add rsa_ext

[Caisin]

pri key encrypt long string
pub key decrypt long data

[tarcieri]

I've mostly seen this referred to as RSA in Electronic CodeBook (ECB) mode.

It's a suboptimal way to use RSA as it needs to perform a modpow operation per "block", and generally discouraged.

A better option is to use hybrid encryption, which uses something like RSA-KEM to establish a shared symmetric key, then symmetric cryptography to actually encrypt the message.

HPKE provides a standard protocol for this, although unfortunately we don't yet provide RSA-KEM support: #323

[Caisin]

I know what you mean
because this encryption and decryption method is provided in other programming languages
when we interact with other third-party systems, they use this way to verify encryption and decryption of apis,
in order to adapt to their api, we have to use this way to encrypt and decrypt,
I searched for a long time in crates.io and github, but did not find,
so I implement it myself.
I believe that there will be others who have the same needs as me
so I create this pull request

[tarcieri]

because this encryption and decryption method is provided in other programming languages

That's not a justification for including this feature. The term for that is cargo culting.

I would personally prefer not to include such a feature in the rsa crate because:

• It is a non-standard construction without an associated standards document like an RFC
• It is badly designed and including it is something of an endorsement
• Hybrid encryption should be the preferred solution to this problem

However, feel free to publish your own crate that implements this construction.