diff --git a/features/mbedtls/targets/TARGET_STM/md5_alt.c b/features/mbedtls/targets/TARGET_STM/md5_alt.c
index 9b4a701ee86..1d3918e9571 100644
--- a/features/mbedtls/targets/TARGET_STM/md5_alt.c
+++ b/features/mbedtls/targets/TARGET_STM/md5_alt.c
@@ -70,15 +70,15 @@ void mbedtls_md5_starts( mbedtls_md5_context *ctx )
     }
 }
 
-void mbedtls_md5_process( mbedtls_md5_context *ctx, const unsigned char data[64] )
+void mbedtls_md5_process( mbedtls_md5_context *ctx, const unsigned char data[MBEDTLS_MD5_BLOCK_SIZE] )
 {
-    HAL_HASH_MD5_Accumulate(&ctx->hhash_md5, (uint8_t *)data, 64);
+    HAL_HASH_MD5_Accumulate(&ctx->hhash_md5, (uint8_t *)data, MBEDTLS_MD5_BLOCK_SIZE);
 }
 
 void mbedtls_md5_update( mbedtls_md5_context *ctx, const unsigned char *input, size_t ilen )
 {
     size_t currentlen = ilen;
-    // store mechanism to handle 64 bytes per 64 bytes
+    // store mechanism to handle MBEDTLS_MD5_BLOCK_SIZE bytes per MBEDTLS_MD5_BLOCK_SIZE bytes
     if (currentlen == 0){ // only change HW status is size if 0
         if(ctx->hhash_md5.Phase == HAL_HASH_PHASE_READY)
         {
@@ -87,18 +87,18 @@ void mbedtls_md5_update( mbedtls_md5_context *ctx, const unsigned char *input, s
           HASH->CR |= HASH_ALGOSELECTION_MD5 | HASH_CR_INIT;
         }
         ctx->hhash_md5.Phase = HAL_HASH_PHASE_PROCESS;
-    } else if (currentlen < (64-ctx->sbuf_len)) {
+    } else if (currentlen < (MBEDTLS_MD5_BLOCK_SIZE-ctx->sbuf_len)) {
         // only buffurize
         memcpy(ctx->sbuf+ctx->sbuf_len, input, currentlen);
         ctx->sbuf_len += currentlen;
     } else {
         // fill buffer and process it
-        memcpy(ctx->sbuf + ctx->sbuf_len, input, (64-ctx->sbuf_len));
-        currentlen -= (64-ctx->sbuf_len);
+        memcpy(ctx->sbuf + ctx->sbuf_len, input, (MBEDTLS_MD5_BLOCK_SIZE-ctx->sbuf_len));
+        currentlen -= (MBEDTLS_MD5_BLOCK_SIZE-ctx->sbuf_len);
         mbedtls_md5_process(ctx, ctx->sbuf);
         // now process every input as long as it is %4 bytes
         size_t iter = currentlen / 4;
-        HAL_HASH_MD5_Accumulate(&ctx->hhash_md5, (uint8_t *)(input+64-ctx->sbuf_len), (iter*4));
+        HAL_HASH_MD5_Accumulate(&ctx->hhash_md5, (uint8_t *)(input+MBEDTLS_MD5_BLOCK_SIZE-ctx->sbuf_len), (iter*4));
         // sbuf is now fully accumulated, now copy 1 / 2 or 3 remaining bytes
         ctx->sbuf_len = currentlen % 4;
         if (ctx->sbuf_len !=0) {
@@ -112,7 +112,7 @@ void mbedtls_md5_finish( mbedtls_md5_context *ctx, unsigned char output[16] )
     if (ctx->sbuf_len > 0) {
         HAL_HASH_MD5_Accumulate(&ctx->hhash_md5, ctx->sbuf, ctx->sbuf_len);
     }
-    mbedtls_zeroize( ctx->sbuf, 64);
+    mbedtls_zeroize( ctx->sbuf, MBEDTLS_MD5_BLOCK_SIZE);
     ctx->sbuf_len = 0;
     __HAL_HASH_START_DIGEST();
 
diff --git a/features/mbedtls/targets/TARGET_STM/md5_alt.h b/features/mbedtls/targets/TARGET_STM/md5_alt.h
index 7998167a3d9..5c8daf96791 100644
--- a/features/mbedtls/targets/TARGET_STM/md5_alt.h
+++ b/features/mbedtls/targets/TARGET_STM/md5_alt.h
@@ -31,16 +31,18 @@
 extern "C" {
 #endif
 
+#define MBEDTLS_MD5_BLOCK_SIZE (64) // must be a multiple of 4
 /**
  * \brief          MD5 context structure
  * \note     HAL_HASH_MD5_Accumulate cannot handle less than 4 bytes, unless it is the last call to the  function
- *           A 64 bytes buffer is used to save values and handle the processing 64 bytes per 64 bytes
+ *           A MBEDTLS_MD5_BLOCK_SIZE bytes buffer is used to save values and handle the processing
+ *           MBEDTLS_MD5_BLOCK_SIZE bytes per MBEDTLS_MD5_BLOCK_SIZE bytes
  *           If MD5_finish is called and sbuf_len>0, the remaining bytes are accumulated prior to the call to HAL_HASH_MD5_Finish
  */
 typedef struct
 {
   HASH_HandleTypeDef hhash_md5;/*!< ST HAL HASH struct */
-  unsigned char sbuf[64]; /*!< 64 buffer to store values so that algorithm is caled once the buffer is filled */
+  unsigned char sbuf[MBEDTLS_MD5_BLOCK_SIZE]; /*!< MBEDTLS_MD5_BLOCK_SIZE buffer to store values so that algorithm is caled once the buffer is filled */
   unsigned char sbuf_len; /*!< number of bytes to be processed in sbuf */
 }
 mbedtls_md5_context;