Save encrypted files in binary format

Default to the more space-efficient binary encoding for newly encrypted files
instead of the traditional base64 encoding, eliminating the 33% overhead.

The new option 'encryption.use_legacy_encoding' allows to force the legacy
encoding format if needed. Files encoded in the old format remain readable.

Based on https://github.com/owncloud/encryption/pull/224 and
https://github.com/owncloud/core/pull/38249 by karakayasemi.

Signed-off-by: plumbeo <plumbeo@users.noreply.github.com>

5 files changed, 95 insertions, 36 deletions

diff --git a/apps/encryption/lib/Crypto/Crypt.php b/apps/encryption/lib/Crypto/Crypt.php
index 93120068c6a..f8ba3d69b80 100644
--- a/apps/encryption/lib/Crypto/Crypt.php
+++ b/apps/encryption/lib/Crypto/Crypt.php
@@ -77,6 +77,9 @@ class Crypt {
 	public const HEADER_START = 'HBEGIN';
 	public const HEADER_END = 'HEND';
 
+	// default encoding format, old Nextcloud versions used base64
+	public const BINARY_ENCODING_FORMAT = 'binary';
+
 	/** @var ILogger */
 	private $logger;
 
@@ -96,6 +99,11 @@ class Crypt {
 	private $supportLegacy;
 
 	/**
+	 * Use the legacy base64 encoding instead of the more space-efficient binary encoding.
+	 */
+	private bool $useLegacyBase64Encoding;
+
+	/**
 	 * @param ILogger $logger
 	 * @param IUserSession $userSession
 	 * @param IConfig $config
@@ -107,6 +115,7 @@ class Crypt {
 		$this->config = $config;
 		$this->l = $l;
 		$this->supportLegacy = $this->config->getSystemValueBool('encryption.legacy_format_support', false);
+		$this->useLegacyBase64Encoding = $this->config->getSystemValueBool('encryption.use_legacy_base64_encoding', false);
 	}
 
 	/**
@@ -215,12 +224,15 @@ class Crypt {
 			throw new \InvalidArgumentException('key format "' . $keyFormat . '" is not supported');
 		}
 
-		$cipher = $this->getCipher();
-
 		$header = self::HEADER_START
-			. ':cipher:' . $cipher
-			. ':keyFormat:' . $keyFormat
-			. ':' . self::HEADER_END;
+			. ':cipher:' . $this->getCipher()
+			. ':keyFormat:' . $keyFormat;
+
+		if ($this->useLegacyBase64Encoding !== true) {
+			$header .= ':encoding:' . self::BINARY_ENCODING_FORMAT;
+		}
+
+		$header .= ':' . self::HEADER_END;
 
 		return $header;
 	}
@@ -234,10 +246,11 @@ class Crypt {
 	 * @throws EncryptionFailedException
 	 */
 	private function encrypt($plainContent, $iv, $passPhrase = '', $cipher = self::DEFAULT_CIPHER) {
+		$options = $this->useLegacyBase64Encoding ? 0 : OPENSSL_RAW_DATA;
 		$encryptedContent = openssl_encrypt($plainContent,
 			$cipher,
 			$passPhrase,
-			0,
+			$options,
 			$iv);
 
 		if (!$encryptedContent) {
@@ -424,6 +437,8 @@ class Crypt {
 			$password = $this->generatePasswordHash($password, $cipher, $uid);
 		}
 
+		$binaryEncoding = isset($header['encoding']) && $header['encoding'] === self::BINARY_ENCODING_FORMAT;
+
 		// If we found a header we need to remove it from the key we want to decrypt
 		if (!empty($header)) {
 			$privateKey = substr($privateKey,
@@ -435,7 +450,9 @@ class Crypt {
 			$privateKey,
 			$password,
 			$cipher,
-			0
+			0,
+			0,
+			$binaryEncoding
 		);
 
 		if ($this->isValidPrivateKey($plainKey) === false) {
@@ -470,10 +487,11 @@ class Crypt {
 	 * @param string $cipher
 	 * @param int $version
 	 * @param int|string $position
+	 * @param boolean $binaryEncoding
 	 * @return string
 	 * @throws DecryptionFailedException
 	 */
-	public function symmetricDecryptFileContent($keyFileContents, $passPhrase, $cipher = self::DEFAULT_CIPHER, $version = 0, $position = 0) {
+	public function symmetricDecryptFileContent($keyFileContents, $passPhrase, $cipher = self::DEFAULT_CIPHER, $version = 0, $position = 0, bool $binaryEncoding = false) {
 		if ($keyFileContents == '') {
 			return '';
 		}
@@ -493,7 +511,8 @@ class Crypt {
 		return $this->decrypt($catFile['encrypted'],
 			$catFile['iv'],
 			$passPhrase,
-			$cipher);
+			$cipher,
+			$binaryEncoding);
 	}
 
