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Full Discussion: UNIX.com response times
Contact Us Post Here to Contact Site Administrators and Moderators UNIX.com response times Post 303005553 by bakunin on Thursday 19th of October 2017 03:46:54 PM
Old 10-19-2017
Quote:
Originally Posted by rbatte1
Is that partly because encryption includes compression?
This is not the case. In fact, SSL works like this (short introduction to encryption theory):

First, we need to establish the difference between asymmetric and symmetric encryption methods.

In symmetric encryption a cipher is used to encrypt as well as decrypt the message. The cipher is shared between the sender and the receiver beforehand. Advantage: keys can be smaller (typically 128-bit or 256-bit) and it allows for two-way communication. Disadvantage: whoever knows the cipher can encode as well as decode it.

Asymmetric encryption works with two different ciphers: one (the "public" key) is used (only!) to encrypt the message. To decrypt it one needs the other "private" cipher. You can send around your public key without caring for who knows it, because only the encryption is possible. As long as you keep your private key to yourself you alone can decrypt anything encrypted with your public key. Advantage: you don't need to share the (private) key with anyone. Disadvantage: allows only a one-way communication and uses significantly larger keys (1024 or 2048 bit for RSA nowadays).

The most common asymmetric algorithms are RSA and elliptic curves (ECC). RSA is based on the fact that integer factorisation is difficult and expensive computation-wise. Basically you build the product of two very large prime numbers: the product is easy to calculate (and published) but without knowing the factors it is difficult to compute them (the private key) from the product. ECC computes the discrete logarithm of a random elliptic curve element. The elliptic curve is built over a Galois field (not the real numbers) and the discrete logarithm is computed in respect to a point at infinity.

As asymmetric encryption only works one-way, how is it used for information exchange, say, between a web server and the browser? The idea is to use a handshake-procedure to establish a session:

1) Server sends his public key to client.
2) Client creates a symmetric session key, encrypts it with the public key of the server and sends it back
3) Server decrypts the session key and
4) both client and server use this symmetric key for the duration of the session

All these algorithms do NOT compress anything at all. In fact they are neutral to the amount of data being transferred.

I hope this helps.

bakunin
This User Gave Thanks to bakunin For This Post:
 

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LWP::RobotUA(3pm)					User Contributed Perl Documentation					 LWP::RobotUA(3pm)

NAME
LWP::RobotUA - a class for well-behaved Web robots SYNOPSIS
use LWP::RobotUA; my $ua = LWP::RobotUA->new('my-robot/0.1', 'me@foo.com'); $ua->delay(10); # be very nice -- max one hit every ten minutes! ... # Then just use it just like a normal LWP::UserAgent: my $response = $ua->get('http://whatever.int/...'); ... DESCRIPTION
This class implements a user agent that is suitable for robot applications. Robots should be nice to the servers they visit. They should consult the /robots.txt file to ensure that they are welcomed and they should not make requests too frequently. But before you consider writing a robot, take a look at <URL:http://www.robotstxt.org/>. When you use a LWP::RobotUA object as your user agent, then you do not really have to think about these things yourself; "robots.txt" files are automatically consulted and obeyed, the server isn't queried too rapidly, and so on. Just send requests as you do when you are using a normal LWP::UserAgent object (using "$ua->get(...)", "$ua->head(...)", "$ua->request(...)", etc.), and this special agent will make sure you are nice. METHODS
The LWP::RobotUA is a sub-class of LWP::UserAgent and implements the same methods. In addition the following methods are provided: $ua = LWP::RobotUA->new( %options ) $ua = LWP::RobotUA->new( $agent, $from ) $ua = LWP::RobotUA->new( $agent, $from, $rules ) The LWP::UserAgent options "agent" and "from" are mandatory. The options "delay", "use_sleep" and "rules" initialize attributes private to the RobotUA. If "rules" are not provided, then "WWW::RobotRules" is instantiated providing an internal database of robots.txt. It is also possible to just pass the value of "agent", "from" and optionally "rules" as plain positional arguments. $ua->delay $ua->delay( $minutes ) Get/set the minimum delay between requests to the same server, in minutes. The default is 1 minute. Note that this number doesn't have to be an integer; for example, this sets the delay to 10 seconds: $ua->delay(10/60); $ua->use_sleep $ua->use_sleep( $boolean ) Get/set a value indicating whether the UA should sleep() if requests arrive too fast, defined as $ua->delay minutes not passed since last request to the given server. The default is TRUE. If this value is FALSE then an internal SERVICE_UNAVAILABLE response will be generated. It will have an Retry-After header that indicates when it is OK to send another request to this server. $ua->rules $ua->rules( $rules ) Set/get which WWW::RobotRules object to use. $ua->no_visits( $netloc ) Returns the number of documents fetched from this server host. Yeah I know, this method should probably have been named num_visits() or something like that. :-( $ua->host_wait( $netloc ) Returns the number of seconds (from now) you must wait before you can make a new request to this host. $ua->as_string Returns a string that describes the state of the UA. Mainly useful for debugging. SEE ALSO
LWP::UserAgent, WWW::RobotRules COPYRIGHT
Copyright 1996-2004 Gisle Aas. This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself. perl v5.14.2 2012-02-11 LWP::RobotUA(3pm)
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