Repository navigation
Expand file tree
/
Copy pathch11_blockchain.html
More file actions
1414 lines (1411 loc) · 70.9 KB
/
Copy pathch11_blockchain.html
File metadata and controls
1414 lines (1411 loc) · 70.9 KB
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta name="generator" content="Asciidoctor 2.0.20">
<title>The Blockchain</title>
<link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Open+Sans:300,300italic,400,400italic,600,600italic%7CNoto+Serif:400,400italic,700,700italic%7CDroid+Sans+Mono:400,700">
<style>
/*! Asciidoctor default stylesheet | MIT License | https://asciidoctor.org */
/* Uncomment the following line when using as a custom stylesheet */
/* @import "https://fonts.googleapis.com/css?family=Open+Sans:300,300italic,400,400italic,600,600italic%7CNoto+Serif:400,400italic,700,700italic%7CDroid+Sans+Mono:400,700"; */
html{font-family:sans-serif;-webkit-text-size-adjust:100%}
a{background:none}
a:focus{outline:thin dotted}
a:active,a:hover{outline:0}
h1{font-size:2em;margin:.67em 0}
b,strong{font-weight:bold}
abbr{font-size:.9em}
abbr[title]{cursor:help;border-bottom:1px dotted #dddddf;text-decoration:none}
dfn{font-style:italic}
hr{height:0}
mark{background:#ff0;color:#000}
code,kbd,pre,samp{font-family:monospace;font-size:1em}
pre{white-space:pre-wrap}
q{quotes:"\201C" "\201D" "\2018" "\2019"}
small{font-size:80%}
sub,sup{font-size:75%;line-height:0;position:relative;vertical-align:baseline}
sup{top:-.5em}
sub{bottom:-.25em}
img{border:0}
svg:not(:root){overflow:hidden}
figure{margin:0}
audio,video{display:inline-block}
audio:not([controls]){display:none;height:0}
fieldset{border:1px solid silver;margin:0 2px;padding:.35em .625em .75em}
legend{border:0;padding:0}
button,input,select,textarea{font-family:inherit;font-size:100%;margin:0}
button,input{line-height:normal}
button,select{text-transform:none}
button,html input[type=button],input[type=reset],input[type=submit]{-webkit-appearance:button;cursor:pointer}
button[disabled],html input[disabled]{cursor:default}
input[type=checkbox],input[type=radio]{padding:0}
button::-moz-focus-inner,input::-moz-focus-inner{border:0;padding:0}
textarea{overflow:auto;vertical-align:top}
table{border-collapse:collapse;border-spacing:0}
*,::before,::after{box-sizing:border-box}
html,body{font-size:100%}
body{background:#fff;color:rgba(0,0,0,.8);padding:0;margin:0;font-family:"Noto Serif","DejaVu Serif",serif;line-height:1;position:relative;cursor:auto;-moz-tab-size:4;-o-tab-size:4;tab-size:4;word-wrap:anywhere;-moz-osx-font-smoothing:grayscale;-webkit-font-smoothing:antialiased}
a:hover{cursor:pointer}
img,object,embed{max-width:100%;height:auto}
object,embed{height:100%}
img{-ms-interpolation-mode:bicubic}
.left{float:left!important}
.right{float:right!important}
.text-left{text-align:left!important}
.text-right{text-align:right!important}
.text-center{text-align:center!important}
.text-justify{text-align:justify!important}
.hide{display:none}
img,object,svg{display:inline-block;vertical-align:middle}
textarea{height:auto;min-height:50px}
select{width:100%}
.subheader,.admonitionblock td.content>.title,.audioblock>.title,.exampleblock>.title,.imageblock>.title,.listingblock>.title,.literalblock>.title,.stemblock>.title,.openblock>.title,.paragraph>.title,.quoteblock>.title,table.tableblock>.title,.verseblock>.title,.videoblock>.title,.dlist>.title,.olist>.title,.ulist>.title,.qlist>.title,.hdlist>.title{line-height:1.45;color:#7a2518;font-weight:400;margin-top:0;margin-bottom:.25em}
div,dl,dt,dd,ul,ol,li,h1,h2,h3,#toctitle,.sidebarblock>.content>.title,h4,h5,h6,pre,form,p,blockquote,th,td{margin:0;padding:0}
