[crypto] Check for all-zeros result from X25519 key exchange

RFC7748 states that it is entirely optional for X25519 Diffie-Hellman
implementations to check whether or not the result is the all-zero
value (indicating that an attacker sent a malicious public key with a
small order).  RFC8422 states that implementations in TLS must abort
the handshake if the all-zero value is obtained.

Return an error if the all-zero value is obtained, so that the TLS
code will not require knowledge specific to the X25519 curve.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/crypto/x25519.c

@@ -59,6 +59,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #include <stdint.h>
 #include <string.h>
 #include <assert.h>
+#include <errno.h>
 #include <ipxe/init.h>
 #include <ipxe/x25519.h>
 
@@ -781,10 +782,11 @@ static void x25519_reverse ( struct x25519_value *value ) {
  * @v base		Base point
  * @v scalar		Scalar multiple
  * @v result		Point to hold result (may overlap base point)
+ * @ret rc		Return status code
  */
-void x25519_key ( const struct x25519_value *base,
+int x25519_key ( const struct x25519_value *base,
-		  const struct x25519_value *scalar,
+		 const struct x25519_value *scalar,
-		  struct x25519_value *result ) {
+		 struct x25519_value *result ) {
 	struct x25519_value *tmp = result;
 	union x25519_quad257 point;
 
@@ -805,4 +807,7 @@ void x25519_key ( const struct x25519_value *base,
 	/* Reverse result */
 	bigint_done ( &point.value, result->raw, sizeof ( result->raw ) );
 	x25519_reverse ( result );
+
+	/* Fail if result was all zeros (as required by RFC8422) */
+	return ( bigint_is_zero ( &point.value ) ? -EPERM : 0 );
 }

src/include/ipxe/errfile.h

@@ -407,6 +407,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #define ERRFILE_efi_rng		      ( ERRFILE_OTHER | 0x005c0000 )
 #define ERRFILE_efi_shim	      ( ERRFILE_OTHER | 0x005d0000 )
 #define ERRFILE_efi_settings	      ( ERRFILE_OTHER | 0x005e0000 )
+#define ERRFILE_x25519		      ( ERRFILE_OTHER | 0x005f0000 )
 
 /** @} */
 

src/include/ipxe/x25519.h

@@ -84,8 +84,8 @@ extern void x25519_multiply ( const union x25519_oct258 *multiplicand,
 extern void x25519_invert ( const union x25519_oct258 *invertend,
 			    union x25519_quad257 *result );
 extern void x25519_reduce ( union x25519_quad257 *value );
-extern void x25519_key ( const struct x25519_value *base,
+extern int x25519_key ( const struct x25519_value *base,
-			 const struct x25519_value *scalar,
+			const struct x25519_value *scalar,
-			 struct x25519_value *result );
+			struct x25519_value *result );
 
 #endif /* _IPXE_X25519_H */

src/tests/x25519_test.c

@@ -136,6 +136,8 @@ struct x25519_key_test {
 	struct x25519_value expected;
 	/** Number of iterations */
 	unsigned int count;
+	/** Key exchange is expected to fail (i.e. produce all-zeroes) */
+	int fail;
 };
 
 /**
@@ -143,14 +145,16 @@ struct x25519_key_test {
  *
  * @v name		Test name
  * @v COUNT		Number of iterations
+ * @v FAIL		Expected failure status
  * @v BASE		Base point
  * @v SCALAR		Scalar multiple
  * @v EXPECTED		Expected result
  * @ret test		X25519 key exchange test
  */
-#define X25519_KEY_TEST( name, COUNT, BASE, SCALAR, EXPECTED )		\
+#define X25519_KEY_TEST( name, COUNT, FAIL, BASE, SCALAR, EXPECTED )	\
 	static struct x25519_key_test name = {				\
 		.count = COUNT,						\
+		.fail = FAIL,						\
 		.base = { .raw = BASE },				\
 		.scalar = { .raw = SCALAR },				\
 		.expected = { .raw = EXPECTED },			\
@@ -259,6 +263,7 @@ static void x25519_key_okx ( struct x25519_key_test *test,
 	struct x25519_value scalar;
 	struct x25519_value actual;
 	unsigned int i;
+	int rc;
 
 	/* Construct input values */
 	memcpy ( &base, &test->base, sizeof ( test->base ) );
@@ -272,7 +277,12 @@ static void x25519_key_okx ( struct x25519_key_test *test,
 
