Catch the Change

Old Tag Fails

The receiver recomputes the toy check and rejects the changed message with the old tag.

highlighted = computed this step

Old tag fails on changed bytes

When the receiver sums key, changed message, and old tag, the fold is 0x0800, not all ones.

bad fold=0x0800\text{bad fold}=0x0800
Changed message with old tagThe old tag no longer makes the changed message fold to all ones.Changed plus old tag - computerowindexvalueword00x0f0eword10x7061word20x793dword30x3930word40xd622raw sum0x000207fefolded0x0800result0xf7ff

The check rejects

The valid fold would be 0xffff. This one differs, so the toy receiver rejects.

accepted=0\text{accepted}=0
Changed message with old tagThe old tag no longer makes the changed message fold to all ones.Changed plus old tag - computerowindexvalueword00x0f0eword10x7061word20x793dword30x3930word40xd622raw sum0x000207fefolded0x0800result0xf7ff

The old tag was for different bytes

The old tag was made for the original message, not the changed message.

same message=0\text{same message}=0
Changed message with old tagThe old tag no longer makes the changed message fold to all ones.Changed plus old tag - computerowindexvalueword00x0f0eword10x7061word20x793dword30x3930word40xd622raw sum0x000207fefolded0x0800result0xf7ff

Summary

A tag check catches this changed message before accepting it.

reject=1\text{reject}=1
Changed message with old tagThe old tag no longer makes the changed message fold to all ones.Changed plus old tag - computerowindexvalueword00x0f0eword10x7061word20x793dword30x3930word40xd622raw sum0x000207fefolded0x0800result0xf7ff