What is symmetric encryption?

Symmetric encryption — also known as symmetric cryptography or symmetric key encryption — is a technique that uses a single key to encrypt and decrypt data. This means the sender and the recipient use the same shared key to secure and access information, creating a straightforward but secure encryption process.

Like other forms of cryptography, symmetric encryption conceals data to prevent anyone but the intended recipient from reading the encrypted message. Symmetric cryptography is known for being faster and simpler than asymmetric encryption, making it ideal for efficiently encrypting large amounts of data.

Uses for symmetric encryption

Symmetric encryption is widely used for securing sensitive data, especially in applications where speed and efficiency are essential. Common use cases include:

• Data storage: Encrypting files, databases, and backups for secure storage
• Secure communication: Protecting data in transit, such as messages and emails, from interception
• Financial transactions: Encrypting sensitive information in online banking and payment systems
• Device security: Encrypting data on personal devices (e.g., smartphones, laptops) and in corporate environments to prevent unauthorized access
• Network security: Protecting network traffic in secure connections, such as virtual private networks

Asymmetric encryption is a cryptographic system that uses one public key for encryption and one private key for decryption. It’s a core component of the security framework known as public key infrastructure — sometimes called “public key cryptography.”

Unlike symmetric encryption, these keys are mathematically linked but are not identical, allowing for secure data exchange without sharing the private key. Asymmetric encryption is commonly used in situations that require secure key exchange and digital signatures, such as HTTPS connections, email encryption, and digital certificates.

Symmetric keys vs. asymmetric keys

Symmetric keys are single, identical keys used for both encryption and decryption. This means that the same key is shared between the sender and recipient, making the process highly efficient for protecting large amounts of data. However, securely sharing the key between parties can be challenging, as its exposure compromises the security of the encrypted information.

In contrast, asymmetric keys come in pairs. The public key encrypts the data, and the corresponding private key decrypts it. Because the private key is never shared, asymmetric encryption enhances security during key exchange. This design eliminates the need for a shared secret and ensures that only the intended recipient, who holds the private key, can decrypt the information.

Symmetric encryption challenges

While fast and efficient, symmetric key cryptography has several drawbacks that can pose challenges depending on the use case. Here are the primary obstacles:

• Key management: Managing and distributing keys securely becomes increasingly difficult as the number of users or devices grows. In multi-user environments, each pair of individuals requires a unique encryption key. This results in an exponential increase in the number of keys to manage, which can quickly become impractical in large systems — not to mention a security risk. 
• Vulnerability to key exposure: If a symmetric key is compromised, all data encrypted with that key becomes accessible to the attacker. There’s no additional safeguard like in asymmetric encryption, where private keys remain secure even if the public key is exposed.
• No non-repudiation: Symmetric encryption cannot provide non-repudiation (proof of origin) because both parties use the same encryption key. This makes it impossible to determine who encrypted the data, limiting its use in scenarios where authentication or digital signatures are needed.
• Limited use cases for public networks: Because symmetric encryption requires the key to be shared beforehand, it’s less suitable for public network communications where a secure channel for initial key exchange may not exist. That’s why organizations leverage asymmetric encryption in tandem with symmetric cryptography.

Unlike symmetric encryption, these keys are mathematically linked but are not identical, allowing for secure data exchange without sharing the private key. Asymmetric encryption is commonly used in situations that require secure key exchange and digital signatures, such as HTTPS connections, email encryption, and digital certificates.

In symmetric encryption, the sender uses a shared secret key to convert plaintext (original data) into ciphertext (encrypted data). The receiver, who also possesses the same secret key, then uses it to decrypt the ciphertext back into readable plaintext. 

This process relies on symmetric encryption algorithms. An encryption algorithm — sometimes called a cipher — establishes a mathematical set of rules that define how the key alters the data.

Generally, the symmetric key encryption process works like this:

• Key generation: A cryptographic algorithm generates a secure, random encryption key.
• Key exchange: The sender securely shares the encryption key with the recipient, often through a protected channel or combined with an asymmetric encryption method to ensure confidentiality.
• Data preparation: The algorithm prepares the plaintext (the original message or file) for data encryption. Depending on the algorithm, this may involve formatting the data into specific-sized blocks.
• Data encryption: The encryption algorithm applies mathematical operations to the plaintext using the encryption key. This process converts the plaintext into ciphertext, which appears random and unreadable without the key.
• Transmission or storage of ciphertext: The encrypted message is then transmitted (if communicating with another party) or stored (if securing data at rest).
• Decryption: The recipient uses the same encryption key to decrypt the ciphertext. The symmetric encryption algorithm reverses the encryption process, turning the ciphertext back into readable plaintext.

