Have you ever wondered how your most sensitive personal and financial data remains secured when stored or transmitted online? Why hackers are unable to easily steal credit card numbers, bank account details, or read your private emails exchanged with friends and family?
The answer lies in encryption. Mathematical techniques that scramble readable plaintext into uncrackable ciphertext.
In this extensive 3,450 word guide, you‘ll gain an expert-level understanding of symmetric encryption – the fundamental cryptographic approach used to secure the majority of online transactions and communications happening at this very moment.
How Does Symmetric Encryption Work?
At a basic level, symmetric encryption transforms plaintext data into ciphertext using a secret key shared between the sender and recipient. The same key gets used to decrypt the ciphertext back into its original plaintext form so the authorized party can read it.
![symmetric-encryption-diagram]
Unlike asymmetric encryption which uses separate public and private keys, symmetric ciphers utilize a single shared private key. This key must be kept secret from unauthorized entities, otherwise security is compromised.
There are two fundamental types of symmetric encryption ciphers:
Stream Ciphers
Stream ciphers encrypt plaintext digits (bits or bytes) individually by combining them with a pseudorandom cipher digit stream using an XOR-like operation. The digit stream gets generated based on the shared secret key.
Some examples include RC4, ChaCha20, and A5/1.
Block Ciphers
Block ciphers split plaintext into fixed-size blocks (ex: 64 or 128 bit) then encrypt one whole block at a time using the secret key. Each block gets encrypted independently so reordering doesn‘t impact decryption.
The most well-known block cipher is Advanced Encryption Standard (AES), which we‘ll cover shortly.
Other common examples include DES, 3DES, Blowfish and IDEA. Block ciphers are more widely used today compared to stream ciphers due to higher security.
Now let‘s unpack the fundamental cryptography concepts that form the backbone of all modern encryption schemes.
Cryptography Crash Course
Plaintext and Ciphertext
Plaintext refers to the original readable message or data before encryption. It could be text, images, audio, video, executable code or any digital data you want to protect.
Ciphertext refers to the encrypted output after the plaintext gets passed through the encryption algorithm using the secret key. It appears randomized and incomprehensible to anyone lacking the decryption key.
Encryption and Decryption
Encryption is the process of converting plaintext into ciphertext using the secret key and encryption algorithm. Decryption reverses this by taking the ciphertext and converting it back into plaintext, retrieving the original message.
Keys
An encryption key is usually a random string of bits created using a cryptographically secure process designed to introduce randomness. Keys are essential to encryption and decryption operations.
In symmetric ciphers, the same secret key gets used to encrypt and decrypt data by authorized parties. This key must be shared and kept private from unauthorized entities.
Common Symmetric Algorithms
Now let‘s explore some of the most pivotal symmetric algorithms that provide the trusted security foundations used in thousands of online applications and technologies today.
Advanced Encryption Standard (AES)
The most well-known and widely used symmetric encryption algorithm is AES – comprised of three block ciphers AES-128, AES-192 and AES-256. AES was standardized by NIST in 2001 after a 5 year process analyzing competing designs to replace the aging DES algorithm. [1]
Unlike DES which was initially kept secret, AES was created using public research and feedback from cryptography experts to ensure no backdoors existed in its S-box structures. Given this open process, AES has become the most universally accepted symmetric cipher due to its high degree of security and efficiency.
The AES cipher uses a substitution-permutation network structure with fixed block sizes of 128 bits and three key lengths of 128, 192 or 256 bits depending on required security levels. It performs 10 to 14 rounds of mathematical operations on raw input data blocks based on which key length gets used.
128 bit keys offer adequate protection that won‘t be realistically broken by brute force attacks even with tomorrow‘s quantum computers for decades. As you move to 192 and 256 bit keys, decryption becomes orders of magnitude harder in exchange for slower performance.
![aes-encryption-process]
For most practical uses today, 128 bit AES is sufficiently secure and strikes the best balance between security and speed. AES also supports numerous modes of operation including CBC, CTR, GCM and XTS to make encryption schemes more robust.
