To secure Internet communications, several mechanisms, known collectively as public-key cryptography, have been developed and implemented to protect sensitive data during transmission over the Internet. Public-key cryptography consists of data encryption, key exchange, digital signatures, and digital certificates.
Encryption is a process wherein a cryptographic algorithm is used to encode information to safeguard it from anyone except the intended recipient. There are two types of keys used for encryption:
Symmetric-key encryption is where the same algorithm (key) is used to encrypt and decrypt the message. This form of encryption provides minimal security because the key is simple, and therefore easy to decipher. However, transfer of data that is encrypted with a symmetric key is fast because the computation required to encrypt and decrypt the message are minimal.
Public/private keys, also known as asymmetric keys, are a pair of keys that are made up of public and private components to encrypt and decrypt messages. Typically, the message is encrypted by the sender with a private key, and decrypted by the recipient with the sender’s public key, although this may vary. It is quite possible to use a recipient’s public key to encrypt a message, who then uses his private key to decrypt the message.
The algorithms used to create public and private keys are more complex, and therefore harder to decipher. However, public/private key encryption requires more computation, sends more data over the connection, and noticeably slows the transfer of data.
The solution for reducing computation overhead and speeding transactions without sacrificing security is to use a combination of both symmetric key and public/private key encryption in what is known as a key exchange.
For large amounts of data, a symmetric key is used to encrypt the original message. The sender then uses either his private key or the recipient’s public key to encrypt the symmetric key. Both the encrypted message and the encrypted symmetric key are sent to the recipient. Depending on what key was used to encrypt the message (public or private) the recipient uses the opposite to decrypt the symmetric key. Once the key has been exchanged, the recipient uses the symmetric key to decrypt the message.
Digital signatures are used for tamper detection and non-repudiation. Digital signatures are created with a mathematical algorithm that generates a unique, fixed-length string of numbers from a text message; the result is called a hash or message digest.
To ensure message integrity, the message digest is encrypted by the signer’s private key, then sent to the recipient along with information about the hashing algorithm. The recipient decrypts the message with the signer’s public key. This process also regenerates the original message digest. If the digests match, the message proves to be intact and tamper free. If they do not match, the data has either been modified in transit or the data was signed by an imposter.
Further, the digital signature provides non-repudiation—senders are prevented from denying, or repudiating, that they sent the message, because the sender’s private key encrypted the message. Obviously, if the private key has been compromised (stolen or deciphered), the digital signature is worthless for non-repudiation.
Certificates are like passports: once you have been assigned one, the authorities have all your identification information in the system. Immigration control can access your information as you travel from country to country. Like a passport, the certificate is used to verify the identity of one entity (server, router, Web site, and so on) to another.
There are two types of certificates:
Server certificates – A server certificate authenticates the server that holds it. Certificates are issued by a trusted third-party Certificate Authority (CA), much like the U.S. Department of State issues passports. The CA validates the holder’s identity, and embeds the holder’s public key and other identification information into the digital certificate. Certificates also contain the digital signature of the issuing CA, verifying the integrity of the data contained therein and validating its use.
CA certificates – Also known as trusted root certificates, CA certificates are used by servers when they function as a client, such as during remote procedure calls (RPCs). When connecting to a remote server for RPCs, Adaptive Server verifies that the CA that signed the remote server’s certificate is a “trusted” CA listed in its own CA trusted roots file. If it is not, the connection fails.
The combination of these mechanisms protect data transmitted over the Internet from eavesdropping and tampering. These mechanisms also protect users from impersonation, where one entity pretends to be another (spoofing), or where a person or an organization says it is set up for a specific purpose when the real intent is to capture private information (misrepresentation).