a{color:#2156a5;text-decoration:underline;line-height:inherit}
a:hover,a:focus{color:#1d4b8f}
a img{border:0}
p{line-height:1.6;margin-bottom:1.25em;text-rendering:optimizeLegibility}
p aside{font-size:.875em;line-height:1.35;font-style:italic}
h1,h2,h3,#toctitle,.sidebarblock>.content>.title,h4,h5,h6{font-family:"Open Sans","DejaVu Sans",sans-serif;font-weight:300;font-style:normal;color:#ba3925;text-rendering:optimizeLegibility;margin-top:1em;margin-bottom:.5em;line-height:1.0125em}
h1 small,h2 small,h3 small,#toctitle small,.sidebarblock>.content>.title small,h4 small,h5 small,h6 small{font-size:60%;color:#e99b8f;line-height:0}
h1{font-size:2.125em}
h2{font-size:1.6875em}
h3,#toctitle,.sidebarblock>.content>.title{font-size:1.375em}
h4,h5{font-size:1.125em}
h6{font-size:1em}
hr{border:solid #dddddf;border-width:1px 0 0;clear:both;margin:1.25em 0 1.1875em}
em,i{font-style:italic;line-height:inherit}
strong,b{font-weight:bold;line-height:inherit}
small{font-size:60%;line-height:inherit}
code{font-family:"Droid Sans Mono","DejaVu Sans Mono",monospace;font-weight:400;color:rgba(0,0,0,.9)}
ul,ol,dl{line-height:1.6;margin-bottom:1.25em;list-style-position:outside;font-family:inherit}
ul,ol{margin-left:1.5em}
ul li ul,ul li ol{margin-left:1.25em;margin-bottom:0}
ul.circle{list-style-type:circle}
ul.disc{list-style-type:disc}
ul.square{list-style-type:square}
ul.circle ul:not([class]),ul.disc ul:not([class]),ul.square ul:not([class]){list-style:inherit}
ol li ul,ol li ol{margin-left:1.25em;margin-bottom:0}
dl dt{margin-bottom:.3125em;font-weight:bold}
dl dd{margin-bottom:1.25em}
blockquote{margin:0 0 1.25em;padding:.5625em 1.25em 0 1.1875em;border-left:1px solid #ddd}
blockquote,blockquote p{line-height:1.6;color:rgba(0,0,0,.85)}
@media screen and (min-width:768px){h1,h2,h3,#toctitle,.sidebarblock>.content>.title,h4,h5,h6{line-height:1.2}
h1{font-size:2.75em}
h2{font-size:2.3125em}
h3,#toctitle,.sidebarblock>.content>.title{font-size:1.6875em}
h4{font-size:1.4375em}}
table{background:#fff;margin-bottom:1.25em;border:1px solid #dedede;word-wrap:normal}
table thead,table tfoot{background:#f7f8f7}
table thead tr th,table thead tr td,table tfoot tr th,table tfoot tr td{padding:.5em .625em .625em;font-size:inherit;color:rgba(0,0,0,.8);text-align:left}
table tr th,table tr td{padding:.5625em .625em;font-size:inherit;color:rgba(0,0,0,.8)}
table tr.even,table tr.alt{background:#f8f8f7}
table thead tr th,table tfoot tr th,table tbody tr td,table tr td,table tfoot tr td{line-height:1.6}
h1,h2,h3,#toctitle,.sidebarblock>.content>.title,h4,h5,h6{line-height:1.2;word-spacing:-.05em}
h1 strong,h2 strong,h3 strong,#toctitle strong,.sidebarblock>.content>.title strong,h4 strong,h5 strong,h6 strong{font-weight:400}
.center{margin-left:auto;margin-right:auto}
.stretch{width:100%}
.clearfix::before,.clearfix::after,.float-group::before,.float-group::after{content:" ";display:table}
.clearfix::after,.float-group::after{clear:both}
:not(pre).nobreak{word-wrap:normal}
:not(pre).nowrap{white-space:nowrap}
:not(pre).pre-wrap{white-space:pre-wrap}
:not(pre):not([class^=L])>code{font-size:.9375em;font-style:normal!important;letter-spacing:0;padding:.1em .5ex;word-spacing:-.15em;background:#f7f7f8;border-radius:4px;line-height:1.45;text-rendering:optimizeSpeed}
pre{color:rgba(0,0,0,.9);font-family:"Droid Sans Mono","DejaVu Sans Mono",monospace;line-height:1.45;text-rendering:optimizeSpeed}
pre code,pre pre{color:inherit;font-size:inherit;line-height:inherit}
pre>code{display:block}
pre.nowrap,pre.nowrap pre{white-space:pre;word-wrap:normal}
