diff --git a/apps/testing/appinfo/info.xml b/apps/testing/appinfo/info.xml
index 9b833778a52ea..ee19663cfef4c 100644
--- a/apps/testing/appinfo/info.xml
+++ b/apps/testing/appinfo/info.xml
@@ -18,6 +18,7 @@
 	</types>
 
 	<commands>
+		<command>OCA\Testing\Command\SetUnencryptedSize</command>
 		<command>OCA\Testing\Command\StaticHunt</command>
 	</commands>
 
diff --git a/apps/testing/composer/composer/autoload_classmap.php b/apps/testing/composer/composer/autoload_classmap.php
index ee7424a0ea809..7f2e8e4d68001 100644
--- a/apps/testing/composer/composer/autoload_classmap.php
+++ b/apps/testing/composer/composer/autoload_classmap.php
@@ -9,6 +9,7 @@
     'Composer\\InstalledVersions' => $vendorDir . '/composer/InstalledVersions.php',
     'OCA\\Testing\\AlternativeHomeUserBackend' => $baseDir . '/../lib/AlternativeHomeUserBackend.php',
     'OCA\\Testing\\AppInfo\\Application' => $baseDir . '/../lib/AppInfo/Application.php',
+    'OCA\\Testing\\Command\\SetUnencryptedSize' => $baseDir . '/../lib/Command/SetUnencryptedSize.php',
     'OCA\\Testing\\Command\\StaticHunt' => $baseDir . '/../lib/Command/StaticHunt.php',
     'OCA\\Testing\\Controller\\ConfigController' => $baseDir . '/../lib/Controller/ConfigController.php',
     'OCA\\Testing\\Controller\\LockingController' => $baseDir . '/../lib/Controller/LockingController.php',
diff --git a/apps/testing/composer/composer/autoload_static.php b/apps/testing/composer/composer/autoload_static.php
index 9ea8e6dc0a8f0..4f842897e7fa2 100644
--- a/apps/testing/composer/composer/autoload_static.php
+++ b/apps/testing/composer/composer/autoload_static.php
@@ -24,6 +24,7 @@ class ComposerStaticInitTesting
         'Composer\\InstalledVersions' => __DIR__ . '/..' . '/composer/InstalledVersions.php',
         'OCA\\Testing\\AlternativeHomeUserBackend' => __DIR__ . '/..' . '/../lib/AlternativeHomeUserBackend.php',
         'OCA\\Testing\\AppInfo\\Application' => __DIR__ . '/..' . '/../lib/AppInfo/Application.php',
+        'OCA\\Testing\\Command\\SetUnencryptedSize' => __DIR__ . '/..' . '/../lib/Command/SetUnencryptedSize.php',
         'OCA\\Testing\\Command\\StaticHunt' => __DIR__ . '/..' . '/../lib/Command/StaticHunt.php',
         'OCA\\Testing\\Controller\\ConfigController' => __DIR__ . '/..' . '/../lib/Controller/ConfigController.php',
         'OCA\\Testing\\Controller\\LockingController' => __DIR__ . '/..' . '/../lib/Controller/LockingController.php',
diff --git a/apps/testing/lib/Command/SetUnencryptedSize.php b/apps/testing/lib/Command/SetUnencryptedSize.php
new file mode 100644
index 0000000000000..d2bb1f10e7434
--- /dev/null
+++ b/apps/testing/lib/Command/SetUnencryptedSize.php
@@ -0,0 +1,51 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OCA\Testing\Command;
+
+use OCP\Files\IRootFolder;
+use Symfony\Component\Console\Command\Command;
+use Symfony\Component\Console\Input\InputArgument;
+use Symfony\Component\Console\Input\InputInterface;
+use Symfony\Component\Console\Output\OutputInterface;
+
+/**
+ * Overwrite the unencrypted size stored in the file cache, e.g. to simulate
+ * file cache entries written by older versions.
+ */
+class SetUnencryptedSize extends Command {
+	public function __construct(
+		private IRootFolder $rootFolder,
+	) {
+		parent::__construct();
+	}
+
+	#[\Override]
+	protected function configure(): void {
+		$this
+			->setName('testing:set-unencrypted-size')
+			->setDescription('Overwrite the unencrypted size of a file in the file cache')
+			->addArgument('user', InputArgument::REQUIRED, 'Owner of the file')
+			->addArgument('path', InputArgument::REQUIRED, 'Path of the file relative to the home of the user')
+			->addArgument('size', InputArgument::REQUIRED, 'Unencrypted size to store');
+	}
+
+	#[\Override]
+	protected function execute(InputInterface $input, OutputInterface $output): int {
+		$node = $this->rootFolder
+			->getUserFolder($input->getArgument('user'))
+			->get($input->getArgument('path'));
+
+		$node->getStorage()->getCache()->update($node->getId(), [
+			'unencrypted_size' => (int)$input->getArgument('size'),
+		]);
+
+		return self::SUCCESS;
+	}
+}
diff --git a/build/integration/encryption_features/encryption.feature b/build/integration/encryption_features/encryption.feature
index fed221e1a72ad..cb173726162d5 100644
--- a/build/integration/encryption_features/encryption.feature
+++ b/build/integration/encryption_features/encryption.feature
@@ -52,6 +52,25 @@ Feature: encryption
     Then the HTTP status code should be "200"
     And Downloaded content should be "the second content"
 
