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### Re: Codebusters C

Posted: **September 23rd, 2018, 9:51 pm**

by **Umaroth**

Since it isn't specified in the rules, does anyone know how test writers unwittingly giving us non invertible Hill cipher matrices will be prevented? This would probably cause a lot of frustration among teams.

### Re: Codebusters C

Posted: **September 23rd, 2018, 10:07 pm**

by **Name**

Since it isn't specified in the rules, does anyone know how test writers unwittingly giving us non invertible Hill cipher matrices will be prevented? This would probably cause a lot of frustration among teams.

For a 2x2 cipher in order to be invertable ad-bc mod 26 has to be coprime to 26

Under the current rules I don't think we have to invert 3x3s?

### Re: Codebusters C

Posted: **September 26th, 2018, 2:55 pm**

by **Jacobi**

Since it isn't specified in the rules, does anyone know how test writers unwittingly giving us non invertible Hill cipher matrices will be prevented? This would probably cause a lot of frustration among teams.

For a 2x2 cipher in order to be invertable ad-bc mod 26 has to be coprime to 26

Under the current rules I don't think we have to invert 3x3s?

Eh... I would say yes.

As someone said, "Prepare for everything so you're ready for everything."

If no one said that, someone should have.

### Re: Codebusters C

Posted: **October 4th, 2018, 4:58 pm**

by **jlordhe**

Do you have to be extremely good at math for this event?

### Re: Codebusters C

Posted: **October 4th, 2018, 6:04 pm**

by **Unome**

Do you have to be extremely good at math for this event?

Not for most ciphers. There are a few that require significant math background (e.g. Hill cipher).

### Re: Codebusters C

Posted: **October 4th, 2018, 6:47 pm**

by **TheChiScientist**

Do you have to be extremely good at math for this event?

Not for most ciphers. There are a few that require significant math background (e.g. Hill cipher).

Most ciphers just require studying and memorization. Make sure you know your alphabet very well.

As Unome said only a few require actual number crunching.

### Re: Codebusters C

Posted: **October 5th, 2018, 4:00 pm**

by **cryptoderes**

Can someone do a quick walkthrough on how to solve an RSA cipher?

### Re: Codebusters C

Posted: **October 5th, 2018, 4:31 pm**

by **UTF-8 U+6211 U+662F**

Can someone do a quick walkthrough on how to solve an RSA cipher?

Does

this help?

### Re: Codebusters C

Posted: **October 6th, 2018, 12:24 pm**

by **cryptoderes**

Can someone do a quick walkthrough on how to solve an RSA cipher?

Does

this help?

Ah yes, thank you so much!

### Re: Codebusters C

Posted: **October 6th, 2018, 1:53 pm**

by **UTF-8 U+6211 U+662F**

Can someone do a quick walkthrough on how to solve an RSA cipher?

Does

this help?

Ah yes, thank you so much!

Just make sure you know encryption and decryption