From: "Frykholm, Niklas" <nfrykholm@...>
Date: 2001-10-15T18:01:16+09:00
Subject: [ruby-talk:22558] Re: OSSL opinion

>Out of interest could you elaborate on what the things noted below
>"means":
>
>> 	- allows you to plugin any backend (provider)
>>
>Whats the idea behind this? Whats the main advantages?

The idea is that Java specifies an API that is independant of the
implementation. Then you can plug-in a particular implementation (called a
Cryptographic Service Provider, CSP) without having to change your code. You
can have several CSPs installed that provide different algorithms, different
speed of implementation, etc. If you then request a "Blowfish" algorithm,
Java checks all your CSPs to see if you have someone installed that supports
this algorithm. (You can also request to get the algorithm from a particular
CSP.)

The advantages are flexibility and interoperability. Not only Java does
this. Microsoft has a crypto API (MSCAPI) that lets you plug-in a backend,
Netscape uses another API (PKCS #11) which lets you change the backend. (As
usual, there are many standards to choose from :) I haven't worked with
these API's directly and am not sure whether you can have multiple CSPs
running simultaneously, as in Java, but you can do such things as writing a
CSP that lets the encryption be done by a smartcard instead of by software
on the computer. 

>>             + modes of operation (ECB, OCB, CTR...)
>>
>What are these acronyms?

Block-cipher algorithms (AES, Blowfish, DES, etc...) only work at one block
of data at a time, i.e. they take a chunk (typically 64 or 128 bits) and
encrypt it as another chunk of the same size.

If you want to encrypt something larger than a block (such as a file), you
must specify how this should be done. The simplest is to just encrypt each
block of the file. This is called Electronic Codebook Mode (ECB). You also
need to specify the length of the file (since it may not be an integral
number of blocks) in some way.

ECB has some disadvantages. For example, if the same block is encrypted
twice, the encryption looks the same. So if you send me the answer to three
questions

	YES	YES	NO

and we assume that each answer is encoded in a 64 bit structure (or whatever
the block size is) --- this is inefficient, but bear with me --- then
encoded with ECB it would look something like this

	H8GF...  H8GF...  PUt7...

From this, an attacker could conclude that you have given the same answer to
the two first questions and a different answer to the third question, i.e.
he will know that you either answered NO NO YES or YES YES NO.

The other modes deal with this problem. You will find descriptions of them
in standard cryptographics texts or on the internet (search for "modes of
operation" or something similar).

>Please excuse my ignorance...

We are all ignorant about something.

-- Niklas