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Aes GcmEdit

AES-GCM, or Advanced Encryption Standard in Galois/Counter Mode, is a widely deployed authenticated encryption mode that provides both confidentiality and data integrity in a single, efficient operation. Built on the block cipher Advanced Encryption Standard and the Galois/Counter Mode of operation (Galois/Counter Mode), it is designed to be fast in hardware and software alike, making it a common choice for securing communications and data storage. In practice, AES-GCM encrypts plaintext to produce ciphertext and, at the same time, computes an authentication tag to detect tampering, ensuring that altered data is detectable upon decryption.

The mode is favored for its combination of performance and security guarantees. It is used in many security protocols and standards, including Transport Layer Security, IPsec, and SSH, and is supported by a wide ecosystem of cryptographic libraries and hardware accelerators. The authentication tag, typically 128 bits in strength but configurable in practice, helps prevent forgery or modification of the ciphertext and any associated data (AAD).

Overview

AES-GCM combines two ideas: counter-mode encryption to provide confidentiality, and a Galois field-based universal hash to provide integrity. While AES supplies a strong, widely analyzed block cipher, GCM supplies a mechanism to authenticate both the ciphertext and optional associated data in a way that is tightly integrated with encryption. This integration allows a single pass over the data to both encrypt and authenticate, which improves performance on modern processors that support AES instructions and hardware acceleration.

Key concepts in AES-GCM include: - Key management: AES-GCM uses a secret key (commonly 128, 192, or 256 bits) shared between communicating parties. The key must be kept secure to preserve the confidentiality and integrity guarantees. - Nonce (IV): Each encryption operation uses a unique nonce. Reusing a nonce with the same key is catastrophic for security, because it enables an adversary to recover plaintext and forge messages. The design of nonce handling is a central practical concern in deployments. - Counter mode: Within AES-GCM, a counter-mode construction is used to turn AES into a stream-like cipher for encryption. This enables parallelizable encryption and decryption. - GHASH: The authentication component uses a polynomial in a Galois field (GHASH) to produce an authentication tag that binds the ciphertext and any AAD. This tag helps detect tampering. - Authentication tag: The tag verifies integrity. It is typically 128 bits in strength but can be chosen to be shorter in some configurations, with trade-offs in security margins.

These facets come together to give AES-GCM its AEAD (authenticated encryption with associated data) properties. For a deeper dive into the underlying math and construction, see GHASH and Authenticated encryption with associated data.

Technical details

Encryption and decryption process

  • Encryption: A unique nonce is chosen for each message. The mode encrypts the plaintext using a counter-derived keystream based on AES, while simultaneously computing the authentication tag using GHASH. The output consists of the ciphertext and the tag.
  • Decryption: The recipient recomputes the authentication tag from the received ciphertext and any AAD. If the tag verification passes, the plaintext is recovered; otherwise, tampering is indicated and the data should be discarded.

Nonce management and security

  • Nonces must be unique per key for every encryption operation. Reuse can lead to catastrophic security failures, including the exposure of plaintexts and forged messages.
  • Common practice is to use a 96-bit (12-byte) nonce with a counter, or to derive nonces deterministically from a per-message counter combined with a per-key seed.
  • Libraries and protocols often implement nonce management patterns to reduce the risk of reuse, such as counter exhaustion checks and explicit nonce generation mechanisms.

Performance and implementations

  • AES-GCM excels on modern processors that provide hardware acceleration for Advanced Encryption Standard, such as AES-NI on x86 architectures.
  • It is widely implemented in cryptographic libraries and security protocols. Notable environments include the SSL/TLS stack in OpenSSL, LibreSSL, and BoringSSL, as well as protocol implementations like TLS and IPsec.
  • Alternative AEAD modes, such as ChaCha20-Poly1305, are preferred in some contexts (for example, when hardware support for AES is limited), but AES-GCM remains the standard in many Internet security deployments due to its maturity and performance on common hardware.

Security considerations and debates

AES-GCM is robust when used correctly, but it has well-known failure modes primarily related to nonce misuse. If a nonce is ever reused with the same key, attackers can often recover plaintext or forge messages, which undermines both confidentiality and integrity. Consequently, practical guidance emphasizes strict nonce management, including ensuring nonces are never repeated for a given key and that nonce generation is auditable and resistant to collision.

The choice between AES-GCM and other AEAD schemes is a subject of ongoing discussion in security communities. Some environments favor alternatives like ChaCha20-Poly1305 for reasons including software simplicity, constant-time properties, and performance on platforms lacking AES hardware acceleration. Others rely on the maturity, ecosystem support, and interoperability of AES-GCM, particularly in widely adopted standards like TLS and IPsec.

Questions about policy and governance around cryptography—such as debates over backdoors, key escrow, or government access—intersect with AES-GCM in the sense that secure, well-vetted cryptographic primitives underpin secure communications. The technical community generally emphasizes that the strongest security comes from proven designs with careful key and nonce management, rather than weakened algorithms or bypasses that could jeopardize privacy and commerce.

Applications and adoption

  • In TLS, AES-GCM has become a dominant mode for securing web traffic, contributing to confidentiality and integrity guarantees for HTTPs connections.
  • In IPsec and other VPN technologies, AES-GCM is used to protect data in transit across networks.
  • In secure shell implementations, AES-GCM provides authenticated encryption for remote sessions and file transfers.
  • In data storage, AES-GCM is used to protect artifacts at rest in systems that require both confidentiality and integrity.

Common implementations aim to provide secure defaults, with careful nonce handling and explicit tag verification during decryption. Developers also consider interoperability with legacy systems that may use older modes or different tag lengths, balancing security with compatibility.

See also