Add the key-refresh layer

Derives each block key from the transformations that leave the decision
statistic invariant: a global sign per frame entry, a relabeling of the
frame entries applied to keys and codebook together, and a permutation
of which user holds which row. The legitimate error rate is unchanged
at 0.257 across 24 blocks while a refresh that installs fresh
orthogonal keys reaches 0.710, and a key recovered by known plaintext
returns to the random-guess level one block later.

data/refresh.csv

@@ -0,0 +1,25 @@
+block,legit_invariant,legit_naive,eve_ser
+0,0.2575,0.982775,0.9998575
+1,0.256835,0.7427116667,0.9998133333
+2,0.2566516667,0.8483041667,0.9999883333
+3,0.2572983333,0.778605,0.9996666667
+4,0.2569683333,0.6199566667,0.9979908333
+5,0.2571308333,0.6580941667,0.9997825
+6,0.2572883333,0.7788558333,0.9997916667
+7,0.2577975,0.6935075,0.9999575
+8,0.2573458333,0.8400325,0.99839
+9,0.25766,0.6547933333,0.9999866667
+10,0.2565575,0.4783583333,0.9995791667
+11,0.2578183333,0.65481,0.99993
+12,0.2573583333,0.8477883333,0.9997283333
+13,0.2571408333,0.6548475,0.9999741667
+14,0.2571491667,0.6579591667,0.9999975
+15,0.2574591667,0.4778091667,0.9994458333
+16,0.2568716667,0.76187,0.9999083333
+17,0.2572141667,0.8045925,0.9994866667
+18,0.2573291667,0.8065691667,0.9997483333
+19,0.2571316667,0.6892025,0.9992841667
+20,0.2576716667,0.5278108333,0.9980375
+21,0.25724,0.742345,0.9999641667
+22,0.2570333333,0.6545633333,0.9997633333
+23,0.25745,0.68938,0.99989

data/refresh_kpa.csv

@@ -0,0 +1,7 @@
+n_frames,ser_same_block,ser_next_block
+2,0.2777190625,0.998930625
+4,0.2615640625,0.998644375
+8,0.2594196875,0.998631875
+16,0.2582471875,0.99867
+32,0.2579625,0.99871875
+64,0.257455,0.9987190625