What is symmetric encryption?

Symmetric encryption is a method that uses a single key for both encryption and decryption. This type of symmetric cryptography offers a number of benefits, including robust data security, fast encryption and decryption speeds, and a decent level of quantum resistance when used with larger key sizes.

Symmetric encryption ensures that only the intended recipient of an encrypted payload can access the data. Thanks to its relative simplicity and speed, it is well-suited to cybersecurity tasks that involve large amounts of data.

In this guide, I will walk you through symmetric encryption, explaining what it is, how it works, the different key sizes people tend to implement it with (such as AES-128 or AES-256), and the best uses for this type of single-key encryption. I will also compare it to asymmetric encryption (also known as public key cryptography) to explain the key differences.

To explain the limitations of symmetric cryptography, I will discuss the need for secure key management, along with common key exchange methods that services use when they need to share a symmetric key securely between users over the internet.

What is symmetric encryption?

Symmetric encryption uses a single key to both encrypt and decrypt data.

When used for secure communications, both the sender and recipient must share the key to access the encrypted information. This creates risk if they share the key insecurely, and creates the need to stack symmetric cryptography with a secure key exchange method (often referred to as the key distribution problem).

It is also worth noting that symmetric encryption can secure data that is not shared. For example, you could use it in scenarios where only you need access to the key, which removes the need for secure key exchange altogether. Examples of this kind of single-use symmetric encryption include encrypted data storage, disk encryption (using programs like BitLocker or FileVault), and secure password management.

When symmetric encryption protects data, it scrambles readable information (known as plaintext) into an unreadable format called ciphertext. The same key then decrypts the ciphertext and restores access to the original data.

Because it uses a single key, symmetric encryption is generally faster and more efficient than other encryption technologies. This is why services often use it for tasks that involve large amounts of data. Common examples include file encryption, databases, and secure communications such as VPNs.

See also: Common encryption types explained: A guide to protocols and algorithms

Types of symmetric encryption (AES, 3DES, block vs stream)

Various algorithms use symmetric encryption; while they differ in design, they all have one thing in common: they use a single key for both encrypting and decrypting data. This means that symmetric encryption is not just one single method; it is a system used by a wide range of algorithms and protocols.

Different algorithms use different approaches to symmetric encryption, but they all tend to fall into two categories: block ciphers and stream ciphers.

Block Cipher vs. Stream Cipher

These two terms refer to how different symmetric ciphers encrypt data.

With a block cipher, the algorithm scrambles data in fixed-size chunks; the word “block” refers to this process in the cipher’s name. This type of symmetric encryption works well for encrypting data at rest. For example, systems use it to encrypt files, hard drives, or databases where information is stored.

A stream cipher gets its name because it nibbles at data bit by bit, encrypting one bit or one byte at a time as a continuous stream, rather than processing the entire message in fixed blocks. Systems use this type of cipher to encrypt data in transit, such as real-time data transmission tasks like video streaming, where you can’t afford to wait for the whole file to be encrypted and sent in one large chunk.

In modern apps and systems, the algorithm you will encounter most often is AES.

Advanced Encryption Standard (AES)

The Advanced Encryption Standard (AES) is the most widely used symmetric encryption algorithm today. The cybersecurity industry considers it the gold standard and uses it in everything from VPNs and TLS connections to banking systems and secure file storage.

AES stands for Advanced Encryption Standard, and it is widely used because it offers strong security and reliability. AES is a block cipher and supports multiple key sizes, including 128-bit, 192-bit, and 256-bit. While AES-128 is already considered secure, AES-256 provides a higher level of protection, and services use it for highly sensitive data.

Triple DES (3DES)

Triple DES (3DES) is an older symmetric encryption algorithm that improves on the original DES standard by applying the encryption process three times. This increases its effective bit strength to around 112 bits.

Although 3DES was once widely used, experts now consider it insecure and are phasing it out in favor of more secure and efficient algorithms like AES.

