Java IO.pdf - Nguyen Dang Binh

Java I/O
them your encryption (public) key. Even if the key is intercepted, this only allows the
interceptor to send you encrypted messages. It does not allow them to decode encrypted
messages intended for you.
The most famous public key cipher is the patented [5] RSA cipher, named after its inventors,
Ronald L. Rivest, Adi Shamir, and Leonard M. Adleman. RSA has the particularly nice
property that either key can be used for encryption or decryption. Generally, you'll keep one
key secret (your private key) and publish the corresponding key. People can send encrypted
messages to you using your public key that you decrypt with your private key. Furthermore,
by encrypting either a message or a hash code of the message with your private key, which
may then be decrypted with your public key, you can digitally sign messages. Any message
that can be successfully decrypted with your public key may be presumed to have come from
you, because only you could have encrypted it with your private key in the first place. (Of
course, if someone steals your private key, all bets are off.)
10.4.3 Block Versus Stream Ciphers
Encryption algorithms may also be divided into block and stream ciphers. A block cipher
always encrypts a fixed number of bytes with each pass. For example, DES encrypts eightbyte
blocks. If the data you're encrypting is not an integral multiple of the block size, the data
must be padded with extra bytes to round up to the block size. Stream ciphers, by contrast, act
on each bit or byte individually in the order it appears in the stream; padding is not required.
Block ciphers can operate in a variety of modes that use various algorithms to determine how
the result of the encryption of one block of data influences the encryption of subsequent
blocks. This ensures that identical blocks of plaintext do not produce identical blocks of
ciphertext, a weakness code breakers can exploit. To ensure that messages that start with the
same plaintext (for example, many email messages or form letters) don't also start with the
same ciphertext (also a weakness code breakers can exploit), these modes require a nonsecret
initialization vector, generally of the same size as a block, in order to begin the encoding.
Initialization vectors are not secret and are generally passed in the clear with the encrypted
data.
10.4.4 Key Management
Storing keys securely is a difficult problem. If the key is stored in hardware like a smart card,
it can be stolen. If the key is stored in a file on a disk, the disk can be stolen. Many basic PC
protection schemes are based on OS- or driver-level operations that refuse to mount the disk
without the proper password, but simply using a new OS (or driver or custom hardware)
allows the key or unencrypted data to be read off the disk.
Ideally, keys should not be stored anywhere except in a human being's memory. Human
beings, however, have a hard time remembering arbitrary 56-bit keys like
0x78A53666090BCC, much less more secure 64 -, 112-, or 128-bit keys. Therefore, keys
humans have to remember are generally stored as a string of text called a password. Even
then, the password is vulnerable to a rubber hose attack. Truly secure systems like those used
to protect bank vaults require separate passwords remembered by two or more individuals.
5 The patent expires September 20, 2000.
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