Electrical computers and digital processing systems: support – Multiple computer communication using cryptography – Particular communication authentication technique
Reexamination Certificate
1998-06-23
2001-03-27
Hayes, Gail (Department: 2131)
Electrical computers and digital processing systems: support
Multiple computer communication using cryptography
Particular communication authentication technique
Reexamination Certificate
active
06209093
ABSTRACT:
BACKGROUND OF THE DISCLOSURE
1. Field of the Invention
The invention relates to cryptography, particularly a technique for: generating, for a given message to be signed, an authentic cryptographic signature that can be privately authenticated, by a recipient of the signed message, as having originated from a signor of the message; and appropriately authenticating such a signature.
2. Description of the Prior Art
Over the centuries, for as long as information has been communicated between two individuals, information has been susceptible to third-party interception, eavesdropping, compromise and/or corruption. Clearly, the problem of securely protecting information from such acts has existed for quite a long time.
Traditionally, this problem has been handled through the development, over the years, of increasingly sophisticated cryptographic techniques. One class of these techniques involves the use of key-based ciphers. In particular, through a key-based cipher, sequences of intelligible data, i.e., plaintext, that collectively form a message are each mathematically transformed, through an enciphering algorithm, into seemingly unintelligible data, i.e., so-called ciphertext. Not only must the transformation be completely reversible, i.e., two way in the sense that the ciphertext must be invertible back to its corresponding original plaintext but also on a 1:1 basis, i.e., each element of plaintext can only be transformed into one and only one element of ciphertext. In addition, a particular cipher that generated any given ciphertext must be sufficiently secure from cryptanalysis. To provide a requisite level of security, a unique key is selected which defines only one unique corresponding cipher, i.e., precluding, to the extent possible, a situation where multiple differing keys each yields reversible transformations between the same plaintext-ciphertext correspondence. The strength of any cryptographic technique and hence the degree of protection it affords from third-party intrusion is directly proportional to the time required, by a third-party, to perform cryptanalysis, e.g., with a key-based cipher to successfully convert the ciphertext into its corresponding plaintext without prior knowledge of the key. While no encryption technique is completely impervious from cryptanalysis, an immense number of calculations and an extremely long time interval required therefor—given the computing technology then available—required to break a cipher without prior knowledge of its key effectively renders many techniques, for all practical intents and purposes, sufficiently secure to warrant their widespread adoption and use. In that regard, as recently as a few years ago, if a cipher was of such complexity that it required on the order of man-years or more to break, in view of the state of the processing technology then available to do so, the underlying cryptographic technique was viewed by many as rendering a sufficient decree of security to warrant its use.
Public-key algorithms are one form of a key-based cipher. In such an algorithm, each communicating party generates a public-private key pair. Each party posts his(her) public key to a publicly accessible bulletin board, server or other facility, but maintains the corresponding private key in secret. In essence, an originating party desiring to encrypt a plaintext message and transmit it to another party, i.e., a destination party, both using the same public-key algorithm, will first access the public key of the destination party, encrypt the plaintext message using that public key into a ciphertext message and transmit the ciphertext message to the destination party. After receipt of the ciphertext message, the destination party, using his(her) private key, will then decrypt the message to recover the original plaintext. The keys are precisely computed, through use of very specific algorithms, to provide a requisite level of security while guaranteeing complete reversibility.
While public-key cryptographic systems can provide extremely secure encryption, to the point where breaking a public-key cipher is simply infeasible given the sheer number of operations potentially required to do so, such systems do have drawbacks that can limit their use. A principal drawback with a public-key system is its dependence on individual keys and a modulus that each carries a rather long bit sequence. For example, a modulus can easily be 1024 bits in length, while an individual key can be formed of a sequence of hundreds of bits. In some applications, such as cryptographic application programs where such keys can be readily stored, indexed and accessed as needed, the key length presents few, if any, practical problems for a user. For other applications, such long key sequences, even if converted to alphanumeric data, can still yield exceedingly long character strings that preclude easy manual entry by a user. In fact, the source of the extreme security of a public-key system lies in its use of very long bit sequences both for the keys and the modulus. If the modulus were to be appreciably shortened, then an encrypted message could be easily broken by cryptanalysis and hence the security of the underlying system readily compromised.
Computing technology continues to rapidly evolve. Processors, once unheard of just a few years ago in terms of their high levels of sophistication and speed, are becoming commercially available at ever decreasing prices. Consequently, processing systems, such as personal computers and workstations, that were previously viewed as not possessing sufficient processing power to break many so-called “secure” cryptographic ciphers are now, given their current power and sophistication, providing third parties with the necessary capability to effectively break those same ciphers. What may have taken years of continual computing a decade ago can now be accomplished in a very small fraction of that time. Hence, as technology evolves, the art of cryptography advances in lockstep in a continual effort to develop increasingly sophisticated cryptographic techniques that withstand correspondingly intensifying cryptanalysis.
Totally apart from cryptography, during at least the past decade, computer software manufacturers have been and continue to be subject to considerable unauthorized use of their products by unlicensed third parties. This is due, in part, to the relative ease with which a distribution media, such as diskettes or a CD-ROM, containing a software program can be duplicated. In an effort to thwart such unauthorized use, a relatively long alphanumeric indicia is often distributed with each legitimate copy of a packaged software product and must be entered by a user, when prompted during user installation of that copy on a computer. In particular, the copy includes an installation program which is first loaded and executed by the user to initiate and properly sequence through the entire installation process. Typically, at an early point in the installation process, the program will prompt the user of the copy being installed to manually enter the indicia. The indicia may contain, e.g., ten or more digits. In the case of distribution via compact discs (CDs), the indicia is printed on a label affixed to each case containing a CD. With diskette based distribution, the indicia is often printed on a certificate or other insert included within each software package. In any event, once the user has fully entered the indicia and has so signaled the program, typically by clicking an “OK” button (or the like) displayed on a monitor, the installation program will attempt to validate that indicia in an effort to determine whether the specific copy being installed is a licensed version or not. If the indicia is validated, the installation process proceeds; otherwise, it prematurely terminates. The underlying premise is that each user (i.e., licensee), who has legally obtained a valid copy, will possess the entire packaging as provided by the manufacturer and hence will have a valid indicia; but, an unauthorized user who simply obtains
Montgomery Peter L.
Venkatesan Ramarathnam R.
DiLorenzo Anthony
Hayes Gail
Michaelson Peter L.
Michaelson & Wallace
Microsoft Corporation
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