@secret = "7X9gx9E3Qx4EiUdB63nc" # Your secret key
@timestamp = "2021-05-26 14:00:00" # The exact (French) time at which the API request was made
@response = "WwBOU6s8DaMWmYdctBJwfuoujFgVygBUjhsbdf8eWqQ"

def decrypt_response(response)
  key = OpenSSL::Digest::SHA256.hexdigest(@timestamp + @secret)
  decipher = OpenSSL::Cipher.new("aes-256-cbc")
  decipher.decrypt
  decipher.key = key[0..31]
  decipher.iv = generate_iv
  json = decipher.update(Base64.decode64(response)) + decipher.final
  JSON.parse(json)
end

def generate_signature
  key = OpenSSL::Digest::SHA256.hexdigest(@secret)
  decipher = OpenSSL::Cipher.new("aes-256-cbc")
  decipher.encrypt
  decipher.key = key[0..31]
  decipher.iv = generate_iv
  Base64.encode64(decipher.update(@timestamp) + decipher.final).strip
end

private

def generate_iv
  full_iv = OpenSSL::Digest::SHA256.hexdigest(
    OpenSSL::Digest::SHA256.hexdigest(
      @secret
    )
  )
  full_iv.byteslice(0..15)
end

decrypt_response(@response)
# => {status: "SUCCESS"}

generate_signature
# => "bDop0cbjKpkySlpvnNGvBMg7PuYFFgPPqTTS2RAHoY0="