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Base64Encode(ByRef Bin, n=0)
{
   Static Chars="ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
   m := VarSetCapacity(Bin)
   VarSetCapacity(out,Round(m*1.3))
   Loop % n<1 || n>m ? m : n
      A := *(&Bin+A_Index-1)
      ,m := Mod(A_Index,3)
      ,b := m=1 ? A << 16 : m=2 ? b+(A<<8) : b+A
      ,out .= m ? "" : SubStr(Chars,((b>>18)&63)+1,1) SubStr(Chars,((b>>12)&63)+1,1) SubStr(Chars,((b>>6)&63)+1,1) SubStr(Chars,(b&63)+1,1)
      Return out (m ? SubStr(Chars,((b>>18)&63)+1,1) SubStr(Chars,((b>>12)&63)+1,1) (m=1 ? "==" : SubStr(Chars,((b>>6)&63)+1,1) "=") : "")
}

Base64Decode(ByRef Bin, Code, IsString=0)
{
   Static Chars="ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
   StringReplace Code, Code, =,, All
   VarSetCapacity(Bin, 3*StrLen(Code)//4, 0)
   pos := 0, Address := &Bin
   Loop Parse, Code
   {
      m := A_Index&3
      ,d := InStr(Chars,A_LoopField,1) - 1
      ,b := m ? (m=1 ? d<<18 : b+(d<<24-6*m)) : b+d
      IfEqual,m,0
         Numput(b>>16,Address+0,Pos++,"UChar")
         ,Numput(255 & b>>8,Address+0,Pos++,"UChar")
         ,Numput(255 & b,Address+0,Pos++,"UChar")
   }
   c1 := b>>16, c2 := 255 & b>>8, c3 := 0
   Loop % !!m+(m&1)
      NumPut(c%A_Index%,Address+0,Pos++,"UChar")
   If IsString
      VarSetCapacity(Bin,-1)
}

Base64Encode(ByRef Bin,Length = "")
{
 static CharSet := "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
 If (Length = "")
  Length := StrLen(Bin) << !!A_IsUnicode
 VarSetCapacity(Output,Ceil(Length / 3) * 4), Index := 0, pBin := &Bin
 Loop, % Length // 3
  Temp1 := (*(pBin ++) << 16) | (*(pBin ++) << 8) | (*(pBin ++)), Output .= SubStr(CharSet,((Temp1 >> 18) & 63) + 1,1) . SubStr(CharSet,((Temp1 >> 12) & 63) + 1,1) . SubStr(CharSet,((Temp1 >> 6) & 63) + 1,1) . SubStr(CharSet,(Temp1 & 63) + 1,1)
 Temp2 := Mod(Length,3)
 If (Temp2 = 0)
  Return, Output
 Index := pBin - &Bin, Temp1 := NumGet(Bin,Index,"UChar") << 16
 If (Temp2 = 1)
  Return, Output . SubStr(CharSet,((Temp1 >> 18) & 63) + 1,1) . SubStr(CharSet,((Temp1 >> 12) & 63) + 1,1) . "=="
 Temp1 |= NumGet(Bin,Index + 1,"UChar") << 8
 Return, Output . SubStr(CharSet,((Temp1 >> 18) & 63) + 1,1) . SubStr(CharSet,((Temp1 >> 12) & 63) + 1,1) . SubStr(CharSet,((Temp1 >> 6) & 63) + 1,1) . "="
}

Benchmark:                    Base64 encoding with 1000 character strings
Iterations:                   1000
Control Run:                  0.818053 ms.    (0.000818 ms. per run)
Original Encoder (rseding91): 4233.641312 ms. (4.233641 ms. per run)
Optimised Encoder (Uberi):    1833.870210 ms. (1.833870 ms. per run)

