What are the disadvantage of rsa algorithm?

Answer 1

A disadvantage of using public-key cryptography for encryption is speed. There are many secret-key encryption methods that are significantly faster than any currently available public-key encryption method. Nevertheless, public-key cryptography can be used with secret-key cryptography to get the best of both worlds. For encryption, the best solution is to combine public- and secret-key systems in order to get both the security advantages of public-key systems and the speed advantages of secret-key systems. Such a protocol is called a digital envelope, which is explained in more detail in Question 2.2.4.

Public-key cryptography may be vulnerable to impersonation, even if users' private keys are not available. A successful attack on a certification authority (see Question 4.1.3.14) will allow an adversary to impersonate whomever he or she chooses by using a public-key certificate from the compromised authority to bind a key of the adversary's choice to the name of another user.

In some situations, public-key cryptography is not necessary and secret-key cryptography alone is sufficient. These include environments where secure secret key distribution can take place, for example, by users meeting in private. It also includes environments where a single authority knows and manages all the keys, for example, a closed banking system. Since the authority knows everyone's keys already, there is not much advantage for some to be "public" and others to be "private." Note, however, that such a system may become impractical if the number of users becomes large; there are not necessarily any such limitations in a public-key system.

Public-key cryptography is usually not necessary in a single-user environment. For example, if you want to keep your personal files encrypted, you can do so with any secret key encryption algorithm using, say, your personal password as the secret key. In general, public-key cryptography is best suited for an open multi-user environment.

Public-key cryptography is not meant to replace secret-key cryptography, but rather to supplement it, to make it more secure. The first use of public-key techniques was for secure key establishment in a secret-key system [DH76]; this is still one of its primary functions. Secret-key cryptography remains extremely important and is the subject of much ongoing study and research. Some secret-key cryptosystems are discussed in the sections on block ciphers and stream ciphers.