HPAVC/HPAVC CD-ROM.iso/pc/BF_SDK11.ZIP/ASM.TXT

Text File | 1996-06-10 | 11.4 KB | 276 lines

                                    BF-SDK
                                    ~~~~~~
                                     V1.1

                              Assemblerinterface
                              ~~~~~~~~~~~~~~~~~~

                          (c) 1996 Cedric Reinartz


This documentation helps you to use the assembler interface in your own
program. A typical program structure would be:

   - declaration of some variables
   - include BFE_ASM.INC
   - definition of your data (.DATA)
   - your code (.CODE) where you can use the BF-Macros

In the following only the first and last part will be explained. Please
have a look also in BFE_ASM.INC, BFENG386.ASM and ASM_DEMO.ASM to fully
understand the code.

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

VARIABLES
~~~~~~~~~


NOPUB

With this variable you can decide if BFENG386.ASM will be included or if
BFENG386.OBJ will be used. If "noPub" is not defined the functions in BFENG386
will be exported and you can (must) use BFENG386.OBJ. In this case you can not
influence the behavior of BFENG386 further.
Notice that BFENG386.OBJ which is shipped with this package is compiled with
defaults preset in BFENG386.ASM. If you want to use the OBJ with different
options you have to change the defaults and recompile.
An example for this kind of use is the generation of the pascal unit in this
package (BLOWFISH.TPU).
If "noPub" is defined (the value doesn't matter) you include BFENG386.ASM in
your own code. In this case there are variables which influence how
BFENG386.ASM will be compiled:
Default: not defined

-------------------------------------------------------------------------------

USESMALL

If "useSmall" is not defined the .model large directive will be used. This
will result in CALLs generated as FAR by the assembler. If you want NEAR
CALLs (for speed reasons as example) you have to define "useSmall" (value
doesn't matter).
Default: not defined

-------------------------------------------------------------------------------

NOLOOP

If "noLOOP" is defined with the value "1" all the encryption rounds are placed
after each other. This is very memory consuming, but eliminates the need for
LOOPs and thus gives more speed. Any other value will generate a LOOP
construction which is much smaller and about 20% slower than the enrolled
version.
Use this for TSR's and device drivers which handle slow devices.
Note: This variable must be defined.
Default: 1

-------------------------------------------------------------------------------

RNDS

With "rnds" you can define how many rounds Blowfish should go. There
are 3 possible values:

       1:  There are always 16 Rounds to go
       2:  There are always 32 Rounds to go
       3:  You switch between 16 and 32 Rounds by software

The different values are all due to speed and memory reasons. A "3" is the most
flexible one. If 16 Rounds are set with "rnds equ 3" the code is a bit slower
and slightly bigger than using "rnds equ 1".
If you use "rnds equ 3" in conjunction with "noLOOP equ 0" you are able to set
the number of rounds from 2 to 32. Remember that the security enhances with a
increasing number of rounds (you should use at least 14), while the speed
decreases with increasing rounds.
Note: This variable must be defined.
Default: 3

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

MACROS
~~~~~~


_InitCrypt seg,off,len

 Function   : Initialises the Boxes
 Argument   : seg: segment of the password
              off: offset of the password
              len: length of the password
 Return     : none
 Affects    : all but DS
 Description: A Password with a maximum of 56 Bytes is used to encrypt the
              Boxes. These encrypted boxes are vital for the further encryption
              process with _(CBC)Encrypt and _(CBC)Decrypt. If you want to
              change the password at runtime you have to restore the original
              contents of the boxes before you can use this function again
              (See _GetBoxes and _SetBoxes).

-------------------------------------------------------------------------------

_WeakKey

 Function   : Test if the generated Boxes are weak
 Argument   : none
 Return     : AX = Status (1=weak, 0=good)
 Affects    : AX, BX, CX, DX, SI, DI, direction Flag
 Description: After "_InitCrypt" you should test the Boxes with this function.
              If they provide a weakness which a cryptoanalyst could use to
              break the cipher a "1" is returned. In this case you should
              reload the original boxes and let the user choose a different
              password.

-------------------------------------------------------------------------------

_Encrypt seg,off,len

 Function   : Encrypts a plaintext string using the ECB method
 Argument   : seg: segment of the plaintext
              off: offset of the plaintext
              len: length of the plaintext
 Return     : none
 Affects    : all but DS
 Description: This function encrypts (multiple) blocks of 8 Bytes of plaintext
              using the ECB method (blocks are not chained). The generated
              ciphertext will overwrite the plaintext. Thus the seg:off
              argument will point to the ciphertext after the encryption. The
              length of the plaintext must be a multiple of 8. If it is not,
              plaintext bytes will be left at the end of the ciphertext.

