Generate private key with passphrase

Trick copyright: The Book of Secret Knowledge <https://github.com/trimstray/the-book-of-secret-knowledge>

# _ciph: aes128, aes256
# _len: 2048, 4096
( _ciph="aes128" ; _fd="private.key" ; _len="2048" ; \
openssl genrsa -${_ciph} -out ${_fd} ${_len} )

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