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<?php
/**
* Signature class file.
*
* @package Activitypub
*/
namespace Activitypub;
use Activitypub\Signature\Http_Message_Signature;
use Activitypub\Signature\Http_Signature_Draft;
/**
* ActivityPub Signature Class.
*
* @author Matthias Pfefferle
* @author Django Doucet
*/
class Signature {
/**
* Initialize the class.
*/
public static function init() {
\add_filter( 'http_request_args', array( self::class, 'sign_request' ), 0, 2 ); // Ahead of all other filters, so signature is set.
\add_filter( 'http_response', array( self::class, 'maybe_double_knock' ), 10, 3 );
}
/**
* Generate a new RSA key pair for signing HTTP requests.
*
* Does not persist anything — callers are responsible for storing the keys.
*
* @since 9.1.0
*
* @return array The key pair with 'private_key' and 'public_key', both null on failure.
*/
public static function generate_key_pair() {
$config = array(
'digest_alg' => 'sha512',
'private_key_bits' => 2048,
'private_key_type' => \OPENSSL_KEYTYPE_RSA,
);
$key = \openssl_pkey_new( $config );
$private_key = null;
$detail = array();
if ( $key ) {
\openssl_pkey_export( $key, $private_key );
$detail = \openssl_pkey_get_details( $key );
}
// Check if keys are valid.
if (
empty( $private_key ) || ! \is_string( $private_key ) ||
! isset( $detail['key'] ) || ! \is_string( $detail['key'] )
) {
return array(
'private_key' => null,
'public_key' => null,
);
}
return array(
'private_key' => $private_key,
'public_key' => $detail['key'],
);
}
/**
* Get the key pair stored in an option, migrating a legacy pair or generating a new one on first use.
*
* @since 9.1.0
*
* @param string $option_key The option name the key pair is stored in.
* @param callable|null $legacy_callback Optional. Callback that returns a legacy key pair to migrate, or false. Default null.
*
* @return array The key pair with 'private_key' and 'public_key'.
*/
public static function get_key_pair( $option_key, $legacy_callback = null ) {
$key_pair = \get_option( $option_key );
if ( $key_pair ) {
return $key_pair;
}
$key_pair = $legacy_callback ? $legacy_callback() : false;
if ( ! $key_pair ) {
$key_pair = self::generate_key_pair();
// Only persist valid keys.
if ( empty( $key_pair['private_key'] ) ) {
return $key_pair;
}
}
// `update_option()` also overwrites a corrupted-but-present row, which `add_option()` would silently skip.
\update_option( $option_key, $key_pair );
return $key_pair;
}
/**
* Sign an HTTP Request.
*
* @param array $args An array of HTTP request arguments.
* @param string $url The request URL.
*
* @return array Request arguments with signature headers.
*/
public static function sign_request( $args, $url ) {
// Bail if there's nothing to sign with.
if ( ! isset( $args['key_id'], $args['private_key'] ) ) {
return $args;
}
if ( '1' === \get_option( 'activitypub_rfc9421_signature' ) && self::could_support_rfc9421( $url ) ) {
$signature = new Http_Message_Signature();
} else {
$signature = new Http_Signature_Draft();
}
return $signature->sign( $args, $url );
}
/**
* Verifies the http signatures
*
* On success the verified keyId is returned (a truthy string), so callers can bind it to
* the activity actor without re-parsing headers, which cannot tell which signature label
* actually validated. Pass/fail callers should branch on {@see is_wp_error()} as before.
*
* @since 9.0.0 Returns the verified keyId on success instead of `true`.
*
* @param \WP_REST_Request|array $request The request object or $_SERVER array.
*
* @return string|\WP_Error The verified keyId on success, WP_Error on failure.
*/
public static function verify_http_signature( $request ) {
if ( \is_object( $request ) ) { // REST Request object.
$body = $request->get_body();
$headers = $request->get_headers();
$headers['(request-target)'][0] = \strtolower( $request->get_method() ) . ' ' . self::get_route( $request );
} else {
$headers = self::format_server_request( $request );
$headers['(request-target)'][0] = \strtolower( $headers['request_method'][0] ) . ' ' . $headers['request_uri'][0];
}
$signature = isset( $headers['signature_input'] ) ? new Http_Message_Signature() : new Http_Signature_Draft();
return $signature->verify( $headers, $body ?? null );
}
/**
* Extract the signing keyId that {@see Signature::verify_http_signature()} would verify against.
