Three Notes, Three Layers

The story

During a training exercise, the blue team intercepted three notes passed between two "agents". Each note unlocks the next one, and the last one hides the flag. None of this is strong cryptography: it is a ladder of encodings and toy ciphers, and the point is to learn to recognise each of them.

Your task

Read stage1.txt, stage2.txt and stage3.txt in order. Every note, once decoded, tells you how to handle the next one. Stage 1 is a classic Caesar cipher on the letters of an English sentence (digits and punctuation are untouched). You are not told the shift.

Submit the flag from the last note. It looks like IMB{...} and is the whole text after the FLAG= marker.

Toolbox, with one tiny example each

• Caesar: shift every letter by the same amount along the alphabet. Shifting Hello by 3 gives Khoor, and shifting back by 3 undoes it. There are only 25 useful shifts, so you can try them all.
• Hex: two hexadecimal digits per byte. 48 65 6c 6c 6f is the text Hello.
• Base64: a text-safe encoding of bytes. SGVsbG8= is the text Hello.
• XOR with a repeating key: byte i of the text is XORed with byte i mod k of a key of length k. For example A (0x41) XOR b (0x62) is 0x23. XOR is its own inverse: ciphertext XOR key gives the plaintext back, and ciphertext XOR plaintext gives the key.

Everything can be done with a few lines of Python or JavaScript in Code Lab.