Why is the security of modern cryptography based on the secrecy of the key, rather than the secrecy of the classical cryptography algorithm? __ algorithm

The current password must be tested by countless people, proving that there is no trap.

If you are not public, no one believes in the security of your algorithm.
Many algorithms are later proven to be unsafe.

Simply put, the three main foundations of modern cryptography are:
1, the introduction of large prime numbers;
3, the irreversible nature of the algorithm;
2, based on computer operation rather than hand calculation.
Based on these three points, in modern cryptography, the "encryption algorithm" is far more complex than the "to encrypt content" itself, and in classical algorithms, the encryption algorithm and the strength of the encrypted content is roughly the same, or in the same order of magnitude.

I remember not all modern cryptographic algorithms rely on large primes, and DES, the elliptic curve method is not as good as it seems.
Modern encryption algorithm is the most important thing is the irreversibility of the algorithm, you know the algorithm, according to ciphertext guessing plaintext is also impossible (such as Des needs 2^47 Group of civilized text to be able to untie).
and the classical code system is originally through the encryption algorithm itself rather than the password to encrypt, for example, Caesar password is c+k, see ciphertext, and then know the encryption algorithm, it is easy to guess the results to

Because modern cryptography is based on the computer, the replacement of the key is convenient, because an algorithm of the key can be randomly assigned to generate, but the replacement algorithm is not convenient, a cryptographic technology products the most support of the cryptographic algorithm is limited, there are several, can not develop a cryptographic products, what algorithms can be supported. Therefore, secret key secrecy becomes important, because the mature algorithm standard is public for the convenience of social application.