# Selected Cipher Schemes and Their Uses

Tom Kelliher, CS 325

Sept. 19, 2011

### Outline

1. DES.

2. AES.

3. Public key principles.

4. Uses.

### Coming Up

Discussion of individual Perl/CGI assignment and project.

# DES

1. IBM-designed and NSA-analyzed.
1. 64-bit block cipher. Symmetric.

2. Uses simple arithmetic and logical operations.

3. Slow -- hardware implementations were available. (But faster than asymmetric.)

4. Sixteen rounds of substitution and transposition.

2. Controversy:
1. 128 bit keys to 64 to 56 (parity bits).

2. Design of S-boxes.

3. Differential cryptanalysis shows that DES is optimal.''

This technique was known to IBM and the NSA much earlier.

## Marginally Increased Strength: Double DES

1. Product cipher -- apply DES twice, with two keys.

2. With two keys, should have strength of , right?

3. Wrong -- strength is , using Meet in the Middle'' attack.

Requires two plaintext, ciphertext pairs.

## Reasonably Increased Strength: Triple DES

1. Apply DES three times, with three keys:

Strength is . Meet in the middle'' defeats one stage.

2. Apply DES three times, with two keys:

Strength is just .

Why a decrypt stage? Consider the case -- single DES.

# AES

1. Replaces DES as a US standard.

2. Winner of a contest.'' Designed by two Dutch cryptographers (Rijndael).

Vetted by NSA.

3. 128-bit block cipher. Symmetric.

4. Uses simple arithmetic and logical operations.

5. Fast and easy to implement.

6. Variable key length: 128, 192, 256.

Key lengths of 192 and 256 are approved for US Top Secret level data.

7. Number of rounds is a function of key length: 9, 11, 13.

Decreasing the number of rounds weakens AES. To date, best known attacks are with 7, 8, 9 rounds, respectively. Too close for comfort?

What do we mean by breaking encryption?

# Public Key Encryption

General principles:

1. Asymmetric.

2. .

Also: .

3. 10,000 times slower than private key.

4. RSA based on finding the two prime factors of a large number.

# Uses

Elements:

1. Symmetric cipher for private data transfer.

Key distribution?

2. Asymmetric for initial privacy, authentication.

E for privacy, D for authentication.

How can someone'' securely send you a document?

How do I convince you of my identity?

How do I securely send you a document, convincing you it's from me?

3. Hash (MD5) for digestification and digital signature.

Requirements for secure information transfer between two parties. Information should be:

1. Unforgeable.

2. Authentic.

3. Unalterable.

4. Non-reusable.

How do we use the former elements so as to provide these features?

Digital certificate:

1. An originator's identity and public key.

2. CA certifies.

3. Digest and sign by CA.