Require that users explicitly specify the desired algorithm. In my
opinion, we are not in a position to specify the default cipher.

When OpenSSL::PKCS7.encrypt is given only two arguments, it uses
"RC2-40-CBC" as the symmetric cipher algorithm. 40-bit RC2 is a US
export-grade cipher and considered insecure.

Although this is technically a breaking change, the impact should be
minimal. Even when OpenSSL is compiled with RC2 support and the macro
OPENSSL_NO_RC2 is not defined, it will not actually work on modern
systems because RC2 is part of the legacy provider.

https://github.com/ruby/openssl/commit/439f456bfa