Use a Polybius square to get

threehphsevenharmonyofournfivey

3hph7harmonyo4n5y

Split numbers in pairs

14 04 21 17 18 j 09 16 18 04 l 12 19 04 l 16 20 22 j 21 18

Convert numbers to letters using a=0

OEVRSjJQSElMTElQUWjVS

We now have 21 characters which is a bit odd. If you convert from Base64 you get the prefix and a good part of the keyword.

Let’s split things a bit to try to find out what is wrong

OEVRSjJQSElMTElQ

WjVS

We have to dismiss the U for now because we need 16 characters

Convert these from Base64

8EQJ2PHILLIP

Z5R

We know the keyword should be PHILLIPS which explain the U in the original string.

Now we need to brute the first letter and first numbers of the suffix. The winning combination is N8. So the good Base64 string would be

OEVRSjJQSElMTElQU044WjVS

Convert Base64 to Ascii

8EQJ2PHILLIPSN8Z5R

Make a 3×6 grid

Split into two groups of 3×3

Read the first one going down from the upper right 6

6dfj8crea

Read the second one going up from the lower left t

tivef2y9d

6dfj8creativef2y9d