 	/**
@@ -610,14 +629,16 @@ class Crypt {
 	 * @param string $iv
 	 * @param string $passPhrase
 	 * @param string $cipher
+	 * @param boolean $binaryEncoding
 	 * @return string
 	 * @throws DecryptionFailedException
 	 */
-	private function decrypt($encryptedContent, $iv, $passPhrase = '', $cipher = self::DEFAULT_CIPHER) {
+	private function decrypt(string $encryptedContent, string $iv, string $passPhrase = '', string $cipher = self::DEFAULT_CIPHER, bool $binaryEncoding = false): string {
+		$options = $binaryEncoding === true ? OPENSSL_RAW_DATA : 0;
 		$plainContent = openssl_decrypt($encryptedContent,
 			$cipher,
 			$passPhrase,
-			0,
+			$options,
 			$iv);
 
 		if ($plainContent) {
@@ -728,4 +749,8 @@ class Crypt {
 			throw new MultiKeyEncryptException('multikeyencryption failed ' . openssl_error_string());
 		}
 	}
+
+	public function useLegacyBase64Encoding(): bool {
+		return $this->useLegacyBase64Encoding;
+	}
 }
diff --git a/apps/encryption/lib/Crypto/Encryption.php b/apps/encryption/lib/Crypto/Encryption.php
index 58cefcbe087..b44472fd04a 100644
--- a/apps/encryption/lib/Crypto/Encryption.php
+++ b/apps/encryption/lib/Crypto/Encryption.php
@@ -103,11 +103,7 @@ class Encryption implements IEncryptionModule {
 	/** @var DecryptAll  */
 	private $decryptAll;
 
-	/** @var int unencrypted block size if block contains signature */
-	private $unencryptedBlockSizeSigned = 6072;
-
-	/** @var int unencrypted block size */
-	private $unencryptedBlockSize = 6126;
+	private bool $useLegacyBase64Encoding = false;
 
 	/** @var int Current version of the file */
 	private $version = 0;
@@ -184,6 +180,11 @@ class Encryption implements IEncryptionModule {
 		$this->user = $user;
 		$this->isWriteOperation = false;
 		$this->writeCache = '';
+		$this->useLegacyBase64Encoding = true;
+
+		if (isset($header['encoding'])) {
+			$this->useLegacyBase64Encoding = $header['encoding'] !== Crypt::BINARY_ENCODING_FORMAT;
+		}
 
 		if ($this->session->isReady() === false) {
 			// if the master key is enabled we can initialize encryption
@@ -229,6 +230,7 @@ class Encryption implements IEncryptionModule {
 
 		if ($this->isWriteOperation) {
 			$this->cipher = $this->crypt->getCipher();
+			$this->useLegacyBase64Encoding = $this->crypt->useLegacyBase64Encoding();
 		} elseif (isset($header['cipher'])) {
 			$this->cipher = $header['cipher'];
 		} else {
@@ -237,7 +239,13 @@ class Encryption implements IEncryptionModule {
 			$this->cipher = $this->crypt->getLegacyCipher();
 		}
 
-		return ['cipher' => $this->cipher, 'signed' => 'true'];
+		$result = ['cipher' => $this->cipher, 'signed' => 'true'];
+
+		if ($this->useLegacyBase64Encoding !== true) {
+			$result['encoding'] = Crypt::BINARY_ENCODING_FORMAT;
+		}
+
+		return $result;
 	}
 
 	/**
@@ -325,8 +333,8 @@ class Encryption implements IEncryptionModule {
 			$remainingLength = strlen($data);
 
 			// If data remaining to be written is less than the
-			// size of 1 6126 byte block
-			if ($remainingLength < $this->unencryptedBlockSizeSigned) {
+			// size of 1 unencrypted block
+			if ($remainingLength < $this->getUnencryptedBlockSize(true)) {
 
