Symmetric encryption with a shared key: how it protects your data

When you use the internet, your messages, passwords, and personal data are constantly transmitted over the network. To protect this information from cybercriminals, a special encryption system exists – symmetric encryption. This encryption method with symmetric keys is based on a single secret code used for both encoding and decoding information.

Historically, symmetric encryption algorithms were first used by government agencies and military institutions to secure their communications. Today, this principle forms the foundation of security for millions of computer systems worldwide.

How the symmetric key system actually works

Imagine you want to send a secret message to a friend. The simplest way is to agree on a special “code” that only you two know. This principle underpins the symmetric encryption scheme using one key.

The process begins with “plaintext” – your original message or data. When you pass it through an encryption algorithm (so-called encryption), the text is transformed into a meaningless sequence of characters – “ciphertext.” Anyone who intercepts your message can see this ciphertext, but it cannot be read without the correct key.

The recipient, possessing the same secret key, runs the ciphertext through a reverse algorithm and retrieves the original message.

The security of the entire system depends on one thing: how hard is it to guess the correct key? A computer trying to brute-force all possible 128-bit keys would need billions of years of continuous operation. If the key length is extended to 256 bits, the task becomes practically impossible even for the most powerful modern computers. 256-bit keys are considered secure even against hypothetical attacks using quantum computers.

Two methods of encoding: blocks and streams

There are two main ways symmetric encryption works in practice.

Block ciphers divide your data into fixed-size portions – for example, 128-bit blocks. Each block is encrypted separately using the same algorithm and key. It’s like cutting a message into identical pieces and sealing each piece in a separate envelope with the same code.

Stream ciphers work differently – they encode data not in large chunks but one bit at a time. It’s similar to writing a message letter by letter, encrypting each letter immediately before sending.

Why symmetric is not the same as asymmetric

In the world of encryption, there is another fundamental method – asymmetric encryption, and there are key differences between them.

The main difference: symmetric encryption uses one key for both (encryption and decryption), while asymmetric uses two completely different keys.

In asymmetric systems, one key is called the public key (public) – known to everyone. The other is the private key – kept secret by you. If someone encrypts data with your public key, only your private key can decrypt it.

However, asymmetric algorithms have their cost: they operate much slower than symmetric ones and require more computational resources. Additionally, to provide the same level of security, public and private keys must be significantly longer than symmetric keys.

Where symmetric encryption is used on your computer

If you’ve ever used a messenger for secure communication or uploaded files to cloud storage, you’ve already worked with symmetric encryption.

The most popular example is AES (Advanced Encryption Standard). This algorithm is used everywhere: from secure messaging apps to data protection on cloud servers. AES can operate in software or be embedded directly into computer chips.

The most common version is AES-256, where 256 indicates the key length in bits. Such keys are considered practically unbreakable.

Interesting fact: when people hear about “cryptography” in the context of blockchain and Bitcoin, they often think of encryption. In reality, these systems do not use encryption but a specialized digital signature algorithm called ECDSA (Elliptic Curve Digital Signature Algorithm). This algorithm creates unique “signatures” to verify authenticity but does not encrypt the data itself. ECDSA is based on elliptic curve mathematics, which can be used for many tasks, but ECDSA itself is not suitable for encryption.

TLS: when symmetric and asymmetric work together

One of the biggest problems with symmetric encryption arises when you need to share a secret key with someone over an insecure channel. If the key is intercepted, all security is lost.

To solve this, modern web protocols use a hybrid approach. They first establish a secure channel using asymmetric encryption (to securely exchange keys), then switch to fast symmetric encryption for transmitting large volumes of data.

The most prominent example of such a hybrid system is the TLS (Transport Layer Security) protocol, which protects most of the modern internet. When you visit a secure website (the address starts with https://), you are using TLS.

Why symmetric encryption is still the best for fast operation

Symmetric encryption has several undeniable advantages that make it indispensable in modern systems:

• Speed: Symmetric algorithms operate much faster than asymmetric ones, making them ideal for encrypting large amounts of information.

• Simplicity: Systems with a single key are easier to implement and require less computational power.

• Security flexibility: To increase security, you just need to increase the key length. Each additional bit exponentially increases the difficulty of brute-force attacks.

However, this advantage has a downside – the problem of key distribution. When a symmetric key is transmitted over an unsecured channel, it risks being intercepted. If an unauthorized person gains access to your symmetric key, all data encrypted with that key becomes accessible to them.

Risks of implementation errors

It is important to understand that a mathematically secure algorithm can be broken due to programming errors during its implementation. Although a very long key makes brute-force attacks impossible, coding mistakes often create “backdoors” that attackers can exploit. That’s why security audits and vulnerability testing are critical for any cryptographic system.

Conclusion

Symmetric encryption with a single key remains one of the most important tools for protecting data in the digital world. From securing your internet traffic to safeguarding files on cloud services, this method of encoding provides speed, simplicity, and reliability. Although it is often combined with asymmetric encryption to solve the problem of secure key exchange, symmetric encryption remains the backbone of modern computer security.