Modern Cryptography for Absolute Beginner

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Jeffrey Cohen @jeffcohen cohen.jeff@gmail.com Modern Cryptography for the absolute beginner

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Alan Turing

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Secret Writing Substitution Transposition Steganography Cryptography Code Cipher From "The Code Book," by Simon Singh. Doubleday, 1999

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Cryptography is not Security

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Cryptography There are two primary use cases for digital cryptography: • Verification • Secrecy

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There are two uses cases for verification: • Message tampering • Authorship Cryptography

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How can we verify that a message was transmitted perfectly without any accidental changes? Use Case: Message Tampering

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Use Case: Parity Bits

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C A T Content: Use Case: Parity Bits

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01000011 01000001 01010100 Content: Use Case: Parity Bits

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01000011 01000001 01010100 Even Parity: Content: 11000011 01000001 11010100 Use Case: Parity Bits

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01000011 01000001 01010100 Odd Parity: Content: 01000011 11000001 01010100 Use Case: Parity Bits

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Check Digits

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1. Sum the digits in the odd-numbered positions, then multiply by 3. 2. Add the digits in the even-numbered positions to the previous result. 3. Divide by 10, and keep the remainder. 4. If the remainder is not 0,   subtract the remainder from 10. Check Digits

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Data Verification Checksum Digest Hash Fingerprint MD5 SHA-1 SHA-256 bcrypt These are all synonyms!

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Use Case: Password Security Password: bcrypt hash: swordfish $20A6@3AC194F02... Reversing this process is impossible.

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Symmetric Encryption

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Hello Symmetric Encryption

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Hello Ifmmp "Advance by 1" Symmetric Encryption

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Hello Ifmmp "Advance by 1" Symmetric Encryption

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Hello Ifmmp Fortunately, this is reversible Symmetric Encryption

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Hello Ifmmp Fortunately, this is reversible DES  AES-256 Blowfish Symmetric Encryption

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Hello Ifmmp But how do we transmit the key? Symmetric Encryption

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Symmetric Encryption

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Each key transforms data. They are called a pair because they mathematically exactly reverse the effect of the other key. Public Key Cryptography

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One key is arbitrarily selected to be the public key, and the other will be the private key. Public Key Cryptography

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Hello ?????? Public Key Cryptography

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Hello x9dak Public Key Public Key Cryptography

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Public Key Hello x9dak Private Key Public Key Cryptography

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Mr. A Mr. B Use Case: Secret Message

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Mr. A wants to send a secret message to Mr. B. Mr. A Mr. B Mr. B, Meet me at noon for lunch. Your friend, Mr. A Use Case: Secret Message

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Mr. A Mr. B STEP 1: Mr. A encrypts the message with Mr. B's public key. Li%8aja^@*9cmakA P91&*C9Naxw8723h Yolq&6209CKn02K Mr. B, Meet me at noon for lunch. Your friend, Mr. A Use Case: Secret Message

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Mr. A Mr. B STEP 2: Mr. B decrypts the message with Mr. B's private key. Li%8aja^@*9cmakA P91&*C9Naxw8723h Yolq&6209CKn02K Mr. B, Meet me at noon for lunch. Your friend, Mr. A Mr. B, Meet me at noon for lunch. Your friend, Mr. A Use Case: Secret Message

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A1. Mr. A calculates the content's digest. A2. Mr. A encrypts the digest with his  private key. This is the "wax seal".  B1. Mr. B decrypts the digest value using  Mr. A's public key. B2. Mr. B independently calculates the  digest of the received content. B3. Mr. B expects the digests to match! Use Case: Authenticity

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ssh-keygen -t rsa -b 4096 Wait a Minute!

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ssh-keygen -t rsa -b 4096 Wait a Minute!

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ssh-keygen -t rsa -b 4096 This won't work! RSA can only encrypt messages shorter than the "key length". Wait a Minute!

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Wait a Minute! In reality, we use both! We use asymmetric cryptography to securely transmit a random symmetric key.

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What's Next? Now: PKCS Soon: Elliptic-curve PKCS Future: Quantum implications?

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Modern Cryptography for the absolute beginner Jeffrey Cohen @jeffcohen cohen.jeff@gmail.com