Internet has revolutionized many aspects of our daily lives. Nowadays Internet is used for millions of applications. Many people depend on Internet for several activities like on-line banking, on-line shopping, on-line learning and on-line meetings etc. Huge amount of data travels over the network. Security of data over the network is a critical issue. Making a network secure involves a lot more than just keeping it free from programming errors. Network Security refers to the protection of valuable data against Interception, Interruption, Modification, Fabrication and Non-repudiation. Computer networks are inherently insecure so to protect data over the networks we need some mechanism. Cryptography came into existence to ensure data security. There are various threats to data: Backdoors, denial-of-service attack, direct-access attack, eavesdropping, exploits, indirect-attacks and social engineering and human-error. Cryptography provides protection against security threats. Cryptography means secret writing, the content of original text is scrambled to produce coded text and job of intruders becomes difficult. Secret-writing is the strongest tool of cryptography which protects the data. Cryptography is used to ensure confidentiality, integrity and availability of data by using private and public key cryptography algorithms. Private and Public key algorithms are used to transform original (readable) messages into unreadable jumbles. This paper describes the comparison of Private (symmetric) and Public (asymmetric) key algorithms.