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Rewrite x25519 example program
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Rewrite x25519 example program to fix fatal bug and show
current best practices with the ECDH API.

Signed-off-by: Thomas Daubney <[email protected]>
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@tom-daubney-arm
tom-daubney-arm committed May 30, 2022
1 parent 529bf9d commit d99f8b2
Showing 1 changed file with 59 additions and 73 deletions.
132 changes: 59 additions & 73 deletions programs/pkey/ecdh_curve25519.c
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Original file line number Diff line number Diff line change
Expand Up @@ -34,12 +34,12 @@
#define MBEDTLS_EXIT_FAILURE EXIT_FAILURE
#endif /* MBEDTLS_PLATFORM_C */

#if !defined(MBEDTLS_ECDH_C) || !defined(MBEDTLS_ECDH_LEGACY_CONTEXT) || \
#if !defined(MBEDTLS_ECDH_C) || \
!defined(MBEDTLS_ECP_DP_CURVE25519_ENABLED) || \
!defined(MBEDTLS_ENTROPY_C) || !defined(MBEDTLS_CTR_DRBG_C)
int main( void )
{
mbedtls_printf( "MBEDTLS_ECDH_C and/or MBEDTLS_ECDH_LEGACY_CONTEXT and/or "
mbedtls_printf( "MBEDTLS_ECDH_C and/or "
"MBEDTLS_ECP_DP_CURVE25519_ENABLED and/or "
"MBEDTLS_ENTROPY_C and/or MBEDTLS_CTR_DRBG_C "
"not defined\n" );
Expand All @@ -51,6 +51,8 @@ int main( void )
#include "mbedtls/ctr_drbg.h"
#include "mbedtls/ecdh.h"

#include <string.h>


int main( int argc, char *argv[] )
{
Expand All @@ -59,8 +61,15 @@ int main( int argc, char *argv[] )
mbedtls_ecdh_context ctx_cli, ctx_srv;
mbedtls_entropy_context entropy;
mbedtls_ctr_drbg_context ctr_drbg;
unsigned char cli_to_srv[32], srv_to_cli[32];
unsigned char cli_to_srv[36], srv_to_cli[33];
const char pers[] = "ecdh";

size_t srv_olen;
size_t cli_olen;
unsigned char secret_cli[32] = { 0 };
unsigned char secret_srv[32] = { 0 };
const unsigned char *p_cli_to_srv = cli_to_srv;

((void) argc);
((void) argv);

Expand All @@ -71,15 +80,17 @@ int main( int argc, char *argv[] )
/*
* Initialize random number generation
*/
mbedtls_printf( " . Seeding the random number generator..." );
mbedtls_printf( " . Seed the random number generator..." );
fflush( stdout );

mbedtls_entropy_init( &entropy );
if( ( ret = mbedtls_ctr_drbg_seed( &ctr_drbg, mbedtls_entropy_func, &entropy,
(const unsigned char *) pers,
sizeof pers ) ) != 0 )
if( ( ret = mbedtls_ctr_drbg_seed( &ctr_drbg, mbedtls_entropy_func,
&entropy,
(const unsigned char *) pers,
sizeof pers ) ) != 0 )
{
mbedtls_printf( " failed\n ! mbedtls_ctr_drbg_seed returned %d\n", ret );
mbedtls_printf( " failed\n ! mbedtls_ctr_drbg_seed returned %d\n",
ret );
goto exit;
}

Expand All @@ -88,28 +99,23 @@ int main( int argc, char *argv[] )
/*
* Client: initialize context and generate keypair
*/
mbedtls_printf( " . Setting up client context..." );
mbedtls_printf( " . Set up client context, generate EC key pair..." );
fflush( stdout );

ret = mbedtls_ecp_group_load( &ctx_cli.grp, MBEDTLS_ECP_DP_CURVE25519 );
if( ret != 0 )
{
mbedtls_printf( " failed\n ! mbedtls_ecp_group_load returned %d\n", ret );
goto exit;
}

ret = mbedtls_ecdh_gen_public( &ctx_cli.grp, &ctx_cli.d, &ctx_cli.Q,
mbedtls_ctr_drbg_random, &ctr_drbg );
ret = mbedtls_ecdh_setup( &ctx_cli, MBEDTLS_ECP_DP_CURVE25519 );
if( ret != 0 )
{
mbedtls_printf( " failed\n ! mbedtls_ecdh_gen_public returned %d\n", ret );
mbedtls_printf( " failed\n ! mbedtls_ecdh_setup returned %d\n", ret );
goto exit;
}

ret = mbedtls_mpi_write_binary( &ctx_cli.Q.X, cli_to_srv, 32 );
ret = mbedtls_ecdh_make_params( &ctx_cli, &cli_olen, cli_to_srv,
sizeof( cli_to_srv ),
mbedtls_ctr_drbg_random, &ctr_drbg );
if( ret != 0 )
{
mbedtls_printf( " failed\n ! mbedtls_mpi_write_binary returned %d\n", ret );
mbedtls_printf( " failed\n ! mbedtls_ecdh_make_params returned %d\n",
ret );
goto exit;
}

