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Document compile-time options.

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Christopher Wellons 2017-03-05 16:26:44 -05:00
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@ -101,3 +101,55 @@ The process for decrypting a file:
4. Initialize ChaCha20 with the shared secret as the key.
5. Decrypt the ciphertext using ChaCha20.
6. Verify `sha256(key + sha256(plaintext))`.
## Compile-time configuration
Various options and defaults can be configured at compile time using C
defines (`-D...`). These also apply to the amalgamation build.
### `ENCHIVE_RANDOM_DEVICE`
For unix-like systems, this is the default source of entropy when
creating keys and IVs. The default value is `/dev/urandom`. You could
set this to `/dev/random`, though that's [pointless][djb] and [a waste
of time][myths]. It can be changed at run time with `--random-device`.
In the future, Enchive may first try `getrandom(2)` / `getentropy(2)`.
### `ENCHIVE_OPTION_RANDOM_DEVICE`
Whether or not the `--random-device` option should be available. This
option is 0 by default on Windows, where Enchive always uses a
[Cryptographic Service Provider][csp].
### `ENCHIVE_OPTION_AGENT`
Whether to expose the `--agent` and `--no-agent` option. This option
is 0 by default on Windows since agents are unsupported.
### `ENCHIVE_AGENT_TIMEOUT`
The default agent timeout in seconds. This can be configured at run
time with an optional argument to `--agent`.
### `ENCHIVE_AGENT_DEFAULT_ENABLED`
Whether or not to enable the agent by default. This can be explicitly
overridden at run time with `--agent` and `--no-agent`.
### `ENCHIVE_KEY_DERIVE_ITERATIONS`
Power-of-two exponent for protection key derivation. Can be configured
at run time with `--iterations`.
### `ENCHIVE_SECKEY_DERIVE_ITERATIONS`
Power-of-two exponent for secret key derivation. Can be configured at
run time with the optional argument to `--derive`.
[myths]: http://www.2uo.de/myths-about-urandom/
[djb]: https://blog.cr.yp.to/20140205-entropy.html
[getrandom]: https://manpages.debian.org/testing/manpages-dev/getrandom.2.en.html
[getentropy]: http://man.openbsd.org/OpenBSD-current/man2/getentropy.2
[csp]: https://msdn.microsoft.com/en-us/library/windows/desktop/aa380246(v=vs.85).aspx