SFTP: restore the line breaks of an OpenSSH private key before use (#24)

Nextcloud 35 reads keys with phpseclib 3, which refuses an OpenSSH-format
key whose line breaks were lost ("Unable to read key"); phpseclib 2 on
Nextcloud 34 read it. The key is fixed when used, so stored locations work
again without re-entering it.

tests/Unit/Backend/ExternalStorageFactoryTest.php

@@ -0,0 +1,35 @@
+<?php
+
+declare(strict_types=1);
+
+namespace OCA\NgBackup\Tests\Unit\Backend;
+
+use OCA\NgBackup\Backend\ExternalStorageFactory;
+use PHPUnit\Framework\TestCase;
+
+/** ExternalStorageFactory::normalizePrivateKey(): keys that phpseclib 2 read, for phpseclib 3 (#24). */
+class ExternalStorageFactoryTest extends TestCase {
+	/** An OpenSSH-format key laid out as ssh-keygen writes it; the body is random, not a real key. */
+	private static function key(): string {
+		$body = base64_encode(random_bytes(1000));
+		return "-----BEGIN OPENSSH PRIVATE KEY-----\n" . chunk_split($body, 70, "\n") . "-----END OPENSSH PRIVATE KEY-----\n";
+	}
+
+	public function testRestoresLostLineBreaks(): void {
+		$key = self::key();
+		$this->assertSame($key, ExternalStorageFactory::normalizePrivateKey($key));
+		$this->assertSame($key, ExternalStorageFactory::normalizePrivateKey(str_replace("\n", ' ', $key)), 'newlines as spaces');
+		$this->assertSame($key, ExternalStorageFactory::normalizePrivateKey(str_replace("\n", '', $key)), 'no newlines');
+		$this->assertSame($key, ExternalStorageFactory::normalizePrivateKey(str_replace("\n", "\r\n", $key)), 'CRLF');
+		$this->assertSame($key, ExternalStorageFactory::normalizePrivateKey(rtrim($key)), 'no trailing newline');
+		$this->assertSame($key, ExternalStorageFactory::normalizePrivateKey(preg_replace('/^/m', '  ', $key)), 'indented');
+	}
+
+	public function testLeavesOtherFormatsAlone(): void {
+		$pem = "-----BEGIN RSA PRIVATE KEY-----\nProc-Type: 4,ENCRYPTED\nDEK-Info: DES-EDE3-CBC,FF44E0A94036766C\n\nAAAA\n-----END RSA PRIVATE KEY-----\n";
+		$this->assertSame($pem, ExternalStorageFactory::normalizePrivateKey($pem));
+		$this->assertSame('not a key', ExternalStorageFactory::normalizePrivateKey('not a key'));
+		$odd = "-----BEGIN OPENSSH PRIVATE KEY-----\\nAAAA\\n-----END OPENSSH PRIVATE KEY-----";
+		$this->assertSame($odd, ExternalStorageFactory::normalizePrivateKey($odd), 'not base64 inside: unchanged');
+	}
+}