Understanding AES Encryption Modes: AES-GCM, AES-CBC, AES-CTR

Introduction

In the realm of encryption, the Advanced Encryption Standard (AES) is a cornerstone of modern cryptography. AES offers multiple modes of operation, each tailored to specific security and performance needs. Among the most widely used modes are AES-GCM (Galois/Counter Mode), AES-CBC (Cipher Block Chaining), and AES-CTR (Counter Mode). This blog post explores these modes in detail, highlighting their differences, performance characteristics, key sizes, resource usage, and ideal use cases.

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AES-GCM (Galois/Counter Mode)

AES-GCM is a state-of-the-art encryption mode that combines confidentiality and integrity in a single operation. It leverages the Counter Mode (CTR) for encryption and the Galois Mode for authentication, making it both efficient and secure. AES-GCM is widely adopted in modern applications, including secure communication protocols like TLS and IPsec.

async function encryptAESGCM(plainText, key) {
  const iv = window.crypto.getRandomValues(new Uint8Array(12)); // 12-byte IV for GCM

  const ciphertext = await window.crypto.subtle.encrypt(
    {
      name: "AES-GCM",
      iv: iv,
    },
    key,
    new TextEncoder().encode(plainText),
  );

  return { ciphertext, iv };
}

How AES-GCM Works:

• Encryption: Data is encrypted using Counter Mode (CTR), which generates a unique keystream for each block.
• Authentication: A Galois Message Authentication Code (GMAC) is computed to ensure the integrity of both the ciphertext and any additional authenticated data (AAD).

Key Features:

• Provides confidentiality, integrity, and authentication.
• Highly efficient and suitable for high-speed applications.
• Requires a 12-byte Initialization Vector (IV) for optimal security.

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AES-CBC (Cipher Block Chaining)

AES-CBC is a classic block cipher mode that uses an Initialization Vector (IV) to encrypt data in fixed-size blocks. Each block of plaintext is XORed with the previous ciphertext block before encryption, ensuring that identical plaintext blocks produce different ciphertext blocks. While AES-CBC provides strong confidentiality, it lacks built-in integrity and authentication mechanisms.

async function encryptAESCBC(plainText, key) {
  const iv = window.crypto.getRandomValues(new Uint8Array(16)); // 16-byte IV for CBC

  const ciphertext = await window.crypto.subtle.encrypt(
    {
      name: "AES-CBC",
      iv: iv,
    },
    key,
    new TextEncoder().encode(plainText),
  );

  return { ciphertext, iv };
}

How AES-CBC Works:

• Encryption: Plaintext is divided into fixed-size blocks. Each block is XORed with the previous ciphertext block and then encrypted.
• Decryption: The process is reversed, using the IV and ciphertext blocks to recover the original plaintext.

Key Features:

• Provides confidentiality but no integrity or authentication.
• Requires padding for plaintexts that are not a multiple of the block size.
• Commonly used in legacy systems and applications like file and database encryption.

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AES-CTR (Counter Mode)

AES-CTR transforms a block cipher into a stream cipher by encrypting successive values of a counter and XORing the resulting keystream with the plaintext. This mode is highly efficient and allows for parallel processing, making it ideal for high-performance applications like real-time data streaming.

async function encryptAESCTR(plainText, key) {
  const counter = window.crypto.getRandomValues(new Uint8Array(16)); // 16-byte counter for CTR

  const ciphertext = await window.crypto.subtle.encrypt(
    {
      name: "AES-CTR",
      counter: counter,
      length: 64, // Length of the counter in bits
    },
    key,
    new TextEncoder().encode(plainText),
  );

  return { ciphertext, counter };
}

How AES-CTR Works:

• Encryption: A counter value is encrypted to produce a keystream, which is XORed with the plaintext to generate ciphertext.
• Decryption: The same counter value is used to regenerate the keystream, which is XORed with the ciphertext to recover the plaintext.

Key Features:

• Provides confidentiality but no integrity or authentication.
• Highly efficient and suitable for parallel processing.
• Ideal for applications requiring low latency, such as streaming media and real-time communication.

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Comparison Table

Feature	AES-GCM	AES-CBC	AES-CTR
Confidentiality	Yes	Yes	Yes
Authentication	Yes	No	No
Integrity	Yes	No	No
Performance	High	Medium	High
Key Size	128, 192, or 256 bits	128, 192, or 256 bits	128, 192, or 256 bits
Resource Usage	Moderate (slightly higher)	Moderate	Low
Usage	IoT, messaging, network protocols	Legacy systems, non-critical applications	High-performance, parallel processing
Use Cases	Secure communication, TLS, IPsec	File encryption, database encryption	Streaming data, real-time encryption

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Conclusion

Choosing the right AES encryption mode depends on your application’s specific requirements:

• AES-GCM is the best choice when confidentiality, integrity, and authentication are all required. It is widely used in secure communication protocols and modern applications.
• AES-CBC is suitable for data-at-rest encryption and legacy systems but lacks built-in integrity and authentication.
• AES-CTR excels in high-performance scenarios where low latency and parallel processing are critical, such as real-time data streaming.

By understanding the strengths and limitations of each mode, you can make informed decisions to enhance your encryption strategy and ensure the security of your data.