diff --git a/src/Simplex/Chat/Mobile/File.hs b/src/Simplex/Chat/Mobile/File.hs
index 4f73e191a..25e694365 100644
--- a/src/Simplex/Chat/Mobile/File.hs
+++ b/src/Simplex/Chat/Mobile/File.hs
@@ -19,7 +19,6 @@ import Data.ByteString (ByteString)
 import qualified Data.ByteString as B
 import qualified Data.ByteString.Lazy as LB
 import qualified Data.ByteString.Lazy.Char8 as LB'
-import Data.Int (Int64)
 import Data.Word (Word8)
 import Foreign.C
 import Foreign.Marshal.Alloc (mallocBytes)
@@ -53,7 +52,7 @@ chatWriteFile path s = do
     <$> runExceptT (CF.writeFile file $ LB.fromStrict s)
 
 data ReadFileResult
-  = RFResult {fileSize :: Int64}
+  = RFResult {fileSize :: Int}
   | RFError {readError :: String}
   deriving (Generic)
 
@@ -65,7 +64,7 @@ cChatReadFile cPath cKey cNonce = do
   key <- B.packCString cKey
   nonce <- B.packCString cNonce
   (r, s) <- chatReadFile path key nonce
-  let r' = LB.toStrict (J.encode r) <> "\NUL"
+  let r' = LB.toStrict $ J.encode r <> "\NUL"
   ptr <- mallocBytes $ B.length r' + B.length s
   putByteString ptr r'
   unless (B.null s) $ putByteString (ptr `plusPtr` B.length r') s
@@ -73,8 +72,9 @@ cChatReadFile cPath cKey cNonce = do
 
 chatReadFile :: FilePath -> ByteString -> ByteString -> IO (ReadFileResult, ByteString)
 chatReadFile path keyStr nonceStr = do
-  either ((,"") . RFError) (\s -> (RFResult $ LB.length s, LB.toStrict s)) <$> runExceptT readFile_
+  either ((,"") . RFError) result <$> runExceptT readFile_
   where
+    result s = let s' = LB.toStrict s in (RFResult $ B.length s', s')
     readFile_ :: ExceptT String IO LB.ByteString
     readFile_ = do
       key <- liftEither $ strDecode keyStr
diff --git a/tests/MobileTests.hs b/tests/MobileTests.hs
index e11496ef4..604a1640e 100644
--- a/tests/MobileTests.hs
+++ b/tests/MobileTests.hs
@@ -1,19 +1,25 @@
 {-# LANGUAGE CPP #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 
+{-# OPTIONS_GHC -fno-warn-orphans #-}
+
 module MobileTests where
 
 import ChatTests.Utils
 import Control.Monad.Except
 import Crypto.Random (getRandomBytes)
+import Data.Aeson (FromJSON (..))
+import qualified Data.Aeson as J
 import Data.ByteString (ByteString)
 import qualified Data.ByteString as B
 import qualified Data.ByteString.Char8 as BS
+import qualified Data.ByteString.Lazy.Char8 as LB
 import Data.Word (Word8)
 import Foreign.C
 import Foreign.Marshal.Alloc (mallocBytes)
 import Foreign.Ptr
 import Simplex.Chat.Mobile
+import Simplex.Chat.Mobile.File
 import Simplex.Chat.Mobile.Shared
 import Simplex.Chat.Mobile.WebRTC
 import Simplex.Chat.Store
@@ -21,7 +27,9 @@ import Simplex.Chat.Store.Profiles
 import Simplex.Chat.Types (AgentUserId (..), Profile (..))
 import Simplex.Messaging.Agent.Store.SQLite (MigrationConfirmation (..))
 import qualified Simplex.Messaging.Crypto as C
+import Simplex.Messaging.Crypto.File (CryptoFileArgs (..))
 import Simplex.Messaging.Encoding.String
+import Simplex.Messaging.Parsers (dropPrefix, sumTypeJSON)
 import System.FilePath ((</>))
 import Test.Hspec
 
@@ -30,8 +38,9 @@ mobileTests = do
   describe "mobile API" $ do
     it "start new chat without user" testChatApiNoUser
     it "start new chat with existing user" testChatApi
-    fit "should encrypt/decrypt WebRTC frames" testMediaApi
-    fit "should encrypt/decrypt WebRTC frames via C API" testMediaCApi
+    it "should encrypt/decrypt WebRTC frames" testMediaApi
+    it "should encrypt/decrypt WebRTC frames via C API" testMediaCApi
+    it "should read/write encrypted files via C API" testFileCApi
 
 noActiveUser :: String
 #if defined(darwin_HOST_OS) && defined(swiftJSON)
@@ -158,6 +167,35 @@ testMediaCApi _ = do
           cLen = fromIntegral len
       ptr <- mallocBytes len
       putByteString ptr frame
-      cKeyStr <- newCString $ BS.unpack keyStr
-      (f cKeyStr ptr cLen >>= peekCString) `shouldReturn` ""
+      cKeyStr <- newCAString $ BS.unpack keyStr
+      (f cKeyStr ptr cLen >>= peekCAString) `shouldReturn` ""
       getByteString ptr cLen
+
+instance FromJSON WriteFileResult where parseJSON = J.genericParseJSON . sumTypeJSON $ dropPrefix "WF"
+
+instance FromJSON ReadFileResult where parseJSON = J.genericParseJSON . sumTypeJSON $ dropPrefix "RF"
+
+testFileCApi :: FilePath -> IO ()
+testFileCApi tmp = do
+  src <- B.readFile "./tests/fixtures/test.pdf"
+  cPath <- newCAString $ tmp </> "test.pdf"
+  let len = B.length src
+      cLen = fromIntegral len
+  ptr <- mallocBytes $ B.length src
+  putByteString ptr src
+  r <- peekCAString =<< cChatWriteFile cPath ptr cLen
+  Just (WFResult (CFArgs key nonce)) <- jDecode r
+  cKey <- encodedCString key
+  cNonce <- encodedCString nonce
+  ptr' <- cChatReadFile cPath cKey cNonce
+  -- the returned pointer contains NUL-terminated JSON string of ReadFileResult followed by the file contents
+  r' <- peekCAString $ castPtr ptr'
+  Just (RFResult sz) <- jDecode r'
+  contents <- getByteString (ptr' `plusPtr` (length r' + 1)) $ fromIntegral sz
+  contents `shouldBe` src
+  sz `shouldBe` len
+  where
+    jDecode :: FromJSON a => String -> IO (Maybe a)
+    jDecode = pure . J.decode . LB.pack
+    encodedCString :: StrEncoding a => a -> IO CString
+    encodedCString = newCAString . BS.unpack . strEncode