int mbedtls_x509_crt_info( char *buf, size_t size, const char *prefix,
                   const mbedtls_x509_crt *crt )
{
    int ret;
    size_t n;
    char *p;
    char key_size_str[BEFORE_COLON];

    p = buf;
    n = size;

    if( NULL == crt )
    {
        ret = mbedtls_snprintf( p, n, "\nCertificate is uninitialised!\n" );
        MBEDTLS_X509_SAFE_SNPRINTF;

        return( (int) ( size - n ) );
    }

    ret = mbedtls_snprintf( p, n, "%scert. version     : %d\n",
                               prefix, crt->version );
    MBEDTLS_X509_SAFE_SNPRINTF;
    ret = mbedtls_snprintf( p, n, "%sserial number     : ",
                               prefix );
    MBEDTLS_X509_SAFE_SNPRINTF;

    ret = mbedtls_x509_serial_gets( p, n, &crt->serial );
    MBEDTLS_X509_SAFE_SNPRINTF;

    ret = mbedtls_snprintf( p, n, "\n%sissuer name       : ", prefix );
    MBEDTLS_X509_SAFE_SNPRINTF;
    ret = mbedtls_x509_dn_gets( p, n, &crt->issuer  );
    MBEDTLS_X509_SAFE_SNPRINTF;

    ret = mbedtls_snprintf( p, n, "\n%ssubject name      : ", prefix );
    MBEDTLS_X509_SAFE_SNPRINTF;
    ret = mbedtls_x509_dn_gets( p, n, &crt->subject );
    MBEDTLS_X509_SAFE_SNPRINTF;

    ret = mbedtls_snprintf( p, n, "\n%sissued  on        : " \
                   "%04d-%02d-%02d %02d:%02d:%02d", prefix,
                   crt->valid_from.year, crt->valid_from.mon,
                   crt->valid_from.day,  crt->valid_from.hour,
                   crt->valid_from.min,  crt->valid_from.sec );
    MBEDTLS_X509_SAFE_SNPRINTF;

    ret = mbedtls_snprintf( p, n, "\n%sexpires on        : " \
                   "%04d-%02d-%02d %02d:%02d:%02d", prefix,
                   crt->valid_to.year, crt->valid_to.mon,
                   crt->valid_to.day,  crt->valid_to.hour,
                   crt->valid_to.min,  crt->valid_to.sec );
    MBEDTLS_X509_SAFE_SNPRINTF;

    ret = mbedtls_snprintf( p, n, "\n%ssigned using      : ", prefix );
    MBEDTLS_X509_SAFE_SNPRINTF;

    ret = mbedtls_x509_sig_alg_gets( p, n, &crt->sig_oid, crt->sig_pk,
                             crt->sig_md, crt->sig_opts );
    MBEDTLS_X509_SAFE_SNPRINTF;

    /* Key size */
    if( ( ret = mbedtls_x509_key_size_helper( key_size_str, BEFORE_COLON,
                                      mbedtls_pk_get_name( &crt->pk ) ) ) != 0 )
    {
        return( ret );
    }

    ret = mbedtls_snprintf( p, n, "\n%s%-" BC "s: %d bits", prefix, key_size_str,
                          (int) mbedtls_pk_get_bitlen( &crt->pk ) );
    MBEDTLS_X509_SAFE_SNPRINTF;

    /*
     * Optional extensions
     */

    if( crt->ext_types & MBEDTLS_X509_EXT_BASIC_CONSTRAINTS )
    {
        ret = mbedtls_snprintf( p, n, "\n%sbasic constraints : CA=%s", prefix,
                        crt->ca_istrue ? "true" : "false" );
        MBEDTLS_X509_SAFE_SNPRINTF;

        if( crt->max_pathlen > 0 )
        {
            ret = mbedtls_snprintf( p, n, ", max_pathlen=%d", crt->max_pathlen - 1 );
            MBEDTLS_X509_SAFE_SNPRINTF;
        }
    }

    if( crt->ext_types & MBEDTLS_X509_EXT_SUBJECT_ALT_NAME )
    {
        ret = mbedtls_snprintf( p, n, "\n%ssubject alt name  : ", prefix );
        MBEDTLS_X509_SAFE_SNPRINTF;

        if( ( ret = x509_info_subject_alt_name( &p, &n,
                                            &crt->subject_alt_names ) ) != 0 )
            return( ret );
    }

    if( crt->ext_types & MBEDTLS_X509_EXT_NS_CERT_TYPE )
    {
        ret = mbedtls_snprintf( p, n, "\n%scert. type        : ", prefix );
        MBEDTLS_X509_SAFE_SNPRINTF;

        if( ( ret = x509_info_cert_type( &p, &n, crt->ns_cert_type ) ) != 0 )
            return( ret );
    }

    if( crt->ext_types & MBEDTLS_X509_EXT_KEY_USAGE )
    {
        ret = mbedtls_snprintf( p, n, "\n%skey usage         : ", prefix );
        MBEDTLS_X509_SAFE_SNPRINTF;

        if( ( ret = x509_info_key_usage( &p, &n, crt->key_usage ) ) != 0 )
            return( ret );
    }

    if( crt->ext_types & MBEDTLS_X509_EXT_EXTENDED_KEY_USAGE )
    {
        ret = mbedtls_snprintf( p, n, "\n%sext key usage     : ", prefix );
        MBEDTLS_X509_SAFE_SNPRINTF;

        if( ( ret = x509_info_ext_key_usage( &p, &n,
                                             &crt->ext_key_usage ) ) != 0 )
            return( ret );
    }

    ret = mbedtls_snprintf( p, n, "\n" );
    MBEDTLS_X509_SAFE_SNPRINTF;

    return( (int) ( size - n ) );
}

When an actor claims to have a given identity, the product does not prove or insufficiently proves that the claim is correct.