# Symmetric Key Algorithm

Symmetric algorithms use a private or secret key that must remain secret between the two parties. Each party pair requires a separate private key.

Therefore, a single user would need a unique secret key for every user with whom she communicates.

Refer to below figure to take a deeper look into multiple Symmetric algorithms, their Advantage and Disadvantages.

Symmetric algorithms include DES, AES, IDEA, Skipjack, Blowfish, Twofish, RC4/RC5/RC6/RC7, and CAST. All these algorithms will be discussed in below figure.

Below figure strengths and weaknesses of symmetric algorithms:-

As mentioned above, Key distribution is the major problem with Symmetric Key encryption. Hence, there are multiple technique used for key distribution with security.

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### Symmetric Key Cryptography Transcript

1. Depend on distributed “shared secret/Private key/Secret Key”
2. Number of keys for n nodes to communicates = n*(n-1)/2
3. DES
• Please note: DES is not an Algorithm “It is a standard that Published “DEA” Algorithm”
• No longer Secure
• 64 bit Plaintext, 64 bit private Key (56 bit is actually key + 8 bit parity bit), 64 bit Ciphertext
• 16 times XOR operations also called as 16 times encryption
• Electronic code book(ECB) mode
• Simple and Least Secure
• Same ciphertest for same plaintext everytime
• Vulnerable to “Codebook” in case of long text
• Must be used only for “small text” e.g. encryption of Database
• Cipher Block Chaining(CBC) mode
• Select Initialization Vector
• Very less chances of getting Ciphertext same
• Error propagate
• Cipher Feedback Mode (CFB)
• Streaming cipher version of CBC
• Uses memory buffer- As buffer full, it is encrypted
• Must not be used for Video and Voice signals
• Output Feedback Mode(OFB)
• Operates in same as CFB
• A combination of Block + Stream cipher
• Instead of chaining; it uses Seed value.
• Future Seed value are determined by running DES algorithm on seed value
• Counter mode(CTR)
• Uses Simple counter instead of Seed
• Most suitable for parallel computing
• Performance is much better than other modes
4. 3DES
• Upto 3 times slower than DES
• A temporary replacement to DES
• DES-EEE3 mode
• Encrypt three times with K1, K2, K3 key
• effective key length = 56*3 = 168
• E(K1,E(K2,E(K3,P)))
• DES-EDE3 mode
• E(K1,D(K2,E(K3,P)))
• also uses three keys but replaces the second encryption operation with a decryption operation
• DES-EEE2 mode
• E(K1,E(K2,E(K1,P)))
• DES-EDE2 mode
• E(K1,D(K2,E(K1,P)))
• DES-EDE1
• only one key
• Backword compatibility purpose
• =DES
5. International Data Encryption Algorithm (IDEA)
• Developed with mindset of insufficient key length
• 128 bit key is broken into 52 x 16 bit subkeys
• Used by Pretty Good Privacy (PGP) email package
• Faster & harder to break than DES
6. Blowfish
• Block Cipher alternative to DES & IDEA
• Variable key length (32 bit to strongest 448 bit)
• Block – 64 bit of plaintext
• much faster than DES and IDEA
• Used in Commercial software & OS, SSH
• Never patented
7. Skipjack
• 64 bit block of text
• 80 bit key and support 4 mode of operation
• Used in Clipper chip
8. RC4
• Used in SSL and WEP
• Stream Cipher
• Variable key size of 40 to 2,048 bits and up to 256 rounds of transformation
9. In terms of speed – RC7 > RC6 >RC5
10. RC5
• Block cipher of 32,64,128 bit size
• Key size 0 to 2040 bits
• e.g. RC5-64/16/16 denotes a 64-bit word (or 128-bit data blocks), 16 rounds of transformation, and a 16-byte (128-bit) key
11. AES
• NIST mandated the use of AES/RSA for all sensitive but unclassified data by the U.S. government.
• Replacement of DES
• Allows 128-bit of blocks, but for RSA block size could be equal to Block Size.
• 128-bit keys require 10 rounds of encryption. 192-bit keys require 12 rounds of encryption. 256-bit keys require 14 rounds of encryption.
12. Twofish
• 128-bit blocks of data & keys up to 256 bits
• Pre-whitening – XORing the plaintext with a separate subkey before the first round of encryption.
• Post-whitening uses a similar operation after the 16th round of encryption.
• Never Patented
13. Strength of Symmetric encryption
• 1000 to 10,000 times faster than Asymmetric key algorithm
• Hard to break
• Cheaper to Implement
14. Weakness of Symmetric encryption
• Key distribution is a major problem – Out of band exchange
• Does not implement non-repudiation & Authentication.
• The algorithm is not scalable
• Number of unique keys needed can cause key management issues
• Keys must be regenerated often
• Only provide confidentiality
• Key compromise occurs if one party is compromised, thereby allowing impersonation
15. Key Distribution i.e. Creation, Distribution, Transmission
• Offline Distribution
• Public key encryption
• Diffie-Hellman
• Susceptible to Man-in-middle Attack unless an organization implements digital signatures or digital certificates for authentication at the beginning of the Diffie-Hellman process

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