PIN translation
Decrypting a PIN block under one key and re-encrypting it under another inside an HSM, so the PIN never appears outside the box.
A PIN typed at a shop till passes through several parties before it reaches the card issuer: the terminal, the acquirer, the card network. Each boundary has its own keys, and at each one the PIN block goes into an HSM that unwraps it under the incoming key and rewraps it under the outgoing one. The format can change along the way too, say from ISO-0 to ISO-4. In Thales payShield land the operations are known simply as CA and CC.
The function is unavoidable and dangerous. Jolyon Clulow showed in 2003 that switching between weaker and stronger formats could serve as an oracle for working out PIN digits, without a single key leaking. Modern HSMs therefore let administrators block format changes that go downhill in strength.