Symmetric-Key Encryption

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Presentation transcript:

Symmetric-Key Encryption CSE 5351: Introduction to Cryptography Reading assignment: Chapter 3 Read sections 3.1-3.2 first (skipping 3.2.2)

Pseudorandom Generators and Stream Ciphers Encryption schemes using pseudorandom generators K&L: Section 3.3

Security of RC4 RC4 is not a truly pseudorandom generator. The key stream generated by RC4 is biased. The second byte is biased toward zero with high probability. The first few bytes are strongly non-random and leak information about the input key. Defense: discard the initial n bytes of the keystream. Called “RC4-drop[n-bytes]”. Recommended values for n = 256, 768, or 3072 bytes. Efforts are under way (e.g. the eSTREAM project) to develop more secure stream ciphers.

The Use of RC4 in WEP 802.11 Frame: WEP is an RC4-based protocol for encrypting data transmitted over an IEEE 802.11 wireless LAN. WEP requires each packet to be encrypted with a separate RC4 key. The RC4 key for each packet is a concatenation of a 40-bit or 104-bit long-term key and a random 24-bit R. RC4 key: Long-term key (40 or 104 bits) R (24) l 802.11 Frame: l Header R Message CRC encrypted

WEP is not secure Mainly because of its way of constructing the key Can be cracked in a minute http://eprint.iacr.org/2007/120.pdf

Stronger Security Notions K&L: Section 3.4

Constructing CPA-Secure Encryption Schemes K&L: Section 3.5

Pseudorandom generator secret one-time pad

Modes of Operations K&L: Section 3.6.2

Counter Mode (CTR)

Output feedback

How is Cipher Feedback (CFB) different from OFB?

Cipher block chaining (CBC)

CBC Chained CBC IV’

Chained CBC

Electronic Code Book (ECB)

Chosen-Ciphertext Attacks K&L Section 3.7

Padding-Oracle Attack: a concrete example of (partial) chosen-ciphertext attacks K&L Section 3.7.2

Cipher block chaining (CBC)