Keysize descriptors
===================

There are a lot of different block ciphers or cryptographic algorithms in general which put several constrains to the number of bits which can be used as key.

Our approach is to find a simple and compact way do specify which lengths are valid and which are not. The system is quite simple, we use a list of patterns (with parameters) and if any matches the keysize is valid, if none matches the keysize is unsupported.

The patterns are:

  * simple list of valid keysizes

  * range of keysizes

  * augmented range of keysizes

simple list of valid keysizes
-----------------------------

The simple keysize list has the following structure:
typedef struct{ /* keysize is valid if listed in items */
	uint8_t  n_items;  /* number of items (value 0 is reserved) */
	uint16_t items[]; /* list of valid lengths */
}keysize_desc_list_t;
First we specify how many keysizes we want to declare valid (this is limited to 255 keysizes but that should not impose any real world constrains). And follow it by the keysizes as 16bit unsigned values.

If you want to declare a lot of keys please check first the other methods since they may give a more compact definition.

range of keysizes
-----------------

This method specifies an entire range of keys a valid using the following structure:
typedef struct{ /* keysize is valid if min<=keysize<=max */
	uint16_t min;
	uint16_t max;
}keysize_desc_range_t;
So all keysizes between min and max (including min and max) are valid. Please note that in most cases also keysizes which are not a multiple of 8 (so are not full bytes) are also matched. If you want to avoid this see the augmented range of keysizes.

augmented range of keysizes
---------------------------

The augmented range of keysizes uses the following structure:
typedef struct{ /* keysize is valid if min<=keysize<=max and if keysize mod distance == offset */
	uint16_t min;
	uint16_t max;
	uint16_t distance;
	uint16_t offset;
}keysize_desc_arg_range_t;
The restriction to a range is the same as with the simpler range of keysizes, but also another restriction is imposed. A valid keysize must have a reminder of offset when divided by distance. So you can limit a keysize to full bytes by simply setting distance to 8 and offset to 0.

the actual descriptor
---------------------

The keysize descriptor is a list of the former patterns. Each pattern is preceded by byte designating the type of pattern and the list is terminated by a NULL byte.

The designator byte can have one of the following values:

0x00
     Terminator byte, signals the end of the list

0x01
     simple list of keysizes

0x02
     simple range of keysizes

0x03
     augmented range of keysizes