In this challenge, we get a gzipped file called perf.data and a minimal description of an environment. Googling this reveals that perf.data is a record format of the perf tool, a Linux profiler. Digging further reveals that the record contains Intel Processor Trace information of the command `convert -font Courier text:- image.png`, which renders input from stdin (presumably the flag) and renders it as a PNG image. Intel Processor Trace information contains all the branching encountered in the command. If we install the exact libraries matching the `perf.data` record, run the same `convert` command against a known plaintext, and compare the branching patterns that `perf` sees during rendering each glyph in `libfreetype` of this known run and the challenge's file, we can reconstruct the flag.

[Read more here](https://blog.vero.site/post/refrain)

Original writeup (https://blog.vero.site/post/refrain).