Tech Note 0003
Minimizing Memory Usage
Tom St Denis

Introduction
------------

For the most part the library can get by with around 20KB of stack and about 32KB of heap even if you use the
public key functions.  If all you plan on using are the hashes and ciphers than only about 1KB of stack is required
and no heap.

To save space all of the symmetric key scheduled keys are stored in a union called "symmetric_key".  This means the 
size of a symmetric_key is the size of the largest scheduled key.  By removing the ciphers you don't use from
the build you can minimize the size of this structure.  For instance, by removing both Twofish and Blowfish the
size reduces to 528 bytes from the 4,256 bytes it would have been (on a 32-bit platform).  Or if you remove
Blowfish and use Twofish with TWOFISH_SMALL defined its still 528 bytes.  Even at its largest the structure is only 
4KB which is normally not a problem for any platform.  


Cipher Name | Size of scheduled key (bytes) |
------------+-------------------------------|
Blowfish    | 4,168                         |
RC5         | 204                           |
RC6         | 176                           |
SAFER+      | 532                           |
Serpent     | 528                           |
Rijndael    | 516                           |
XTEA        | 256                           |
Twofish     | 4,256                         |
Twofish [*] | 193                           |
SAFER [#]   | 217                           |
RC2         | 256                           |
DES         | 256                           |
3DES        | 768                           |
CAST5       | 132                           |
Noekeon     | 32                            |
------------+-------------------------------/
Memory used per cipher on a 32-bit platform.

[*] For Twofish with TWOFISH_SMALL defined
[#] For all 64-bit SAFER ciphers.

Noekeon is a fairly fast cipher and uses very little memory.  Ideally in low-ram platforms all other ciphers should be
left undefined and Noekeon should remain.  While Noekeon is generally considered a secure block cipher (it is insecure
as a hash) CAST5 is perhaps a "runner-up" choice.  CAST5 has been around longer (it is also known as CAST-128) and is 
fairly fast as well.  