Mercurial > dropbear
view libtomcrypt/notes/tech0003.txt @ 1499:2d450c1056e3
options: Complete the transition to numeric toggles (`#if')
For the sake of review, this commit alters only the code; the affiliated
comments within the source files also need to be updated, but doing so
now would obscure the operational changes that have been made here.
* All on/off options have been switched to the numeric `#if' variant;
that is the only way to make this `default_options.h.in' thing work
in a reasonable manner.
* There is now some very minor compile-time checking of the user's
choice of options.
* NO_FAST_EXPTMOD doesn't seem to be used, so it has been removed.
* ENABLE_USER_ALGO_LIST was supposed to be renamed DROPBEAR_USER_ALGO_LIST,
and this commit completes that work.
* DROPBEAR_FUZZ seems to be a relatively new, as-yet undocumented option,
which was added by the following commit:
commit 6e0b539e9ca0b5628c6c5a3d118ad6a2e79e8039
Author: Matt Johnston <[email protected]>
Date: Tue May 23 22:29:21 2017 +0800
split out checkpubkey_line() separately
It has now been added to `sysoptions.h' and defined as `0' by default.
* The configuration option `DROPBEAR_PASSWORD_ENV' is no longer listed in
`default_options.h.in'; it is no longer meant to be set by the user, and
is instead left to be defined in `sysoptions.h' (where it was already being
defined) as merely the name of the environment variable in question:
DROPBEAR_PASSWORD
To enable or disable use of that environment variable, the user must now
toggle `DROPBEAR_USE_DROPBEAR_PASSWORD'.
* The sFTP support is now toggled by setting `DROPBEAR_SFTPSERVER', and the
path of the sFTP server program is set independently through the usual
SFTPSERVER_PATH.
| author | Michael Witten <mfwitten@gmail.com> |
|---|---|
| date | Thu, 20 Jul 2017 19:38:26 +0000 |
| parents | 6dba84798cd5 |
| children |
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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]