Encryption Modes
A brief on various encryption modes, usually for AES, but any block cipher really.
Encryption Modes
Modes of encryption
ECB mode
- Each plaintext block is independently encrypted.
CBC mode
- Input to the algorithm is the XOR of the next block of plaintext and the preceding block of ciphertext.
Cipher Feedback (CFB)
- Input is processed s < b bits at a time.
- Preceding ciphertext is used as input to the algo.
- XORed with the current plaintext to produce the next block of ciphertext.
Output Feedback (OFB)
- Input to the algorithm is the preceding enc output, not the XORed ciphertext.
- Full blocks are used.
Counter (CTR)
- Each block of plaintext is XORed with an increasing counter.
Metrics for modes
- Encryption in parallel?
- Decryption in parallel?
- One plaintext block influencing multiple ciphertext blocks?
- Repeating a block leading to same ciphertext?
- Is only encryption needed for both enc and dec?
- Any other secret info needed to be shared?
ECB
Advantages:
- Parallel computation
Disadvantages:
- Same plaintext blocks lead to same ciphertext blocks
- Both algorithms needed
CBC
Advantages:
- Parallel decryption
- Plaintext blocks affect the succeeding blocks as well
Disadvantages:
- IV must be securely transmitted and be unpredictable
- Enc can only be serial
- Both algorithms needed
CFB
Advantages:
- Parallel decryption
- Only encryption is needed
- Plaintext blocks affect the succeeding blocks as well
Disadvantages:
- IV must be securely transmitted and be unpredictable
- Enc can only be serial
OFB
Advantages:
- Parallel encrpytion/decryption
- Only encryption is needed
- Precomputation using IV/enc is possible.
Disadvantages:
- A plaintext block cannot affect multiple ciphertext blocks
- IV/nonce must be securely transmitted and be unpredictable
CTR
Advantages:
- Parallel encrpytion/decryption
- Only encryption is needed
- Precomputation using IV/enc is possible.
Disadvantages:
- A plaintext block cannot affect multiple ciphertext blocks
- IV/nonce must be securely transmitted and be unpredictable