diff --git a/Crypto/Internal/ByteArray.hs b/Crypto/Internal/ByteArray.hs
index d17c78b..acbd9cb 100644
--- a/Crypto/Internal/ByteArray.hs
+++ b/Crypto/Internal/ByteArray.hs
@@ -22,25 +22,30 @@ import Foreign.Ptr
 import Foreign.ForeignPtr
 
 import Data.ByteString (ByteString)
+import qualified Data.ByteString as B (length)
 import qualified Data.ByteString.Internal as B
 
 class ByteArray ba where
-    byteArrayAlloc :: Int -> (Ptr p -> IO ()) -> IO ba
+    byteArrayAlloc  :: Int -> (Ptr p -> IO ()) -> IO ba
+    byteArrayLength :: ba -> Int
 
 instance ByteArray Bytes where
     byteArrayAlloc = bytesAlloc
+    byteArrayLength = bytesLength
 
 instance ByteArray ByteString where
     byteArrayAlloc sz f = do
         fptr <- B.mallocByteString sz
         withForeignPtr fptr (f . castPtr)
         return $! B.PS fptr 0 sz
+    byteArrayLength = B.length
 
 instance ByteArray SecureMem where
     byteArrayAlloc sz f = do
         out <- allocateSecureMem sz
         withSecureMemPtr out (f . castPtr)
         return out
+    byteArrayLength = secureMemGetSize
 
 byteArrayAllocAndFreeze :: ByteArray a => Int -> (Ptr p -> IO ()) -> a
 byteArrayAllocAndFreeze sz f = unsafeDoIO (byteArrayAlloc sz f)
diff --git a/Crypto/Internal/Memory.hs b/Crypto/Internal/Memory.hs
index a4dda51..b1f55b2 100644
--- a/Crypto/Internal/Memory.hs
+++ b/Crypto/Internal/Memory.hs
@@ -16,6 +16,7 @@ module Crypto.Internal.Memory
     , bytesTemporary
     , bytesCopyTemporary
     , bytesAlloc
+    , bytesLength
     ) where
 
 import Data.Word
@@ -73,3 +74,6 @@ bytesAlloc sz f = do
     ba <- newBytes sz
     withPtr ba f
     return ba
+
+bytesLength :: Bytes -> Int
+bytesLength = sizeofBytes