Terminology (from Schneier, p. 1)
- Encryption: Turning plaintext into ciphertext. Decryption: turning
ciphertext back into plaintext.
- Cryptography: The art and science of keeping messages secure
(practiced by cryptographers)
- Cryptanalysis: Looking for ways to break ciphertext.
- Cryptology: The branch of mathematics encompassing cryptography
and cryptanalysis.
- Steganography: Hiding messages within other messages so that
their existence is a secret.
General types of encryption
- One-time pad: "perfect encryption"
- Substitution cipher; block substitution cipher
- Secret key encryption
- One-way hash
- Public/private key encryption
Encryption issues
- Trapdoors: can you trust the algorithm? Can you examine the
source? Can you trust the cryptoanalysis?
- Random numbers: they're usually not
- Factoring large numbers: it's getting easier
- Applications include: SSL (for secure Web transactions),
PGP (pretty good privacy: public/private key for email), crypt() and
MD5 for Unix passwords, kerberos for trusted host communication...many others!
- The Data Encryption Standard (DES) is the basis for many
encryption systems, including banking and government communication.
It has been implemented in both hardware and software. In practice,
the DES algorithm is repeated 3 times for more security: 3DES or
triple-DES.
- The DES will be gradually replaced by RIJNDAEL, the AES. In
terms of key size (the number of possible keys), AES specifies
3.4x1038 keys for 128 bit,
6.2x1057 keys for 192 bit,
1.1x1077 keys for 256 bit,
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