em em{font-style:normal}
strong strong{font-weight:400}
.keyseq{color:rgba(51,51,51,.8)}
kbd{font-family:"Droid Sans Mono","DejaVu Sans Mono",monospace;display:inline-block;color:rgba(0,0,0,.8);font-size:.65em;line-height:1.45;background:#f7f7f7;border:1px solid #ccc;border-radius:3px;box-shadow:0 1px 0 rgba(0,0,0,.2),inset 0 0 0 .1em #fff;margin:0 .15em;padding:.2em .5em;vertical-align:middle;position:relative;top:-.1em;white-space:nowrap}
.keyseq kbd:first-child{margin-left:0}
.keyseq kbd:last-child{margin-right:0}
.menuseq,.menuref{color:#000}
.menuseq b:not(.caret),.menuref{font-weight:inherit}
.menuseq{word-spacing:-.02em}
.menuseq b.caret{font-size:1.25em;line-height:.8}
.menuseq i.caret{font-weight:bold;text-align:center;width:.45em}
b.button::before,b.button::after{position:relative;top:-1px;font-weight:400}
b.button::before{content:"[";padding:0 3px 0 2px}
b.button::after{content:"]";padding:0 2px 0 3px}
p a>code:hover{color:rgba(0,0,0,.9)}
#header,#content,#footnotes,#footer{width:100%;margin:0 auto;max-width:62.5em;*zoom:1;position:relative;padding-left:.9375em;padding-right:.9375em}
#header::before,#header::after,#content::before,#content::after,#footnotes::before,#footnotes::after,#footer::before,#footer::after{content:" ";display:table}
#header::after,#content::after,#footnotes::after,#footer::after{clear:both}
#content{margin-top:1.25em}
#content::before{content:none}
#header>h1:first-child{color:rgba(0,0,0,.85);margin-top:2.25rem;margin-bottom:0}
#header>h1:first-child+#toc{margin-top:8px;border-top:1px solid #dddddf}
#header>h1:only-child,body.toc2 #header>h1:nth-last-child(2){border-bottom:1px solid #dddddf;padding-bottom:8px}
#header .details{border-bottom:1px solid #dddddf;line-height:1.45;padding-top:.25em;padding-bottom:.25em;padding-left:.25em;color:rgba(0,0,0,.6);display:flex;flex-flow:row wrap}
#header .details span:first-child{margin-left:-.125em}
#header .details span.email a{color:rgba(0,0,0,.85)}
#header .details br{display:none}
#header .details br+span::before{content:"\00a0\2013\00a0"}
#header .details br+span.author::before{content:"\00a0\22c5\00a0";color:rgba(0,0,0,.85)}
#header .details br+span#revremark::before{content:"\00a0|\00a0"}
#header #revnumber{text-transform:capitalize}
#header #revnumber::after{content:"\00a0"}
#content>h1:first-child:not([class]){color:rgba(0,0,0,.85);border-bottom:1px solid #dddddf;padding-bottom:8px;margin-top:0;padding-top:1rem;margin-bottom:1.25rem}
#toc{border-bottom:1px solid #e7e7e9;padding-bottom:.5em}
#toc>ul{margin-left:.125em}
#toc ul.sectlevel0>li>a{font-style:italic}
#toc ul.sectlevel0 ul.sectlevel1{margin:.5em 0}
#toc ul{font-family:"Open Sans","DejaVu Sans",sans-serif;list-style-type:none}
#toc li{line-height:1.3334;margin-top:.3334em}
#toc a{text-decoration:none}
#toc a:active{text-decoration:underline}
#toctitle{color:#7a2518;font-size:1.2em}
@media screen and (min-width:768px){#toctitle{font-size:1.375em}
body.toc2{padding-left:15em;padding-right:0}
#toc.toc2{margin-top:0!important;background:#f8f8f7;position:fixed;width:15em;left:0;top:0;border-right:1px solid #e7e7e9;border-top-width:0!important;border-bottom-width:0!important;z-index:1000;padding:1.25em 1em;height:100%;overflow:auto}
#toc.toc2 #toctitle{margin-top:0;margin-bottom:.8rem;font-size:1.2em}
#toc.toc2>ul{font-size:.9em;margin-bottom:0}
#toc.toc2 ul ul{margin-left:0;padding-left:1em}
#toc.toc2 ul.sectlevel0 ul.sectlevel1{padding-left:0;margin-top:.5em;margin-bottom:.5em}
body.toc2.toc-right{padding-left:0;padding-right:15em}
body.toc2.toc-right #toc.toc2{border-right-width:0;border-left:1px solid #e7e7e9;left:auto;right:0}}
