Comment 29 for bug 392104

A little debugging finds that:
array length: 67
field size: 256
n = 32
array: [4, 65, 4, -26, -5, 56, -82, -53, -122, 32, 102, -86, -64, -59, 84, 5, 110, 1, -49, 38, -7, 3, -97, 122, -36, -18, 99, -126, -83, 83, 34, 12, -38, -84, 43, 83, -38, -25, -58, 9, -30, -37, 108, -43, 35, -118, -15, 53, 104, -26, -45, -51, 3, -83, 100, -119, -108, 25, 75, -37, 39, 9, 50, -121, 105, 68, 96]

The check that throws the exception is the failure of array length to equal (n*2)+1 (65) in this case. The array returned by NSS has two extra bytes. n is calculated from the field size of 256 by adding 7 and shifting right 3. If the field size was 257, it would thus match the array size but this is not a legal EC key size.