Symmetric encryption (or “private key” encryption) is the process of using a single key to both encrypt and decrypt data. It’s called “private key” because the use of a single encryption key necessitates that the key is always kept private.
Symmetric encryption is a widely-used form of cryptography that provides organizations and users with a faster, more readily deployable form of security for large data sets.
Symmetric encryption is often used for high-volume data processing where speed, efficiency, and complexity are important. However, due to its nature as a single-key solution, it presents several security challenges when it comes to actually sharing encrypted data. For this reason, symmetric encryption is often combined with asymmetric encryption (which uses different keys for encryption and decryption) in many modern secure communication protocols.
Symmetric key encryption, also known as symmetric cryptography, is a method of encrypting and decrypting data using the same cryptographic key This key is used for both encryption and decryption, which is why it’s called “symmetric”
In this article, I’ll explain in simple terms what symmetric key encryption is, how it works, its advantages and disadvantages, and some common examples My goal is to provide a beginner-friendly introduction to this fundamental encryption technique
How Does Symmetric Key Encryption Work?
The basic process for symmetric key encryption is:
- Alice wants to send a secret message to Bob
- Alice and Bob agree on a secret key
- Alice encrypts the plaintext message using the secret key
- Alice sends the encrypted ciphertext to Bob
- Bob decrypts the ciphertext back into plaintext using the same secret key
As you can see, the same key is used to encrypt and decrypt the message This is the “symmetric” part of symmetric key encryption
The secret key is often referred to as a “session key”, because it’s randomly generated for each communication session. Both parties need access to the session key in order to achieve secure communication.
Algorithms Used in Symmetric Key Encryption
There are several algorithms commonly used for symmetric key encryption:
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AES (Advanced Encryption Standard) – A very secure block cipher standardized by NIST. AES is widely adopted and used everywhere from HTTPS websites to encrypted hard drives.
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Blowfish – A strong public domain block cipher designed by Bruce Schneier.
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RC4 (Rivest Cipher 4) – A popular stream cipher used in protocols like WEP and WPA. However, RC4 is now considered insecure.
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DES (Data Encryption Standard) – An early block cipher standard. DES is no longer recommended for use because it’s not secure enough.
These algorithms are essentially mathematical functions for scrambling and unscrambling data. Each algorithm has tradeoffs between speed, security, and implementation complexity.
Types of Symmetric Key Encryption
There are two main categories of symmetric key encryption:
Stream Ciphers
- Encrypt data one bit or byte at a time
- Use a pseudorandom keystream to XOR with the plaintext
- Fast performance for real-time encryption
- Examples: RC4, Salsa20
Block Ciphers
- Encrypt data in fixed-size blocks (ex: 128-bit blocks)
- Common modes like CBC (cipher-block chaining) offer more security
- Slower than stream ciphers but widely used
- Examples: AES, Blowfish, DES
Block ciphers are generally preferred over stream ciphers due to their improved security. But stream ciphers can be useful for performance-critical tasks like real-time media streaming.
How Keys Are Exchanged
A challenge with symmetric key encryption is safely exchanging the secret key between participants. Common solutions include:
- Meet in person to exchange keys
- Use asymmetric encryption to send the symmetric key
- Use a key exchange protocol like Diffie-Hellman
- Rely on a public key infrastructure (PKI) and certificates
The secret key must remain confidential in order for symmetric key encryption to be secure. Keys should also be routinely updated and rotated for better security.
Advantages of Symmetric Key Encryption
- Fast performance – Encryption/decryption is very fast, even on large data
- Simple to implement – The algorithms tend to be less complex than public key encryption
- Provides confidentiality – Encrypted data is scrambled and unreadable
These advantages make symmetric key encryption well-suited for things like storage encryption, network data encryption, and message encryption.
Disadvantages of Symmetric Key Encryption
- Key distribution – Safely delivering the secret key is challenging
- No digital signatures – The recipient can’t verify the sender’s identity
- Not scalable – Each pair of participants requires a unique key
Public key encryption addresses some of these disadvantages. However, symmetric key encryption is still widely used in conjunction with public key encryption in many cryptosystems.
