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