Base64Decode(ByRef Bin,Code,IsString = 0)
{
 static CharSet := "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
 StringReplace Code, Code, =,, All
 Length := StrLen(Code), VarSetCapacity(Bin,Ceil((Length / 4) * 3),0), Index := 1, BinPos := 0
 Loop, % Length // 4
  Temp1 := ((InStr(CharSet,SubStr(Code,Index,1),True) - 1) << 18) | ((InStr(CharSet,SubStr(Code,Index + 1,1),True) - 1) << 12) | ((InStr(CharSet,SubStr(Code,Index + 2,1),True) - 1) << 6) | (InStr(CharSet,SubStr(Code,Index + 3,1),True) - 1), NumPut((Temp1 >> 16) | (((Temp1 >> 8) & 255) << 8) | ((Temp1 & 255) << 16),Bin,BinPos,"UInt"), Index += 4, BinPos += 3
 If (Length & 3)
 {
  Temp1 := ((InStr(CharSet,SubStr(Code,Index,1),True) - 1) << 18) | ((InStr(CharSet,SubStr(Code,Index + 1,1),True) - 1) << 12), NumPut(Temp1 >> 16,Bin,BinPos,"UChar")
  If (Length & 1)
   Temp1 |= ((InStr(CharSet,SubStr(Code,Index + 2,1),True) - 1) << 6), NumPut((Temp1 >> 8) & 255,Bin,BinPos + 1,"UChar")
 }
 If IsString
  VarSetCapacity(Bin,-1)
}

Benchmark:                    Base64 decoding with 1000 character strings
Iterations:                   1000
Control Run:                  0.615920 ms.    (0.000616 ms. per run)
Original Decoder (rseding91): 4210.171410 ms. (4.210171 ms. per run)
Optimised Decoder (Uberi):    3145.511597 ms. (3.145512 ms. per run)

Base64Decode(ByRef Bin,Code,IsString = 0)
{
 static CharSet := "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
 StringReplace Code, Code, =,, All
 Length := StrLen(Code), VarSetCapacity(Bin,Ceil((Length / 4) * 3),0), Index := 1, BinPos := 0
 Loop, % Length // 4
  Temp1 := ((InStr(CharSet,SubStr(Code,Index,1),True) - 1) << 18) | ((InStr(CharSet,SubStr(Code,Index + 1,1),True) - 1) << 12) | ((InStr(CharSet,SubStr(Code,Index + 2,1),True) - 1) << 6) | (InStr(CharSet,SubStr(Code,Index + 3,1),True) - 1), NumPut((Temp1 >> 16) | (((Temp1 >> 8) & 255) << 8) | ((Temp1 & 255) << 16),Bin,BinPos,"UInt"), Index += 4, BinPos += 3
 If (Length & 3)
 {
  Temp1 := ((InStr(CharSet,SubStr(Code,Index,1),True) - 1) << 18) | ((InStr(CharSet,SubStr(Code,Index + 1,1),True) - 1) << 12), NumPut(Temp1 >> 16,Bin,BinPos,"UChar")
  If (Length & 1)
   Temp1 |= ((InStr(CharSet,SubStr(Code,Index + 2,1),True) - 1) << 6), NumPut((Temp1 >> 8) & 255,Bin,BinPos + 1,"UChar")
 }

Base64String := "" ;your string in Base64 goes here, in between the quotation marks
Base64Decode(NormalString,Base64String,1)
MsgBox, The decoded string is "%NormalString%". The Base64 string was "%Base64String%"

;this is the function I posted above
Base64Decode(ByRef Bin,Code,IsString = 0)
{
 static CharSet := "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
 StringReplace Code, Code, =,, All
 Length := StrLen(Code), VarSetCapacity(Bin,Ceil((Length / 4) * 3),0), Index := 1, BinPos := 0
 Loop, % Length // 4
  Temp1 := ((InStr(CharSet,SubStr(Code,Index,1),True) - 1) << 18) | ((InStr(CharSet,SubStr(Code,Index + 1,1),True) - 1) << 12) | ((InStr(CharSet,SubStr(Code,Index + 2,1),True) - 1) << 6) | (InStr(CharSet,SubStr(Code,Index + 3,1),True) - 1), NumPut((Temp1 >> 16) | (((Temp1 >> 8) & 255) << 8) | ((Temp1 & 255) << 16),Bin,BinPos,"UInt"), Index += 4, BinPos += 3
 If (Length & 3)
 {
  Temp1 := ((InStr(CharSet,SubStr(Code,Index,1),True) - 1) << 18) | ((InStr(CharSet,SubStr(Code,Index + 1,1),True) - 1) << 12), NumPut(Temp1 >> 16,Bin,BinPos,"UChar")
  If (Length & 1)
   Temp1 |= ((InStr(CharSet,SubStr(Code,Index + 2,1),True) - 1) << 6), NumPut((Temp1 >> 8) & 255,Bin,BinPos + 1,"UChar")
 }
 If IsString
  VarSetCapacity(Bin,-1)
}