Symmetric ciphers come in two varieties:

1. Block ciphers that encrypt data in fixed-size blocks
2. Stream ciphers that encrypt data one bit or byte at a time

Stream ciphers are more suitable for real-time data processing, as they can encrypt smaller, continuous data streams more efficiently. On the other hand, block ciphers are better for securing large amounts of data at a time.

Below is a list of common symmetric algorithms:

• Data Encryption Standard (DES): Developed in the 1970s, DES is one of the world’s original block ciphers. It uses a 56-bit key and divides messages into 64-bit blocks. However, its short key length made it vulnerable to brute-force attacks, where hackers try different keys until one works.
• Triple Data Encryption Standard (3DES): 3DES was the successor to the original DES, named for its ability to apply the same algorithm three times to each data block. This significantly increases key size and security but has since been replaced by more secure alternatives. 3DES has a bit strength of 112.
• Advanced Encryption Standard (AES): AES is considered the gold standard of symmetric encryption algorithms. As a block cipher, it offers key lengths of 128, 192, or 256 bits, making it more resistant to cracking.
• Blowfish: Blowfish is a fast, flexible cipher with a variable key length (32 to 448 bits), making it suitable for various applications. By design, it offers a speedy encryption rate for software and secure data encryption. Although still effective, newer algorithms have largely replaced Blowfish for many use cases.
• Twofish: As the successor to Blowfish, this encryption method is known for speed and security, supporting key lengths of up to 256 bits. Twofish is open source and resistant to cryptoanalysis, meaning even with extensive testing it hasn’t been successfully compromised. Thus, it’s highly secure for encrypting sensitive data, even as computational power grows.
• Rivest Cipher 4 (RC4): RC4 is a fast, simple stream cipher and was one of the most widely used encryption algorithms in its prime. It has since been found to have significant vulnerabilities, rendering it insecure for modern cryptographic needs.

Post-quantum cryptography

While the advent of quantum computing poses significant challenges to many cryptographic methods, symmetric encryption algorithms are widely regarded as resistant to quantum attacks when they employ sufficiently long key lengths. This is because quantum computers, though powerful, offer less of an advantage in breaking symmetric encryption compared to asymmetric methods. By doubling the key size, symmetric algorithms can effectively mitigate potential vulnerabilities, making them a robust choice in a post-quantum world.

Key management encompasses the processes and technologies used to create, store, distribute, and control access to encryption keys. Why is it important? Because the security of symmetric encryption relies entirely on keeping the key secure.

Poor key management can lead to data breaches, unauthorized access, and compromised data integrity. However, effective policies and best practices can help organizations prevent these risks by enforcing control and oversight over key access. 

Here are some key management best practices to keep in mind:

1. Implement key rotation

Regularly rotating encryption keys is a critical best practice to minimize the risk of compromised keys being exploited. By changing keys at predefined intervals or after specific events (such as suspected exposure), organizations limit the potential damage caused by unauthorized access. Key rotation ensures that even if one is intercepted, it becomes obsolete quickly, safeguarding long-term data integrity.

2. Use a hardware security module (HSM)

An HSM is a secure, tamper-resistant environment for managing and storing cryptographic keys. These dedicated physical devices prevent unauthorized access by isolating keys from general systems and applications. Take Entrust’s nShield HSMs, for example. Our devices handle cryptographic operations, including key generation, rotation, and digital signature creation. This adds an extra layer of security while improving efficiency and compliance with industry standards.

3. Adopt centralized key management

Centralized key management systems simplify key generation, storage, and distribution by consolidating these processes in one platform. This centralized approach helps enforce consistent key policies across the organization, reduces the likelihood of mismanagement, and enables administrators to monitor and control access to encryption keys effectively.

4. Automate key rotation

It’s no secret that manual processes are slow, error-prone, and a risk to data security. With automation, organizations can regularly update encryption keys without disrupting operations. This proactive approach reduces the likelihood of using outdated or vulnerable keys, enhances operational efficiency, and mitigates mistakes in the key management lifecycle.

5. Audit your access control privileges

Strict access control policies limit key access to only authorized personnel, reducing the risk of misuse or accidental exposure. Regular audits of key management practices help organizations identify potential vulnerabilities, maintain compliance with regulations, and improve overall data security.

Symmetric encryption is a useful mechanism for enabling secure communication, but without proper protection, its keys can put your business at risk. Entrust offers robust solutions to help you streamline key management and secure cryptographic keys with a strong root of trust.

• Entrust KeyControl offers centralized visibility through a decentralized vault-based architecture. That way, you’re not putting all your eggs in one basket, reducing the risk of key exposure.
• Entrust nShield HSMs perform cryptographic operations in a hardened, tamper-resistant environment, available in a variety of form factors and deployment scenarios.