Mercurial > dropbear
comparison notes/tech0003.txt @ 3:7faae8f46238 libtomcrypt-orig
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| author | Matt Johnston <matt@ucc.asn.au> |
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| date | Mon, 31 May 2004 18:25:41 +0000 |
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| 1 Tech Note 0003 | |
| 2 Minimizing Memory Usage | |
| 3 Tom St Denis | |
| 4 | |
| 5 Introduction | |
| 6 ------------ | |
| 7 | |
| 8 For the most part the library can get by with around 20KB of stack and about 32KB of heap even if you use the | |
| 9 public key functions. If all you plan on using are the hashes and ciphers than only about 1KB of stack is required | |
| 10 and no heap. | |
| 11 | |
| 12 To save space all of the symmetric key scheduled keys are stored in a union called "symmetric_key". This means the | |
| 13 size of a symmetric_key is the size of the largest scheduled key. By removing the ciphers you don't use from | |
| 14 the build you can minimize the size of this structure. For instance, by removing both Twofish and Blowfish the | |
| 15 size reduces to 768 bytes from the 4,256 bytes it would have been (on a 32-bit platform). Or if you remove | |
| 16 Blowfish and use Twofish with TWOFISH_SMALL defined its still 768 bytes. Even at its largest the structure is only | |
| 17 4KB which is normally not a problem for any platform. | |
| 18 | |
| 19 | |
| 20 Cipher Name | Size of scheduled key (bytes) | | |
| 21 ------------+-------------------------------| | |
| 22 Twofish | 4,256 | | |
| 23 Blowfish | 4,168 | | |
| 24 3DES | 768 | | |
| 25 SAFER+ | 532 | | |
| 26 Serpent | 528 | | |
| 27 Rijndael | 516 | | |
| 28 XTEA | 256 | | |
| 29 RC2 | 256 | | |
| 30 DES | 256 | | |
| 31 SAFER [#] | 217 | | |
| 32 RC5 | 204 | | |
| 33 Twofish [*] | 193 | | |
| 34 RC6 | 176 | | |
| 35 CAST5 | 132 | | |
| 36 Noekeon | 32 | | |
| 37 Skipjack | 10 | | |
| 38 ------------+-------------------------------/ | |
| 39 Memory used per cipher on a 32-bit platform. | |
| 40 | |
| 41 [*] For Twofish with TWOFISH_SMALL defined | |
| 42 [#] For all 64-bit SAFER ciphers. | |
| 43 | |
| 44 Noekeon is a fairly fast cipher and uses very little memory. Ideally in low-ram platforms all other ciphers should be | |
| 45 left undefined and Noekeon should remain. While Noekeon is generally considered a secure block cipher (it is insecure | |
| 46 as a hash) CAST5 is perhaps a "runner-up" choice. CAST5 has been around longer (it is also known as CAST-128) and is | |
| 47 fairly fast as well. | |
| 48 | |
| 49 You can easily accomplish this via the "config.pl" script. Simply answer "n" to all of the ciphers except the one you want | |
| 50 and then rebuild the library. [or you can hand edit mycrypt_custom.h] | |
| 51 | |
| 52 |