[FS#405] openvpn-mbedtls can not verify certificate

[LEDE Bugs]

The following task has a new comment added:

FS#405 - openvpn-mbedtls can not verify certificate
User who did this - Adrián Panella (ianchi)

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Just to share the solution that worked for me to switch to mbedtls:

+ change to RSA 2048 
+ change certificate digest from MD5 to SHA256 (use option "default_md = sha256").

The default digest used by easy-rsa key generation scripts was "md5", and it was rejected by mbedtls.
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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=405#comment2325