Symmetric Encryption

Key Takeaways

The global e-commerce market is set to reach $47 trillion by 2030, with billions of people purchase goods and services online each day. But when you shop online, you’re trusting a public network with your sensitive information, like credit card numbers, bank account details and personal data. How do you know it’s secure?  The answer lies in symmetric encryption. Otherwise known as symmetric key cryptography, this fundamental technology is the unsung hero that keeps our personal and financial information safe online. 

In this article, we’ll delve into what symmetric encryption is, how it works, and its applications in the digital world.

What Is Symmetric Encryption?

Symmetric encryption is a subcategory of cryptography that involves encrypting data to protect it from unauthorized access. In simple terms, encryption is the process of converting plaintext (readable data) into ciphertext (scrambled, unreadable data) using a secret key. This secret key is then used to decrypt the ciphertext back into plaintext when the information needs to be accessed. In symmetric key cryptography, the same key is used for both encryption and decryption, making it crucial to keep the key secure.

How Does Symmetric Encryption Work? An Example

Symmetric key cryptography operates by transforming plaintext into ciphertext through a series of mathematical operations. The accompanying cryptographic key ensures that only authorized parties can access the original information. 

If a party doesn’t have access to the key used to encrypt the data, they’ll never be able to read it. The whole process can be broken down into several steps, which we will explain using the concepts of substitution and transposition.

Substitution

Substitution is a cryptographic technique where each element of the plaintext is replaced with another element. For example, in a simple substitution cipher, each letter in the plaintext might be replaced with another letter. The famous Caesar cipher uses the substitution technique. This method is straightforward but it can be made more complex with advanced algorithms, making it harder for unauthorized users to decipher the message.

Transposition

Transposition involves rearranging the order of the characters in the plaintext. Unlike substitution, transposition doesn’t alter the characters themselves but changes their positions. For instance, a transposition cipher can take the first half of a message and swap it with the second half. When combined with substitution, transposition significantly enhances the security of the encrypted message.

An Example

To illustrate how symmetric key cryptography works, let’s consider an e-commerce purchase. When you enter your credit card details on a website, these details are encrypted using a symmetric key. Let’s say your card number is “1234 5678 9012 3456.” The encryption process might involve substituting each digit with another digit and then rearranging them. The resulting ciphertext might look something like “4892 7610 3254 6513.” This ciphertext is then sent to the merchant’s server, where it can only be decrypted by the same symmetric key. The process ensures that your data remains confidential during the transaction.

Symmetric Encryption vs. Asymmetric Encryption

Symmetric encryption doesn’t rely on modern computer science and is the predecessor to asymmetric encryption. With fewer bits and less complex implementation, symmetric encryption requires less overhead, making for a fast and inexpensive cryptographic system.

By comparison, asymmetric encryption is a complex process where two or more users exchange a mix of public and private variables to establish a trusted communication channel. This handshake produces two unique keys: one for the sender to encrypt messages and another for the recipient to decrypt messages.

The complexity of asymmetric encryption algorithms provides robust security for data in transport, but the downside is a slower speed relative to its symmetric counterpart. Conversely, symmetric encryption offers speed but lacks the same security integrity.

	Symmetric Encryption	Asymmetric Encryption
Keys	1 – Private	2 – Public and Private
Bits	128 – 192 – 256	2,048 – 4,096
Speed	Faster	Slower
Overhead	Less Complex and Expensive	More Complex and Expensive
Security	Limited	Strong
Examples	AES, RC4/5/6, 3DES, IDEA	Diffie-Hellman, RSA, ECC, DSA

Combining Symmetric and Asymmetric Encryption

Cryptographers and web developers increasingly bundle symmetric and asymmetric encryption methods to offer encryption services and their users the best of both worlds, between speed and security. This combination, also known as hybrid encryption, usually starts by connecting two users through asymmetric encryption, after which the users can exchange messages with symmetric encryption protected within the asymmetric encrypted channel.

Types of Symmetric Key Encryption

In symmetric key cryptography, a cipher refers to the sequence of substitution and transposition operations applied to transform plaintext into encrypted ciphertext. Symmetric key encryption can be categorized into two primary types based on the method of operation: block ciphers and stream ciphers.

• Block ciphers encrypt data in fixed-size blocks and each block is encrypted independently using the same key. They’re widely used in various cryptographic protocols and systems due to their ability to securely handle large amounts of data.
• Stream ciphers encrypt data one bit or byte at a time. They generate a pseudorandom stream of bits that are combined with the plaintext to produce ciphertext. Stream ciphers are typically faster than block ciphers, making them well-suited for real-time applications like audio and video transmission.

Block Cipher

A block cipher transforms plaintext data into a series of fixed-size blocks of various sizes, using a single encryption key. The output is a sequence of encrypted blocks arranged in a specific order. Upon reaching its destination, the receiver uses the same key to decrypt the ciphertext blocks, converting them back into the original plaintext message. Two of the most well-known block cipher algorithms are the Data Encryption Standard (DES) and the Advanced Encryption Standard (AES).

DES (Data Encryption Standard)

DES was one of the earliest block cipher algorithms and was widely adopted in the 1970s. It encrypts 64-bit blocks of data by using a 56-bit key. Although once considered secure, DES is now deemed vulnerable to brute-force attacks due to its relatively short key length. For the most part, it has been replaced by more secure alternatives such as AES.

AES (Advanced Encryption Standard)

AES was developed as a replacement for DES and is currently the industry standard for symmetric key encryption. It uses key sizes of 128, 192, or 256 bits, offering a much higher level of security. AES encrypts data in 128-bit blocks and is widely used in various applications. These range from securing internet communications to encrypting sensitive data on devices.

