Transcript CS 291 Special Topics on Network Security

CS 285 Network Security
Block Cipher Modes of Operation
Fall 2008
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Introduction
How to encrypt a message with variable lengths
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Decompose the message into blocks, padding if
necessary.
How should the encryption/decryption process of each
individual block interact with each other?
 Modes of operation
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ECB (Electronic Codebook)
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CBC (Cipher Block Chaining)
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CFB (Cipher Feedback)
E
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CFB vs. OFB
CFB
OFB
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OFB (Output Feedback Mode)
E
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CTR (Counter)
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Confidentiality and Integrity Protection
ECB
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Same plaintext blocks produce same ciphertext blocks. This means that the
data pattern is revealed. For example, ECB mode will reveal the image
pattern if used to encrypt image files.
Rearranging the blocks is undetectable.
CBC
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Random IV gurantees that even if the same message is repeated, the
ciphertext is different.
Modifying ciphertext blocks and rearranging ciphertext blocks undetected
are still possible.
CFB
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No integrity protection; Better in detecting alterations than OFB
OFB
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Able to make controlled changes to recovered plaintext. No integrity
protection; not as good as CFB
CTR
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Same as OFB
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Application
ECB
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Block oriented transmission
Not suitable for long messages or
highly structured messages. Good
for single values (e.g. keys)
CBC
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Block-oriented transmission
General-purpose encryption
message authentication code
design
CTR
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Block-oriented transmission
Able to preprocess to generate
one-time pad; Random access;
High performance requirement;
IPsec
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CFB
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Stream-oriented transmission,
 no need for padding;
 ciphertext has the same length
of message;
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pipeline is possible for
encryption, thus good for lowlatency real-time transmission
encryption.
OFB
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Stream-oriented transmission
transmission over noisy channel
Able to preprocess to generate
one-time pad