Which type of symmetric encryption is most used today?

In modern systems, AES is the standard choice for symmetric encryption due to its balance of speed, security, and efficiency.

Older algorithms like DES and 3DES are no longer recommended for most use cases. NIST formally withdrew the Data Encryption Standard (DES) (FIPS 46-3) in 2005, which means new systems should no longer use it.

3DES is also being phased out because it is slower and less secure than AES in modern environments. Authorities officially deprecated it in 2023, which means legacy systems should only use it. Modern use no longer considers it secure enough, and newly developed systems should not use it.

What are the main uses for symmetric encryption?

There are many uses for symmetric encryption and, as mentioned above, this type of cryptography can secure data in transit (think VPNs and Transport Layer Security) and protect data at rest (such as encrypted folders, databases, and servers).

Thanks to its efficiency and fast speeds, symmetric encryption suits use cases where large amounts of data need to be processed quickly. Common uses include full disk encryption, secure file storage, and database protection.

Secure communication protocols also widely use symmetric encryption. For example, during a TLS handshake, asymmetric encryption exchanges a symmetric encryption key.

This type of hybrid system allows secure key exchange using the heavier and slower asymmetric encryption technology, while enabling symmetric cryptography to secure the data itself more quickly.

Here are the main uses for symmetric encryption:

Why do VPNs use symmetric encryption?

VPNs use symmetric encryption to secure data inside a VPN tunnel. This ensures that any data transmitted over a LAN or the internet cannot be decrypted if it is intercepted or monitored (such as by wifi network providers or ISPs).

The benefit of using symmetric encryption to convert a plaintext payload into ciphertext (referred to as the data channel) is that it can encrypt and decrypt large amounts of data quickly. This makes it effective for transmitting large amounts of data between a user’s device and a VPN server – and then back.

For VPN clients to send data privately and quickly, they must first exchange a symmetric encryption key. VPNs achieve this by using asymmetric encryption (public key cryptography) to share the key safely. This hybrid system allows the secret key to be shared safely and privately between remote locations.

Once secure key exchange is achieved (via the VPN control channel), symmetric encryption takes over to encrypt the actual payload. This gives VPNs their advantage, as they are both highly secure against eavesdroppers and fast for activities such as streaming, videoconferencing, or transferring large amounts of data.

By leveraging symmetric encryption alongside a secure public key exchange method, VPNs create a reliable way for individuals, websites, and businesses to protect data against common threats, such as man-in-the-middle (MitM) attacks and unauthorized surveillance, while maintaining fast performance and low latency.

Why does TLS use symmetric encryption?

Transport Layer Security (TLS) is the encryption standard that secures information between end users and websites during HTTPS web browsing sessions. As with VPNs, TLS uses symmetric encryption to turn plaintext data into ciphertext in the most efficient and speedy way possible. The reason is that securing data between clients and servers often involves sending large amounts of data, such as video files, applications, or database queries.

To protect TLS sessions against adversaries (hackers, surveillance agencies, local networks, ISPs, and other eavesdroppers), TLS uses asymmetric encryption to authenticate the server and securely exchange an encryption key.

Once a TLS handshake establishes and confirms the identity of the server, TLS creates a trusted connection between the end user and the intended recipient (website, cloud server, etc). This allows the exchange of data with strong levels of integrity and confidentiality.

Because both parties use the same encryption key to encrypt and decrypt the contents of messages, TLS sessions provide the speed and efficiency that allow end users to communicate effectively without lag and with reliable cybersecurity protections.

How symmetric encryption works (step-by-step)

The easiest way to think about symmetric encryption is like the lockers you use at the swimming pool. Just like a lockbox, safety deposit box, or safe, you use the same key to lock it and unlock it.

1. The encryption key

The first step in the symmetric cryptographic process is to create the secret code (known as the encryption key).

