Simple Cryptography Capture the Flag

[Yoriz]

I assume there is some sort of pattern of how to pick the position of characters from the text file.
The only letters that you seem to know should be there are

f l a g {

which from

flUa8xIpgmFuNeSq5O{

is the character positions

1, 2, 4, 9, 19

And then you have to work out the alogorithm to continue the numbered pattern to find the other characters.
Is that anywhere close to what you are trying to do.
Can you provide more/better information on what you expect the resulting output to be ?

[micseydel]

flUa8xIpgmFuNeSq5O{

is the character positions

1, 2, 4, 9, 19

If you're using 1-based indexing  =p

[ichabod801]

I'm not seeing anything reasonable starting with that sequence (1 or 0 indexed) in the Online Encyclopedia of Integer Sequences.