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3.1 KiB
76 lines
3.1 KiB
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= Usage of hash functions =


===================================




Author: Daniel Otte


email: bg@nerilex.org






0. Foreword


This file will describe how to use the hash function implementations provided


by this library.




1. What a hash function does


A hash function is an algorithm to map an arbitrary long message (in the form


of a bit string) to a fixed length message digest or hash value.


The hash function aims to be collision free, which means that it is not


practicable to find two messages with the same hash value (although this


collision must exist). Also it should not be practicable to construct a


message which maps to a given hash value.






1.1. high frequent parameters:


block size: 512 bits


hash value size: 128 bits, 160 bits, 224 bits, 256 bits, 384 bits, 512 bits




2. Parts of a hash function


* initialization function


* compression algorithm


* finalization function




3. hash function API


The API is not always consistent due to the fact that we tried to optimize the


code for size (flash, heap and stack) and speed (runtime of the different


components).


Generally the API of the implemented block ciphers consists of:




*_init function, which implements the initialisation of the context


*_nextBlock function, which implements the compression algorithm


*_lastBlock function, which implements the the padding algorithm


*_ctx2hash function, which turns a context into an actual hash value


*_ctx_t context type, which can contains the state of a hashing process




3.1 look at the prototypes


Generally the prototypes (defined in the *.h files) will tell you what


parameter means what.




3.1.2 sizes in bits and bytes


Working with cryptographic functions involves working with different


lengths. Some times you want to know it in bits and sometimes in bytes. To


reduce frustration and to avoid bugs we suffix a length parameter with either


_b or _B depending on the meaning. _b means in bits and _B means in bytes


(big b big word).




3.2. *_init function


The *_init function generally takes a pointer to the context as parameter.


This function initializes the context with algorithm specific values.




3.3. *_nextBlock function


The *_nextBlock function is the core of each hash function. It updates the hash


state with a given message block. So this function uses a context pointer and


a message pointer as parameters. The size of a message block is fixed for each


hash function (mostly 512 bit). For the last block of a messages which may be


smaller than the blocksize you have to use the *_lastBlock function described


below.




3.4 *_lastBlock function


The *_lastBlock function finalizes the context with the last bits of a


message. Since the last block is not required to have the blocksize you have


to specify the length of the last block (normally in bits). This function


performs the padding and final processing.




3.5. *_ctx2hash function


The *_ctx2hash function turns a given hash context into an actual hash value.


If multiple sized hash value may be created from a context it is necessary to


give the the size of the hash value as parameter.




