fix concatkdf failures on big endian architectures (cisco#78)

Several of the elements used to compute the digest in ECDH-ES key
agreement computation are represented in binary form as a 32-bit
integer length followed by that number of octets. The 32-bit length
integer is represented in big endian format (the 8 most significant
bits are in the first octet.).

The conversion to a 4 byte big endian integer was being computed
in a manner that only worked on little endian architectures. The
function htonl() returns a 32-bit integer whose octet sequence given
the address of the integer is big endian. There is no need for any
further manipulation.

The existing code used bit shifting on a 32-bit value. In C bit
shifting is endian agnostic for multi-octet values, a right shift
moves most significant bits toward least significant bits. The result
of a bit shift of a multi-octet value on either big or little
archictures will always be the same provided you "view" it as the same
data type (e.g. 32-bit integer). But indexing the octets of that
mulit-octet value will be different depending on endianness, hence the
assembled octets differed depending on endianness.

Issue: cisco#77
Signed-off-by: John Dennis <[email protected]>

src/concatkdf.c

@@ -29,15 +29,9 @@
 ////////////////////////////////////////////////////////////////////////////////
 static uint8_t *_apply_uint32(const uint32_t value, uint8_t *buffer)
 {
-    const uint32_t formatted = htonl(value);
-    const uint8_t data[4] = {
-        (formatted >> 0) & 0xff,
-        (formatted >> 8) & 0xff,
-        (formatted >> 16) & 0xff,
-        (formatted >> 24) & 0xff
-    };
-    memcpy(buffer, data, 4);
+    const uint32_t big_endian_int32 = htonl(value);
 
+    memcpy(buffer, &big_endian_int32, 4);
     return buffer + 4;
 }
 

test/check_concatkdf.c

@@ -60,14 +60,9 @@ static cjose_header_t *_create_otherinfo_header(const uint8_t *apu, const size_t
 
 static bool _cmp_uint32(uint8_t **actual, uint32_t expected)
 {
-    uint32_t value = htonl(expected);
-    uint8_t expectedData[] = {
-        (value >> 0) & 0xff,
-        (value >> 8) & 0xff,
-        (value >> 16) & 0xff,
-        (value >> 24) & 0xff
-    };
-    bool result = (0 == memcmp(*actual, expectedData, 4));
+    uint32_t big_endian_int32 = htonl(expected);
+
+    bool result = (0 == memcmp(*actual, &big_endian_int32, 4));
     (*actual) += 4;
     return result;
 }