Modes Of Operation Flashcards
ECB Electronic Codebook
Each block of plaintext bits is encoded independently using the same key
Secure transmission of single values, suitable for short messages.
Long messages may not be secure.
Cipher Block Chaining CBC
The input to the encryption algorithm is the XOR of the next block of plaintext and the preceding block of cipher text.
More secure than ECB.
provides confidentiality and authentication
Application: general purpose block orientated transmission, authentication
Cipher feedback CFB
Input is processed s bits at a time preceding cipher text is used as input to the encryption algorithm to produce pseudorandom output which is XORED with plaintext to produce next unit of cipher text.
Same to CBC CFB cannot be parallelised.
Advantage that identical plaintext blocks lead to different cipher text blocks.
Application: general purpose stream orientated transmission
Authentication
Output feedback OFB
Similar to CFB except the input to the encryption algorithm is the preceding encryption output and full blocks are used
Has the advantage that identical plaintext blocks lead to different cipher text blocks same as CBC AND CFB
Application: stream orientated transmission over noisy channel
Counter CTR
Each block of plaintext is XORed with an encrypted counter. Counter is incremented for each subsequent block
Same to ECB encryption and decryption using counter mode and be parallelised.
Application: general purpose block orientated transmission
Useful for high speed requirements
Synchronous stream cipher
A key stream is generated independently of the plaintext and cipher text messages and then combined with the plaintext to encrypt or the cipher text to decrypt
Asynchronous stream cipher
A keystream is a function of the key and a fixed number of previous ciphertexts.
ECB algorithms
Encryption = Ci = Ek(Pi)
Decryption = Pi =Dk(Ci)
CBC algorithms
Encryption= Ci = Ek(Pi(+)Ci-1)
Decryption Pi=Dk(Ci)(+)Ci-1
OFB Algorithm
Encryption = Ci = Pi(+)Oi
Decryption Pi=Ci(+)Oi
CFB Algorithm
Encryption = Ci=Pi(+)Oi
Decryption Pi=Ci(+)Oi
Modes that use only encryption
Counter CTR- encrypts counter value to generate a keystream and decryption uses the same keystream
Galois/ counter mode GCM- encrypts the counter values and uses encryption results to generate authentication tag
Passive attack
An attacker eavesdropps on the communication channel or system without altering or affecting the data
Active attack
Attacker actively modifies the data or communication in transit may alter inject or replay messages.
MITM
REPLAY
DDOS
