Quick introduction to reverse engineering for beginners

.text:0054134A push edi
.text:0054134B push 2 ; Origin
.text:0054134D push 0 ; Offset
.text:0054134F push ebp ; File
; move current file position to the end
.text:00541350 call _fseek
.text:00541355 push ebp ; File
.text:00541356 call _ftell ; get current file position
.text:0054135B push 0 ; Origin
.text:0054135D push 0 ; Offset
.text:0054135F push ebp ; File
.text:00541360 mov [esp+2Ch+Str], eax
; move current file position to the start
.text:00541364 call _fseek
This piece of code calculates file size aligned to 64-byte border. This is because this cryptoalgorithm
works with only 64-byte blocks. Its operation is pretty simple: divide file size by 64, forget about remainder
and add 1, then multiple by 64. The following code removes remainder as if value was already divided by 64
and adds 64. It is almost the same.
.text:00541369 mov esi, [esp+2Ch+Str]
.text:0054136D and esi, 0FFFFFFC0h ; reset all lowest 6 bits
.text:00541370 add esi, 40h ; align size to 64-byte border
Allocate buffer with aligned size:
.text:00541373 push esi ; Size
.text:00541374 call _malloc
Call memset(), e,g, clear allocated buffer 2 .
.text:00541379 mov ecx, esi
.text:0054137B mov ebx, eax ; allocated buffer pointer -> to EBX
.text:0054137D mov edx, ecx
.text:0054137F xor eax, eax
.text:00541381 mov edi, ebx
.text:00541383 push ebp ; File
.text:00541384 shr ecx, 2
.text:00541387 rep stosd
.text:00541389 mov ecx, edx
.text:0054138B push 1 ; Count
.text:0054138D and ecx, 3
.text:00541390 rep stosb ; memset (buffer, 0, aligned_size)
Read file via standard C function fread().
.text:00541392 mov eax, [esp+38h+Str]
.text:00541396 push eax ; ElementSize
.text:00541397 push ebx ; DstBuf
.text:00541398 call _fread ; read file
.text:0054139D push ebp ; File
.text:0054139E call _fclose
Call crypt(). This function takes buffer, buffer size (aligned) and password string.
2 malloc() + memset() could be replaced by calloc()
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