Anonymous key quantum cryptography and unconditionally secure quantum bit commitment

A new cryptographic tool, anonymous quantum key technique, is introduced that leads to unconditionally secure key distribution and encryption schemes that can be readily implemented experimentally in a realistic environment. If quantum memory is available, the technique would have many features of public-key cryptography; an identification protocol that does not require a shared secret key is provided as an illustration. The possibility is also indicated for obtaining unconditionally secure quantum bit commitment protocols with this technique. This paper has the same title as my Capri talk but the contents are not identical. The portion on anonymous key is greatly expanded here, while only brief mention is made on quantum bit commitment, a detailed treatment of which is available in Ref. [1]. A classic goal of cryptography is privacy: two parties wish to communicate privately so that an adversary can learn nothing about its content. This was usually achieved through the use of a shared private key, typically a string of binary digits, for encrypting and decrypting the message data. A revolution in cryptography occurred around 1976 with the emergence of public-key cryptography [2], in which knowledge of a public key for encryption would not lead to knowledge of a secret private key for decryption. The concept of digital signature, the binding of a signer to an electronic digital message, was introduced via public-key technique. The idea of using quantum physics for cryptographic purpose was first proposed by Wiesner in the early 1970’s [3]. It came to fruition in the work of Bennett and Brassard [4] on key distribution, culminating in an experimental prototype demonstration [5]. Despite earlier papers on the use of quantum cryptography to achieve other cryptographic goals, it turns out that key distribution is the only viable one so far [6]. Also, the lack of a quantum authentication scheme implies that some standard classical technique has to be employed