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- 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 768 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 768 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) |
- ------------+-------------------------------|
- Twofish | 4,256 |
- Blowfish | 4,168 |
- 3DES | 768 |
- SAFER+ | 532 |
- Serpent | 528 |
- Rijndael | 516 |
- XTEA | 256 |
- RC2 | 256 |
- DES | 256 |
- SAFER [#] | 217 |
- RC5 | 204 |
- Twofish [*] | 193 |
- RC6 | 176 |
- CAST5 | 132 |
- Noekeon | 32 |
- Skipjack | 10 |
- ------------+-------------------------------/
- 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.
- You can easily accomplish this via the "config.pl" script. Simply answer "n" to all of the ciphers except the one you want
- and then rebuild the library. [or you can hand edit tomcrypt_custom.h]
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