I meant an 'intelligent' method but you're quite right, anyway. I was reading the ASC(...MOD...) one incorrectly.
When I sat down and experimented I saw that I misinterpreted. Bummer .

Curses! Well that certainly makes it much harder to discern the length of the original string. It might still be doable, but it would require
that when a number is encoded, the result never starts with a 1 or 2. That way you could check if the number begins 1 or 2, if so, isolate the
3 digit code, if not, then isolate the 2 digit code.

I think that if all ASCII characters are potentially allowed (specifically things like .,?!$* etc) that will probably kill off any ability to decode it.

Can you double check the output you gave? I'm assuming that the words are alpha only with upper and lower case. Examples I've tried
so far seem to all produce a 3 digit number for each letter either beginning with 1 or 2. The length of the output you've given isn't
divisible by three.
If I'm right, then I believe there is a method, but it does involve brute force (of a kind). It appears initially that for any given string length
each letter corresponds to a specific number depending upon it's position regardless of the other letters around it. I'd basically have to
calculate a massive lookup table.
Assuming I can deduce the length of the string by len(string)/3, then I can use the lookup table to deduce the values for each 3-digit number.

This is all dependant on the fact that the number you gave is missing a digit or two, though

No, it's fine the way you've done it.
A major step in deciphering it would be to know the total length of the three strings. Even if you were able to deduce that somehow
you'd still be unable to determine what part of the concatenated string is Password0, Password1, and UserName. I thought there was
a clue in the encoding string, but that was before I realised all three strings went into the same variable.

I personally like VB, but I mainly use VBA with Excel. I'll have a go at this later. I think it's doable from a glance, but I'll need to play with it.

In VB, if the strings are numbers, the string + string adds the numbers, but, if they aren't numbers, it "concatenates" them.
Thus, for strings, in VB, the + and & operators do the same thing.

Note that I am not using VB.net, but VB 5.0. There is a world of difference between the two. VB.net is similar to Java. Standard VB is an older language, much less strict as far as syntax goes.

string & string -> puts the two strings together, in the order they are typed, to form one longer string
Asc(string) -> gives the ASCII value of the first character in the string
Mid(string, integer, [integer]) -> returns the section of the string starting at the position defined by the first integer, and of length defined by the second integer (if it is not present, it returns the text from the starting point to the end of the string)
sqr(integer) -> returns the square root of the number

VB does have a few nice things about it: It isn't case-sensitive, you can leave out spaces, etc., and it corrects it without prompting; furthermore, there are no compilers for Java that have as much freedom for the user interface. There are also many different ways of achieving the same result, from simple to complex, long to short.

I have to agree, however, that there could be some improvements.

For x = 0 To 1
    CompiledString = CompiledString & Password(x).Text
Next

CompiledString = CompiledString & UserName.Text

For x = 1 To Len(CompiledString)
    Output = Output & Int(Asc(Mid(CompiledString, x, 1)) + Asc(Mid(CompiledString, Int(((x + Sqr(Len(Output))) Mod (Len(CompiledString) - 1)) + 1), 1)))
Next

I will give high praise to anyone who thinks they know how to retrieve the original information and actually can.

For those without VB 5.0 compilers, here are the results:

UserName.text = afoiadfjanwief
Password(0).Text = wedraixdn
Password(1).Text = rhescaiufenxe
Output = 21320613321521521313720022519321622014020622920020522023114221614781235202229213137230211142153220216199211202222221210230215201203226196197207223199211229225203216203211219194205222206211214220220200199224218202224217206217202221219201201217196207224222203212212152217201203210216206212207214198224203209206134

UserName.text = aerjxosadmxxicwwmdxMALW
Password(0).Text = adxneuXddwriox
Password(1).Text = XAwjixnckefexnIXern
Output = 21320613321521521313720022519321622014020622920020522023114221614781235202229213137230211142153220216199211202222221210230215201203226196197207223199211229225203216203211219194205222206211214220220200199224218202224217206217202221219201201217196207224222203203207220221207203199205185165221212202230229204208204198220230183189218202210185166233226216235207199216221222206219229192197221191175173188211206240221216214188209185239211204239229208207221179166196197170201234220198218202206208176234203205229230219206220221210220230150153177201207197221224207208211203162219215225240215198226220211201240187153177201207197221224207237199215185165228226225225229218216201222178185186160185201234220198218202206185230221202220219219227206201220239219173208178179186188211206240221216214188209208185224205224239219199227178178188217210193198202231198197201233220225222235185165228225240215198226220211201240187138164188211210217211215223208215197219223225231225187184238215205240187164183188199201240220190176201214229211206225226208176234203205229230219212200221220219173208178179186184198214226230212232185200209239234210210239207165239183170196221201139217206202152209184197213219218198215198210225143220212132225221235221131229230141149239178165190184

What gave

Output = 1811291842231331601942231331901811982221911291831291921982151981692092151401201292322052191872062231402101551412332202062296421515764156213212213233232205215133212139147172125199213206224148181218228215199218147227172207225216185205140184149211230198235116171219211142191208211