This preserves the size in bits of the key negotiated for the algorithm used by a now-expired control tunnel, per its description, and the description specifically notes DES uses 56 bits and 3DES uses 168 bits, while AES's key size varies with what was actually negotiated. It's the historical strength record paired with HistEncryptAlgo, letting an admin confirm not just which algorithm but exactly how strong a variant a since-closed tunnel used. A history entry showing AES with only 128 bits when a partner's contract specified AES-256 would be concrete evidence of a compliance gap during that connection's lifetime.