This secret needs to be mathematically secure against adversaries, which means that it needs to be complex. To achieve data security and integrity against brute-force attacks, computers must use complex math to create a long, random string of numbers and letters that serves as the key.

The strength of this key depends heavily on computational entropy. This is a measure of how random and unpredictable the key is. Higher entropy makes it much harder for attackers to guess the key using brute-force methods.

The security of a key is often measured in terms of bit strength, which reflects how difficult it is to break using brute-force attacks. For example, AES-128 or AES-256, with the latter being more reliable due to the larger bit size of the key.

Once that is done, anybody who has that key can use it to unlock the payload – no matter what it happens to be.

2. Encrypting the payload

Once the key has been created, it can turn unprotected data (known as plaintext) into scrambled information that nobody can understand (known as ciphertext). For this to happen, the computer uses the key to run a complex algorithm.

Modern algorithms such as Advanced Encryption Standard (AES) use a substitution-permutation network to securely scramble data, which is more advanced than older designs such as the Feistel cipher used in legacy systems like DES.

The mathematical process that scrambles the data ties directly to the key, so that only users with the exact key can recover the information.

These modern designs have strong cryptanalysis resistance, meaning they withstand advanced mathematical attacks used by security researchers and malicious actors.

In addition, modern systems often leverage tools such as HKDF (HMAC-based Key Derivation Function) to derive strong encryption keys from shared secrets. This ensures that even weak user inputs (weak passwords, for example) can transform into secure keys that keep user data secure.

3. Transmitting data

Once data has been turned into ciphertext, it can be transmitted securely between users. Nobody on the local network, the Internet Service Provider, backbone servers along the route, or even endpoints can read the data unless they have the key.

In many systems, message authentication codes (MACs) are also used alongside symmetric encryption to ensure data integrity. This provides an additional step for recipients to verify that the message has not been altered during transmission (MitM attacks, spoofing, etc).

This means you can send data from one office to another, but only the intended recipient – the colleague who has the key – can decrypt it and gain access to the plaintext.

4. The final step: Decryption

The final step in the symmetric encryption process is simply to regain access to encrypted data using the key. If the payload is stored on your own computer or on an external hard drive, you can use the key in your possession to decrypt it. However, if you want to share the data with another user, such as in the transmission phase above, you will first need to share the key securely. This is where secure key exchange methods come in.

How do users share a symmetric key securely?

The main caveat with symmetric cryptography is that it requires you to already have the key. This raises the question: what happens when two users want to send each other data remotely, for example, between different offices or even across different countries?

This problem is often referred to as the key distribution problem, and is one of the main challenges in symmetric cryptography.

Sending the shared key in a direct message would be dangerous because it would give anybody who intercepts that message the ability to also open the encrypted payload. One option is to meet up in person to share the key, or to send the key in a physical form, such as a hard drive or USB dongle, through the post.

For most people, this kind of in-person key exchange or physical handover is simply too impractical. Websites and end users do not know each other, and they need to exchange keys quickly for normal day-to-day use. This is why the encryption system is split into two parts:

  • Share the key securely
  • Use the symmetric key to actually encrypt and decrypt shared data

In order to achieve this, symmetric encryption is paired with a slower and more robust form of encryption called asymmetric encryption. This alternative form of encryption uses public key cryptography to enable secure sharing of secrets between users.

For example, methods such as Ephemeral Diffie-Hellman (EDH or ECDH) are often used to generate temporary session keys with perfect forward secrecy (PFS). This means that even if a key is compromised later, any data transmitted in previous sessions cannot be decrypted.

Note that in protocols such as TLS, a temporary value known as the pre-master secret is used during the handshake process to help generate the final symmetric session key.

Symmetric vs Asymmetric Encryption

Unlike symmetric encryption, which uses the same key for encryption and decryption, asymmetric encryption uses a more complex system involving two keys: a public key that can be shared freely, and a private key that is mathematically linked to the public key.

