Behaves identical to the legacy counterpart rsa_save_privatekey_mem() except that it allows the caller to specify the cipher used to encrypt the private key (the legacy function always uses des-ede3-cbc).
rsa_save_privatekey_mem_cipher( privkey var, rsa var, passphrase, ciphername )
ciphername = an OpenSSL cipher identifier that specifies the cipher to be used. Note that OpenSSL only permits a subset of its supported ciphers to be used for RSA key encryption. For example, only CBC mode ciphers are permitted.