diff --git a/tests/compat.sh b/tests/compat.sh
index 0eae1eab3..80c2d31a3 100755
--- a/tests/compat.sh
+++ b/tests/compat.sh
@@ -15,6 +15,10 @@
 
 set -u
 
+# Limit the size of each log to 10 GiB, in case of failures with this script
+# where it may output seemingly unlimited length error logs.
+ulimit -f 20971520
+
 # initialise counters
 TESTS=0
 FAILED=0
diff --git a/tests/scripts/mbedtls_test.py b/tests/scripts/mbedtls_test.py
index ac2912d4c..6ac68a4fb 100755
--- a/tests/scripts/mbedtls_test.py
+++ b/tests/scripts/mbedtls_test.py
@@ -79,7 +79,7 @@ class TestDataParser(object):
         split_colon_fn = lambda x: re.sub(r'\\' + split_char, split_char, x)
         if len(split_char) > 1:
             raise ValueError('Expected split character. Found string!')
-        out = map(split_colon_fn, re.split(r'(?<!\\)' + split_char, inp_str))
+        out = list(map(split_colon_fn, re.split(r'(?<!\\)' + split_char, inp_str)))
         out = [x for x in out if x]
         return out
 
@@ -99,11 +99,11 @@ class TestDataParser(object):
 
             # Check dependencies
             dependencies = []
-            line = data_f.next().strip()
+            line = next(data_f).strip()
             match = re.search('depends_on:(.*)', line)
             if match:
                 dependencies = [int(x) for x in match.group(1).split(':')]
-                line = data_f.next().strip()
+                line = next(data_f).strip()
 
             # Read test vectors
             line = line.replace('\\n', '\n')
@@ -115,7 +115,7 @@ class TestDataParser(object):
                 err_str_fmt = "Number of test arguments({}) should be even: {}"
                 raise TestDataParserError(err_str_fmt.format(args_count, line))
             grouped_args = [(args[i * 2], args[(i * 2) + 1])
-                            for i in range(len(args)/2)]
+                            for i in range(int(len(args)/2))]
             self.tests.append((name, function_name, dependencies,
                                grouped_args))
 
@@ -261,21 +261,21 @@ class MbedTlsTest(BaseHostTest):
         data_bytes += bytearray([function_id, len(parameters)])
         for typ, param in parameters:
             if typ == 'int' or typ == 'exp':
-                i = int(param)
-                data_bytes += 'I' if typ == 'int' else 'E'
+                i = int(param, 0)
+                data_bytes += b'I' if typ == 'int' else b'E'
                 self.align_32bit(data_bytes)
                 data_bytes += self.int32_to_big_endian_bytes(i)
             elif typ == 'char*':
                 param = param.strip('"')
                 i = len(param) + 1  # + 1 for null termination
-                data_bytes += 'S'
+                data_bytes += b'S'
                 self.align_32bit(data_bytes)
                 data_bytes += self.int32_to_big_endian_bytes(i)
-                data_bytes += bytearray(list(param))
-                data_bytes += '\0'   # Null terminate
+                data_bytes += bytearray(param, encoding='ascii')
+                data_bytes += b'\0'   # Null terminate
             elif typ == 'hex':
                 binary_data = self.hex_str_bytes(param)
-                data_bytes += 'H'
+                data_bytes += b'H'
                 self.align_32bit(data_bytes)
                 i = len(binary_data)
                 data_bytes += self.int32_to_big_endian_bytes(i)
@@ -310,7 +310,7 @@ class MbedTlsTest(BaseHostTest):
 
         param_bytes, length = self.test_vector_to_bytes(function_id,
                                                         dependencies, args)
-        self.send_kv(length, param_bytes)
+        self.send_kv(''.join('{:02x}'.format(x) for x in length), ''.join('{:02x}'.format(x) for x in param_bytes))
 
     @staticmethod
     def get_result(value):
diff --git a/tests/ssl-opt.sh b/tests/ssl-opt.sh
index fc0e1d029..238ed97d0 100755
--- a/tests/ssl-opt.sh
+++ b/tests/ssl-opt.sh
@@ -21,6 +21,10 @@
 
 set -u
 
+# Limit the size of each log to 10 GiB, in case of failures with this script
+# where it may output seemingly unlimited length error logs.
+ulimit -f 20971520
+
 if cd $( dirname $0 ); then :; else
     echo "cd $( dirname $0 ) failed" >&2
     exit 1
diff --git a/tests/suites/target_test.function b/tests/suites/target_test.function
index e4c3e30de..d430d9d5d 100644
--- a/tests/suites/target_test.function
+++ b/tests/suites/target_test.function
@@ -59,10 +59,29 @@ int verify_dependencies( uint8_t count, uint8_t * dep_p )
     return( DEPENDENCY_SUPPORTED );
 }
 
+/**
+ * \brief       Receives hex string on serial interface, and converts to a byte.
+ *
+ * \param none
+ *
+ * \return      unsigned int8
+ */
+uint8_t receive_byte()
+{
+    uint8_t byte;
+    uint8_t c[3];
+    char *endptr;
+    c[0] = greentea_getc();
+    c[1] = greentea_getc();
+    c[2] = '\0';
+
+    assert( unhexify( &byte, c ) != 2 );
+    return( byte );
+}
 
 /**
  * \brief       Receives unsigned integer on serial interface.
- *              Integers are encoded in network order.
+ *              Integers are encoded in network order, and sent as hex ascii string.
  *
  * \param none
  *
@@ -71,10 +90,17 @@ int verify_dependencies( uint8_t count, uint8_t * dep_p )
 uint32_t receive_uint32()
 {
     uint32_t value;
-    value =  (uint8_t)greentea_getc() << 24;
-    value |= (uint8_t)greentea_getc() << 16;
-    value |= (uint8_t)greentea_getc() << 8;
-    value |= (uint8_t)greentea_getc();
+    const uint8_t c[9] = { greentea_getc(),
+                           greentea_getc(),
+                           greentea_getc(),
+                           greentea_getc(),
+                           greentea_getc(),
+                           greentea_getc(),
+                           greentea_getc(),
+                           greentea_getc(),
+                           '\0'
+                         };
+    assert( unhexify( &value, c ) != 8 );
     return( (uint32_t)value );
 }
 
@@ -132,7 +158,7 @@ uint8_t * receive_data( uint32_t * data_len )
     greentea_getc(); // read ';' received after key i.e. *data_len
 
     for( i = 0; i < *data_len; i++ )
-        data[i] = greentea_getc();
+        data[i] = receive_byte();
 
     /* Read closing braces */
     for( i = 0; i < 2; i++ )