 Internet Security
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| 04/01/02 |
A cryptographic encryption algorithm, also known as cipher, transforms
a "plain text" (e.g., human readable) pt and outputs cipher text
ct as the output, ct = cipher(pt, key), so that it is
possible to re-generate the pt from the ct through a
companion decryption algorithm. Note that we said "for example,
human readable" and not "that is, human readable" as an explanation for the
phrase "plain text". In other words, the so-called "plain text" may
be human un-readable binary data that is ready-to-use by a computer.
Ciphers use keys together with plain text as the input to produce cipher text. It is in the key that the security of a modern cipher lies, not in the details of the algorithm.
Roughly speaking, computationally infeasible means that a certain computation that we are talking about takes way too long (hundreds of years) to compute using the fastest of (super) computers.
Suppose our key is a 128-bit number. There are
340,282,366,920,938,463,463,374,607,431,768,211,456
128-bit numbers starting from zero (i.e., 128 bits of 0). To recover a particular key by brute force, one must, on average, search half the key space:
170,141,183,460,469,231,731,687,303715,884,105,728.
If we use 1,000,000,000 machines that could try 1,000,000,000 keys/sec, it would take all these machines longer than the universe as we know it has existed to find the key.
This is not the same thing as saying that computational infeasibility is the same idea as Turing-incomputable. Nor is it the same thing as saying that you cannot make a lucky guess, or heuristically arrive at a possible answer, and then systematically verify that the guessed answer is indeed the correct answer, all done within a matter of seconds on a lowly PC. Here is an example: Microsoft Windows NT uses the DES encryption algorithm in storing the passwords. Brute-forcing such a scrambled password to compute the plain text password can take, according to Microsoft, "about a billion years." But the L0pht team (www.l0pht.com) claims that L0phtCrack breaks Windows NT passwords in about one week, running in the background on a Pentium 200 based PC.
In the context of cryptography, the factorization of an arbitrarily large number N, into its constituent primes, determining the powers n2, n3, n5, n7, etc. of the primes, is computationally infeasible -- as far as we know.
N = 2n2 * 3 n3* 5 n5 * 7 n7* ...
Based on this, the decryption is computationally infeasible.
A one-way hash function takes a variable-length input sequence of bytes and converts it into a fixed-length sequence. The fixed length is considerably shorter than the typical length of the input, and hence the function is a hash function. The "one way" means that the function is designed to be computationally infeasible to reverse the process, that is, to discover, with no prior information other than the hash, a string that hashes to a given value.
The nature of all hash functions is that there must exist multiple input sequences that map to the same hash. The inverse is a mathematical relation, not a mathematical function. But, a good hash functions have the following properties: It is rare to find two strings, from the expected set of typically used strings, that would produce the same hash value. A slight change in an input string causes the hash value to change drastically. E.g., if we flip one bit in the input string, at least half of the bits in the hash value flip as a result, then the hash is a good one.
One-way hash functions are also known as message digests (MD), fingerprints, or compression functions. The most popular one-way hash algorithms are MD4 and MD5 (both producing a 128-bit hash value), and SHA, also known as SHA1 (producing a 160-bit hash value).
Symmetric-key cryptography is an encryption system in which the sender and receiver of a message share a single, common key to encrypt and decrypt the message. Symmetric-key systems are simpler and faster, but their main drawback is that the two parties must somehow exchange the key in a secure way. Symmetric-key cryptography is sometimes also called secret-key cryptography.
If ct = encryption (pt, key), then pt = decryption (ct, key).
The most popular symmetric-key system is the DES, short for Data Encryption Standard. DES was developed in 1975 and standardized by ANSI in 1981 as ANSI X.3.92. DES encrypts data in 64-bit blocks using a 56-bit key. The algorithm transforms the input in a series of steps into a 64-bit output.
IDEA (International Data Encryption Algorithm) is a block cipher which uses a 128-bit length key to encrypt successive 64-bit blocks of plain text. The procedure is quite complicated using subkeys generated from the key to carry out a series of modular arithmetic and XOR operations on segments of the 64-bit plaintext block. The encryption scheme uses a total of fifty-two 16-bit subkeys.
Blowfish is a symmetric block cipher that can be used as a drop-in replacement for DES or IDEA. It takes a variable-length key, from 32 bits to 448 bits, making it ideal for both domestic and exportable use. Blowfish is unpatented and license-free, and is available free for all uses.
