The common assumption about digital signatures is that they disallow any kind of modification on signed data. However, a more flexible approach is often needed and has been advocated lately, one in which some restricted modifications may still occur, without invalidating the data. This is made possible by offering signatures which are homomorphic with respect to some operation on the message domain. Starting from the signature(s) of some data instance( s), computed by the data owner, anybody else can derive the signature corresponding to a new data instance, if obtained only via some accepted operation from the previous one(s). More, updated signatures should be indistinguishable from the ones computed by the data owner and this updating step should be applicable as many times as needed. This paper deals with the signing of insert-only collections, in which element insertions are accepted but no removals should occur. Newly inserted elements do not have to be signed or known by the initial signer. We propose two techniques: one which transposes the insert-only problem into a delete-only one (which is already solved), and another technique based on zero-knowledge proofs. We also give performance measures and discuss applications.