diff --git a/library/chachapoly.c b/library/chachapoly.c
index 77d5477..696d97b 100644
--- a/library/chachapoly.c
+++ b/library/chachapoly.c
@@ -263,22 +263,22 @@
     /* The lengths of the AAD and ciphertext are processed by
      * Poly1305 as the final 128-bit block, encoded as little-endian integers.
      */
-    len_block[ 0] = (unsigned char)( ctx->aad_len       );
-    len_block[ 1] = (unsigned char)( ctx->aad_len >>  8 );
-    len_block[ 2] = (unsigned char)( ctx->aad_len >> 16 );
-    len_block[ 3] = (unsigned char)( ctx->aad_len >> 24 );
-    len_block[ 4] = (unsigned char)( ctx->aad_len >> 32 );
-    len_block[ 5] = (unsigned char)( ctx->aad_len >> 40 );
-    len_block[ 6] = (unsigned char)( ctx->aad_len >> 48 );
-    len_block[ 7] = (unsigned char)( ctx->aad_len >> 56 );
-    len_block[ 8] = (unsigned char)( ctx->ciphertext_len       );
-    len_block[ 9] = (unsigned char)( ctx->ciphertext_len >>  8 );
-    len_block[10] = (unsigned char)( ctx->ciphertext_len >> 16 );
-    len_block[11] = (unsigned char)( ctx->ciphertext_len >> 24 );
-    len_block[12] = (unsigned char)( ctx->ciphertext_len >> 32 );
-    len_block[13] = (unsigned char)( ctx->ciphertext_len >> 40 );
-    len_block[14] = (unsigned char)( ctx->ciphertext_len >> 48 );
-    len_block[15] = (unsigned char)( ctx->ciphertext_len >> 56 );
+    len_block[ 0] = MBEDTLS_BYTE_0( ctx->aad_len );
+    len_block[ 1] = MBEDTLS_BYTE_1( ctx->aad_len );
+    len_block[ 2] = MBEDTLS_BYTE_2( ctx->aad_len );
+    len_block[ 3] = MBEDTLS_BYTE_3( ctx->aad_len );
+    len_block[ 4] = MBEDTLS_BYTE_4( ctx->aad_len );
+    len_block[ 5] = MBEDTLS_BYTE_5( ctx->aad_len );
+    len_block[ 6] = MBEDTLS_BYTE_6( ctx->aad_len );
+    len_block[ 7] = MBEDTLS_BYTE_7( ctx->aad_len );
+    len_block[ 8] = MBEDTLS_BYTE_0( ctx->ciphertext_len );
+    len_block[ 9] = MBEDTLS_BYTE_1( ctx->ciphertext_len );
+    len_block[10] = MBEDTLS_BYTE_2( ctx->ciphertext_len );
+    len_block[11] = MBEDTLS_BYTE_3( ctx->ciphertext_len );
+    len_block[12] = MBEDTLS_BYTE_4( ctx->ciphertext_len );
+    len_block[13] = MBEDTLS_BYTE_5( ctx->ciphertext_len );
+    len_block[14] = MBEDTLS_BYTE_6( ctx->ciphertext_len );
+    len_block[15] = MBEDTLS_BYTE_7( ctx->ciphertext_len );
 
     ret = mbedtls_poly1305_update( &ctx->poly1305_ctx, len_block, 16U );
     if( ret != 0 )