second one can only turn anticlockwise (from C to B to A).”

He names the one who locks it Anna, and she has an exclusive on one key — the private key. The second key is the public one, which is copied and distributed.

“So. Anna has her private key that can turn from A to B to C. And everyone else has her public-key that can turn from C to B to A.” This opens up the possibility of locking up what you don’t have the power to unlock.

"First of all, imagine you want to send Anna a very personal document. You put the document in the box and use a copy of her public-key to lock it. Remember, Anna’s public-key only turns anticlockwise, so you turn it to position A. Now the box is locked. The only key that can turn from A to B is Anna’s private key, the one she’s kept for herself."

Anyone with the public key can make sure the box is locked, and only the person in possession of the private key can unlock it. Back to the world of algorithms, only the private key can decrypt what was encrypted by the public key. But it also has the possibility of allowing the public key to decrypt what was decrypt what was encrypted with the private key.

That opens up the possibility of attaching digital signatures, which Vryonis explains as follows:

"Someone delivers me this box and he says it’s from Anna. I don’t believe him, but I pick Anna’s public-key from the drawer where I keep all the public-keys of my friends, and try it. I turn right, nothing. I turn left and the box opens! “Hmm”, I think. “This can only mean one thing: the box was locked using Anna’s private key, the one that only she has.”

In that scenario, the lock that is only possible from the private key guarantees that the sender is the one represented, which is the function of the digital signature. It would be like an unbroken seal on a letter formed by the person’s signet ring used in the days of quill pens.

Accordingly, the asymmetric key offers more possible functions than the symmetric key system. Anyone with the public key can secure their data transmission to be decrypted only by the one in possession of the private key.

Plus anyone who receives data encrypted by the private key can trust the source. That preserves the integrity of the files and the validation of origin for digital communication, both of which are essential for functional and secure digital interactions.


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Ariella Brown
Ariella Brown

Ariella Brown has written about technology and marketing, covering everything from analytics to virtual reality since 2010. Before that she earned a PhD in English, taught college level writing and launched and published a magazine in both print and digital format.Now she is a full-time writer, editor, and marketing consultant.Links to her blogs, favorite quotes, and photos can be found here at Write Way Pro. Her portfolio is at