-------------------------------------------------------------------------------

_Decrypt seg,off,len

 Function   : Decrypts a ciphertext string using the ECB method
 Argument   : seg: segment of the ciphertext
              off: offset of the ciphertext
              len: length of the ciphertext
 Return     : none
 Affects    : all but DS
 Description: This function decrypts (multiple) blocks of 8 Bytes of cipher-
              text using the ECB method (blocks are not chained). The generated
              plaintext will overwrite the ciphertext. Thus the seg:off
              argument will point to the plaintext after the decryption. The
              length of the ciphertext must be a multiple of 8. If it is not,
              ciphertext bytes will be left at the end of the plaintext.

-------------------------------------------------------------------------------

_CBCEncrypt seg,off,len,segcl,offcl,segch,offch

 Function   : Encrypts a plaintext string using the CBC method
 Argument   : seg  : segment of the plaintext
              off  : offset of the plaintext
              len  : length of the plaintext
              segcl: segment of the IV (low DWord)
              offcl: offset of the IV (low DWord)
              segch: segment of the IV (high DWord)
              offch: offset of the IV (high DWord)
 Return     : none
 Affects    : all but DS
 Description: This function encrypts (multiple) blocks of 8 Bytes of plaintext
              using the CBC method (blocks are chained using an initialisation
              vector IV). The generated ciphertext will overwrite the plain-
              text. Thus the seg:off argument will point to the ciphertext
              after the encryption. The length of the plaintext must be a
              multiple of 8. If it is not, plaintext bytes will be left at the
              end of the ciphertext. The IV must not be secret. For different
              encryptions you can choose the same IV but if possible you should
              use different ones to improve security.
              Note: you need the original IV for decryption.

-------------------------------------------------------------------------------

_CBCDecrypt seg,off,len,segcl,offcl,segch,offch

 Function   : Decrypts a ciphertext string using the CBC method
 Argument   : seg  : segment of the ciphertext
              off  : offset of the ciphertext
              len  : length of the ciphertext
              segcl: segment of the IV (low DWord)
              offcl: offset of the IV (low DWord)
              segch: segment of the IV (high DWord)
              offch: offset of the IV (high DWord)
 Return     : none
 Affects    : all but DS
 Description: This function decrypts (multiple) blocks of 8 Bytes of ciphertext
              using the CBC method (blocks are chained using an initialisation
              vector IV). The generated plaintext will overwrite the cipher-
              text. Thus the seg:off argument will point to the plaintext
              after the decryption. The length of the ciphertext must be a
              multiple of 8. If it is not, ciphertext bytes will be left at the
              end of the plaintext.
              Note: The IV is the same used on encryption.

-------------------------------------------------------------------------------

_ClearBoxes

 Function   : Clears the Boxes
 Argument   : none
 Return     : none
 Affects    : EAX, CX, DI
 Description: Clears the contents of the P- and S-Boxes. This should be used
              before you exit your program to make sure that no one can get
              your Key.

-------------------------------------------------------------------------------

_SetRounds rnds

 Function   : Set number of rounds. Returns number of rounds.
 Argument   : rnds: # of rounds = 16, 32 or 2-32. See variable RNDS
 Return     : AX = number of rounds
 Affects    : AX
 Description: You always have to give a Argument. If BFENG386.ASM was compiled
              with "rnds equ 3" you are able to set the number of rounds
              between 2 and 32. This function always returns the number of
              rounds actual used. If you supply an invalid argument the
              default is used and returned. For more details see variable
              RNDS.

-------------------------------------------------------------------------------

_GetBoxes seg,off

 Function   : Copies the actual P- and S-Boxes to a user buffer
 Arguments  : seg: segment of the user buffer
              off: offset of the user buffer
 Return     : none
 Affects    : BX, CX, DX, SI, DI, direction flag
 Description: Use this to save the original Boxes. If you want to change the
              Password at runtime you have to restore these Boxes first.
              You can also save the encrypted Boxes and restore them if needed.
              In this way you are able to swap Passwords at high speed.
              The needed buffer size is 1042 for rnds=1 and 1058 for rnds=2 or
              rnds=3

-------------------------------------------------------------------------------

_SetBoxes seg,off

 Function   : Copies a user buffer to the P- and S-Boxes
 Arguments  : seg: segment of the user buffer
              off: offset of the user buffer
 Return     : none
 Affects    : BX, CX, DX, SI, DI, direction flag
 Description: Use this to restore the Boxes with previously saved ones.
              See description of "_GetBoxes".

-------------------------------------------------------------------------------

_GetBoxPointer

 Function   : Returns a Pointer to the beginning of PBox in DX:AX
 Arguments  : none
 Return     : DX:AX = pointer to the Boxes
 Affects    : AX, DX
 Description: If the _DATA segments are combined, DX will be the same as DS.
              For a reason I do not know DX:AX is sometimes wrong, so verify
              this! You can use this function to initialise DS


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