怎样对邮件正文分段加密及解密解决思路

$mailcont = "邮件正文………超过1024字节………邮件正文";    $crtpath = "/usr/nc-home/webpost/mailbox/d00/1/y/u/yushuai.niu/.user/certkey/wxd.crt";    exec("chmod 777 {$crtpath}");    $fp = fopen($crtpath, "r");    $crt = fread($fp, 8192);    fclose($fp);    $pubKey = openssl_get_publickey($crt);    //从证书中解析公钥    $keyData = openssl_pkey_get_details($pubKey);    $blocks = str_split($mailcont, 1000);    foreach ($blocks as $block) {    //公钥加密    if (!openssl_public_encrypt($block, $chrtext, $keyData['key'])) {    echo "
" . openssl_error_string() . "
";    }    $chrtext .= $chrtext;    }    echo "chrtext-->>>$chrtext
";    $keypath = "/usr/nc-home/webpost/mailbox/d00/1/y/u/yushuai.niu/.user/secretkey/wxd.key";    exec("chmod 777 {$keypath}");    $fp = fopen($keypath, "r");    $key = fread($fp, 8192);    fclose($fp);    $cipher = openssl_pkey_get_private($key);    openssl_private_decrypt($chrtext, $dcyCont, $cipher);    echo "dcyCont-->>>$dcyCont
";