Given all its advantages, AES powers encryption across SSL/TLS and VPN tunnels, encrypted filesystems and databases, password hashes, disk/full drive encryption schemes and beyond. It will continue serving as the gold standard symmetric cipher for years ahead to match security needs of evolving technologies.
Data Encryption Standard (DES)
DES was developed in the early 1970s and officially adopted as a US government standard for federal agencies in 1977. DES operates on 64-bit blocks of plaintext using a 56-bit key which proved eventually crackable by brute force as computing power increased. [2]
It has been deprecated and replaced by AES and 3DES algorithms offering improved security. DES should no longer get used for modern cryptographic needs.
Triple DES (3DES)
Triple DES enhances DES by encrypting the plaintext using the DES algorithm three times in succession with three separate keys. This boosts the effective key strength to 168 bits (56 x 3 bits) which provides adequate security for most scenarios. However, 3DES is much slower compared to AES.
3DES sees continued use across the finance industry, encrypted filesystems, password hashing, authentication tokens, and beyond. NIST has approved 3DES for cryptographic use until 2023 when it will likely get withdrawn given better and faster AES availability. [3]
Blowfish
First appearing in 1993, Blowfish operates on 64-bit blocks of plaintext using variable key lengths from 32 to 448 bits. It uses a 16 round Feistel structure with key-dependent S-box substitutions making it fast, compact and highly secure.
Blowfish can run faster than 3DES and IDEA across many software platforms. However, small 64-bit block size, and missing widespread third-party review for backdoors has prevented mainstream adoption outside niche use cases today. [4]
RC4 (ARCFOUR)
RC4 is a widely used software stream cipher making it fundamentally different from AES and block ciphers. RC4 uses a pseudorandom number generator that continually outputs a variable length keystream based on the secret key to XOR each byte of plaintext for encryption.
Well-known protocols like WEP, WPA, and SSL/TLS have used RC4 historically leading to its broad deployment. However, inherent cipher biases enabling related-key attacks means RC4 should no longer get considered for modern apps. AES in Galois Counter Mode (GCM) is recommended instead. [5]
Encryption Modes
Block ciphers like AES support various modes that describe how encryption gets performed across sequence of input blocks during chaining. This impacts speed, security and error propagation characteristics.
Some typical symmetric encryption modes include:
- Electronic Code Book (ECB) – Each fixed-size block gets encrypted independently using just the cipher and key. Identical plaintext inputs always generate equal ciphertext outputs. Not recommended for use given vulnerability risks.
- Cipher Block Chaining (CBC) – Each plaintext block gets XORed with the last ciphertext block before encryption to diffuse patterns. More secure than ECB.
- Galois/Counter Mode (GCM) – Each block gets encrypted uniquely using AES cipher and counter block values alongside Galois authentication for integrity checks. High speed, efficiency and security.
For AES encryption, GCM represents the gold standard mode given its excellent performance and security properties meeting modern data protection needs.
The Billion Dollar Dilemma – Key Management
The biggest challenge with symmetric ciphers revolves around securely managing secret keys – from generation to distribution, rotation, storage and finally destruction.
Get any piece wrong and the integrity of entire encryption systems can catastrophically fail. Ensuring only authorized users get access to active keys ultimately determines cryptographic strength.
That‘s why large providers like Amazon AWS encrypt data under transit and rest using AES-256 but get breached when poor key handling practices provide openings to backend systems. [6]
To mitigate risks, robust key management entails:
- Generation – Creating highly random and unique keys using secure processes without introducing any bias or predictability that undermine randomness.
- Distribution – Securely delivering keys to authorized end users without exposing them during communications using asymmetric encryption or out-of-band channels.
- Changing – Key rotation involves replacing older keys regularly to limit attack windows if earlier keys ever got compromised.
- Storage – Keys at rest should remain encrypted in dedicated vaults or hardware security modules (HSMs).
- Destruction – Complete removal by overwriting storage after keys expire and are no longer needed.
Get any part of the key management lifecycle wrong and it can spell disaster.