@media screen and (min-width:1280px){body.toc2{padding-left:20em;padding-right:0}
#toc.toc2{width:20em}
#toc.toc2 #toctitle{font-size:1.375em}
#toc.toc2>ul{font-size:.95em}
#toc.toc2 ul ul{padding-left:1.25em}
body.toc2.toc-right{padding-left:0;padding-right:20em}}
#content #toc{border:1px solid #e0e0dc;margin-bottom:1.25em;padding:1.25em;background:#f8f8f7;border-radius:4px}
#content #toc>:first-child{margin-top:0}
#content #toc>:last-child{margin-bottom:0}
#footer{max-width:none;background:rgba(0,0,0,.8);padding:1.25em}
#footer-text{color:hsla(0,0%,100%,.8);line-height:1.44}
#content{margin-bottom:.625em}
.sect1{padding-bottom:.625em}
@media screen and (min-width:768px){#content{margin-bottom:1.25em}
.sect1{padding-bottom:1.25em}}
.sect1:last-child{padding-bottom:0}
.sect1+.sect1{border-top:1px solid #e7e7e9}
#content h1>a.anchor,h2>a.anchor,h3>a.anchor,#toctitle>a.anchor,.sidebarblock>.content>.title>a.anchor,h4>a.anchor,h5>a.anchor,h6>a.anchor{position:absolute;z-index:1001;width:1.5ex;margin-left:-1.5ex;display:block;text-decoration:none!important;visibility:hidden;text-align:center;font-weight:400}
#content h1>a.anchor::before,h2>a.anchor::before,h3>a.anchor::before,#toctitle>a.anchor::before,.sidebarblock>.content>.title>a.anchor::before,h4>a.anchor::before,h5>a.anchor::before,h6>a.anchor::before{content:"\00A7";font-size:.85em;display:block;padding-top:.1em}
#content h1:hover>a.anchor,#content h1>a.anchor:hover,h2:hover>a.anchor,h2>a.anchor:hover,h3:hover>a.anchor,#toctitle:hover>a.anchor,.sidebarblock>.content>.title:hover>a.anchor,h3>a.anchor:hover,#toctitle>a.anchor:hover,.sidebarblock>.content>.title>a.anchor:hover,h4:hover>a.anchor,h4>a.anchor:hover,h5:hover>a.anchor,h5>a.anchor:hover,h6:hover>a.anchor,h6>a.anchor:hover{visibility:visible}
#content h1>a.link,h2>a.link,h3>a.link,#toctitle>a.link,.sidebarblock>.content>.title>a.link,h4>a.link,h5>a.link,h6>a.link{color:#ba3925;text-decoration:none}
#content h1>a.link:hover,h2>a.link:hover,h3>a.link:hover,#toctitle>a.link:hover,.sidebarblock>.content>.title>a.link:hover,h4>a.link:hover,h5>a.link:hover,h6>a.link:hover{color:#a53221}
details,.audioblock,.imageblock,.literalblock,.listingblock,.stemblock,.videoblock{margin-bottom:1.25em}
details{margin-left:1.25rem}
details>summary{cursor:pointer;display:block;position:relative;line-height:1.6;margin-bottom:.625rem;outline:none;-webkit-tap-highlight-color:transparent}
details>summary::-webkit-details-marker{display:none}
details>summary::before{content:"";border:solid transparent;border-left:solid;border-width:.3em 0 .3em .5em;position:absolute;top:.5em;left:-1.25rem;transform:translateX(15%)}
details[open]>summary::before{border:solid transparent;border-top:solid;border-width:.5em .3em 0;transform:translateY(15%)}
details>summary::after{content:"";width:1.25rem;height:1em;position:absolute;top:.3em;left:-1.25rem}
.admonitionblock td.content>.title,.audioblock>.title,.exampleblock>.title,.imageblock>.title,.listingblock>.title,.literalblock>.title,.stemblock>.title,.openblock>.title,.paragraph>.title,.quoteblock>.title,table.tableblock>.title,.verseblock>.title,.videoblock>.title,.dlist>.title,.olist>.title,.ulist>.title,.qlist>.title,.hdlist>.title{text-rendering:optimizeLegibility;text-align:left;font-family:"Noto Serif","DejaVu Serif",serif;font-size:1rem;font-style:italic}
table.tableblock.fit-content>caption.title{white-space:nowrap;width:0}
.paragraph.lead>p,#preamble>.sectionbody>[class=paragraph]:first-of-type p{font-size:1.21875em;line-height:1.6;color:rgba(0,0,0,.85)}