Common Examples and Applications
- Disk and file encryption (ex: VeraCrypt, BitLocker)
- Secure internet protocols (ex: TLS, HTTPS, SSH, PGP)
- Encrypted databases
- Encrypted messaging (ex: Signal, WhatsApp)
- Blockchain encryption (ex: AES used in Bitcoin)
Symmetric key encryption provides a fast and simple way to protect data in transit and at rest. It powers much of the encryption we rely on every day.
Symmetric cryptography continues to be foundational to security, despite the rise of public key encryption. Combining symmetric and asymmetric techniques allows us to benefit from the strengths of each.
References
- Symmetric encryption on Wikipedia
- Introduction to Cryptography by Christof Paar (video series)
- Practical Cryptography for Developers Book by Svetlin Nakov
How Does Symmetric Encryption Work?
Generally, encryption works by translating plain text into encoded (cipher) text using an algorithm and a secret key that is, theoretically, computationally infeasible to crack.
The general process of encryption works as follows:
- Key Generation: A secret key is generated using advanced mathematical equations that is used to encode data. This key is shared between sender and receiver to maintain the obfuscation of the data.
- Encryption: The encoding process uses complex algorithms and random environmental data (called “entropy”) to transform the original data is usually complex and involves multiple rounds of transformation to ensure the ciphertext is not easily decipherable without the key.
- Decryption: Upon receiving the data, the recipient uses the same key to decode the data–essentially, reversing the process with the key serving as the information needed to “unlock” that data.
Additionally, symmetric encryption algorithms will usually come in one of two forms:
- Stream Ciphers: Stream ciphers encrypt plaintext messages one bit at a time. They create an arbitrarily long keystream of bits, which is then combined with the plaintext bits one by one to produce the ciphertext.
- Block Ciphers: Block ciphers take a chunk, or block, of data and transform it. It then works through the data set block by block. Some block ciphers will repeatedly encrypt blocks for added security.
How Does Symmetric Encryption Compare to Asymmetric Encryption?
As the name implies, symmetric encryption works through a sense of symmetry between both ends (encryption and decryption) of the process. Asymmetric encryption is the opposite–it uses a different key for each process..
Accordingly, the biggest challenge in symmetric encryption is the secure distribution of the key. Since the same key is used by all parties involved, it must be shared… which presents its own problems in terms of securing keys.
Conversely, asymmetric encryption doesn’t require this kind of sharing. Only one key (the public key) is shared, and it is only used for encoding data and cannot be used to compromise encrypted data.
Asymmetric Encryption – Simply explained
FAQ
What is a symmetric key encryption?
What is the difference between asymmetric and symmetric key encryption?
What is an example of asymmetric key encryption?
Can symmetric key be decrypted?
What is symmetric encryption?
A Definition & Explanation To put this in the simplest terms possible, symmetric encryption is a type of encryption that uses the same key to encrypt and decrypt data. Both the sender and the recipient have identical copies of the key, which they keep secret and don’t share with anyone.
What is symmetric key cryptography?
The symmetric key cryptographic method employs one secret key for the operations of encryption and decryption. Substitution techniques provide two significant approaches, wherein elements (letters, characters) from the plaintext message are replaced with new elements according to the rules based on the secret key.
What is asymmetric key encryption?
In symmetric key encryption, the same key used to encrypt the data is used to decrypt the data. In asymmetric key encryption, one key is used to only encrypt the data (the public key) and another key is used to decrypt (the private key). First, let’s look at asymmetric key encryption with a simple analogy.
Why do we need a symmetric key encryption algorithm?
Scrambling plaintext into ciphertext is essential to ensure data cannot be read or used by the wrong people. Learn the basics of symmetric key encryption algorithms here. Cryptography is one area of information security that is well known but often not well understood.
