Get the Whitepaper called RSA Asymmetric Polymorphic Shellcode. It discusses how to encrypt and decrypt the opcodes of the shellcode, how the program that decrypts the shellcode was built and how to get the opcodes, and much more.
You can download it from the following link: https://packetstormsecurity.com/files/download/141335/rsaasymmetric-polymorphic.pdf
Source: https://packetstormsecurity.com/files/141335/RSA-Asymmetric-Polymorphic-Shellcode.html