This relationship between the two keys is based on computational infeasibility, meaning it would take an impractical amount of time and resources to reverse the process and derive the private key.

This system is reliable because the private and public keys are linked using complex mathematics, which also enables features such as non-repudiation, where a sender cannot deny having sent a message.

Asymmetric encryption is reliable not just because of key length, but because of its high level of cryptanalysis resistance, making it nearly impossible to find mathematical ‘cracks’ in the code.

How asymmetric encryption works

The easiest way to visualize this system is to think of the public key as the address that anybody can drop a letter into. This public key is made public so that anybody can deliver a message to you.

The private key is like the key you use to open the back of the post box and pick up the mail inside. This method makes it ideal for tasks that need to happen across long distances, where systems need instant key exchange to safely exchange values used to generate the final ephemeral session keys.

Algorithms then use these symmetric keys, rely on a substitution-permutation network to provide high cryptanalysis resistance, generate them for a single session, and discard them to maintain the security of the encrypted data.

Why is asymmetric encryption used with symmetric encryption

The primary drawback of this dual key system is that it is much more complex mathematically. For it to work, it relies on heavier mathematical processes, often using predefined parameters such as a Diffie-Hellman group during the key exchange phase, which makes it much slower than symmetric encryption.

This is why asymmetric encryption does not encrypt actual payloads: it takes too long to encrypt and decrypt larger amounts of data. This makes it unsuitable for bulk data encryption.

By using asymmetric encryption just to deliver the symmetric key, two users can interact privately and share encrypted payloads quickly. As soon as the public key exchange method completes the initial handshake (leveraging concepts such as a pre-master secret – a temporary shared secret used to create the final key – and perfect forward secrecy (PFS), which protects past sessions from later decryption), users can engage in bulk data encryption at a distance.

This kind of hybrid encryption approach commonly establishes secure sessions for HTTPS, TLS, VPNs, and other protocols, where strong key entropy and sufficient bit strength are critical to ensuring long-term data security.

Advantages and disadvantages of symmetric encryption

Symmetric encryption is a reliable cryptographic system preferred for its ability to handle bulk data encryption. I have listed the advantages below to give you an idea of when to use symmetric encryption:

Advantages of symmetric key encryption

  1. Fast encryption speed: Because the math behind it is less complex, symmetric encryption is much faster than asymmetric encryption. This makes it the perfect choice when encrypting large amounts of data at rest or in transit.
  2. Low overheads: Symmetric encryption doesn’t take much processing power from a computer or phone. This makes it a great solution for securing data on mobile devices or other devices with limited processing power and memory. It also works well for hardware-based encryption in mobile devices, where bit strength must be balanced with battery life.
  3. Uses less bandwidth: When symmetric encryption is in use, the final payload is generally smaller in size compared to some other encryption methods. This helps save bandwidth when sending data across a network or the internet.
  4. Excellent compatibility: Symmetric encryption works with almost every hardware and software platform. This makes it suitable for use across a wide variety of platforms and for a wide variety of tasks that require encryption.
  5. Improved data integrity: Symmetric encryption can generate Message Authentication Codes (MACs) to prove that data wasn’t tampered with during its journey. This protects against spoofing and MitM attacks.
  6. PCI DSS Compliance: Symmetric encryption allows organizations to meet key PCI DSS requirements by protecting cardholder data and related sensitive information. This enables secure transactions to occur online using strong, standardized algorithms such as AES.

Disadvantages of Symmetric Encryption

  • The key sharing problem. A primary problem with symmetric encryption revolves around sharing the lone encryption key. Because both users need the same key, systems must transfer it somehow. If attackers intercept that key while it handles data in transit, the entire system is compromised.
  • The single key risk. Symmetric encryption uses a single key. That makes it critical to keep that key safe. If it gets lost or stolen, all encrypted data could be compromised. This is why strong cryptographic entropy remains so crucial when systems generate keys, and why secure key rotation principles help reduce long-term risk.
  • Key management challenges at scale. As systems grow, managing encryption keys becomes more difficult. Larger environments require more keys, which increases complexity. This is where the key management lifecycle becomes important, covering how systems create, store, use, rotate, and eventually destroy keys.
  • No built-in non-repudiation. Symmetric encryption means that both users share the same key. As a result, it cannot prove the origin of an encrypted message. This makes it unsuitable for tasks that require authentication, such as digital signatures.

