RandomPassword.cs

using System;
using Waher.Content;
using Waher.Script;
using Waher.Script.Abstraction.Elements;
using Waher.Script.Functions.Vectors;
using Waher.Script.Model;
using Waher.Script.Objects;
using Waher.Script.Statistics.Functions;
 
namespace Waher.IoTGateway.ScriptExtensions.Functions
{
    public class RandomPassword : FunctionZeroVariables
    {
        public RandomPassword(int Start, int Length, Expression Expression)
            : base(Start, Length, Expression)
        {
        }
 
        public override string FunctionName => nameof(RandomPassword);
 
        public override bool IsAsynchronous => false;
 
        public static string CreateRandomPassword(int NrBytes, int NrBuckets)
        {
            if (NrBytes < 16)
                throw new ArgumentOutOfRangeException(nameof(NrBytes), "Must be at least 16.");
 
            if (NrBuckets < 0)
                throw new ArgumentOutOfRangeException(nameof(NrBuckets), "Must be non-negative.");
 
            byte[] Proposal;
            double[] H;
            double MinH;
 
            do
            {
                Proposal = Gateway.NextBytes(NrBytes);
 
                if (NrBuckets > 1)
                {
                    H = Histogram.Compute(Proposal, NrBuckets, 0, 256);
                    MinH = Min.CalcMin(H, null);
                }
                else MinH = 1;
            }
            while (MinH == 0);
 
            return Base64Url.Encode(Proposal);
        }
 
        public static string CreateRandomPassword()
        {
            return CreateRandomPassword(32, 12);
 
            //
            // This condition approximately removes 1 in 2 generated passwords, 
            // if NrBytes=32, NrBuckets=12, reducing the strength from 256 bits to 255 bits.
            //
            // In a test of 100'000 randomly generated passwords, 42'973 passed
            // (~43%), 57027 failed (~57%).
            //
            // Sample script:
            //
            //     NrPass:=0;
            //     NrFail:=0;
            //     foreach x in 1..100000 do
            //     (
            //         Bin:=Base64Decode(Base64Encode(Gateway.NextBytes(32)));
            //         H:=Histogram([foreach x in Bin : x],0,256,12);
            //         if (Min(H[1])>0) then NrPass++ else NrFail++
            //     );
            //     {"NrPass":NrPass,"NrFail":NrFail}
            //
        }
 
    public override IElement Evaluate(Variables Variables)
        {
            return new StringValue(CreateRandomPassword(32, 12));
        }
    }
}