 				// Set writeCache to contents of $data
 				// The writeCache will be carried over to the
@@ -343,14 +351,14 @@ class Encryption implements IEncryptionModule {
 			} else {
 
 				// Read the chunk from the start of $data
-				$chunk = substr($data, 0, $this->unencryptedBlockSizeSigned);
+				$chunk = substr($data, 0, $this->getUnencryptedBlockSize(true));
 
 				$encrypted .= $this->crypt->symmetricEncryptFileContent($chunk, $this->fileKey, $this->version + 1, $position);
 
 				// Remove the chunk we just processed from
 				// $data, leaving only unprocessed data in $data
 				// var, for handling on the next round
-				$data = substr($data, $this->unencryptedBlockSizeSigned);
+				$data = substr($data, $this->getUnencryptedBlockSize(true));
 			}
 		}
 
@@ -374,7 +382,7 @@ class Encryption implements IEncryptionModule {
 			throw new DecryptionFailedException($msg, $hint);
 		}
 
-		return $this->crypt->symmetricDecryptFileContent($data, $this->fileKey, $this->cipher, $this->version, $position);
+		return $this->crypt->symmetricDecryptFileContent($data, $this->fileKey, $this->cipher, $this->version, $position, !$this->useLegacyBase64Encoding);
 	}
 
 	/**
@@ -462,15 +470,27 @@ class Encryption implements IEncryptionModule {
 	 * get size of the unencrypted payload per block.
 	 * Nextcloud read/write files with a block size of 8192 byte
 	 *
+	 * Encrypted blocks have a 22-byte IV and 2 bytes of padding, encrypted and
+	 * signed blocks have also a 71-byte signature and 1 more byte of padding,
+	 * resulting respectively in:
+	 *
+	 *  8192 - 22 - 2 = 8168 bytes in each unsigned unencrypted block
+	 *  8192 - 22 - 2 - 71 - 1 = 8096 bytes in each signed unencrypted block
+	 *
+	 * Legacy base64 encoding then reduces the available size by a 3/4 factor:
+	 *
+	 *  8168 * (3/4) = 6126 bytes in each base64-encoded unsigned unencrypted block
+	 *  8096 * (3/4) = 6072 bytes in each base64-encoded signed unencrypted block
+	 *
 	 * @param bool $signed
 	 * @return int
 	 */
 	public function getUnencryptedBlockSize($signed = false) {
-		if ($signed === false) {
-			return $this->unencryptedBlockSize;
+		if ($this->useLegacyBase64Encoding) {
+			return $signed ? 6072 : 6126;
+		} else {
+			return $signed ? 8096 : 8168;
 		}
-
-		return $this->unencryptedBlockSizeSigned;
 	}
 
 	/**
diff --git a/apps/encryption/tests/Crypto/CryptTest.php b/apps/encryption/tests/Crypto/CryptTest.php
index 60548a0f1dc..ef4e46085a4 100644
--- a/apps/encryption/tests/Crypto/CryptTest.php
+++ b/apps/encryption/tests/Crypto/CryptTest.php
@@ -139,9 +139,9 @@ class CryptTest extends TestCase {
 	 */
 	public function dataTestGenerateHeader() {
 		return [
-			[null, 'HBEGIN:cipher:AES-128-CFB:keyFormat:hash:HEND'],
-			['password', 'HBEGIN:cipher:AES-128-CFB:keyFormat:password:HEND'],
-			['hash', 'HBEGIN:cipher:AES-128-CFB:keyFormat:hash:HEND']
+			[null, 'HBEGIN:cipher:AES-128-CFB:keyFormat:hash:encoding:binary:HEND'],
+			['password', 'HBEGIN:cipher:AES-128-CFB:keyFormat:password:encoding:binary:HEND'],
+			['hash', 'HBEGIN:cipher:AES-128-CFB:keyFormat:hash:encoding:binary:HEND']
 		];
 	}
 
@@ -305,15 +305,17 @@ class CryptTest extends TestCase {
 	 * test parseHeader()
 	 */
 	public function testParseHeader() {
-		$header = 'HBEGIN:foo:bar:cipher:AES-256-CFB:HEND';
+		$header = 'HBEGIN:foo:bar:cipher:AES-256-CFB:encoding:binary:HEND';
 		$result = self::invokePrivate($this->crypt, 'parseHeader', [$header]);
 
