@c acl_blockcipher.texi @section Block ciphers A block cipher is a algorithm which turns an input of fixed length into an output of the same length (enciphering or encrypting). The transformation is specified by a key which has to be of a fixed length, or a length of a given set or range. Generally there is also an algorithm which turns the output back to the previous input (deciphering or decrypting) when supplied with the same key. @subsection List of available block ciphers This is a list of the currently supported block ciphers: @itemize @bullet @item AES (Advanced Encryption Standard) @item Camellia @item CAST5 @item CAST6 @item CS-Cipher @item DES (Data Encryption Standard) @item Khazad @item Noekeon @item Present @item RC5 @item RC6 @item Seed @item Serpent (AES finalist) @item Shacal1 @item Shacal2 @item Skipjack @item TDES (Tripple DES) @item Threefish @item XTEA @end itemize @subsection high frequent parameters: @table @asis @item block size 64 bits, 128 bits @item key size 64 bits, 80 bits, 128 bits, 192 bits, 256 bits (note that some block ciphers use different sizes) @end table @subsection Parts of a block cipher @itemize @bullet @item encryption algorithm @item decryption algorithm @item mostly a set of subkeys @item mostly a keyschedule which generates the subkeys from the supplied key. @end itemize As we can see here a block cipher normally has an algorithm besides the encryption and decryption algorithm, which we call keyschedule. Mostly the encryption and decryption algorithm consist of multiple rounds, where each round (and sometimes between rounds) subkeys are needed to modify the data. This subkeys are generated by the keyschedule and stored in a state or context variable. Note that not all algorithms need a pregenerated context, sometimes it is easy to generate the subkeys "on the fly" so there is not always the need of a context variable. In this case instead of a context the actual key is passed to the encryption and decryption function. @subsection API of block ciphers 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: @table @code @item *_init function, which implements the keyschedule @item *_enc function, which implements the encryption algorithm @item *_dec function, which implements the decryption algorithm @item *_free function, which frees memory allocated for the keyschedule @item *_ctx_t context type, which can contain a keyschedule and other information @end table @subsubsection @code{*_init} function The @code{*_init} function generally takes a pointer to the key as first parameter. For ciphers where the keysize is not fixed the second parameter gives the keysize (in bits regularly) and the last parameter points to the context variable to fill. For some ciphers there are additional parameters like the number of rounds, these parameters generally occur before the context pointer. @subsubsection @code{*_enc} and @code{*_dec} functions The encryption and decryption function of a specific algorithm normally do not differ in their parameters. Generally these functions take a pointer to the block to operate on. Some ciphers allow to specify two blocks, where the first one will be written to and the second will contain the source block. The two blocks may overlap or be the same. Most ciphers have only one block pointer. The block specified by the pointer is encrypted (if the @code{*_enc} function is called) or decrypted (if the @code{*_dec} function is called). The last parameter specifies either the key direct (with a pointer to it) or is a pointer to a context created with the @code{*_init} function. It is guaranteed that the context is in the same state as before the *_enc or @code{*_dec} function call. Most @code{*_enc} and @code{*_dec} functions do not modify the context at all, but some do for reducing dynamic memory requirements. So here are some limitations to the reentrant property. @subsubsection @code{*_free} function A @code{*_free} function is only provided where needed (so most ciphers do not have it). It is used to free memory dynamically allocated by the @code{*_init} function. @subsubsection *_ctx_t type A variable of the @code{*_ctx_t} type may hold information needed by the @code{*_enc} or @code{*_dec} function. It is initialized by the @code{*_init} function. If dynamic memory is allocated by the @code{*_init} function also a @code{*_free} function is provided which frees the allocated memory. An initialized @code{*_ctx_t} variable may not be copied as it may contains pointers to itself. @section Block cipher abstraction layer (BCAL) The BlockCipeherAbstractionLayer (BCAL) is an abstraction layer which allows usage of all implemented block ciphers in a simple way. It abstracts specific function details and is suitable for implementations which want to be flexible in the choosing of specific block ciphers. Another important aspect is that this abstraction layer enables the implementation of block cipher operating modes independently from concrete ciphers. It is very simple to use and reassembles the API used to implement individual ciphers. The main component is a block cipher descriptor which contains the details of the individual ciphers. Care should be taken when choosing a specific keysize. It may be the case that the chosen keysize is not compatible with the chosen block cipher. @subsection Parts of BCAL The BCAL is split up in different parts: @itemize @bullet @item BCAL declaration for BCAL decriptors @item algorithm specific definitions of BCAL decriptors @item BCAL basic context type @item BCAL basic functions @end itemize @subsubsection BCAL declaration for BCAL decriptors The BCAL descriptor is a structure which is usually placed in FLASH or ROM since modification is unnecessary. It contains all information required to use the according block cipher. @verbatim typedef struct { uint8_t type; /* 1==block cipher */ uint8_t flags; PGM_P name; uint16_t ctxsize_B; uint16_t blocksize_b; bc_init_fpt init; bc_enc_fpt enc; bc_dec_fpt dec; bc_free_fpt free; PGM_VOID_P valid_keysize_desc; } bcdesc_t; /* block cipher descriptor type */ @end verbatim @table @var @item type should be set to @samp{1} to indicate that this descriptor is for a block cipher. @item flags defines what kind of init function is provided and what kind of