RSA and "Shared Secrets"

R is for Rivest, S for Shamir and A for Adleman. These are the three inventors of the most commonly used public-key cryptosystem on the web today. Every time you click on any URL that starts with https, your browser and the remote web server exchange a "shared secret" using RSA, which is the commonly used public-key cryptosystem on the web today. The beauty of the public-key method is that the shared secret is subsequently used to encrypt/decrypt sensitive information using what are called symmetric-key algorithms while an eavesdropper cannot discover this secret.

Increasing Use of Internet. The Internet today is a truly global marketplace for ideas and commerce, with a wide variety of goods and services available from any kind of computer, around the world. The Census Bureau of the U.S. Department of Commerce estimates that retail e-commerce sales, in the U.S. alone, amounted to more than 12 billion dollars for the second quarter of 2003. According to the U.S. Securities and Exchange Commission, over 25% of all U.S. retail stock trades now take place on the Internet. Worldwide, more than 100 million households bank online.

Transactions and Security. Secure communication is an intrinsic requirement for such transactions. Whether exchanging financial, business or personal information, people want to know with whom they are communicating (authentication) and they wish to ensure that the information is neither modified (data integrity) nor disclosed (confidentiality) in transit. Encryption algorithms like AES, Triple-DES and RC4 can be used for confidentiality; and keyed-hash algorithms like HMAC-SHA can provide source authentication and data integrity. These algorithms are classified as symmetric-key because they require the sender and receiver to agree on the same Key.

"Introductions" to Public-key or Asymmetric Cryptography. Key agreement over a public network like the Internet is challenging because, typically, the communicating parties have not met prior to their first interaction. Public-key or asymmetric cryptography solves this problem. Each communicating party has a pair of keys--one of which can be made public without revealing any information about the other key, which is kept private. Messages encrypted with a public-key can only be decrypted by the corresponding private-key. For example, a bank can send customers its public-key over an insecure channel. Customers can then use the public-key to encrypt a message that only the bank can decrypt. They still need to ensure that the public-key does indeed belong to the bank. This assurance is provided by digitally signed documents, called certificates that bind an entity's name and other attributes to its public-key. Certificates are issued by trusted third parties called certificate authorities. The infrastructure necessary to create, distribute, verify and, if necessary, revoke certificates is called a public-key infrastructure (PKI).

Combining Public-key Cryptography and Symmetric-key Operations. The powerful advantage of public-key cryptography comes at a price: public-key operations are much more computationally intensive than symmetric-key operations. For this reason, Internet security protocols like SSL, SSH, S/MIME and IPsec employ a combination of both. They use slower public-key algorithms to establish a shared secret between the sender and receiver. This secret is used to derive keys for the much faster symmetric-key algorithms used for confidentiality and integrity of bulk communication.