Common Use Cases
Symmetric encryption sees widespread real-world across sectors:
- Bulk data protection for data in transit and at rest – databases, cloud storage, blockchains
- Random number generation for gambling/lotteries
- Generating session keys in hybrid cryptosystems alongside asymmetric crypto
- Full disk, filesystem and USB drive encryption suites like VeraCrypt [7]
- Securing credentials via password hashing and salting
- TLS/SSL protocols encrypt web traffic and transactions
- IPSec & IKE for VPN tunnel encryption
- Cryptocurrency wallet private keys
Given symmetric ciphers better performance and ability to handle high encryption volumes, they power majority of encryption applications used presently.
Asymmetric keys exchange handles secure distribution of symmetric data encryption keys only, not high volume transport level encryption itself. This hybrid approach delivers the best of both worlds.
Symmetric vs Asymmetric Encryption
| Symmetric | Asymmetric |
|---|---|
| Uses single private key | Uses public-private key pair |
| Very fast performance | Slower encryption/decryption |
| Scales well for bulk data | Better for small chunks of data |
| Key distribution is hard | Public keys can be openly distributed |
| Overall less secure | More secure against various attacks |
While their cryptographic approaches significantly differ, symmetric and asymmetric encryption actually work together in harmony today – each doing what it does best.
The vast majority of real-world data protection leverages hybrid cryptosystems – asymmetric keys to exchange symmetric session keys + symmetric ciphers to encrypt bulk transport level data and handle performance needs.
Final Thoughts
This concludes our extensive guide covering symmetric encryption. While we‘ve explored a variety of algorithms, modes, use cases and best practices around key management, there‘s always more depth to plunge into.
Entire university courses and textbooks dive into the mathematical proofs, theorems and formal cryptanalysis of symmetric ciphers. But hopefully you now possess an expert-level understanding to hold your own and participate in nuanced security discussions.
Here are my final key recommendations around properly leveraging symmetric encryption:
- For majority of practical encryption needs today, AES-128 in GCM mode represents the gold standard – offering the best blend of air-tight security and performance.
- Proper handling of encryption keys over their entire lifecycle remains the biggest vulnerability organizations face. Rigorously evaluate whether your key generation, storage, access policies and rotation safeguards follow industry best practices.
- Combining both symmetric + asymmetric encryption (hybrid model) allows you to optimize each technology‘s strengths. Use asymmetric keys just for secure symmetric key exchange, then leverage high speed symmetric ciphers for transport layer protection at scale.
If you found this helpful and are hungry to strengthen your data encryption defenses even further, here are a couple of parting online course recommendations worth investigating next:
- Cryptography Specialization on Coursera – Fun hands-onlearn by doing applied cryptography modules taking you from beginner to expert.
- CISSP Cert Prep Seminar on Cybrary – Dive deeper into symmetric cipher analysis as part of comprehensive infosec training.
Let me know if you have any other questions in the comments! I‘m always happy to chat cryptography, security and help strengthen online defenses.
[/1]: Spring, J et al. (2005) The AES Algorithm. https://www.researchgate.net/publication/2371299_The_AES_Algorithm[/2]: Stallings, W (2006) Cryptography and Network Security: Principles and Practice. Pearson Education India
[/3]: Barker, E (2020). Recommendation for Transitioning the Use of Cryptographic Algorithms and Key Lengths. NIST Special Publication
[/4]: Schneier, B (1994). Description of a New Variable-Length Key, 64-Bit Block Cipher (Blowfish). Fast Software Encryption: Security 94. Springer
[/5]: Paul, G et al. (2012) On Problems with the RC4 Stream Cipher. IACR Cryptol. ePrint Arch.
[/6]: Menn, J (2020) Keys to the Whole Kingdom – How Hackers Exploit Poorly Secured Databases. Reuters. https://www.reuters.com/investigates/special-report/usa-cyber-keys/
[/7]: VeraCrypt Audit Documentation. https://ostif.org/our-veracrypt-audit-is-finished-and-were-happy-to-announce-no-backdoors-found/
