Symmetric Encryption

Symmetric Cryptosystems
- Block Ciphers
- Streaming Ciphers
Symmetric Block Modes

Symmetric Cryptosystems

Symmetric cryptography uses the same key for both encryption and decryption. The sender and receiver must share this key, keeping it secret from others.

Session key - single key is used to encrypt and decrypt data.
Both parties must have this key.

Symmetric encryption is generally faster and less computationally intensive compared to asymmetric cryptography. It is also effective for encrypting large volumes of data.

Block Ciphers

Algorithm	Cipher Type	Block Size	Number of Rounds	Key Size	Description
DES (Data Encryption Standard)	Block Cipher	64-bit	16 rounds	56-bit	Legacy block cipher; once widely used but now considered insecure.
3DES (Triple DES)	Block Cipher	64-bit	48 rounds (3x16)	168-bit	3DES repeats DES process 3 times, hence 56*3 = 168-bit key size.
IDEA (International Data Encryption Algorithm)	Block Cipher	64-bit	8 rounds	128-bit	128-bit key size makes it harder to break, but is not widely utilized.
AES (Advanced Encryption Standard)	Block Cipher	128-bit	10/12/14 rounds	128/192/256-bit	Current encryption standard; highly secure and widely used.
Blowfish	Block Cipher	64-bit	16 rounds	32-448 bit (variable)	Developed as DES replacement; fast and flexible block cipher with variable key length.
Twofish	Block Cipher	128-bit	16 rounds	128/192/256-bit	A finalist in the AES competition, released as open source along with Blowfish.

A few notes:

64-bit block size, but in reality its 56-bits because 8-bits is for overhead/parity.
The larger the block size, the faster the encryption.
Number of rounds refers to how many rounds the data went through encryption. It is a pretty good clue as to how robust encryption is.
Blowfish and twofish were both released as open source products.

Streaming Ciphers

Also known as RC Cipher Suites, the streaming ciphers comprises a range of ciphers, from block to stream, with varying levels of flexibility and security.

Rivest Cipher, created by Ron Rivest.
Some are now considered insecure (like RC4)
Others have had broader applications in encryption history (like RC5 and RC6).

Unlike block ciphers, streaming ciphers encrypt one bit a time. It is wideless in encrypting wireless networks.

Algorithm	Cipher Type	Block Size	Number of Rounds	Key Size	Description
RC2	Block Cipher	64-bit	Variable rounds	8-128 bit (variable)	Early block cipher; adjustable key size, mostly used in legacy applications.
RC4	Stream Cipher	N/A	Variable	40-2048 bit (variable)	Used in SSL and WEP; now considered insecure due to vulnerabilities in key scheduling.
RC5	Block Cipher	32/64/128-bit	Variable rounds	0-2040 bit (variable)	Highly flexible block cipher with variable block size, key size, and rounds.
RC6	Block Cipher	128-bit	20 rounds	128/192/256-bit	Extended version of RC5; designed for high security and was a finalist in the AES competition.

A few notes:

RC1 was never published.
RC2 was considered weak and was skipped over.
RC3 was cracked before it was even released to the public.
RC4, RC5, and RC6 are commonly used in networks today.

Symmetric Block Modes

Symmetric block ciphers can operate in various modes that define how blocks of plaintext are transformed into ciphertext. Different modes address specific requirements and security concerns.

Electronic Codebook

ECB (Electronic Codebook) mode can show identical blocks for identical plaintext inputs, compromising security.

As an example, if we encrypt the image above, we’ll get a scrambled image that still shows some patterns. Although it is encrypted, the image is still recognizable.

Advantages:

Simple and fast; allows random access to blocks.

Disadvantages:

Patterns in plaintext are retained in ciphertext
Identical plaintext blocks result in identical ciphertext blocks
This makes it vulnerable to analysis

Cipher Block Chaining (CBC)

Each block of plaintext is XORed with the previous ciphertext block before encryption. The first block uses an initialization vector (IV).

Initialization vector

Same size as the other blocks.
The first block is applied with XOR using the IV.
The output will then be used as the IV for the next block, repeating the cycle.

Advantages:

Adds randomness to encryption
Even identical plaintext blocks yield different ciphertext due to chaining.

Disadvantages:

Requires sequential processing
Small changes in plaintext can affect the whole chain
Complicates decryption if data is corrupted

Cipher Feedback (CFB)

Turns block ciphers into self-synchronizing stream ciphers. The previous ciphertext block (or IV) is encrypted and then XORed with the plaintext to produce ciphertext.

Advantages:

Provides flexibility in block size
Allows for partial block updates
Ideal for real-time encryption

Disadvantages:

More complex error propagation
If an error occurs, it affects multiple blocks

Output Feedback (OFB)

Similar to CFB, but the encrypted output (keystream) is generated independently of the plaintext and then XORed with the plaintext to produce ciphertext.