Encrypt data using an external private key?

Hi everyone,
I’m currently working on a web service implementation. Our external service receives an attribute encrypt using AES/ECB/PKCS5PADDING, but I don’t find any method of the Crypto class that allow me to use an external private key to encrypt data. Is any way to resolve this?
Thanks,
https://developer.salesforce.com/docs/atlas.en-us.200.0.apexcode.meta/apexcode/apex_classes_restful_crypto.htm#apex_System_Crypto_encrypt
https://www.linkedin.com/pulse/encrypting-salesforce-data-aes-di%C3%A9ffrei-quadros