 		$this->assertTrue(is_array($result));
-		$this->assertSame(2, count($result));
+		$this->assertSame(3, count($result));
 		$this->assertArrayHasKey('foo', $result);
 		$this->assertArrayHasKey('cipher', $result);
+		$this->assertArrayHasKey('encoding', $result);
 		$this->assertSame('bar', $result['foo']);
 		$this->assertSame('AES-256-CFB', $result['cipher']);
+		$this->assertSame('binary', $result['encoding']);
 	}
 
 	/**
@@ -324,18 +326,20 @@ class CryptTest extends TestCase {
 	public function testEncrypt() {
 		$decrypted = 'content';
 		$password = 'password';
+		$cipher = 'AES-256-CTR';
 		$iv = self::invokePrivate($this->crypt, 'generateIv');
 
 		$this->assertTrue(is_string($iv));
 		$this->assertSame(16, strlen($iv));
 
-		$result = self::invokePrivate($this->crypt, 'encrypt', [$decrypted, $iv, $password]);
+		$result = self::invokePrivate($this->crypt, 'encrypt', [$decrypted, $iv, $password, $cipher]);
 
 		$this->assertTrue(is_string($result));
 
 		return [
 			'password' => $password,
 			'iv' => $iv,
+			'cipher' => $cipher,
 			'encrypted' => $result,
 			'decrypted' => $decrypted];
 	}
@@ -349,7 +353,7 @@ class CryptTest extends TestCase {
 		$result = self::invokePrivate(
 			$this->crypt,
 			'decrypt',
-			[$data['encrypted'], $data['iv'], $data['password']]);
+			[$data['encrypted'], $data['iv'], $data['password'], $data['cipher'], true]);
 
 		$this->assertSame($data['decrypted'], $result);
 	}
@@ -391,8 +395,9 @@ class CryptTest extends TestCase {
 	 */
 	public function testDecryptPrivateKey($header, $privateKey, $expectedCipher, $isValidKey, $expected) {
 		$this->config->method('getSystemValueBool')
-			->with('encryption.legacy_format_support', false)
-			->willReturn(true);
+			->withConsecutive(['encryption.legacy_format_support', false],
+					  ['encryption.use_legacy_base64_encoding', false])
+			->willReturnOnConsecutiveCalls(true, false);
 
 		/** @var \OCA\Encryption\Crypto\Crypt | \PHPUnit\Framework\MockObject\MockObject $crypt */
 		$crypt = $this->getMockBuilder(Crypt::class)
diff --git a/config/config.sample.php b/config/config.sample.php
index 54d3d46070a..77e587acf83 100644
--- a/config/config.sample.php
+++ b/config/config.sample.php
@@ -1799,6 +1799,15 @@ $CONFIG = [
 'cipher' => 'AES-256-CTR',
 
 /**
+ * Use the legacy base64 format for encrypted files instead of the more space-efficient
+ * binary format. The option affects only newly written files, existing encrypted files
+ * will not be touched and will remain readable whether they use the new format or not.
+ *
+ * Defaults to ``false``
+ */
+'encryption.use_legacy_base64_encoding' => false,
+
+/**
  * The minimum Nextcloud desktop client version that will be allowed to sync with
  * this server instance. All connections made from earlier clients will be denied
  * by the server. Defaults to the minimum officially supported Nextcloud desktop
diff --git a/lib/private/Files/Stream/Encryption.php b/lib/private/Files/Stream/Encryption.php
index e1233e76b93..0f1838c97c8 100644
--- a/lib/private/Files/Stream/Encryption.php
+++ b/lib/private/Files/Stream/Encryption.php
@@ -259,7 +259,6 @@ class Encryption extends Wrapper {
 		$this->cache = '';
 		$this->writeFlag = false;
 		$this->fileUpdated = false;
-		$this->unencryptedBlockSize = $this->encryptionModule->getUnencryptedBlockSize($this->signed);
 
 		if (
 			$mode === 'w'
@@ -284,6 +283,7 @@ class Encryption extends Wrapper {
 			$accessList = $this->file->getAccessList($sharePath);
 		}
 		$this->newHeader = $this->encryptionModule->begin($this->fullPath, $this->uid, $mode, $this->header, $accessList);
+		$this->unencryptedBlockSize = $this->encryptionModule->getUnencryptedBlockSize($this->signed);
 
 		if (
 			$mode === 'w'