# Brainteaser Rep to anyone who even attemps an answer (within reason)

I'm trying to make a quiz to help revise for a module at uni and have come across this in the revision notes, anybody fancy a guess?.

An Internet banking system allows customers to select their own 6-digit

security number. Two digits are selected at random by the bank for the

client to enter as an authentication mechanism each time the customer

logs in. Assuming an eavesdropper is able to read the digits entered by

the customer what is the least possible number of logins the

eavesdropper would have to see in order to learn the security number?

An Internet banking system allows customers to select their own 6-digit

security number. Two digits are selected at random by the bank for the

client to enter as an authentication mechanism each time the customer

logs in. Assuming an eavesdropper is able to read the digits entered by

the customer what is the least possible number of logins the

eavesdropper would have to see in order to learn the security number?

Banned

bolton.ac.uk/Qua…pdf

Original Poster

A 1 login

B 3 logins

C 15 logins

D 30 logins

I'm leaning towards once myself, maybe a trick question?. Either that or a badly written question:)

Banned

Original Poster

yup I'm sitting it on tuesday i'm making a multiple choice quiz on my iphone to help me remember. the first quarter of the exam is multiple choice & I have answered virtually all the rest just this one thats throwing me.

Original Poster

Thats what I think it just seems to obvious though thats what was throwing me.:thinking:

Gonna go with one.

Banned

ide deffo go with one, i think its one of those that throw a load of bits in to make it appear more than it is

Original Poster

Cheers all think I'm defo going for one If they where using a everypossible 2 combination of 0-9 surely it would be 99.

Cheers again all have some rep i'm off to bed.

Banned

Banned

no because ther is only 6 digits - lol

if it was about the two numbers and they are random it would be 36

2 digits of a 6 digit code each time..

if cust is given 2 digits each log in then it would be 3 min, assuming he gets 2 different each time!!!!

maybe...

4 2 2 [ ? ] [ ? ] 1

Or does it mean that only two digits appear on the screen like so:

4 [ ? ] 2 [ ? ] [ ? ] [? ]

and they have to enter the rest?

Or am I missing the point entirely?

Basically I think it says please enter for example digit 3 and 5 of your security number

Banned

then if completely random it would take 36 goes to cover all possibilities

God, I'm an idiot. Thanks for that.

I get asked this quite often when I use my card online.

If that's the case, then I would agree and say it would be 3 times. If the only information on the screen are the two presumably disguised numbers, I think the minimum would be 3 and then would require a lot of luck, too.

Yer but it asks for the least possible which is 3

Banned

the least possible, but numbers are still random, i dont think it has anything to do with the two numbers

Assuming an eavesdropper is able to read the digits entered by the customerI'm with Sassie

No, not at all. Let's say for example the code was: 1 2 3 4 5 6 - when you get to the screen which requires your code, you will be asked for two numbers. In this case, let's say they asked for the 3rd and 5th number, which in this simple example would be 3 and 5. The next time they may ask for the 1st and 6th one, which will obviously be 1 and 6. Then on the third time they may ask you for the 2nd and 4th, which will be 2 and 4.

If this happened in the above sequence, they would know that your entire 6 digit code is: 1 2 3 4 5 6 after 3 viewings.

However... I guess it could only need one time, as let's say on the first occasion it asked for digits 3 and 5, it might ask for the same digits the next time. So if you were extremely jammy, you could do it in one, however you wouldn't get the entire code, just 2 digits.

Yer so if they asked for the number of logins that would guarantee the security number then I agree. However they are asking for the least possible number it has nothing to do with the randomness. The least possible logins occurs when each consecutive login has two different numbers from the security numbers which after 3 attempts would give the answer.

Which isnt the question.. it asks you how many times to know the sequrity number, implying the whole 6 digits.

so yes it is 3.

Banned

pmsl, an evesdropper they said, not someone sat on her shoulder looking at what digits they where asking for

Banned

random means random, not 1 of each number

What are you talking about? It's asking hypothetically how many times they would have to see someone enter the code.

Banned

Are you saying they can't see what number the bank ask for? Surely then you could never be sure, because if there was a repeat digit involved you wouldn't know if it was a new digit or where it was in the sequence.

Sorry, didn't see this post.

As it asks for the entire code, then I agree, it's 3. I was just saying that you could possibly gain access by seeing them enter 2 digits once, even though it's unrelated to the question. ;-)

Wrong because there are no guarantees that all the numbers will come up in a

randomsequence it could ask for the same number over and over again. Highly improbable but still possible.3

6 digit security, 2 digits asked for on each attempt, therefore 3 attempts is the MINIMUM number of times that would be required to see the entire code.