Private Const gc_MaxLongValue As Long = 2147483647
 
Private Function RSA_Mod( _
    ByVal vBasis As Long, _
    ByVal vExp As Long, _
    ByVal vMod As Long _
  ) As Long
 
 
'''    * a ... Basis (Zahl, die potenziert werden soll)
'''    * c ... Exponent
'''    * b ... Binärdarstellung von c, wobei das höchstwertige Bit an der 
' Stelle 0 steht
'''            und das niedrigstwertige Bit an der Stelle n; (b hat n+1 Ziffern)
'''    * m ... Modulus
 
 
'''res = 1
''' for i=1..n
'''    res = res ^ 2 Mod m
'''    if b_i = 1
'''       res = (res * a) Mod m
'''    end-if
''' end-for
 
  Dim numBits As Long
  Dim bits_Exp() As Long
 
On Error GoTo err_handler
 
  numBits = Int(Log(vExp) / Log(2) + 1#)
 
  ReDim bits_Exp(1 To numBits)
 
  Dim tmp As Long
  Dim i As Long
  tmp = vExp
  'lowest bit --> highest position
  For i = numBits To 1 Step -1
    bits_Exp(i) = tmp Mod 2
    tmp = tmp \ 2
  Next i
 
  RSA_Mod = 1
  For i = 1 To numBits
 
    If (CDbl(RSA_Mod) * CDbl(RSA_Mod)) > gc_MaxLongValue Then
 
      RSA_Mod = split_mod(RSA_Mod, RSA_Mod, vMod)
 
    Else
      RSA_Mod = (RSA_Mod * RSA_Mod) Mod vMod
    End If
 
    If bits_Exp(i) = 1 Then
 
      If (CDbl(RSA_Mod) * CDbl(vBasis)) > gc_MaxLongValue Then
        RSA_Mod = split_mod(RSA_Mod, vBasis, vMod)
      Else
        RSA_Mod = (RSA_Mod * vBasis) Mod vMod
      End If
 
    End If
 
  Next i
 
 
 
 
  GoTo endproc
err_handler:
  Debug.Print Err.Description
  Debug.Assert False
 
  GoTo endproc
err_debug:
  Resume
endproc:
 
End Function
 
 
Private Function split_mod( _
    ByVal vf1 As Long, _
    ByVal vf2 As Long, _
    ByVal vMod As Long _
  ) As Long
 
  Dim split As Long
  Dim sum As Long
  Dim j As Long
  Dim i As Long
 
  Dim dbltmp As Double
 
  On Error GoTo err_handler
 
 
  dbltmp = CDbl(vf1) * CDbl(vf2)
 
  split = Int(Log(dbltmp) / Log(2) + 1#) ^ 2
 
  ReDim f1(1 To split) As Long
  ReDim f2(1 To split) As Long
 
  sum = 0
 
  f1(1) = vf1 \ split
  sum = f1(1)
  For j = 2 To split - 1
    f1(j) = f1(1)
    sum = sum + f1(1)
  Next j
  f1(split) = vf1 - sum
 
  f2(1) = vf2 \ split
  sum = f2(1)
  For j = 2 To split - 1
    f2(j) = f2(1)
    sum = sum + f2(1)
  Next j
  f2(split) = vf2 - sum
 
  sum = 0
  For i = 1 To split
    For j = 1 To split
      sum = (sum + (f1(i) * f2(j))) Mod vMod
    Next j
  Next i
  split_mod = sum
 
 
  GoTo endproc
err_handler:
  Debug.Print Err.Description
  Debug.Assert False
 
  GoTo endproc
err_debug:
  Resume
endproc:
 
End Function

Gruß
Dirk

--
?Get it right the first time