Why does PuTTY use its own private key format when log into SSH server?

Just a little bit curious, why PuTTY use its own v开发者_开发知识库ersion of private key format to do SSH?

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The author of PuTTY gives two main reasons for having the custom key format on this page.

In short:

• PuTTY's format stores the public half of the key in plaintext, which allows PuTTY to send the public key to the server automatically.

• The key is fully tamperproofed with the help of a Message Authentication Code.