Add a simple function to get the group id from a key object.

This information is available via mbedtls_ecp_export, but that function
consumes a lot of memory, which is a waste if all you need is to identify
the curve.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

Sometimes you don't need to have all the parts of a key pair object. Relax
the behavior of mbedtls_ecp_keypair so that you can extract just the parts
that you need.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

Set the public key in a key pair. This complements mbedtls_ecp_read_key and
the functions can be used in either order.

Document the need to call check functions separately.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

For when you calculate or import a private key, and then need to calculate
the public key.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

It gave uncrustify trouble
(uncrustify/uncrustify#4044)

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

Directly export the public part of a key pair without having to go through
intermediate objects (using mbedtls_ecp_point_write_binary would require a
group object and a point object).

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

This eliminates the use of MBEDTLS_PRIVATE in sample programs to access
fields of an mbedtls_ecp_keypair structure.

When displaying elliptic curve points, the program now display the
coordinates in the standard form instead of the internal representation.

The auxiliary function show_ecp_key is present in three programs. It's more
complex than the previous code which was also triplicated. There's no good
place for such auxiliary functions that don't belong in the library and are
used in multiple sample programs.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

All of ECP requires the bignum module and there is no plan to change that,
so guarding a few bits of code is just noise.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>