Encrypt and decrypt using rot13 and rot7

So I'm writing a program that encrypts and decrypts a text consisting of letters and other symbols.

Specification
Allow the user to enter a text. All characters are allowed.

Encrypt text.
Use the two keys Rot13 and rot7
Change the key every five characters
Save the encrypted text in a separate string
Print the encrypted string on the screen

Decrypt text
Use the same keys as above
Print the decrypted string on the screen

Anyone that could help me?
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