decrypt and encrypt functions are provided. @table @asis @item bit 0 if clear (@samp{0}) designates an init function with fixed key length, so the length parameter is omitted (@code{init(void* ctx, void* key)}). if set (@samp{1}) designates an init function which requires an explicit keysize argument (@code{init(void*ctx, uint16_t length_b, void* key)}). @item bit 1 if clear (@samp{0}) designates that the encryption function transforms the plaintext block in place to the ciphertext (@code{enc(void* block, void* ctx)}). if set (@samp{1}) designates that the encryption function offers a dedicated pointers for input and output. The two regions may be the same (@code{enc(void* out, void* in, void*ctx)}). @item bit 2 if clear (@samp{0}) designates that the decryption function transforms the ciphertext block in place to the plaintext (@code{dec(void* block, void* ctx)}). if set (@samp{1}) designates that the decryption function offers a dedicated pointers for input and output. The two regions may be the same (@code{dec(void* out, void* in, void*ctx)}). @end table @item name is a pointer to a zero terminated ASCII string giving the name of the implemented primitive. On targets with Harvard-architecture the string resides in code memory (FLASH, ROM, ...). @item ctxsize_B is the number of bytes which should be allocated for the context variable. @item blocksize_b is the number of bits on which the encrypt and decrypt function work on. @item init is a pointer to the init function (see @samp{flags} how the init function should be called). If there is no init function this field is NULL. @item enc is a pointer to the encryption function (see @samp{flags} how the encryption function should be called). @item dec is a pointer to the decryption function (see @samp{flags} how the decryption function should be called). @item free is a pointer to the free function or NULL if there is no free function. @item valid_keysize_desc is a pointer to a keysize descriptor structure which is used to validate that the chosen keysize is valid @end table @subsubsection BCAL-Basic context Besides the context types for individual ciphers there is a generic context type for BCAL. This is the context to use when using BCAL based functions. The BCAL context has the following structure: @verbatim typedef struct{ bcdesc_t* desc_ptr; uint16_t keysize; void* ctx; } bcgen_ctx_t; @end verbatim @table @code @item desc_ptr a pointer to the BCAL descriptor @item keysize the chosen keysize @item ctx pointer to the cipher specific context @end table @subsubsection BCAL-Basic BCAL-Basic provides the basic features of an block cipher on top of the BCAL. To use it you simply have to include the algorithms you want to use, the BCAL descriptor file and of course the BCAL-Basic implementation. The following functions are provided: @table @code @item bcal_cipher_init @code{uint8_t bcal_cipher_init(const bcdesc_t* cipher_descriptor, const void* key, uint16_t keysize_b, bcgen_ctx_t* ctx)} this function initializes a BCAL context based on the given BCAL descriptor pointer (first parameter) with a given key (second parameter) of a given length (third parameter). The context to initialize is designated by the pointer passed as fourth parameter. If everything works fine @samp{0} is returned. In the case something fails the following codes are returned: @table @samp @item 1 The specified keysize is not available with this cipher @item 2 It was not possible to allocate enough memory to hold the key. (This is returned when there is no actual init function and you ran out of memory) @item 3 It was not possible to allocate enough memory to hold the context variable for the selected cipher. @end table @item bcal_cipher_free @code{void bcal_cipher_free(bcgen_ctx_t* ctx)} this function frees the memory allocated by the init function and should be called whenever you are finished with BCAL context. It automatically also calls the @code{free} function if necessary. @item bcal_cipher_enc @code{void bcal_cipher_enc(void* block, const bcgen_ctx_t* ctx)} this function encrypts a block in-place using a given BCAL contex. @item bcal_cipher_dec @code{void bcal_cipher_dec(void* block, const bcgen_ctx_t* ctx)} this function decrypts a block in-place using a given BCAL contex. @item bcal_cipher_getBlocksize_b @code{uint16_t bcal_cipher_getBlocksize_b(const bcdesc_t* desc)} this function returns the block size of a given cipher by using the BCAL descriptor (to which a pointer must be passed). @item bcal_cipher_getKeysizeDesc @code{PGM_VOID_P bcal_cipher_getKeysizeDesc(const bcdesc_t* desc)} this function returns a pointer to the keysize descriptor of a given cipher by using the BCAL descriptor (to which a pointer must be passed). @end table @subsection 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: @itemize @bullet @item simple list of valid keysizes @item range of keysizes @item augmented range of keysizes @end itemize @subsubsection simple list of valid keysizes The simple keysize list has the following structure: @verbatim 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; @end verbatim 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. @subsubsection range of keysizes This method specifies an entire range of keys a valid using the following structure: @verbatim typedef struct{ /* keysize is valid if min<=keysize<=max */ uint16_t min; uint16_t max; }keysize_desc_range_t; @end verbatim So all keysizes between @code{min} and @code{max} (including @code{min} and @code{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. @subsubsection augmented range of keysizes The augmented range of keysizes uses the following structure: @verbatim 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; @end verbatim 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 @code{offset} when divided by @code{distance}. So you can limit a keysize to full bytes by simply setting @code{distance} to @samp{8} and @code{offset} to @samp{0}. @subsubsection 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 @code{NULL} byte. The designator byte can have one of the following values: @table @samp @item 0x00 Terminator byte, signals the end of the list @item 0x01 simple list of keysizes @item 0x02 simple range of keysizes @item 0x03 augmented range of keysizes @end table