Use a more compact encoding of bad points
Base 10 is horrible, base 256 is much better.
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/library/ecp.c b/library/ecp.c
index fcd3e9e..ae94d79 100644
--- a/library/ecp.c
+++ b/library/ecp.c
@@ -2747,6 +2747,10 @@
#if defined(MBEDTLS_ECP_MONTGOMERY_ENABLED)
#if defined(MBEDTLS_ECP_DP_CURVE25519_ENABLED)
+/* The following constants are defined in ecp_curves.c */
+extern const mbedtls_mpi mbedtls_ecp_x25519_bad_point_1;
+extern const mbedtls_mpi mbedtls_ecp_x25519_bad_point_2;
+
/*
* Check that the input point is not one of the low-order points.
* This is recommended by the "May the Fourth" paper:
@@ -2756,10 +2760,9 @@
static int ecp_check_pubkey_x25519( const mbedtls_mpi *X, const mbedtls_mpi *P )
{
int ret;
- mbedtls_mpi XmP, bad;
+ mbedtls_mpi XmP;
mbedtls_mpi_init( &XmP );
- mbedtls_mpi_init( &bad );
/* Reduce X mod P so that we only need to check values less than P.
* We know X < 2^256 so we can proceed by subtraction. */
@@ -2772,25 +2775,21 @@
if( mbedtls_mpi_cmp_int( &XmP, 1 ) <= 0 ) /* takes care of 0 and 1 */
return( MBEDTLS_ERR_ECP_INVALID_KEY );
- MBEDTLS_MPI_CHK( mbedtls_mpi_read_string( &bad, 10,
- "325606250916557431795983626356110631294008115727848805560023387167927233504" ) );
- if( mbedtls_mpi_cmp_mpi( &XmP, &bad ) == 0 )
+ if( mbedtls_mpi_cmp_mpi( &XmP, &mbedtls_ecp_x25519_bad_point_1 ) == 0 )
return( MBEDTLS_ERR_ECP_INVALID_KEY );
- MBEDTLS_MPI_CHK( mbedtls_mpi_read_string( &bad, 10,
- "39382357235489614581723060781553021112529911719440698176882885853963445705823" ) );
- if( mbedtls_mpi_cmp_mpi( &XmP, &bad ) == 0 )
+ if( mbedtls_mpi_cmp_mpi( &XmP, &mbedtls_ecp_x25519_bad_point_2 ) == 0 )
return( MBEDTLS_ERR_ECP_INVALID_KEY );
- MBEDTLS_MPI_CHK( mbedtls_mpi_sub_int( &bad, P, 1 ) );
- if( mbedtls_mpi_cmp_mpi( &XmP, &bad ) == 0 )
+ /* Final check: check if XmP + 1 is P (final because it changes XmP!) */
+ MBEDTLS_MPI_CHK( mbedtls_mpi_add_int( &XmP, &XmP, 1 ) );
+ if( mbedtls_mpi_cmp_mpi( &XmP, P ) == 0 )
return( MBEDTLS_ERR_ECP_INVALID_KEY );
ret = 0;
cleanup:
mbedtls_mpi_free( &XmP );
- mbedtls_mpi_free( &bad );
return( ret );
}
diff --git a/library/ecp_curves.c b/library/ecp_curves.c
index 9a32ac1..9c1d1b4 100644
--- a/library/ecp_curves.c
+++ b/library/ecp_curves.c
@@ -83,14 +83,13 @@
#define ECP_MPI_INIT(s, n, p) {s, (n), (mbedtls_mpi_uint *)(p)}
+#define ECP_MPI_INIT_ARRAY(x) \
+ ECP_MPI_INIT(1, sizeof(x) / sizeof(mbedtls_mpi_uint), x)
+
#define ECP_POINT_INIT_XY_Z0(x, y) { \
- ECP_MPI_INIT(1, sizeof(x) / sizeof(mbedtls_mpi_uint), x),\
- ECP_MPI_INIT(1, sizeof(y) / sizeof(mbedtls_mpi_uint), y), \
- ECP_MPI_INIT(1, 0, NULL) }
+ ECP_MPI_INIT_ARRAY(x), ECP_MPI_INIT_ARRAY(y), ECP_MPI_INIT(1, 0, NULL) }
#define ECP_POINT_INIT_XY_Z1(x, y) { \
- ECP_MPI_INIT(1, sizeof(x) / sizeof(mbedtls_mpi_uint), x),\
- ECP_MPI_INIT(1, sizeof(y) / sizeof(mbedtls_mpi_uint), y), \
- ECP_MPI_INIT(1, 1, mpi_one) }
+ ECP_MPI_INIT_ARRAY(x), ECP_MPI_INIT_ARRAY(y), ECP_MPI_INIT(1, 1, mpi_one) }
#if defined(MBEDTLS_ECP_DP_SECP192R1_ENABLED) || \
defined(MBEDTLS_ECP_DP_SECP224R1_ENABLED) || \
@@ -4703,6 +4702,28 @@
return( ret );
}
+
+/*
+ * Constants for the two points other than 0, 1, -1 (mod p) in
+ * https://cr.yp.to/ecdh.html#validate
+ * See ecp_check_pubkey_x25519().
+ */
+static const mbedtls_mpi_uint x25519_bad_point_1[] = {
+ BYTES_TO_T_UINT_8( 0xe0, 0xeb, 0x7a, 0x7c, 0x3b, 0x41, 0xb8, 0xae ),
+ BYTES_TO_T_UINT_8( 0x16, 0x56, 0xe3, 0xfa, 0xf1, 0x9f, 0xc4, 0x6a ),
+ BYTES_TO_T_UINT_8( 0xda, 0x09, 0x8d, 0xeb, 0x9c, 0x32, 0xb1, 0xfd ),
+ BYTES_TO_T_UINT_8( 0x86, 0x62, 0x05, 0x16, 0x5f, 0x49, 0xb8, 0x00 ),
+};
+static const mbedtls_mpi_uint x25519_bad_point_2[] = {
+ BYTES_TO_T_UINT_8( 0x5f, 0x9c, 0x95, 0xbc, 0xa3, 0x50, 0x8c, 0x24 ),
+ BYTES_TO_T_UINT_8( 0xb1, 0xd0, 0xb1, 0x55, 0x9c, 0x83, 0xef, 0x5b ),
+ BYTES_TO_T_UINT_8( 0x04, 0x44, 0x5c, 0xc4, 0x58, 0x1c, 0x8e, 0x86 ),
+ BYTES_TO_T_UINT_8( 0xd8, 0x22, 0x4e, 0xdd, 0xd0, 0x9f, 0x11, 0x57 ),
+};
+const mbedtls_mpi mbedtls_ecp_x25519_bad_point_1 = ECP_MPI_INIT_ARRAY(
+ x25519_bad_point_1 );
+const mbedtls_mpi mbedtls_ecp_x25519_bad_point_2 = ECP_MPI_INIT_ARRAY(
+ x25519_bad_point_2 );
#endif /* MBEDTLS_ECP_DP_CURVE25519_ENABLED */
#if defined(MBEDTLS_ECP_DP_CURVE448_ENABLED)
