LESSON 9: SECRET - KEY ENCRYPTION

Secret-key encryption, also known as symmetric encryption, involves the use of a shared key for both encryption by the transmitter and decryption by the receiver. Secret-keyencryption works inthe following way: Anne wishes to send a purchase order (PO) to Bob in such a way that only Bob can read it. Anne encrypts the PO (the plaintext) with an encryption key and sends the encrypted PO (the cipher text) to Bob.Encryption scrambles the message, renderingit unreadable to anyone but the intended recipient.

Bob decrypts the cipher text with thedecryption key and reads the PO. Note that in secret-key encryption, the encryption key and decryption key are the same (see Fig.). The transmitter uses a cryptographic secret “key” to encrypt the message, and the recipient must use the same key to

decipher or decryptit. A widely adopted implementation of secret-key en-cryption is data encryption standard (DES).

Although secret-key encryption is useful in many cases, it has signifi-cant limitations. Allparties must know and trust each othercompletely, and have in their possession a protected copy of the key. If the transmitter and receiver are in separate sites, they must trust not being overheard during face-to-face meetings or over a public messaging system (a phone system, a postal service) when the secret key is being exchanged. Anyone

DecryptBob

does not scale well to a

business envi-ronment

where a company deals

with thousands of

online customers.

Further, secret-key

encryption is

impractical for

exchanging messages

with a large group of

previously unknown

parties over a public

network. For in-stance,

in order for a merchant

to conduct transactions

securely with Internet

subscribers, each

consumer would need

a distinct secret key as-

signed by the merchant

and transmitted over a

separate secure channel

such as a telephone,

adding to the overall

cost. Hence, given the

difficulty of providing

secure key

management, it is hard

to see secret-key

encryption becoming a

dominant player in

electronic commerce.

If secret encryption

cannot ensure safe

electronic commerce,

what can? The solution

to widespread open

network security is a

newer, more sophis-

ticated form of

encryption, first devel-

oped in the 1970s,

known as public-key

encryption.

Public-Key EncryptionPublic-key encryption,

also known as

asymmetric

encryption, uses two

keys: one key to

encrypt the message

and a different key to

decrypt the message.

The two keys are

mathematically related

so that data encrypted

with one key only be

decrypted

using the other.

who over-hears or intercepts the key intransit can later use that key to read all en-crypted messages.

Since shared keys must be securely distributed to each communicating party, secret-key encryption suffers from the problem ofkey distribution-generation,

transmission, and storage of keys. Secure key distribution is cumbersome in large networks and

Unlike secret-keyencryption, whichuses a single keyshared by two (ormore) parties,public-keyencryption uses apair of keys foreach party. One ofthe two keys is

“public” and theother is “private.”The public key canbe made known toother parties; theprivate key mustbe keptconfidential andmust be known

only to its owner. Both keys, however,need to be protected against modification.

The best known public-key encryption algorithm is RSA (named after its inventors Rivest, Shamir, and Adleman). In the RSA method, each participant creates two unique keys, a “public key,” which is published in a sort of public directory, and a “private key,” which is kept secret. The two keys work to-gether; whatever data one of the keys “locks,” only the other can unlock.

For example, if an individual wants tosend a snoop-proofemail mes-sage to a friend, she simplylooks up his public key and uses that key to en-crypt her text. When the friend receives the e-mail, he uses his private key to convert the encrypted message on his computer screen back to the

sender’s original message in clear text. Since only thebona fide author ofan encrypted message has knowledge of the private key, a successful decryp-tion using the corresponding public key verifies the identity of the author and ensures message integrity. Even if a would-becriminal intercepts the message on its way to the intended recipient, that criminal has no way of deciphering the message without the private key.

Figure illustrates what a public key looks like. This long string is ac-tually a number represented in hexadecimal. The computer handles the hard work of manipulating the large numbers used in the math ofencrypt-ing and decrypting messages.

Table compares secret- and public key systems. Both types of sys-tems offer advantages and disadvantages. Often, the two are

combined to form a hybrid system toexploit the strengths of each method. To determine which type of encryption best meets its needs, an organization first has to identify its security requirements and operating environment. Public-key encryption is particularly useful when the parties wishing to communicate cannot rely on eachother or do not share a common key. This is often the case in online commerce.

Another prominentpublic key method being used in online commerce today is called Digital Signatures.

Comparing Secret key And public keyEncryption methods.

FeaturesSecret keyPublic Key

Pair of keys

One key is private, one is public

Key Management Simple but difficult Need digital

certificates & third parties.