Is symmetric encryption safe?

Symmetric encryption remains safe as long as modern standards and practices govern its implementation. I recommend this type of private key cryptography with strong algorithms like AES rather than outdated ones like DES. In addition, the key length must be adequate to prevent brute forcing. Most experts agree that bit sizes like 128-bit or 256-bit provide strong long-term security. However, AES-256 is preferable for highly sensitive data sets.

Finally, services must handle key management securely. Nowadays, automated tools and systems manage this. They help prevent key exposure from human error or weak controls.

Symmetric cryptography only becomes risky when services use weak algorithms or short keys. It also becomes risky when they expose or poorly manage the encryption key.

Common threats and weaknesses

The primary threats and weaknesses of symmetric encryption include brute force attacks, key leakage, and poor implementation (such as using outdated algorithms or inadequate key lengths). In most cases, these risks arise when best practices are not followed, rather than from the encryption itself.

Future of symmetric encryption

Symmetric encryption will remain a central part of data security and cybersecurity as organizations prepare for the impact of quantum computing. Unlike some asymmetric systems, symmetric algorithms such as AES offer greater resistance to quantum attacks. However, it is important to acknowledge that they are not immune to the threat of cryptographically relevant quantum computers.

Quantum risk and future-proofing

Quantum computers introduce risks for current encryption standards because of algorithms like Grover’s algorithm. This is predicted to reduce the efficacy of symmetric keys. As a result, a 128-bit key could offer security closer to 64-bit in a quantum scenario. This effectively reduces the strength of the key, making it significantly easier to brute force.

Because of this, most security experts are now recommending the use of larger key sizes, such as AES-256, as part of a quantum-resistant strategy.

Anybody interested in quantum migration timelines and the regulatory changes expected over the next half a decade should refer to the National Institute of Standards and Technology (NIST). NIST continues to provide guidance on cryptographic standards.

While much of its current post-quantum cryptography (PQC) work focuses on asymmetric encryption, symmetric encryption remains part of the long-term strategy. It offers relative resistance to quantum attacks when systems implement it with sufficient key length and strong key entropy.

One of the biggest emerging risks is known as “Harvest Now, Decrypt Later” (HNDL). This refers to attackers collecting encrypted data today with the intention of decrypting it in the future using quantum computers.

Understanding HNDL and the need to migrate high-risk systems or data sets is crucial. This applies to any organization or individual that processes, stores, or uses long-lived data sets that must remain confidential for many years. Data most at risk includes financial records, healthcare data, government communications, intellectual property, and long-term archives or backups.

Understanding how symmetric encryption fits into modern cybersecurity is essential, especially when combined with secure key exchange methods and strong encryption standards like AES.

Glossary of important encryption terms

Have any of the words or acronyms above led to confusion? I want this guide to be accessible to people in all walks of life, not just those who understand cryptography. That is why I have made the extra effort to explain each of the terms above in a little more detail. This should help to complete your understanding of the subject matter.

