Sustitution cyphers: why they should not be used & and how to crack them Frequency Analysis Andrea Tino This document uses graphic assets from Freepik Cyphers A cypher is an algorithm which takes an input text called plain text, and generates another text, called cypher text, by transforming the priginal one. The plain text is not limited its length can vary. Every cypher needs a key to work. The key allows the plain text to be encrypted into a cypher text and it The role of Cryptography Security is a branch of Computer Science, Soﬅware Engineering and Mathematics which deals with problems related to confining access to data only to those who have the proper set of permissions. Encryption is a crucial part of this story and it consists in a broad collection of techniques used to guarantee that secrets can safely be delivered on non-secure channels. Encryption is used today in order to deliver private information over the Internet: the biggest network we have in the world. Thanks to cryptography, we do many things over the Internet: we can pay bills, we can use our credit cards, check our emails and safely use social networks. Security is a very important maer today. Everything we have is moving to the Cloud and all our data are stored over the Internet. In order to protect all this information, Substitution cyphers Cyphers can work in several different ways. The oldest, and today least secure, type of cyphers employ a technique called: key-replacement. also allows the reverse process to take place: decryption. The key has a fixed size, it cannot vary. To successfully decrypt a message, it is necessary to use the same exact key that was used to encrypt the message. That is why the key must be kept safe and distributed (safely), only to authorized people. The most famous cypher and oldest known in hostory is Caesar’s cypher: the Roman Emperor used to send messages to his generals by encrypting them with a substitution cypher always with the same key known to him and his closest men in the military. As it is possible to see, this mechanism allows secret messages to be exchanged even if the message ends up in the wrong hands. However they key must be not be sent over a non-safe channel. we must encrypt our data and be sure nobody can access it except us. This is done automatically by the services we use: email accounts, smart-phones, etc. A B C D E F G H I J K L M N O P Q R S T U V W X Y Z A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Plain text #$%!?+0^!@ ?+0^!!?+0^ !@?+0^!?+0 ^!@?+0^!!@ #$%!!?+0^! Cypher text ENCRYPT DECRYPT ALICE BOB Message Message #$%!?+0^!@ ?+0^!!?+0^ !@?+0^!?+0 ^!@?+0^!!@ #$%!!?+0^! Encrypted message Original message TRUDY Hey Alice, it was Trudy who ate your cake yesterday! Don’t tell her I told you. Khb Dmlfh, lw zdv Wuxgb zkr dwh brxu fdnh bhvwhugdb! Grq’w whoo khu L wrog brx. Bob sends the message but before it encrypts the message using Caesar’s Cypher. Khb Dmlfh, lw zdv Wuxgb zkr dwh brxu fdnh bhvwhugdb! Grq’w whoo khu L wrog brx. hb Dmlfh, lw dv Wuxgb zkr wh brxu fdnh hvwhugdb! rq’w whoo khu Bob sends Alice the message over Wi-Fi. Since this is not a secure channel, Bob decides to encrypt the message. Trudy is sniffing Bob and Alice’s communications and she successfully acquires the message. Alice is Bob’s friend, and he will send her an important secret message that only she should read. Trudy tries to read the message content, but she realizes she cannot as it is encrypted. E F G H J M A B C D E F G I J O A B C D English language Cypher text D = A H = E M = J Trudy knows Alice and Bob communicate in English, so she calculates how many times each leer appears in the cypher-text and builds the frequency histogram. Later, she compares the cypher-text histogram with the one relative to the English language. The highest and lowest bars give Trudy indication on the main associations.

Workshop on Cryptography - Frequency Analysis (basic)

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Page 1: Workshop on Cryptography - Frequency Analysis (basic)

Sustitution cyphers: why they should not be used & and how to crack them

Frequency AnalysisAndrea Tino

This document uses graphic assets from Freepik

CyphersA cypher is an algorithm which takes an input text called plain text, and generates another text, called cypher text, by transforming the priginal one. The plain text is not limited its length can vary.

Every cypher needs a key to work. The key allows the plain text to be encrypted into a cypher text and it

The role of CryptographySecurity is a branch of Computer Science, So�ware Engineering and Mathematics which deals with problems related to confining access to data only to those who have the proper set of permissions.

Encryption is a crucial part of this story and it consists in a broad collection of techniques used to

guarantee that secrets can safely be delivered on non-secure channels. Encryption is used today in order to deliver private information over the Internet: the biggest network we have in the world.

Thanks to cryptography, we do many things over the Internet: we can pay bills, we can use our credit cards, check our emails and safely use social networks.

Security is a very important ma�er today. Everything we have is moving to the Cloud and all our data are stored over the Internet. In order to protect all this information,

Substitution cyphersCyphers can work in several different ways. The oldest, and today least secure, type of cyphers employ a technique called: key-replacement.

also allows the reverse process to take place: decryption. The key has a fixed size, it cannot vary.

To successfully decrypt a message, it is necessary to use the same exact key that was used to encrypt the message. That is why the key must be kept safe and distributed (safely), only to authorized people.

The most famous cypher and oldest known in hostory is Caesar’s cypher: the Roman Emperor used to send messages to his generals by encrypting them with a substitution cypher always with the same key known to him and his closest men in the military.

As it is possible to see, this mechanism allows secret messages to be exchanged even if the message ends up in the wrong hands. However they key must be not be sent over a non-safe channel.

we must encrypt our data and be sure nobody can access it except us. This is done automatically by the services we use: email accounts, smart-phones, etc.

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

A B CD E F G H I J K L M N O P Q R S T U V W X Y Z

Plain text

#$%!?+0^!@?+0^!!?+0^!@?+0^!?+0^!@?+0^!!@#$%!!?+0^!

Cypher text

ENCRYPT

DECRYPT

ALICEBOB Message

Message

#$%!?+0^!@?+0^!!?+0^!@?+0^!?+0^!@?+0^!!@#$%!!?+0^!

Encrypted message

Original message

TRUDY

Hey Alice, it was Trudy who ate your cake yesterday!Don’t tell her I told you.

Khb Dmlfh, lw zdv Wuxgb zkr dwh brxu fdnh bhvwhugdb!Grq’w whoo khu L wrog brx.

Bob sends the message but before it encrypts the message using Caesar’s Cypher.

Khb Dmlfh, lw zdv Wuxgb zkr dwh brxu fdnh bhvwhugdb!Grq’w whoo khu L wrog brx.

Bob sends Alice the message over Wi-Fi. Since this is not a secure channel, Bob decides to encrypt the message.

Trudy is sniffing Bob and Alice’s communications and she successfully acquires the message.

Alice is Bob’s friend, and he will send her an important secret message that only she should read.

Trudy tries to read the message content, but she realizes she cannot as it is encrypted.

E F G H J MA B C DE F G I J OA B C D

English language Cypher text

D = A

H = E

M = J

Trudy knows Alice and Bob communicate in English, so she calculates how many times each le�er appears in the cypher-text and builds the frequency histogram.

Later, she compares the cypher-text histogram with the one relative to the English language. The highest and lowest bars give Trudy indication on the main associations.