*
* The returned keyId is only trustworthy if it identifies the key the signature is
* actually checked with, so this mirrors the verifier's header choice rather than
* scanning headers in an arbitrary order:
*
* - When a `Signature-Input` header is present the RFC 9421 verifier is used, so the
* keyId is taken from there and a draft `Signature` header (which the verifier ignores)
* is not consulted. The RFC 9421 verifier accepts whichever of several signature labels
* validates, so a `Signature-Input` carrying more than one keyId is ambiguous: we cannot
* know in advance which key will verify and must not guess, so `null` is returned.
* - Otherwise the draft HTTP Signatures form is used, taking the first `keyId` from the
* `Signature` header or, failing that, the `Authorization` header — matching the draft
* verifier, which reads `signature ?? authorization`.
*
* @since 9.0.0
*
* @param \WP_REST_Request $request The request object.
*
* @return string|null The keyId, or null when none is present or the choice is ambiguous.
*/
public static function get_key_id( $request ) {
$signature_input = $request->get_header( 'signature-input' );
if ( $signature_input ) {
/*
* keyid is a `;`-delimited parameter whose value may be quoted or unquoted.
* Anchoring on `;` (or string start) avoids matching a `keyid=` substring inside
* another parameter's value. Count every label's keyId: more than one is ambiguous.
*/
$count = \preg_match_all( '/(?:^|;)\s*keyid="?([^";,\s]+)/i', $signature_input, $matches );
return 1 === $count ? $matches[1][0] : null;
}
// A draft signature may arrive in the Signature header or, less commonly, Authorization.
$signature = $request->get_header( 'signature' );
if ( ! $signature ) {
$signature = $request->get_header( 'authorization' );
}
if ( $signature && \preg_match( '/keyId="([^"]+)"/i', $signature, $matches ) ) {
return $matches[1];
}
return null;
}
/**
* If a request with RFC-9421 signature fails, we try again with the Draft Cavage signature.
*
* @param array $response HTTP response.
* @param array $args HTTP request arguments.
* @param string $url The request URL.
*
* @return array The HTTP response.
*/
public static function maybe_double_knock( $response, $args, $url ) {
// Bail if it didn't use an RFC-9421 signature or there's nothing to sign with.
if ( ! isset( $args['key_id'], $args['private_key'], $args['headers']['Signature-Input'] ) ) {
return $response;
}
$response_code = \wp_remote_retrieve_response_code( $response );
// Fall back to Draft Cavage signature for any 4xx responses.
if ( $response_code >= 400 && $response_code < 500 ) {
unset( $args['headers']['Signature'], $args['headers']['Signature-Input'], $args['headers']['Content-Digest'] );
self::rfc9421_add_unsupported_host( $url );
$args = ( new Http_Signature_Draft() )->sign( $args, $url );
$response = \wp_safe_remote_request( $url, $args );
}
return $response;
}
/**
* Formats the $_SERVER to resemble the WP_REST_REQUEST array,
* for use with verify_http_signature().
*
* @param array $server The $_SERVER array.
*
* @return array $request The formatted request array.
*/
public static function format_server_request( $server ) {
$headers = array();
foreach ( $server as $key => $value ) {
$key = \str_replace( 'http_', '', \strtolower( $key ) );
$headers[ $key ][] = \wp_unslash( $value );
}
return $headers;
}
/**
* Returns route.
*
* @param \WP_REST_Request $request The request object.
*
* @return string
*/
private static function get_route( $request ) {
// Check if the route starts with "index.php".
if ( \str_starts_with( $request->get_route(), '/index.php' ) || ! \rest_get_url_prefix() ) {
$route = $request->get_route();
} else {
$route = '/' . \rest_get_url_prefix() . '/' . \ltrim( $request->get_route(), '/' );
}
// Fix route for subdirectory installations.
$path = \wp_parse_url( \get_home_url(), PHP_URL_PATH );
if ( \is_string( $path ) ) {
$path = \trim( $path, '/' );
}
if ( $path ) {
$route = '/' . $path . $route;
}
/*
* Append the query string. Peers sign the full request-target including
* the query (see Http_Signature_Draft::sign()), so the reconstructed
* value has to match byte-for-byte. Use the raw REQUEST_URI instead of
* re-encoding the parsed query params, re-encoding could change the
* percent-encoding or parameter order and break the signature.