 	/* Calculate key */
 	for ( i = 0 ; i < test->count ; i++ ) {
-		x25519_key ( &base, &scalar, &actual );
+		rc = x25519_key ( &base, &scalar, &actual );
+		if ( test->fail ) {
+			okx ( rc != 0, file, line );
+		} else {
+			okx ( rc == 0, file, line );
+		}
 		memcpy ( &base, &scalar, sizeof ( base ) );
 		memcpy ( &scalar, &actual, sizeof ( scalar ) );
 	}
@@ -461,7 +471,7 @@ X25519_INVERT_TEST ( invert_5,
  * Scalar: 0xa546e36bf0527c9d3b16154b82465edd62144c0ac1fc5a18506a2244ba449ac4
  * Result: 0xc3da55379de9c6908e94ea4df28d084f32eccf03491c71f754b4075577a28552
  */
-X25519_KEY_TEST ( rfc7748_1, 1,
+X25519_KEY_TEST ( rfc7748_1, 1, 0,
 	BASE ( 0xe6, 0xdb, 0x68, 0x67, 0x58, 0x30, 0x30, 0xdb, 0x35, 0x94,
 	       0xc1, 0xa4, 0x24, 0xb1, 0x5f, 0x7c, 0x72, 0x66, 0x24, 0xec,
 	       0x26, 0xb3, 0x35, 0x3b, 0x10, 0xa9, 0x03, 0xa6, 0xd0, 0xab,
@@ -479,7 +489,7 @@ X25519_KEY_TEST ( rfc7748_1, 1,
  * Scalar: 0x4b66e9d4d1b4673c5ad22691957d6af5c11b6421e0ea01d42ca4169e7918ba0d
  * Result: 0x95cbde9476e8907d7aade45cb4b873f88b595a68799fa152e6f8f7647aac7957
  */
-X25519_KEY_TEST ( rfc7748_2, 1,
+X25519_KEY_TEST ( rfc7748_2, 1, 0,
 	BASE ( 0xe5, 0x21, 0x0f, 0x12, 0x78, 0x68, 0x11, 0xd3, 0xf4, 0xb7,
 	       0x95, 0x9d, 0x05, 0x38, 0xae, 0x2c, 0x31, 0xdb, 0xe7, 0x10,
 	       0x6f, 0xc0, 0x3c, 0x3e, 0xfc, 0x4c, 0xd5, 0x49, 0xc7, 0x15,
@@ -497,7 +507,7 @@ X25519_KEY_TEST ( rfc7748_2, 1,
  * Scalar: 0x0900000000000000000000000000000000000000000000000000000000000000
  * Result: 0x422c8e7a6227d7bca1350b3e2bb7279f7897b87bb6854b783c60e80311ae3079
  */
-X25519_KEY_TEST ( rfc7748_3, 1,
+X25519_KEY_TEST ( rfc7748_3, 1, 0,
 	BASE ( 0x09, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
 	       0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
 	       0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
@@ -521,7 +531,7 @@ X25519_KEY_TEST ( rfc7748_3, 1,
  * to avoid a pointlessly slow test cycle in the common case of
  * running tests under Valgrind.
  */
-X25519_KEY_TEST ( rfc7748_4_100, 100,
+X25519_KEY_TEST ( rfc7748_4_100, 100, 0,
 	BASE ( 0x09, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
 	       0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
 	       0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
@@ -535,6 +545,24 @@ X25519_KEY_TEST ( rfc7748_4_100, 100,
 		   0x1d, 0x26, 0x09, 0xc9, 0x2e, 0x5a, 0x8f, 0x1d, 0xeb, 0xe2,
 		   0x15, 0x0a ) );
 
+/* Base:   2^255 - 19 + 1 (deliberately malicious public key)
+ * Scalar: 0x000102030405060708090a0b0c0d0e0f000102030405060708090a0b0c0d0e0f
+ * Result: Failure (all zeros)
+ */
+X25519_KEY_TEST ( malicious, 1, 1,
+	BASE ( 0xee, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+	       0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+	       0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+	       0xff, 0x7f ),
+	SCALAR ( 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09,
+		 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f, 0x00, 0x01, 0x02, 0x03,
+		 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d,
+		 0x0e, 0x0f ),
+	EXPECTED ( 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+		   0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+		   0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+		   0x00, 0x00 ) );
+
 /**
  * Perform X25519 self-tests
  *
@@ -562,6 +590,7 @@ static void x25519_test_exec ( void ) {
 	x25519_key_ok ( &rfc7748_2 );
 	x25519_key_ok ( &rfc7748_3 );
 	x25519_key_ok ( &rfc7748_4_100 );
+	x25519_key_ok ( &malicious );
 }
 
 /** X25519 self-test */