crypto/rsa: check RSA private/public key size before using it

[xin-tsla]

Hi All,
The issue this PR tries to solve is that when private/public key size is zero, and use the private key to sign a token or public key to verify, it returns an error: crypto/rsa: message too long for RSA key size, which does not reflect the root cause.
The solution in the PR is to check the private/public key size before using the private/public key, and returns the error crypto/rsa: private/public key size zero if the private/public key size is zero to have a better error information and help to investigate.

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Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Message from Xin Zhang:

Patch Set 10:

(1 comment)

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