diff --git a/tests/include/test/helpers.h b/tests/include/test/helpers.h
index 36ec8e6..69d882c 100644
--- a/tests/include/test/helpers.h
+++ b/tests/include/test/helpers.h
@@ -54,7 +54,29 @@
 int mbedtls_test_platform_setup( void );
 void mbedtls_test_platform_teardown( void );
 
-int mbedtls_test_unhexify( unsigned char *obuf, const char *ibuf );
+/**
+ * \brief          This function translates an ASCII string encoding an
+ *                 hexadecimal number into the encoded hexadecimal number. The
+ *                 hexadecimal number is represented as an array of
+ *                 unsigned char.
+ *
+ * \note           The output buffer can be the same as the input buffer. For
+ *                 any other overlapping of the input and output buffers, the
+ *                 behavior is undefined.
+ *
+ * \param obuf     Output buffer.
+ * \param obufmax  Size in number of bytes of \p obuf.
+ * \param ibuf     Input buffer.
+ * \param len      The number of unsigned char written in \p obuf. This must
+ *                 not be \c NULL.
+ *
+ * \return         \c 0 on success.
+ * \return         \c -1 if the output buffer is too small or the input string
+ *                 is not a valid ASCII encoding of an hexadecimal number.
+ */
+int mbedtls_test_unhexify( unsigned char *obuf, size_t obufmax,
+                           const char *ibuf, size_t *len );
+
 void mbedtls_test_hexify( unsigned char *obuf,
                           const unsigned char *ibuf,
                           int len );
diff --git a/tests/src/helpers.c b/tests/src/helpers.c
index 08d88a5..b9abf19 100644
--- a/tests/src/helpers.c
+++ b/tests/src/helpers.c
@@ -41,38 +41,49 @@
 #endif /* MBEDTLS_PLATFORM_C */
 }
 
-int mbedtls_test_unhexify( unsigned char *obuf, const char *ibuf )
+static int ascii2uc(const char c, unsigned char *uc)
 {
-    unsigned char c, c2;
-    int len = strlen( ibuf ) / 2;
-    TEST_HELPER_ASSERT( strlen( ibuf ) % 2 == 0 ); /* must be even number of bytes */
+    if( ( c >= '0' ) && ( c <= '9' ) )
+        *uc = c - '0';
+    else if( ( c >= 'a' ) && ( c <= 'f' ) )
+        *uc = c - 'a' + 10;
+    else if( ( c >= 'A' ) && ( c <= 'F' ) )
+        *uc = c - 'A' + 10;
+    else
+        return( -1 );
+
+    return( 0 );
+}
+
+int mbedtls_test_unhexify( unsigned char *obuf,
+                           size_t obufmax,
+                           const char *ibuf,
+                           size_t *len )
+{
+    unsigned char uc, uc2;
+
+    *len = strlen( ibuf );
+
+    /* Must be even number of bytes. */
+    if ( ( *len ) & 1 )
+        return( -1 );
+    *len /= 2;
+
+    if ( (*len) > obufmax )
+        return( -1 );
 
     while( *ibuf != 0 )
     {
-        c = *ibuf++;
-        if( c >= '0' && c <= '9' )
-            c -= '0';
-        else if( c >= 'a' && c <= 'f' )
-            c -= 'a' - 10;
-        else if( c >= 'A' && c <= 'F' )
-            c -= 'A' - 10;
-        else
-            TEST_HELPER_ASSERT( 0 );
+        if ( ascii2uc( *(ibuf++), &uc ) != 0 )
+            return( -1 );
 
-        c2 = *ibuf++;
-        if( c2 >= '0' && c2 <= '9' )
-            c2 -= '0';
-        else if( c2 >= 'a' && c2 <= 'f' )
-            c2 -= 'a' - 10;
-        else if( c2 >= 'A' && c2 <= 'F' )
-            c2 -= 'A' - 10;
-        else
-            TEST_HELPER_ASSERT( 0 );
+        if ( ascii2uc( *(ibuf++), &uc2 ) != 0 )
+            return( -1 );
 
-        *obuf++ = ( c << 4 ) | c2;
+        *(obuf++) = ( uc << 4 ) | uc2;
     }
 
-    return len;
+    return( 0 );
 }
 
 void mbedtls_test_hexify( unsigned char *obuf,
@@ -117,6 +128,7 @@
 unsigned char *mbedtls_test_unhexify_alloc( const char *ibuf, size_t *olen )
 {
     unsigned char *obuf;
+    size_t len;
 
     *olen = strlen( ibuf ) / 2;
 
@@ -125,8 +137,7 @@
 
     obuf = mbedtls_calloc( 1, *olen );
     TEST_HELPER_ASSERT( obuf != NULL );
-
-    (void) mbedtls_test_unhexify( obuf, ibuf );
+    TEST_HELPER_ASSERT( mbedtls_test_unhexify( obuf, *olen, ibuf, &len ) == 0 );
 
     return( obuf );
 }
diff --git a/tests/suites/host_test.function b/tests/suites/host_test.function
index c57fa07..a459eed 100644
--- a/tests/suites/host_test.function
+++ b/tests/suites/host_test.function
@@ -277,8 +277,13 @@
         {
             if ( verify_string( &val ) == 0 )
             {
-                *int_params_store = mbedtls_test_unhexify(
-                                        (unsigned char *) val, val );
+                size_t len;
+
+                TEST_HELPER_ASSERT(
+                  mbedtls_test_unhexify( (unsigned char *) val, strlen( val ),
+                                         val, &len ) == 0 );
+
+                *int_params_store = len;
                 *out++ = val;
                 *out++ = (char *)(int_params_store++);
             }
diff --git a/tests/suites/target_test.function b/tests/suites/target_test.function
index f7a9f04..8354b96 100644
--- a/tests/suites/target_test.function
+++ b/tests/suites/target_test.function
@@ -70,12 +70,16 @@
 {
     uint8_t byte;
     uint8_t c[3];
-    char *endptr;
+    size_t len;
+
     c[0] = greentea_getc();
     c[1] = greentea_getc();
     c[2] = '\0';
 
-    TEST_HELPER_ASSERT( mbedtls_test_unhexify( &byte, c ) != 2 );
+    TEST_HELPER_ASSERT( mbedtls_test_unhexify( &byte, sizeof( byte ),
+                                               c, &len ) == 0 );
+    TEST_HELPER_ASSERT( len != 2 );
+
     return( byte );
 }
 
@@ -90,6 +94,7 @@
 uint32_t receive_uint32()
 {
     uint32_t value;
+    size_t len;
     const uint8_t c_be[8] = { greentea_getc(),
                               greentea_getc(),
                               greentea_getc(),
@@ -101,7 +106,11 @@
                              };
     const uint8_t c[9] = { c_be[6], c_be[7], c_be[4], c_be[5], c_be[2],
                            c_be[3], c_be[0], c_be[1], '\0' };
-    TEST_HELPER_ASSERT( mbedtls_test_unhexify( (uint8_t*)&value, c ) != 8 );
+
+    TEST_HELPER_ASSERT( mbedtls_test_unhexify( (uint8_t*)&value, sizeof( value ),
+                                               c, &len ) == 0 );
+    TEST_HELPER_ASSERT( len != 8 );
+
     return( value );
 }