Backport 2.28: Doc: Add note on special use of A in ecp group structure

[ivq]

Description

Backport of #6999.
The changes made on #6999 are:

1. mbedtls_ecp_group_a_is_minus_3(): 2.28 does not have MBEDTLS_PRIVATE.
2. Added macro inline in ecp.h.

PR checklist

• changelog provided
• backport not required as this is a backport
• tests not required

[tom-cosgrove-arm]

Why is the build failing on Windows?

[2023-08-09T14:49:57.888Z] "c:\windows\workspace\mbed-tls-pr-head_PR-8046-head\src\ALL_BUILD.vcxproj" (default target) (1) ->

[2023-08-09T14:49:57.888Z] "C:\Windows\workspace\mbed-tls-pr-head_PR-8046-head\src\programs\x509\cert_req.vcxproj" (default target) (10) ->

[2023-08-09T14:49:57.888Z] (ClCompile target) -> 

[2023-08-09T14:49:57.888Z]   C:\Windows\workspace\mbed-tls-pr-head_PR-8046-head\src\include\mbedtls/ecp.h(1031): error C2054: expected '(' to follow 'inline' [C:\Windows\workspace\mbed-tls-pr-head_PR-8046-head\src\programs\x509\cert_req.vcxproj]

[2023-08-09T14:49:57.888Z]   C:\Windows\workspace\mbed-tls-pr-head_PR-8046-head\src\include\mbedtls/ecp.h(1032): error C2085: 'mbedtls_ecp_group_a_is_minus_3' : not in formal parameter list [C:\Windows\workspace\mbed-tls-pr-head_PR-8046-head\src\programs\x509\cert_req.vcxproj]

[2023-08-09T14:49:57.888Z]   C:\Windows\workspace\mbed-tls-pr-head_PR-8046-head\src\include\mbedtls/ecp.h(1032): error C2143: syntax error : missing ';' before '{' [C:\Windows\workspace\mbed-tls-pr-head_PR-8046-head\src\programs\x509\cert_req.vcxproj]

[tom-cosgrove-arm]

Ah: some versions of MSVC don't support inline!!! https://stackoverflow.com/a/24736425/7761803

[tom-cosgrove-arm]

This is done by mbedtls/build_info.h in development, but library/common.h in 2.28 - looking at other files in include/ in 2.28, might be easiest just to add the following to ecp.h

#if (defined(__ARMCC_VERSION) || defined(_MSC_VER)) && \
    !defined(inline) && !defined(__cplusplus)
#define inline __inline
#endif

[ivq]

This is done by mbedtls/build_info.h in development, but library/common.h in 2.28 - looking at other files in include/ in 2.28, might be easiest just to add the following to ecp.h

#if (defined(__ARMCC_VERSION) || defined(_MSC_VER)) && \
    !defined(inline) && !defined(__cplusplus)
#define inline __inline
#endif

Thanks. Added

[gilles-peskine-arm]

LGTM

[tom-cosgrove-arm]

Why is there no change to ecp.c? I would expect something like

diff --git a/library/ecp.c b/library/ecp.c
index 2d80b6f33..e2ce197d2 100644
--- a/library/ecp.c
+++ b/library/ecp.c
@@ -1528,7 +1528,7 @@ static int ecp_double_jac(const mbedtls_ecp_group *grp, mbedtls_ecp_point *R,
     mbedtls_mpi_init(&M); mbedtls_mpi_init(&S); mbedtls_mpi_init(&T); mbedtls_mpi_init(&U);
 
     /* Special case for A = -3 */
-    if (grp->A.p == NULL) {
+    if (mbedtls_ecp_group_a_is_minus_3(grp)) {
         /* M = 3(X + Z^2)(X - Z^2) */
         MBEDTLS_MPI_CHK(mbedtls_mpi_mul_mod(grp, &S,  &P->Z,  &P->Z));
         MBEDTLS_MPI_CHK(mbedtls_mpi_add_mod(grp, &T,  &P->X,  &S));
@@ -2806,7 +2806,7 @@ static int ecp_check_pubkey_sw(const mbedtls_ecp_group *grp, const mbedtls_ecp_p
     MBEDTLS_MPI_CHK(mbedtls_mpi_mul_mod(grp, &RHS, &pt->X,   &pt->X));
 
     /* Special case for A = -3 */
-    if (grp->A.p == NULL) {
+    if (mbedtls_ecp_group_a_is_minus_3(grp)) {
         MBEDTLS_MPI_CHK(mbedtls_mpi_sub_int(&RHS, &RHS, 3));  MOD_SUB(RHS);
     } else {
         MBEDTLS_MPI_CHK(mbedtls_mpi_add_mod(grp, &RHS, &RHS, &grp->A));

[gilles-peskine-arm]

@tom-cosgrove-arm to minimize risk

[tom-cosgrove-arm]

@tom-cosgrove-arm to minimize risk

Okay, that's fine then

[tom-cosgrove-arm]

LGTM