+  # Files written by older versions use the base64 encoding and may have no
+  # unencrypted size in the file cache, so it has to be recalculated from the
+  # blocks of the file.
+  Scenario: Download a base64 encoded file with an unknown unencrypted size
+    Given invoking occ with "config:system:set encryption.use_legacy_base64_encoding --value true --type boolean"
+    And the command was successful
+    And user "user0" exists
+    And As an "user0"
+    And User "user0" adds a file of 20000 bytes to "/legacy.bin"
+    And invoking occ with "config:system:delete encryption.use_legacy_base64_encoding"
+    And the command was successful
+    And invoking occ with "app:enable testing"
+    And the command was successful
+    And invoking occ with "testing:set-unencrypted-size user0 /legacy.bin 0"
+    And the command was successful
+    When Downloading file "/legacy.bin"
+    Then the HTTP status code should be "200"
+    And File "/legacy.bin" should have prop "d:getcontentlength" equal to "20000"
+
   # With "part_file_in_storage" disabled the part file is written to the user
   # home while the target lives on another storage, so the upload has to read the
   # part file back to move it over. A part file never has a file cache entry, so
diff --git a/lib/private/Files/Storage/Wrapper/Encryption.php b/lib/private/Files/Storage/Wrapper/Encryption.php
index 04ae28be70f97..748b81f482230 100644
--- a/lib/private/Files/Storage/Wrapper/Encryption.php
+++ b/lib/private/Files/Storage/Wrapper/Encryption.php
@@ -444,6 +444,8 @@ protected function fixUnencryptedSize(string $path, int $size, int $unencryptedS
 			return 0;
 		}
 
+		// the block size depends on the encoding the module reads from the header
+		$encryptionModule->begin($this->getFullPath($path), $this->uid, 'r', $header, []);
 		$signed = isset($header['signed']) && $header['signed'] === 'true';
 		$unencryptedBlockSize = $encryptionModule->getUnencryptedBlockSize($signed);
 
@@ -476,7 +478,6 @@ protected function fixUnencryptedSize(string $path, int $size, int $unencryptedS
 		fclose($stream);
 
 		// we have to decrypt the last chunk to get it actual size
-		$encryptionModule->begin($this->getFullPath($path), $this->uid, 'r', $header, []);
 		$decryptedLastChunk = $encryptionModule->decrypt($lastChunkContentEncrypted, $lastChunkNr . 'end');
 		$decryptedLastChunk .= $encryptionModule->end($this->getFullPath($path), $lastChunkNr . 'end');
 
diff --git a/tests/lib/Files/Storage/Wrapper/EncryptionTest.php b/tests/lib/Files/Storage/Wrapper/EncryptionTest.php
index e6adab2ec119d..4775242fd88f3 100644
--- a/tests/lib/Files/Storage/Wrapper/EncryptionTest.php
+++ b/tests/lib/Files/Storage/Wrapper/EncryptionTest.php
@@ -394,6 +394,68 @@ public static function dataTestVerifyUnencryptedSize(): array {
 		];
 	}
 
+	/**
+	 * The unencrypted block size depends on the encoding stored in the file header,
+	 * which the encryption module only knows once begin() has read the header.
+	 */
+	public function testFixUnencryptedSizeUsesBlockSizeOfFileEncoding(): void {
+		$begun = false;
+		$module = $this->createMock(IEncryptionModule::class);
+		$module->method('begin')->willReturnCallback(function () use (&$begun) {
+			$begun = true;
+			return [];
+		});
+		$module->method('getUnencryptedBlockSize')->willReturnCallback(function (bool $signed) use (&$begun) {
+			return $begun ? 6072 : 8096;
+		});
+		$module->method('decrypt')->willReturn(str_repeat('a', 42));
+		$module->method('end')->willReturn('');
+
+		$header = ['cipher' => 'AES-256-CTR', 'signed' => 'true', 'oc_encryption_module' => 'UNIT_TEST_MODULE'];
+		$content = str_pad(Util::HEADER_START, $this->headerSize, '-') . str_repeat('x', 2 * 8192 + 100);
+		$stream = fopen('php://memory', 'r+');
+		fwrite($stream, $content);
+		rewind($stream);
+		$cache = $this->createMock(ICache::class);
+		$cache->method('get')->with('legacy.txt')->willReturn(['fileid' => 42]);
+		$sourceStorage = $this->createMock(\OC\Files\Storage\Storage::class);
+		$sourceStorage->method('fopen')->with('legacy.txt', 'r')->willReturn($stream);
+		$sourceStorage->method('getCache')->willReturn($cache);
+
+		$expected = 2 * 6072 + 42;
+		$cache->expects($this->once())->method('update')->with(42, ['unencrypted_size' => $expected]);
+
+		$instance = $this->getMockBuilder(Encryption::class)
+			->setConstructorArgs(
+				[
+					[
+						'storage' => $sourceStorage,
+						'root' => 'foo',
+						'mountPoint' => '/',
+						'mount' => $this->mount
+					],
+					$this->encryptionManager,
+					$this->util,
+					$this->logger,
+					$this->file,
+					null,
+					$this->keyStore,
+					$this->mountManager,
+					$this->arrayCache,
+				]
+			)
+			->onlyMethods(['getHeader', 'getHeaderSize', 'getEncryptionModule'])
+			->getMock();
+		$instance->method('getHeader')->willReturn($header);
+		$instance->method('getHeaderSize')->willReturn($this->headerSize);
+		$instance->method('getEncryptionModule')->willReturn($module);
+
+		$this->assertEquals(
+			$expected,
+			$this->invokePrivate($instance, 'fixUnencryptedSize', ['legacy.txt', strlen($content), 0])
+		);
+	}
+
 	/**
 	 *
 	 * @param string $source