*/
// phpcs:ignore WordPress.Security.ValidatedSanitizedInput
$query = (string) \wp_parse_url( $_SERVER['REQUEST_URI'] ?? '', \PHP_URL_QUERY );
if ( '' !== $query ) {
$route .= '?' . $query;
}
return $route;
}
/**
* Check if RFC-9421 signature could be supported.
*
* @param string $url The URL to check.
*
* @return bool True, if RFC-9421 signature could be supported, false otherwise.
*/
private static function could_support_rfc9421( $url ) {
$host = \wp_parse_url( $url, \PHP_URL_HOST );
$list = \get_option( 'activitypub_rfc9421_unsupported', array() );
if ( isset( $list[ $host ] ) ) {
if ( $list[ $host ] > \time() ) {
return false;
}
unset( $list[ $host ] );
\update_option( 'activitypub_rfc9421_unsupported', $list );
}
return true;
}
/**
* Set RFC-9421 signature unsupported for a given host.
*
* @param string $url The URL to set.
*/
private static function rfc9421_add_unsupported_host( $url ) {
$list = \get_option( 'activitypub_rfc9421_unsupported', array() );
$host = \wp_parse_url( $url, \PHP_URL_HOST );
$list[ $host ] = \time() + MONTH_IN_SECONDS;
\update_option( 'activitypub_rfc9421_unsupported', $list, false );
}
/**
* Compute the collection digest for a specific instance.
*
* Implements FEP-8fcf: Followers collection synchronization.
* The digest is created by XORing together the individual SHA256 digests
* of each follower's ID.
*
* @see https://codeberg.org/fediverse/fep/src/branch/main/fep/8fcf/fep-8fcf.md
*
* @param array $collection The user ID whose followers to compute.
*
* @return string|false The hex-encoded digest, or false if no followers.
*/
public static function get_collection_digest( $collection ) {
if ( empty( $collection ) || ! \is_array( $collection ) ) {
return false;
}
// Initialize with zeros (64 hex chars = 32 bytes = 256 bits).
$digest = \str_repeat( '0', 64 );
foreach ( $collection as $item ) {
// Compute SHA256 hash of the follower ID.
$hash = \hash( 'sha256', $item );
// XOR the hash with the running digest.
$digest = self::xor_hex_strings( $digest, $hash );
}
return $digest;
}
/**
* XOR two hexadecimal strings.
*
* Used for FEP-8fcf digest computation.
*
* @param string $hex1 First hex string.
* @param string $hex2 Second hex string.
*
* @return string The XORed result as a hex string.
*/
public static function xor_hex_strings( $hex1, $hex2 ) {
$result = '';
// Ensure both strings are the same length (should be 64 chars for SHA256).
$length = \max( \strlen( $hex1 ), \strlen( $hex2 ) );
$hex1 = \str_pad( $hex1, $length, '0', STR_PAD_LEFT );
$hex2 = \str_pad( $hex2, $length, '0', STR_PAD_LEFT );
// XOR each pair of hex digits.
for ( $i = 0; $i < $length; $i += 2 ) {
$byte1 = \hexdec( \substr( $hex1, $i, 2 ) );
$byte2 = \hexdec( \substr( $hex2, $i, 2 ) );
$result .= \str_pad( \dechex( $byte1 ^ $byte2 ), 2, '0', STR_PAD_LEFT );
}
return $result;
}
/**
* Parse a Collection-Synchronization header (FEP-8fcf).
*
* Parses the signature-style format used by the Collection-Synchronization header.
*
* @see https://codeberg.org/fediverse/fep/src/branch/main/fep/8fcf/fep-8fcf.md
*
* @param string $header The header value.
*
* @return array|false Array with parsed parameters (collectionId, url, digest), or false on failure.
*/
public static function parse_collection_sync_header( $header ) {
if ( empty( $header ) ) {
return false;
}
// Parse the signature-style format: key="value", key="value".
$params = array();
if ( \preg_match_all( '/(\w+)="([^"]*)"/', $header, $matches, PREG_SET_ORDER ) ) {
foreach ( $matches as $match ) {
$params[ $match[1] ] = $match[2];
}
}
// Validate required fields for FEP-8fcf.
if ( empty( $params['collectionId'] ) || empty( $params['url'] ) || empty( $params['digest'] ) ) {
return false;
}
return $params;
}
}