D. J. Bernstein
Fighting patents

US patent 6141420, Vanstone Mullin Agnew, elliptic-curve cryptography

Text of the patent
This patent has a priority date of 29 July 1994 (see the "parent case" section), so it expires 29 July 2014.

This patent is known as a patent on elliptic-curve point compression. However, this patent has never been fought in court and will clearly not survive in court, for two reasons:

The contents of the patent are as follows.

Claims 1 through 28 are for encryption using an elliptic curve over a finite field of characteristic 2 with elements represented on a normal basis.

Claim 29 is for point compression: "A method of transferring the coordinates of a point on an algebraic curve defined by a function of two variables between a pair of correspondents connected by a data comunications link comprising the steps of forwarding from one correspondent to another a coordinate of said point, providing at said other correspondent parameters of said algebraic curve, and computing at said other correspondent said other coordinate from said one coordinate and said algebraic curve." This claim includes many systems published more than a year before the patent was filed, and is therefore invalid. For example, in the CRYPTO '85 article introducing elliptic-curve cryptography, Victor Miller suggested transmitting only the x coordinate for elliptic-curve Diffie-Hellman:

Finally, it should be remarked, that even though we have phrased everything in terms of points on an elliptic curve, that, for the key exchange protocol (and other uses as one-way functions), that only the x-coordinate needs to be transmitted. The formulas for multiples of a point cited in the first section make it clear that the x-coordinate of a multiple depends only on the x-coordinate of the original point.
This is the compression mechanism used for ECDH in Curve25519.

Claims 30 through 41 are on more specific techniques involving "forwarding with said one coordinate identifying information of said other coordinate." (Exception: claim 36 is claim 29 for elliptic curves over finite fields of characteristic 2.) This is a waste of time in the context of elliptic-curve Diffie-Hellman, but is useful for elliptic-curve signatures. Bodo Moeller points to page 171 of the Harper-Menezes-Vanstone paper "Public-key cryptosystems with very small key lengths" at Eurocrypt '92, published more than a year before the filing of this patent:

The key length can be shortened to n+1 bits as follows. Observe first that the change of variables (x,y) -> (x,xz) transforms equation (1) to z^2 + z = x + a + bx^(-2). (3) Given the x-coordinate of a point P = (xbar,ybar), we can compute the right hand side of (3). Then (3) has precisely 2 solutions, namely z' and z'+1, and these solutions can be easily found. We can then select the correct solution zbar (and hence reconstruct ybar as ybar = xbar zbar) if we know the least significant bit of zbar. Thus to transmit P it is sufficient to transmit xbar and the least significant bit of ybar/xbar.

Claims 42 through 52 are on some secret-key encryption mechanisms.