Computer viruses, hackers, intrusions and ther computer crimes have recently become a serious security problem in information systems. Digital signatures are useful to defend against these threats, especially against computer viruses. This is because a modification of a file can be detected by checking the consistency of the originai file with its accompanying digital signature. But an executable program might have been infected with the viruses before the signature was created. In this case, the infection cannot be detected by signature verification and the origin of the infection cannot be specified either. In this paper, we propose a signature scheme in which one can sign right after the creation of an executable program. That is, when a user compiles a source program, the compiler automatically creates both the executable program and its signature. Thus viruses cannot infect the executable programs without detection. Moreover, we can specify the creator of contaminated executable programs. In our signature scheme, a signature is created from a set of secret integers stored in a compiler, which is calculated from a compiler-maker's secret key. Each compiler is possessed by only one user and it is used only when a secret value is fed into it. In this way a signature of an executable program and the compiler-owner are linked to each other. Despite these measures, an executable program could run abnormally because of an infection in prepro-cessing step, e.g. an infection of library files or included files. An infection of these files is detected by ordinary digital signatures. The proposed signature scheme together with digital signature against infection in the preprocessing step enables us to specify the origin of the infection. The name of the signature creator is not necessary for detecting an infection. So, an owner's public value is not searched in our scheme, and only a public value of a compiler-maker is required for signature verification. Furthermore, no one can use a compiler owned by another to create a proper signature.

In this paper a new type of digital proxy signature is proposed. The proxy signature allows a designated person, called a proxy signer, to sign on behalf of an original signer. Classification of the proxy signatures is shown from the point of view of the degree of delegation, and the necessary conditions of a proxy signature are clarified. The proposed proxy signature scheme is based on either the discrete logarithm problem or the problem of taking the square root modulo of a composite number. Compared to the consecutive execution of the ordinary digital signature schemes, it has a direct from, and a verifier does not need a public key of a user other than the original signer in the verification stage. Moreover, it requires less computational work than the consecutive execution of the signature schemes. Due to this efficiency together with the delegation property, an organization, e.g. a software company, can very efficiently create many signatures of its own by delegating its signing power to multiple employees. Another attractive feature is that the proxy signature based on the discrete logarithm problem is highly applicable to other ordinary signature schemes based on the same problem, For instance, designated confirmer proxy signatures can be constructed. As a stronger form of proxy signature for partial delegation, another type of proxy signature scheme is proposed in which even an original signer cannot create a proxy signature. Furthermore, using a proposed on-line proxy updating protocol, the orignal signer can revoke proxies of dishonest proxy signers.