AES-256-GCM encryption/decryption class in PHP

AesGcmCrypto.php

<?php

declare(strict_types=1);

class AesGcmCrypto {

	private const CIPHER = 'aes-256-gcm';

	/** 96-bit IV is standard for GCM. */
	private const IV_LENGTH = 12;

	/** Tag with 128-bits. */
	private const TAG_LENGTH = 16;

	/**
	 * @param string $key 32-byte (256-bit) secret key.
	 */
	public function __construct(
		private string $key,
	) {
		if (strlen($this->key) !== 32) {
			throw new \InvalidArgumentException(
				'Key must be 32 bytes (256 bits)',
			);
		}
	}

	/**
	 * Encrypts plaintext with optional AAD.
	 *
	 * @param ?string $aad Optional additional authenticated data.
	 * @return string Base64-encoded string containing IV + ciphertext + tag.
	 */
	public function encrypt(string $plaintext, string $aad = ''): string {

		$iv = random_bytes(self::IV_LENGTH);
		$tag = '';

		$ciphertext = openssl_encrypt(
			$plaintext,
			self::CIPHER,
			$this->key,
			OPENSSL_RAW_DATA,
			$iv,
			$tag,
			$aad,
			self::TAG_LENGTH,
		);

		if ($ciphertext === false) {
			throw new \RuntimeException('Encryption failed');
		}

		// Combine IV + ciphertext + tag for transmission.
		return base64_encode($iv . $ciphertext . $tag);

	}

	/**
	 * Decrypts base64-encoded encrypted message with optional AAD.
	 *
	 * @param string $encoded Base64-encoded IV + ciphertext + tag.
	 * @param ?string $aad Optional additional authenticated data.
	 * @return string Decrypted plaintext.
	 */
	public function decrypt(string $encoded, string $aad = ''): string {

		$data = base64_decode($encoded, true);

		if (
			$data === false
			|| strlen($data) < (self::IV_LENGTH + self::TAG_LENGTH)
		) {
			throw new \InvalidArgumentException('Invalid encrypted input');
		}

		$iv = substr($data, 0, self::IV_LENGTH);
		$tag = substr($data, -self::TAG_LENGTH);
		$ciphertext = substr($data, self::IV_LENGTH, -self::TAG_LENGTH);

		$plaintext = openssl_decrypt(
			$ciphertext,
			self::CIPHER,
			$this->key,
			OPENSSL_RAW_DATA,
			$iv,
			$tag,
			$aad,
		);

		if ($plaintext === false) {
			throw new \RuntimeException(
				'Decryption failed or authentication failed',
			);
		}

		return $plaintext;

	}

}