In short: Telent is transmitted codes, SSH is encrypted transfer.
Use Telnet to access a remote computer that is used to TCP / IP protocol to control your network devices from a building when the equivalent of shouting in your user name and password. Soon someone will be listening, and they will make use of your lack of safety awareness. The traditional network service programs such as: ftp, pop, and telnet is insecure in nature, because they Chuansong the network clear text passwords and data, with ulterior motives are very easy to intercept the passwords and data. Moreover, these security authentication service program also has its weaknesses, is vulnerable to a "middleman" (man-in-the-middle) attacks in this way. The so-called "middleman" in the attack, is the "middleman" posing as the real server receives the data you passed to the server, and then pretending to pass your data to the real server. Server and data transfer between your being a "middleman", after a rigged changed hands, there will be a very serious problem.
SSH is an alternative Telnet and other remote console management applications, industry standards. SSH commands are encrypted and kept confidential in several ways. SSH has many functions, It can replace telnet, it can be ftp, pop, or even to provide a safe ppp "channel." SSH (Secure SHell) So far there are two incompatible versions - SSH1 and SSH2.
SSH1 is divided into two versions 1.3 and 1.5. SSH1 using DES, 3DES, Blowfish, and RC4 symmetric encryption algorithm such as protection of data security transmission, while the symmetric key encryption algorithm is non-symmetric encryption algorithm by (RSA) to complete the exchange. SSH1 using cyclic redundancy check code (CRC) to ensure data integrity, but this method was found to be defective.
SSH2 RSA avoid patent issues, and fix the CRC of the defect. SSH2 with the Digital Signature Algorithm (DSA) and Diffie-Hellman (DH) algorithm instead of RSA to complete the exchange of symmetric keys, confirm with the message code (HMAC) to replace the CRC. SSH2 also increased the AES and Twofish encryption algorithms such as symmetric.