ASN1 tests: Match negative INTEGERs with the actual library behavior

mbedtls_asn1_get_int() and mbedtls_asn1_get_mpi() behave differently
on negative INTEGERs (0200). Don't change the library behavior for now
because this might break interoperability in some applications. Change
the test function to the library behavior.

Fix the test data with negative INTEGERs. These test cases were
previously not run (they were introduced but deliberately deactivated
in 27d806fab4). The test data was
actually wrong: ASN.1 uses two's complement, which has no negative 0,
and some encodings were wrong. Now the tests have correct data, and
the test code rectifies the expected data to match the library
behavior.

tests/suites/test_suite_asn1parse.data

@@ -172,27 +172,15 @@ get_integer:"020100":"0":0
 INTEGER 0, extra leading 0
 get_integer:"02020000":"0":0
 
-INTEGER -0
-depends_on:SUPPORT_NEGATIVE_INTEGERS
-get_integer:"020180":"0":0
-
 INTEGER 1
 get_integer:"020101":"1":0:
 
 INTEGER 1, extra leading 0
 get_integer:"02020001":"1":0:
 
-INTEGER -1
-depends_on:SUPPORT_NEGATIVE_INTEGERS
-get_integer:"020181":"-1":0
-
 INTEGER 0x7f
 get_integer:"02017f":"7f":0
 
-INTEGER -0x7f
-depends_on:SUPPORT_NEGATIVE_INTEGERS
-get_integer:"0201ff":"-7f":0
-
 INTEGER 0x80
 get_integer:"02020080":"80":0
 
@@ -226,6 +214,12 @@ get_integer:"0281800123456789abcdef0123456789abcdef0123456789abcdef0123456789abc
 INTEGER with 128 value octets (leading 0 in length)
 get_integer:"028200800123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef":"0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef":0
 
+INTEGER -1
+get_integer:"0201ff":"-1":0
+
+INTEGER -0x7f
+get_integer:"020181":"-7f":0
+
 Not INTEGER
 get_integer:"010101":"":MBEDTLS_ERR_ASN1_UNEXPECTED_TAG
 

tests/suites/test_suite_asn1parse.function

@@ -293,6 +293,7 @@ void get_integer( const data_t *input,
 #if defined(MBEDTLS_BIGNUM_C)
     mbedtls_mpi expected_mpi;
     mbedtls_mpi actual_mpi;
+    mbedtls_mpi complement;
     int expected_result_for_mpi = expected_result;
 #endif
     long expected_value;
@@ -303,6 +304,7 @@ void get_integer( const data_t *input,
 #if defined(MBEDTLS_BIGNUM_C)
     mbedtls_mpi_init( &expected_mpi );
     mbedtls_mpi_init( &actual_mpi );
+    mbedtls_mpi_init( &complement );
 #endif
 
     errno = 0;
@@ -314,6 +316,16 @@ void get_integer( const data_t *input,
 #endif
             ) )
     {
+        /* The library returns the dubious error code INVALID_LENGTH
+         * for integers that are out of range. */
+        expected_result_for_int = MBEDTLS_ERR_ASN1_INVALID_LENGTH;
+    }
+    if( expected_result == 0 && expected_value < 0 )
+    {
+        /* The library does not support negative INTEGERs and
+         * returns the dubious error code INVALID_LENGTH.
+         * Test that we preserve the historical behavior. If we
+         * decide to change the behavior, we'll also change this test. */
         expected_result_for_int = MBEDTLS_ERR_ASN1_INVALID_LENGTH;
     }
 
@@ -339,7 +351,34 @@ void get_integer( const data_t *input,
     TEST_EQUAL( ret, expected_result_for_mpi );
     if( ret == 0 )
     {
-        TEST_ASSERT( mbedtls_mpi_cmp_mpi( &actual_mpi , &expected_mpi ) == 0 );
+        if( expected_value >= 0 )
+        {
+            TEST_ASSERT( mbedtls_mpi_cmp_mpi( &actual_mpi,
+                                              &expected_mpi ) == 0 );
+        }
+        else
+        {
+            /* The library ignores the sign bit in ASN.1 INTEGERs
+             * (which makes sense insofar as INTEGERs are sometimes
+             * abused as bit strings), so the result of parsing them
+             * is a positive integer such that expected_mpi +
+             * actual_mpi = 2^n where n is the length of the content
+             * of the INTEGER. (Leading ff octets don't matter for the
+             * expected value, but they matter for the actual value.)
+             * Test that we don't change from this behavior. If we
+             * decide to fix the library to change the behavior on
+             * negative INTEGERs, we'll fix this test code. */
+            unsigned char *q = input->x + 1;
+            size_t len;
+            TEST_ASSERT( mbedtls_asn1_get_len( &q, input->x + input->len,
+                                               &len ) == 0 );
+            TEST_ASSERT( mbedtls_mpi_lset( &complement, 1 ) == 0 );
+            TEST_ASSERT( mbedtls_mpi_shift_l( &complement, len * 8 ) == 0 );
+            TEST_ASSERT( mbedtls_mpi_add_mpi( &complement, &complement,
+                                              &expected_mpi ) == 0 );
+            TEST_ASSERT( mbedtls_mpi_cmp_mpi( &complement,
+                                              &actual_mpi ) == 0 );
+        }
         TEST_ASSERT( p == input->x + input->len );
     }
 #endif
@@ -348,6 +387,7 @@ exit:
 #if defined(MBEDTLS_BIGNUM_C)
     mbedtls_mpi_free( &expected_mpi );
     mbedtls_mpi_free( &actual_mpi );
+    mbedtls_mpi_free( &complement );
 #endif
     /*empty cleanup in some configurations*/ ;
 }