Decrypt a twice-encrypted message using known pairs

Three people pass notes A → B → C → A. Notes are encrypted using a character-substitution cipher (a one-to-one mapping per character).

You are given two known plaintext/ciphertext pairs that allow you to reconstruct the substitution keys:

• From A → B: clearMsgAb and cipheredMsgAb
• From B → C: clearMsgBc and cipheredMsgBc

A third message (intended to be decrypted by A) has been encrypted twice in this order:

1. encrypt with A→B key
2. then encrypt with B→C key Resulting in: cipheredMsgCba

To decrypt cipheredMsgCba, you must:

1. decrypt using the B→C key to obtain an intermediate string
2. decrypt the intermediate string using the A→B key to obtain the final plaintext

Implement a function that returns the final decrypted plaintext.

Inputs:

• string clearMsgAb
• string cipheredMsgAb
• string clearMsgBc
• string cipheredMsgBc
• string cipheredMsgCba

Assumptions (for a well-formed input):

• clearMsgAb.length == cipheredMsgAb.length and clearMsgBc.length == cipheredMsgBc.length .
• The substitution is consistent within each key (if x maps to y once, it always maps to y ).
• Only characters appearing in the known pairs will appear in cipheredMsgCba (so decryption is always possible).