A Variant Invisible Signatures

Invisible signatures (usually, but not characteristically called undeniable ) are a kind of signatures providing more privacy than ordinary ones. 

This is why invisible signatures were introduced in [ChAn90]. (Conversely, if one says that the purpose of digital signature schemes is to imitate handwritten signatures as closely as possible, an invisible signature scheme does too little.) 

A related requirement of invisibility has explicitly been considered with mere authentication, but in a scenario with public keys, in [OkOh91]. 

This last feature gave rise to the original name undeniable . However, it is a feature common to all signature schemes and therefore not characteristic. Moreover, the old terminology is very confusing with the noun undeniability It meant both that signatures cannot be denied and the characteristic feature that they cannot be shown round, which is called invisibility here. 

The schemes described in this section are the historical framework for the schemes and the classification proposed in the remainder of this work. Moreover, some of their components are reused in the new constructions. Therefore, this section does not only describe specifications, although those would be sufficient to delimit the new ideas, but contains sketches of some constructions, too. 

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