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1 Tech Note 0003
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2 Minimizing Memory Usage
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3 Tom St Denis
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4
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5 Introduction
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6 ------------
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7
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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
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9 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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10 and no heap.
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11
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12 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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13 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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14 the build you can minimize the size of this structure. For instance, by removing both Twofish and Blowfish the
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15 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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16 Blowfish and use Twofish with TWOFISH_SMALL defined its still 768 bytes. Even at its largest the structure is only
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17 4KB which is normally not a problem for any platform.
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18
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19
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20 Cipher Name | Size of scheduled key (bytes) |
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21 ------------+-------------------------------|
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22 Twofish | 4,256 |
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23 Blowfish | 4,168 |
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24 3DES | 768 |
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25 SAFER+ | 532 |
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26 Serpent | 528 |
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27 Rijndael | 516 |
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28 XTEA | 256 |
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29 RC2 | 256 |
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30 DES | 256 |
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31 SAFER [#] | 217 |
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32 RC5 | 204 |
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33 Twofish [*] | 193 |
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34 RC6 | 176 |
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35 CAST5 | 132 |
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36 Noekeon | 32 |
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37 Skipjack | 10 |
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38 ------------+-------------------------------/
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39 Memory used per cipher on a 32-bit platform.
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40
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41 [*] For Twofish with TWOFISH_SMALL defined
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42 [#] For all 64-bit SAFER ciphers.
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43
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44 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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45 left undefined and Noekeon should remain. While Noekeon is generally considered a secure block cipher (it is insecure
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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
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47 fairly fast as well.
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48
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49 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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50 and then rebuild the library. [or you can hand edit mycrypt_custom.h]
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51
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52
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