How to decrypt a PKCS7 in Crystal using OpenSSL

PKCS7_decrypt.cr

# shard.yml
# dependencies:
#   openssl_ext:
#     github: spider-gazelle/openssl_ext
#     version: ~> 2.0
require "openssl"
require "openssl_ext"

ciphertext = "-----BEGIN CERTIFICATE-----
MIIBiQYJKoZIhvcNAQcDoIIBejCCAXYCAQAxggEhMIIBHQIBAD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==
-----END CERTIFICATE-----"

public_key = "./keys/public_key.pkcs7.pem"
private_key = "./keys/private_key.pkcs7.pem"

private_key_pem = File.read private_key
private_key_rsa = OpenSSL::PKey::RSA.new(private_key_pem)

public_key_pem = File.read public_key
public_key_x509 = OpenSSL::X509::Certificate.new(public_key_pem)

lib LibCrypto
  type PKCS7 = Void*

  fun pem_read_bio_pkcs7 = PEM_read_bio_PKCS7(bp : Bio*, x : PKCS7*, cb : PasswordCallback, u : Void*) : PKCS7
  fun pkcs7_decrypt = PKCS7_decrypt(p7 : PKCS7, pkey : EvpPKey*, cert : X509, data : Bio*, flags : Int32) : Int32
end

class OpenSSL::PKCS7
  def initialize(@ciphertext : String)
    io = IO::Memory.new(@ciphertext)
    bio = OpenSSL::GETS_BIO.new(io)
    @pkcs7 = LibCrypto.pem_read_bio_pkcs7(bio, nil, nil, nil)
  end

  def decrypt(private_key_rsa : OpenSSL::PKey::RSA, public_key_x509 : OpenSSL::X509::Certificate)
    key = private_key_rsa
    x509 = public_key_x509
    data = OpenSSL::MemBIO.new
    flg = 0

    ret_code = LibCrypto.pkcs7_decrypt(@pkcs7, key, x509, data, 0)
    data.to_string
  end
end

pkcs7 = OpenSSL::PKCS7.new(ciphertext)
puts pkcs7.decrypt(private_key_rsa, public_key_x509)