Mastering Assembly Programming

Encryption/decryption

As we are developing a crypto core here, it is natural to begin with the implementation of the cryptographic functionality first. The following code performs the encryption of data according to the algorithm we previously defined:

f_encrypt:   ; The if statement below, when the condition is TRUE, forces the assembler to produce   ; 32-bit code   if (ACTIVE_TARGET = TARGET_W32_OBJ) |\      (ACTIVE_TARGET = TARGET_W32_DLL) |\      (ACTIVE_TARGET = TARGET_L32_O)      push eax ebx esi edi ecx      lea esi, [data_pointer]      mov esi, [esi]      mov edi, esi      lea ebx, [data_length]      mov ebx, [ebx]      lea ecx, [key]      mov cx, [ecx]      and cl, 0x07   ; Encryption loop   @@:      lodsb      rol al, cl      xor al, ch      stosb      dec ebx      or ebx, 0      jnz @b      pop ecx edi esi ebx eax      ret; In general, ...