Most important encryption terms:

  • Computational entropy. A way of measuring how random an encryption key is. The more random it is, the harder it is for attackers to guess.
  • Substitution-permutation network. A method used by modern encryption algorithms like AES to scramble data in a very structured and secure way.
  • Feistel cipher. An older encryption design used in systems like DES. It is still important historically, but newer methods are generally more secure.
  • Message Authentication Code (MAC). A kind of digital seal that proves a message hasn’t been changed while being sent from one place to another.
  • HKDF (HMAC-based Key Derivation Function). A tool used to turn a shared secret (like a password or key) into a stronger, more secure encryption key.
  • Ephemeral Diffie-Hellman (EDH / ECDH). A method used to create temporary encryption keys that are thrown away after use. This helps keep past data safe even if a key is later compromised.
  • Perfect Forward Secrecy (PFS). A security feature that ensures even if a key is stolen in the future, past communications cannot be decrypted.
  • Pre-master secret. A temporary value used during a secure connection (like HTTPS) to help generate the final encryption key used for the session.
  • Bit strength. A way of describing how strong an encryption key is. Higher numbers (like 256-bit) mean more possible combinations and stronger security.
  • Data at rest. Data that is stored somewhere, like on a hard drive or server, rather than being sent over the internet.
  • Non-repudiation. A property of asymmetric encryption that ensures a sender cannot deny having sent a message, often used in digital signatures.
  • Computational infeasibility. The idea that breaking an encryption system would take an unrealistic amount of time and computing power, even for advanced attackers.
  • Cryptanalysis resistance. A measure of how well an encryption algorithm can withstand attempts to break it using mathematical techniques.
  • Ephemeral session keys. Temporary symmetric keys that are generated for a single session and then discarded to improve security.
  • Diffie-Hellman group. A predefined set of mathematical parameters used during key exchange to securely generate shared secrets.
  • Key entropy. A measure of how random and unpredictable an encryption key is, with higher entropy providing stronger security.

Symmetric key encryption: FAQs

What is the difference between AES-128 and AES-256?

AES-128 and AES-256 are both versions of the Advanced Encryption Standard, but they differ in key length. AES-128 uses a 128-bit key, while AES-256 uses a 256-bit key, making it harder to brute force.

In practice, AES-128 is already considered secure for most use cases and offers slightly better performance. AES-256 provides a higher level of security and is typically used for highly sensitive data or long-term protection.

Can symmetric encryption be broken?

Symmetric encryption can theoretically be broken, but only under certain conditions. In practice, modern algorithms like AES are considered secure because breaking them using brute force would take an impractical amount of time and computing power.

However, symmetric encryption can become vulnerable if weak algorithms are used, keys are too short, or the encryption key is exposed through poor key management. In most real-world cases, breaches occur due to implementation mistakes rather than flaws in the encryption itself.

What is the difference between data at rest and data in transit?

Data at rest refers to information that is stored on a device, such as a hard drive, database, or cloud storage system. Data in transit refers to data that is actively being transmitted across a network, such as over the internet or a local network.

Symmetric encryption is commonly used to protect both types of data, but the risks differ. Data in transit must be protected from interception, while data at rest must be secured against unauthorized access or theft.

What is a substitution-permutation network?

A substitution-permutation network is a mathematical structure used in modern symmetric encryption algorithms like Advanced Encryption Standard (AES).

It works by repeatedly substituting and rearranging bits of data in a structured way, which helps to turn readable plaintext into secure ciphertext.

The goal is to make the relationship between the original data and the encrypted output extremely difficult to reverse, even for attackers using advanced cryptanalysis techniques.

This design is considered more secure than older approaches, such as the Feistel cipher, because it provides stronger resistance against modern attacks.

What is cryptanalysis resistance?

Cryptanalysis resistance refers to how well an encryption algorithm can withstand attempts by experts or attackers to break it using mathematical analysis.

In both symmetric encryption and asymmetric encryption, strong cryptanalysis resistance is essential to ensure that encrypted data cannot be reversed back into plaintext without the correct encryption key.

Modern symmetric algorithms like AES are specifically designed with high cryptanalysis resistance, which helps protect large volumes of data. Asymmetric systems also rely on this principle, but use different mathematical problems to achieve security.

In simple terms, the stronger the cryptanalysis resistance, the harder it is for attackers to find weaknesses and compromise data security.