mbedtls ecdsa privatekey 生成椭圆算法密钥对，打印私钥

打印私钥，主要使用这个函数：
mbedtls_mpi_write_file

int mbedtls_mpi_write_file( const char p, const mbedtls_mpi X,　int radix, FILE *fout );

当fout为NULL，就输出到标准输出了。

When reading from files with mbedtls_mpi_read_file() and writing to files with
mbedtls_mpi_write_file() the buffer should have space
for a (short) label, the MPI (in the provided radix), the newline
characters and the '0'.

int main( int argc, char *argv[] )
{
    int ret = 1;
    int exit_code = MBEDTLS_EXIT_FAILURE;
    mbedtls_ecdsa_context ctx_sign, ctx_verify;
    mbedtls_entropy_context entropy;
    mbedtls_ctr_drbg_context ctr_drbg;
    unsigned char message[100];
    unsigned char hash[32];
    unsigned char sig[MBEDTLS_ECDSA_MAX_LEN];
    size_t sig_len;
    const char *pers = "ecdsa";
    ((void) argv);

    mbedtls_ecdsa_init( &ctx_sign );
    mbedtls_ecdsa_init( &ctx_verify );
    mbedtls_ctr_drbg_init( &ctr_drbg );

    memset( sig, 0, sizeof( sig ) );
    memset( message, 0x25, sizeof( message ) );

    if( argc != 1 )
    {
        mbedtls_printf( "usage: ecdsa\n" );

#if defined(_WIN32)
        mbedtls_printf( "\n" );
#endif

        goto exit;
    }

    /*
     * Generate a key pair for signing
     */
    mbedtls_printf( "\n  . Seeding 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,
                               strlen( pers ) ) ) != 0 )
    {
        mbedtls_printf( " failed\n  ! mbedtls_ctr_drbg_seed returned %d\n", ret );
        goto exit;
    }

    mbedtls_printf( " ok\n  . Generating key pair..." );
    fflush( stdout );

    if( ( ret = mbedtls_ecdsa_genkey( &ctx_sign, ECPARAMS,
                              mbedtls_ctr_drbg_random, &ctr_drbg ) ) != 0 )
    {
        mbedtls_printf( " failed\n  ! mbedtls_ecdsa_genkey returned %d\n", ret );
        goto exit;
    }

    mbedtls_printf( " ok (key size: %d bits)\n", (int) ctx_sign.MBEDTLS_PRIVATE(grp).pbits );

    dump_pubkey( "  + Public key: ", &ctx_sign );
    mbedtls_mpi_write_file( "D   : ", &ctx_sign.MBEDTLS_PRIVATE(d)  , 16, NULL );
exit:

#if defined(_WIN32)
    mbedtls_printf( "  + Press Enter to exit this program.\n" );
    fflush( stdout ); getchar();
#endif

    mbedtls_ecdsa_free( &ctx_verify );
    mbedtls_ecdsa_free( &ctx_sign );
    mbedtls_ctr_drbg_free( &ctr_drbg );
    mbedtls_entropy_free( &entropy );

    mbedtls_exit( exit_code );
}

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