53 lines
2.5 KiB
Plaintext
53 lines
2.5 KiB
Plaintext
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 768 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 768 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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Twofish | 4,256 |
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Blowfish | 4,168 |
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3DES | 768 |
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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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RC2 | 256 |
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DES | 256 |
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SAFER [#] | 217 |
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RC5 | 204 |
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Twofish [*] | 193 |
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RC6 | 176 |
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CAST5 | 132 |
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Noekeon | 32 |
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Skipjack | 10 |
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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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Noekeon is a fairly fast cipher and uses very little memory. Ideally in low-ram platforms all other ciphers should be
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left undefined and Noekeon should remain. While Noekeon is generally considered a secure block cipher (it is insecure
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as a hash) CAST5 is perhaps a "runner-up" choice. CAST5 has been around longer (it is also known as CAST-128) and is
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fairly fast as well.
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You can easily accomplish this via the "config.pl" script. Simply answer "n" to all of the ciphers except the one you want
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and then rebuild the library. [or you can hand edit tomcrypt_custom.h]
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