StringCaseSense, On
Chars := "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"

code:="iVBORw0KGgoAAAANSUhEUgAAAAUAAAAFCAYAAACNbyblAAAAHElEQVQI12P4//8/w38GIAXDIBKE0DHxgljNBAAO9TXL0Y4OHwAAAABJRU5ErkJggg=="



Base64Decode(data, code)
BinWrite("reddot.png", data) ; result is a valid png file



Base64Decode(ByRef bin, code) {
   StringReplace code, code, =,,All
msgbox %   VarSetCapacity(bin, 3*StrLen(code)//4, 0)
   Loop Parse, code
   {
      m := A_Index & 3 ; mod 4
      IfEqual m,0, {
         buffer += DeCode(A_LoopField)
         Append(bin, pos, buffer>>16, 255 & buffer>>8, 255 & buffer)
      }
      Else IfEqual m,1, SetEnv buffer, % DeCode(A_LoopField) << 18
      Else buffer += DeCode(A_LoopField) << 24-6*m
   }
   IfEqual m,0, Return
   IfEqual m,2
        Append(bin, pos, buffer>>16)
   Else Append(bin, pos, buffer>>16, 255 & buffer>>8)
}

Append(ByRef bin, ByRef pos, c1, c2="", c3="", c4="") {
   pos += 0
   Loop 4 {
      IfEqual c%A_Index%,, Break
      DllCall("RtlFillMemory",UInt,&bin+pos, UInt,1, UChar,c%A_Index%)
      pos++
   }
}

DeCode(c) { ; c = a char in Chars ==> position [0,63]
   Global Chars
   Return InStr(Chars,c,1) - 1
}

BinWrite(file, ByRef data, n=0, offset=0)
{
   ; Open file for WRITE (0x40..), OPEN_ALWAYS (4): creates only if it does not exists
   h := DllCall("CreateFile","str",file,"Uint",0x40000000,"Uint",0,"UInt",0,"UInt",4,"Uint",0,"UInt",0)
   IfEqual h,-1, SetEnv, ErrorLevel, -1
   IfNotEqual ErrorLevel,0,Return,0 ; couldn't create the file

   m = 0                            ; seek to offset
   IfLess offset,0, SetEnv,m,2
   r := DllCall("SetFilePointerEx","Uint",h,"Int64",offset,"UInt *",p,"Int",m)
   IfEqual r,0, SetEnv, ErrorLevel, -3
   IfNotEqual ErrorLevel,0, {
      t = %ErrorLevel%              ; save ErrorLevel to be returned
      DllCall("CloseHandle", "Uint", h)
      ErrorLevel = %t%              ; return seek error
      Return 0
   }

   m := VarSetCapacity(data)        ; get the capacity ( >= used length )
   If (n < 1 or n > m)
       n := m
   result := DllCall("WriteFile","UInt",h,"Str",data,"UInt",n,"UInt *",Written,"UInt",0)
   if (!result or Written < n)
       ErrorLevel = -3
   IfNotEqual ErrorLevel,0, SetEnv,t,%ErrorLevel%

   h := DllCall("CloseHandle", "Uint", h)
   IfEqual h,-1, SetEnv, ErrorLevel, -2
   IfNotEqual t,,SetEnv, ErrorLevel, %t%-%ErrorLevel%

   Return Written
}