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### Re: Codebusters C

Posted: **November 19th, 2018, 6:37 pm**

by **Name**

I was reading the rules to make sure I wasn't wasting to much time reading about the state/nats ciphers, and noticed that for the hill cipher it states, "Mathematical Cryptanalysis of the Hill Cipher - either producing a decryption matrix given a 2x2 encryption matrix or computing a decryption matrix given 4 plaintext-ciphertext letter pairs." What does this rule even mean.

Thanks

They can test a hill cipher so that when given a hill cipher matrix they can ask find the inverse key of the matrix or given 4 plaintext crib find the key

So like they can asks find the inverse of

|1 2|

|3 7|

or something like that (usually in order to then decode something)

Or they can ask something like

if the plain text abcd corresponds to the ciphertext efgh find the key used to encrypt/decrypt it

### Re: Codebusters C

Posted: **November 19th, 2018, 9:19 pm**

by **Avalanche**

I was reading the rules to make sure I wasn't wasting to much time reading about the state/nats ciphers, and noticed that for the hill cipher it states, "Mathematical Cryptanalysis of the Hill Cipher - either producing a decryption matrix given a 2x2 encryption matrix or computing a decryption matrix given 4 plaintext-ciphertext letter pairs." What does this rule even mean.

Thanks

They can test a hill cipher so that when given a hill cipher matrix they can ask find the inverse key of the matrix or given 4 plaintext crib find the key

So like they can asks find the inverse of

|1 2|

|3 7|

or something like that (usually in order to then decode something)

Or they can ask something like

if the plain text abcd corresponds to the ciphertext efgh find the key used to encrypt/decrypt it

Thanks, for the help.

### Re: Codebusters C

Posted: **December 14th, 2018, 6:52 am**

by **TheChiScientist**

I was wondering if anyone has any good sources for practice with Spanish Xenocrypts? I can't seem to find anything on the internet...

### Re: Codebusters C

Posted: **December 14th, 2018, 7:29 am**

by **Anomaly**

I was wondering if anyone has any good sources for practice with Spanish Xenocrypts? I can't seem to find anything on the internet...

Just ask someone to create one for you. It only takes a minute to make.

### Re: Codebusters C

Posted: **January 8th, 2019, 11:12 pm**

by **Name**

I haven't ever seen questions related to RSA on any invite test so far, and it seems nearly impossible to encode/decode with only a 5 function calculator. Is the only thing that can be realistically tested is to find the decryption key given the encryption key?

### Re: Codebusters C

Posted: **January 9th, 2019, 7:58 am**

by **dragonfruit35**

I haven't ever seen questions related to RSA on any invite test so far

That’s because RSA isn’t on the list of invy/regs ciphers in the rules- it’s listed in the section to be used at state competitions and Nats

### Re: Codebusters C

Posted: **January 9th, 2019, 12:48 pm**

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

I haven't ever seen questions related to RSA on any invite test so far, and it seems nearly impossible to encode/decode with only a 5 function calculator. Is the only thing that can be realistically tested is to find the decryption key given the encryption key?

Wait, why is it nearly impossible to encode/decode with a 5 function calculator

(Forgive me, I don't really do this event)

### Re: Codebusters C

Posted: **January 9th, 2019, 3:40 pm**

by **Riptide**

I haven't ever seen questions related to RSA on any invite test so far, and it seems nearly impossible to encode/decode with only a 5 function calculator. Is the only thing that can be realistically tested is to find the decryption key given the encryption key?

Wait, why is it nearly impossible to encode/decode with a 5 function calculator

(Forgive me, I don't really do this event)

RSA deals with encrypting/decrypting using exponents, which can get very large. Smaller exponents are doable but I don’t know if 4/5 function calculators can even handle going to such high numbers.

### Re: Codebusters C

Posted: **January 9th, 2019, 3:54 pm**

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

I haven't ever seen questions related to RSA on any invite test so far, and it seems nearly impossible to encode/decode with only a 5 function calculator. Is the only thing that can be realistically tested is to find the decryption key given the encryption key?

Wait, why is it nearly impossible to encode/decode with a 5 function calculator

(Forgive me, I don't really do this event)

RSA deals with encrypting/decrypting using exponents, which can get very large. Smaller exponents are doable but I don’t know if 4/5 function calculators can even handle going to such high numbers.

Presumably they won't use large exponents on the test (otherwise, the code would be very hard to decipher!). However, since RSA uses modular arithmetic, you can use various tricks to get smaller numbers after exponentiation.

### Re: Codebusters C

Posted: **January 13th, 2019, 4:19 pm**

by **TheChiScientist**

Anyone here wants access to an xenocrypt test urgently? I plan to complete a SciOly Codebusters test of pure xenocrypts seeing how difficult they are to find but I don't want to release it without a large number of ciphers if there isn't a large demand.