<?php
/**
 * Settings encryption at rest.
 *
 * @package Bookline
 */

declare( strict_types=1 );

namespace Bookline\Infrastructure\Crypto;

use RuntimeException;

/**
 * Encrypts and decrypts secret setting values using AES-256-GCM.
 *
 * Key material comes from `BOOKLINE_ENCRYPTION_KEY` when defined, otherwise from
 * WordPress authentication salts. Sites that rotate salts should define the
 * constant so existing ciphertext remains readable.
 *
 * @since 0.1.0
 */
final class Encryption {

	private const CIPHER = 'aes-256-gcm';
	private const PREFIX = 'bookline_enc_v1:';

	/**
	 * Derived encryption key (binary).
	 *
	 * @var string
	 */
	private string $key;

	/**
	 * Build an encryption service from key material or WordPress salts.
	 *
	 * @param string|null $key_material Optional raw key material for tests.
	 */
	public function __construct( ?string $key_material = null ) {
		$this->key = $this->derive_key( $key_material ?? $this->resolve_key_material() );
	}

	/**
	 * Encrypt a plaintext secret. Empty strings are returned unchanged.
	 *
	 * @param string $plaintext Plaintext value.
	 * @return string Ciphertext with version prefix, or empty string.
	 *
	 * @throws RuntimeException When OpenSSL fails.
	 *
	 * @since 0.1.0
	 */
	public function encrypt( string $plaintext ): string {
		if ( '' === $plaintext ) {
			return '';
		}

		$iv_length = openssl_cipher_iv_length( self::CIPHER );
		if ( false === $iv_length || $iv_length < 1 ) {
			throw new RuntimeException( 'Unable to determine cipher IV length.' );
		}

		$iv = random_bytes( $iv_length );
		$tag = '';

		$cipher = openssl_encrypt(
			$plaintext,
			self::CIPHER,
			$this->key,
			OPENSSL_RAW_DATA,
			$iv,
			$tag,
			'',
			16
		);

		if ( false === $cipher ) {
			throw new RuntimeException( 'Encryption failed.' );
		}

		return self::PREFIX . bin2hex( $iv . $tag . $cipher );
	}

	/**
	 * Decrypt a ciphertext produced by encrypt(). Plain values pass through.
	 *
	 * @param string $ciphertext Possibly encrypted value.
	 * @return string Plaintext.
	 *
	 * @throws RuntimeException When decryption fails.
	 *
	 * @since 0.1.0
	 */
	public function decrypt( string $ciphertext ): string {
		if ( '' === $ciphertext || ! str_starts_with( $ciphertext, self::PREFIX ) ) {
			return $ciphertext;
		}

		$raw = hex2bin( substr( $ciphertext, strlen( self::PREFIX ) ) );
		if ( false === $raw ) {
			throw new RuntimeException( 'Encrypted payload is not valid hex.' );
		}

		$iv_length = openssl_cipher_iv_length( self::CIPHER );
		if ( false === $iv_length || $iv_length < 1 ) {
			throw new RuntimeException( 'Unable to determine cipher IV length.' );
		}

		$tag_length = 16;
		$min_length = $iv_length + $tag_length + 1;
		if ( strlen( $raw ) < $min_length ) {
			throw new RuntimeException( 'Encrypted payload is truncated.' );
		}

		$iv         = substr( $raw, 0, $iv_length );
		$tag        = substr( $raw, $iv_length, $tag_length );
		$encrypted  = substr( $raw, $iv_length + $tag_length );
		$plaintext  = openssl_decrypt(
			$encrypted,
			self::CIPHER,
			$this->key,
			OPENSSL_RAW_DATA,
			$iv,
			$tag
		);

		if ( false === $plaintext ) {
			throw new RuntimeException( 'Decryption failed.' );
		}

		return $plaintext;
	}

	/**
	 * Whether a stored value looks like Bookline ciphertext.
	 *
	 * @param string $value Stored value.
	 *
	 * @since 0.1.0
	 */
	public function is_encrypted( string $value ): bool {
		return str_starts_with( $value, self::PREFIX );
	}

	/**
	 * Resolve key material from the override constant or WordPress salts.
	 *
	 * @throws RuntimeException When no key material is available.
	 */
	private function resolve_key_material(): string {
		if ( defined( 'BOOKLINE_ENCRYPTION_KEY' ) ) {
			$override = constant( 'BOOKLINE_ENCRYPTION_KEY' );
			if ( is_string( $override ) && '' !== $override ) {
				return $override;
			}
		}

		$parts = [];
		foreach ( [ 'AUTH_KEY', 'SECURE_AUTH_KEY', 'LOGGED_IN_KEY', 'NONCE_KEY', 'AUTH_SALT', 'SECURE_AUTH_SALT' ] as $constant ) {
			if ( defined( $constant ) ) {
				$value = constant( $constant );
				if ( is_string( $value ) && '' !== $value ) {
					$parts[] = $value;
				}
			}
		}

		if ( [] === $parts ) {
			throw new RuntimeException(
				'No encryption key available. Define BOOKLINE_ENCRYPTION_KEY or WordPress salts.'
			);
		}

		return implode( '|', $parts );
	}

	/**
	 * Derive a binary key of fixed length.
	 *
	 * @param string $material Key material.
	 */
	private function derive_key( string $material ): string {
		return hash( 'sha256', $material, true );
	}
}