Symmetric-key cryptography

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Security

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Security Topics

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Cryptography

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Cryptography components

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Encryption and decryption

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In cryptography,

the encryption/decryption algorithms
are public; the keys are secret.

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Symmetric
-
Key Cryptography

Traditional Cipher

Block Cipher

Operation Modes

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Symmetric
-
key cryptography

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In symmetric
-
key cryptography, the
same key is used by the sender (for
encryption) and the receiver (for
decryption). The key is shared.

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:

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In symmetric
-
key cryptography, the
same key is used in both directions.

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:

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Symmetric
-
key cryptography is often
used for long messages.

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:

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Caesar cipher

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Example of monoalphabetic substitution

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In monoalphabetic substitution, the
relationship between a character in the
plaintext to the character in the
ciphertext is always one
-
to
-
one.

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Vigenere cipher

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In polyalphabetic substitution, the
relationship between a character in the
plaintext and a character in the
ciphertext is one
-
to
-
many.

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:

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Transpositional cipher

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Block cipher

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P
-
box

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S
-
box

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Product block

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DES

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General scheme of DES

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Iteration block

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Triple DES

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The DES cipher uses the same concept
as the Caesar cipher, but the
encryption/decryption algorithm is
much more complex due to the sixteen
48
-
bit keys derived from a 56
-
bit key.

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ECB mode

Electronic Code Book Mode

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CBC mode

Cipher Block Chaining Mode

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CFM

Cipher Feedback Mode

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CSM

Stream Cipher Mode

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Cryptanalysis


Some common symmetric
-
key
cryptographic algorithms.

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Public
-
Key Cryptography

RSA

Choosing Public and Private Keys

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Public
-
key cryptography

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Public
-
key algorithms are more
efficient for short messages.

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:

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RSA

Public Key 5 , 119 Private Key 77 , 199