Relative speeds Very fastSlower

UsageUsed for bulk data Used for less

demanding

such as small documents

Digital signatureare used forsendingauthentication.This also meansthat the originatorcannot falselydeny havingsigned the data. Inaddition , a digitalsignature enablesthe computer to

Number of keys Single key

Types of keys Key is

secret

notarize the message , ensuring the recipient that the message has not been forged I transit.

Let us consider the following scenario of a customer, interacting with a merchant , Online mart. When the customer orders something from Online mart, he uses Online mart’s public key to encrypt her confidential information. Online Mart then uses its private key to decrypt the message ( only a private key can unlock a document deciphered with a public key); thus the customer knows that only Online Mart received that data. To ensure further security , the customer can enclose a digital signature , encrypted with her own private key, which Online Mart could decrypt with the customers public key and know that only the particular customercould have sent it. In the other direction Online mart would send confidential information to the customer using her public key, and only she can decrypt it using herprivate key . This shows how digital signature work in combination with public key

encryption to ensure authentication andprivacy.

Technically, How Do Digital Signatures Work?Data is electronically signed by applying the originator’s private key to the data. To increase the speed of the process, the privatekey is applied to a shorter form of the data, called a “hash” or “message digest,” rather than to the entire set of data. The resulting digital signature can be stored or transmitted along with the data. The signature can be verified by any party using the public key of the signer. This featureis very useful, for example, when distributing signed copies of virus-free.software. Any recipient can verifythat the program re-mains virus-free. If the signature verifies properly, then the verifier hascon-fidence that the data was not modified after 1:Jeing signed and that the owner of the public key was the signer.

Digital signatures ensure authentication in the following way. In order to digitallysign a document, a user combines her private key and thedocu-ment and performs a

computation on the composite (key+docurnent) inor-der to generate aunique number called the digital signature. For example, when an electronic document, such as an order form with a credit card number, is run through the digital signature process, the output is a unique “fingerprint” of the document. This “fingerprint” is attached to the original message and further encrypted with the signer’s private key.If a user is communicating with her bank, she sends the result of the second encryp-tion to her bank. The bank then decrypts the document using her public key, and checks to see if the enclosed message has been tampered with by a third party. To verify the

signature, the bank performs a computation in-volving the original document,the purported digital signature, and the cus-tomer’s public key.If the results of the computation generate a matching “fingerprint” of thedocument, the digital signature is verified as genuine; otherwise,the signature may be fraudulent or the message altered.

Digital signatures, variations of whichare being explored by several companies, are the basis for secure commerce. A digitalsignature provides a way to associate the message with the sender, and is the cyberspace equivalent of “signing” for purchases. In this way, consumers can use credit card accounts

over the Internet. Interested readers can refer to [SCHN96] for a more detailed mathematical discussion of digital signatures.

Digital CertificatesAuthentication is further strengthened by the use of digital certificates. Before two parties, Bob and Alice, use public-key encryption to conduct business, each wants to be sure that the other party is authenticated. Before Bob accepts a message with Alice’s digital signature, he wants to be sure that the public key belongs to Alice and not to someone masquerading as Alice on an open network. One way to be sure that the public key belongs to Alice is to receive it over a secure channel directly from Alice. However, in most circumstances this solution is not practical.

An alternative to the use of a secure channel is to use a trusted third party to authenticate that the public key belongs to Alice. Such a party is known as a certificate authority (CA). Once Alice has provided proof of her identity, the certificate authority creates a message containing Alice’s name and her public key. This message, known as a certificate, is digitally signed by the certificate authority. It contains owner identification information, as well as a copy of one of the owner’s public keys. To get the most benefit, thepublic key of the certificate authority should be known to as manypeople as possible. Thus by using one public key (that of a CA) asa trusted third- party means of establishing authentication, disparate parties can engage in electronic commerce with a high degree of trust.

In many ways, digital certificates are the heart of secure electronic transactions. Through the use of a common third party, digital certificates provide an easy and convenient way to ensure that the participants in an electronic commerce transaction can trust each other. For example, in the credit card industry, Visa provides digital certificates to the card-issuing financial institution, and the institution then provides a digital certificate to the cardholder. A similar process takes place for the merchant. At the time of the transaction, each party’s software validates both merchant and cardholder before any information is exchanged. The validation takes place by checking the digital certificates that were both issued by an au-thorized and trusted third party. In short, digital certificates ensure that two computers talking to each other may successfully conduct electronic commerce.

Notes

30 11.676.1