Stream Cipher

The encryption process with a stream cipher is a bit different. It starts with an algorithm that generates a pseudo-random keystream from the encryption key and a unique, randomly generated nonce. This keystream is a random sequence of bits that matches the length of the plaintext. The text is then split into individual bits, which are combined with the keystream, gradually converting the plaintext into ciphertext.

To decrypt the message, the recipient must recreate the keystream used during encryption. This regenerated keystream is then combined with the ciphertext bits to revert the ciphertext to the original plaintext. Stream ciphers encrypt data on a per-bit or per-byte basis, making them faster than block ciphers for certain applications. Two notable stream cipher algorithms are Rivest Cipher 4 (RC4) and Salsa20.

RC4 (Rivest Cipher 4)

RC4 is widely used due to its simplicity and speed. It generates a pseudorandom stream of bits that are combined with the plaintext to produce ciphertext. While RC4 was once popular, several vulnerabilities were discovered and this led to its decline in popularity.

Salsa20

Salsa20 is a modern stream cipher designed to be more secure and efficient. It uses a 256-bit key and a 64-bit nonce to generate a keystream that encrypts data. The cipher is known for its high performance and resistance to cryptographic attacks, making it a preferred choice in many areas.

Block Cipher vs. Stream Cipher: Comparison Table

	Block Cipher	Stream Cipher
Encryption Process	By chunks	Bit-by-bit
Basis	Transposition	Substitution
Complexity	Less Complex	More Complex
Speed	Slower	Faster
Algorithm Modes	ECB and CBC	CFB and OFB

Also read: The State of Blockchain Applications in Cybersecurity | eSecurity Planet

Where is Symmetric Encryption Used?

Symmetric encryption algorithms are best suited for scenarios where data needs to be securely transmitted or stored. They are efficient and require relatively low computational power, making them ideal for use in various real-world applications. Since information is all around us, symmetric encryption finds a plethora of use cases such as:

• Securing Communications: There is a lot of information circulating around the world. Symmetric encryption is widely used for encrypting emails, messages, and data transfers to protect sensitive information from malicious parties.
• Data Storage: Symmetric encryption can help encrypt files and databases to prevent unauthorized access to sensitive data.
• Payment Systems: With symmetric encryption, platforms can ensure the security of financial transactions, such as credit card payments and online banking.
• VPN Technology: VPNs create secure communication channels. They often employ symmetric key encryption to establish secure connections between remote users and corporate networks.

Advantages of Symmetric Encryption

Symmetric encryption offers several benefits, making it a popular choice for securing data. Some of the key advantages are:

• Speed and Efficiency
• Simplicity
• Low Resource Consumption

Speed and Efficiency

Symmetric encryption algorithms are faster than their asymmetric cousins. They require less computational power, making them ideal for real-time applications like video streaming and secure communications.

Simplicity

The use of a single key for both encryption and decryption simplifies the encryption process. This simplicity also makes it easier to implement symmetric encryption in various systems and applications.

Low Resource Consumption

Symmetric encryption algorithms are less resource-intensive, making them suitable for devices with limited computational power, such as smartphones and other devices.

Risks of Symmetric Encryption

While symmetric encryption provides robust security, it is not without its risks. The primary concern lies with the secret key. Some of the main risks include:

• Key Exhaustion
• Attribution Data
• Key Management

Key Exhaustion

Every use of the symmetric encryption key leaks some information that could potentially help attackers reconstruct the key. To mitigate this risk, key hierarchies and regular key rotation are essential.

Attribution Data

Symmetric keys lack embedded metadata like expiry dates or access control details. This calls for better control over key usage and requires a robust encryption key management system.

Key Management

Managing a small number of keys manually is feasible but it becomes impractical with larger organizations. For example, managing millions of EMV payment cards, each with multiple keys, necessitates a specialized key management solution.

Is Symmetric Key Cryptography Secure?

Symmetric key cryptography is generally considered secure but it’s not completely impervious to attacks. Some of the common attack methods include:

Cryptanalysis

Cryptanalysis involves analyzing the ciphertext to discover the key or decrypt the data without the key. While cryptanalysis can be effective against weaker algorithms, modern symmetric encryption algorithms like AES are designed to withstand such attacks.

Brute Force

Brute force attacks involve guessing the correct key by exhausting all possible combinations. The strength of symmetric encryption lies in the length of the key. Longer keys are exponentially more difficult to crack. For example, AES with a 256-bit key would require an enormous amount of computational power and time to brute-force, making it practically infeasible.

Security of Industry Standards

Industry standards like AES are considered secure for the foreseeable future. Even with advanced computing power, a brute force attack on AES-128 would take an estimated 149 trillion years to succeed, making it a reliable choice for securing data.

Closing Thoughts

Symmetric key cryptography plays a vital role in securing online transactions, communications, and data storage. Understanding exactly how this technology works can give you some peace of mind the next time you make an online purchase. While symmetric encryption is not without its challenges, particularly in key management, it remains a cornerstone of modern cybersecurity.

FAQs

What is a Symmetric Key?

A symmetric key is a secret key used in symmetric key cryptography for both encrypting and decrypting data. It must be kept secure to ensure the confidentiality of the encrypted information.

How Many Keys are Used with Symmetric Cryptography?

Symmetric cryptography uses a single key for both encryption and decryption. This key must be securely shared between the communicating parties to prevent a potential data breach.

What’s the Difference Between Symmetric and Asymmetric Cryptography?

The main difference between symmetric and asymmetric cryptography is in the number of keys that are being used. Symmetric cryptography uses a single key for both encryption and decryption, while asymmetric cryptography uses a pair of public and private keys. Symmetric encryption is generally faster but requires secure key management. Asymmetric encryption provides enhanced security but is slower to implement.