Public key cryptography uses two keys -- a public key known to everyone, and a private or secret key known only to the recipient of the message. Public key cryptography was invented in 1976 by Whitfield Diffie and Martin Hellman. For this reason, it is sometimes also called Diffie-Hellman encryption. It is also called asymmetric encryption because it uses two keys instead of one key. The two keys are mathematically related, yet it is computationally infeasible to deduce one from the other.
When John wants to send a secure message to Jane, he uses Jane's public key to encrypt the message. Jane then uses her private key to decrypt it. Anyone with the public key can encrypt a message but not decrypt it with it. Only the person with the private key can decrypt the message. If a message is encrypted with someone's private key, it can only be decrypted with the corresponding public key.
Unfortunately, public-key cryptography is very slow, about 1000 times slower than symmetric key cryptography.
Public-key systems, such as Pretty Good Privacy (PGP), are becoming popular for transmitting information via the Internet. They are extremely secure and relatively simple to use. You, of course, need to know the recipient's public key to encrypt a message. A global registry of public keys is needed, which is one of the promises of the LDAP technology.
The most well-known of the public-key encryption algorithms is RSA, named after its designers Rivest, Shamir, and Adelman. The un-breakability of the algorithm is based on the fact that there is no efficient way to factor very large numbers into their primes.
Then, the public-key is (n, e), and the private key is (n, d). The public key is published in well known places; the private key must be safe guarded. If the number n is small, p and q are easy to discover. Thus, p and q are chosen to be as large as possible, say, more than a hundred digits long. Obviously, p and q should never be revealed, preferably destroyed.
Encryption is done as follows. Consider the entire message to be encrypted as a sequence of bits. Suppose the length of n in bits is b. Split the message into blocks of length b or b-1. A block viewed as a b-bit number should be less than n; if it is not, choose it to be b-1 bits long. Each block is separately encrypted, and the encryption of the entire message is the catenation of the encryption of the blocks. Let m stand for a block viewed as a number. Multiply m with itself e times, and take the modulo n result as c, which is the encryption of m. That is, c = m^e mod n.
Decryption is the "inverse" operation: m = c^d mod n.
The only way known to find d is to know p and q. The e and n are the public key, which is published, while d is the private key, which must be kept secret. The e and d are symmetric in that using either as the encryption key, the other can be used as the decryption key.
In real-world implementations, public keys are rarely used to encrypt actual messages because public-key cryptography is slow. Instead, public-key cryptography is used to distribute symmetric keys, which are then used to encrypt and decrypt actual messages, as follows:
A digital signature is a way to authenticate to a recipient that a received object is indeed that of the sender.
The public key-based communication between Alice and Bob described above is vulnerable to a man-in-the-middle attack.
Let us assume that Mallory, a cracker, not only can listen to the traffic between Alice and Bob, but also can modify, delete, and substitute Alice's and Bob's messages, as well as introduce new ones. Mallory can impersonate Alice when talking to Bob and impersonate Bob when talking to Alice. Here is how the attack works.
A man-in-the-middle attack works because Alice and Bob have no way to verify they are talking to each other. An independent third party that everyone trusts is needed to foil the attack. This third party could bundle the name "Bob" with Bob's public key and sign the package with its own private key. When Alice receives the signed public key from Bob, she can verify the third party's signature. This way she knows that the public key really belongs to Bob, and not Mallory.
A package containing a person's name (and possibly some other information such as an E-mail address and company name) and his public key and signed by a trusted party is called a digital certificate (or digital ID). An independent third party that everyone trusts, whose responsibility is to issue certificates, is called a Certification Authority (CA). A digital certificate is a means of binding the details about an individual or organization to a public key. A digital certificate serves two purposes. First, it provides a cryptographic key that allows another party to encrypt information for the certificate's owner. Second, it provides a measure of proof that the holder of the certificate is who they claim to be - because otherwise, they will not be able to decrypt any information that was encrypted using the key in the certificate.
The recipient of an encrypted message uses the CA's public key to decode the digital certificate attached to the message, verifies it as issued by the CA and then obtains the sender's public key and identification information held within the certificate. With this information, the recipient can send an encrypted reply.
The most widely used standard for digital certificates is X.509, which defines the following structure for public-key certificates:
You can obtain a personal certificate from companies like VeriSign www.verisign.com or Thawte www.thawte.com.
| April 08, 2004 |
| Copyright © 2004 pmateti@cs.wright.edu |