Expand All @@ -118,90 +124,70 @@ int main( int argc, char *argv[] )
/*
* Server: initialize context and generate keypair
*/
mbedtls_printf( " . Setting up server context..." );
mbedtls_printf( " . Server: read params, generate public key..." );
fflush( stdout );

ret = mbedtls_ecp_group_load( &ctx_srv.grp, MBEDTLS_ECP_DP_CURVE25519 );
ret = mbedtls_ecdh_read_params( &ctx_srv, &p_cli_to_srv,
p_cli_to_srv + sizeof( cli_to_srv ) );
if( ret != 0 )
{
mbedtls_printf( " failed\n ! mbedtls_ecp_group_load returned %d\n", ret );
mbedtls_printf( " failed\n ! mbedtls_ecdh_read_params returned %d\n",
ret );
goto exit;
}

ret = mbedtls_ecdh_gen_public( &ctx_srv.grp, &ctx_srv.d, &ctx_srv.Q,
mbedtls_ctr_drbg_random, &ctr_drbg );
ret = mbedtls_ecdh_make_public( &ctx_srv, &srv_olen, srv_to_cli,
sizeof( srv_to_cli ),
mbedtls_ctr_drbg_random, &ctr_drbg );
if( ret != 0 )
{
mbedtls_printf( " failed\n ! mbedtls_ecdh_gen_public returned %d\n", ret );
goto exit;
}

ret = mbedtls_mpi_write_binary( &ctx_srv.Q.X, srv_to_cli, 32 );
if( ret != 0 )
{
mbedtls_printf( " failed\n ! mbedtls_mpi_write_binary returned %d\n", ret );
mbedtls_printf( " failed\n ! mbedtls_ecdh_make_public returned %d\n",
ret );
goto exit;
}

mbedtls_printf( " ok\n" );

/*
* Server: read peer's key and generate shared secret
* Client: read public key
*/
mbedtls_printf( " . Server reading client key and computing secret..." );
mbedtls_printf( " . Client: read public key..." );
fflush( stdout );

ret = mbedtls_mpi_lset( &ctx_srv.Qp.Z, 1 );
ret = mbedtls_ecdh_read_public( &ctx_cli, srv_to_cli,
sizeof( srv_to_cli ) );
if( ret != 0 )
{
mbedtls_printf( " failed\n ! mbedtls_mpi_lset returned %d\n", ret );
goto exit;
}

ret = mbedtls_mpi_read_binary( &ctx_srv.Qp.X, cli_to_srv, 32 );
if( ret != 0 )
{
mbedtls_printf( " failed\n ! mbedtls_mpi_read_binary returned %d\n", ret );
goto exit;
}

ret = mbedtls_ecdh_compute_shared( &ctx_srv.grp, &ctx_srv.z,
&ctx_srv.Qp, &ctx_srv.d,
mbedtls_ctr_drbg_random, &ctr_drbg );
if( ret != 0 )
{
mbedtls_printf( " failed\n ! mbedtls_ecdh_compute_shared returned %d\n", ret );
mbedtls_printf( " failed\n ! mbedtls_ecdh_read_public returned %d\n",
ret );
goto exit;
}

mbedtls_printf( " ok\n" );

/*
* Client: read peer's key and generate shared secret
* Calculate secrets
*/
mbedtls_printf( " . Client reading server key and computing secret..." );
mbedtls_printf( " . Calculate secrets..." );
fflush( stdout );

ret = mbedtls_mpi_lset( &ctx_cli.Qp.Z, 1 );
ret = mbedtls_ecdh_calc_secret( &ctx_cli, &cli_olen, secret_cli,
sizeof( secret_cli ),
mbedtls_ctr_drbg_random, &ctr_drbg );
if( ret != 0 )
{
mbedtls_printf( " failed\n ! mbedtls_mpi_lset returned %d\n", ret );
mbedtls_printf( " failed\n ! mbedtls_ecdh_calc_secret returned %d\n",
ret );
goto exit;
}

ret = mbedtls_mpi_read_binary( &ctx_cli.Qp.X, srv_to_cli, 32 );
ret = mbedtls_ecdh_calc_secret( &ctx_srv, &srv_olen, secret_srv,
sizeof( secret_srv ),
mbedtls_ctr_drbg_random, &ctr_drbg );
if( ret != 0 )
{
mbedtls_printf( " failed\n ! mbedtls_mpi_read_binary returned %d\n", ret );
goto exit;
}

ret = mbedtls_ecdh_compute_shared( &ctx_cli.grp, &ctx_cli.z,
&ctx_cli.Qp, &ctx_cli.d,
mbedtls_ctr_drbg_random, &ctr_drbg );
if( ret != 0 )
{
mbedtls_printf( " failed\n ! mbedtls_ecdh_compute_shared returned %d\n", ret );
mbedtls_printf( " failed\n ! mbedtls_ecdh_calc_secret returned %d\n",
ret );
goto exit;
}

Expand All @@ -210,13 +196,13 @@ int main( int argc, char *argv[] )
/*
* Verification: are the computed secrets equal?
*/
mbedtls_printf( " . Checking if both computed secrets are equal..." );
mbedtls_printf( " . Check if both calculated secrets are equal..." );
fflush( stdout );

ret = mbedtls_mpi_cmp_mpi( &ctx_cli.z, &ctx_srv.z );
if( ret != 0 )
ret = memcmp( secret_srv, secret_cli, srv_olen );
if( ret != 0 || ( cli_olen != srv_olen ) )
{
mbedtls_printf( " failed\n ! mbedtls_ecdh_compute_shared returned %d\n", ret );
mbedtls_printf( " failed\n ! Shared secrets not equal.\n" );
goto exit;
}

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