46 lines
2.3 KiB
Plaintext
46 lines
2.3 KiB
Plaintext
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Tech Note 0003
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Minimizing Memory Usage
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Tom St Denis
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Introduction
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------------
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For the most part the library can get by with around 20KB of stack and about 32KB of heap even if you use the
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public key functions. If all you plan on using are the hashes and ciphers than only about 1KB of stack is required
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and no heap.
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To save space all of the symmetric key scheduled keys are stored in a union called "symmetric_key". This means the
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size of a symmetric_key is the size of the largest scheduled key. By removing the ciphers you don't use from
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the build you can minimize the size of this structure. For instance, by removing both Twofish and Blowfish the
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size reduces to 528 bytes from the 4,256 bytes it would have been (on a 32-bit platform). Or if you remove
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Blowfish and use Twofish with TWOFISH_SMALL defined its still 528 bytes. Even at its largest the structure is only
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4KB which is normally not a problem for any platform.
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Cipher Name | Size of scheduled key (bytes) |
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------------+-------------------------------|
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Blowfish | 4,168 |
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RC5 | 204 |
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RC6 | 176 |
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SAFER+ | 532 |
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Serpent | 528 |
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Rijndael | 516 |
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XTEA | 256 |
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Twofish | 4,256 |
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Twofish [*] | 193 |
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SAFER [#] | 217 |
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RC2 | 256 |
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DES | 256 |
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3DES | 768 |
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CAST5 | 132 |
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------------+-------------------------------/
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Memory used per cipher on a 32-bit platform.
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[*] For Twofish with TWOFISH_SMALL defined
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[#] For all 64-bit SAFER ciphers.
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Following this chart its ideal that in extremely low memory platforms that all of the ciphers are disabled and CAST5 is
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left. CAST5 is a fairly fast cipher on all platforms which makes it ideally suited. It should be noted that the
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SAFER and SAFER+ keys are formed of arrays of unsigned char. So in effect on platforms where "unsigned long" is
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8 bytes SAFER would have the smallest key (CAST5 would come out to 264 bytes). In this case I would recommend
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SAFER-SK128.
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