.admonitionblock>table{border-collapse:separate;border:0;background:none;width:100%}
.admonitionblock>table td.icon{text-align:center;width:80px}
.admonitionblock>table td.icon img{max-width:none}
.admonitionblock>table td.icon .title{font-weight:bold;font-family:"Open Sans","DejaVu Sans",sans-serif;text-transform:uppercase}
.admonitionblock>table td.content{padding-left:1.125em;padding-right:1.25em;border-left:1px solid #dddddf;color:rgba(0,0,0,.6);word-wrap:anywhere}
.admonitionblock>table td.content>:last-child>:last-child{margin-bottom:0}
.exampleblock>.content{border:1px solid #e6e6e6;margin-bottom:1.25em;padding:1.25em;background:#fff;border-radius:4px}
.sidebarblock{border:1px solid #dbdbd6;margin-bottom:1.25em;padding:1.25em;background:#f3f3f2;border-radius:4px}
.sidebarblock>.content>.title{color:#7a2518;margin-top:0;text-align:center}
.exampleblock>.content>:first-child,.sidebarblock>.content>:first-child{margin-top:0}
.exampleblock>.content>:last-child,.exampleblock>.content>:last-child>:last-child,.exampleblock>.content .olist>ol>li:last-child>:last-child,.exampleblock>.content .ulist>ul>li:last-child>:last-child,.exampleblock>.content .qlist>ol>li:last-child>:last-child,.sidebarblock>.content>:last-child,.sidebarblock>.content>:last-child>:last-child,.sidebarblock>.content .olist>ol>li:last-child>:last-child,.sidebarblock>.content .ulist>ul>li:last-child>:last-child,.sidebarblock>.content .qlist>ol>li:last-child>:last-child{margin-bottom:0}
.literalblock pre,.listingblock>.content>pre{border-radius:4px;overflow-x:auto;padding:1em;font-size:.8125em}
@media screen and (min-width:768px){.literalblock pre,.listingblock>.content>pre{font-size:.90625em}}
@media screen and (min-width:1280px){.literalblock pre,.listingblock>.content>pre{font-size:1em}}
.literalblock pre,.listingblock>.content>pre:not(.highlight),.listingblock>.content>pre[class=highlight],.listingblock>.content>pre[class^="highlight "]{background:#f7f7f8}
.literalblock.output pre{color:#f7f7f8;background:rgba(0,0,0,.9)}
.listingblock>.content{position:relative}
.listingblock code[data-lang]::before{display:none;content:attr(data-lang);position:absolute;font-size:.75em;top:.425rem;right:.5rem;line-height:1;text-transform:uppercase;color:inherit;opacity:.5}
.listingblock:hover code[data-lang]::before{display:block}
.listingblock.terminal pre .command::before{content:attr(data-prompt);padding-right:.5em;color:inherit;opacity:.5}
.listingblock.terminal pre .command:not([data-prompt])::before{content:"$"}
.listingblock pre.highlightjs{padding:0}
.listingblock pre.highlightjs>code{padding:1em;border-radius:4px}
.listingblock pre.prettyprint{border-width:0}
.prettyprint{background:#f7f7f8}
pre.prettyprint .linenums{line-height:1.45;margin-left:2em}
pre.prettyprint li{background:none;list-style-type:inherit;padding-left:0}
pre.prettyprint li code[data-lang]::before{opacity:1}
pre.prettyprint li:not(:first-child) code[data-lang]::before{display:none}
table.linenotable{border-collapse:separate;border:0;margin-bottom:0;background:none}
table.linenotable td[class]{color:inherit;vertical-align:top;padding:0;line-height:inherit;white-space:normal}
table.linenotable td.code{padding-left:.75em}
table.linenotable td.linenos,pre.pygments .linenos{border-right:1px solid;opacity:.35;padding-right:.5em;-webkit-user-select:none;-moz-user-select:none;-ms-user-select:none;user-select:none}
pre.pygments span.linenos{display:inline-block;margin-right:.75em}
.quoteblock{margin:0 1em 1.25em 1.5em;display:table}
.quoteblock:not(.excerpt)>.title{margin-left:-1.5em;margin-bottom:.75em}
.quoteblock blockquote,.quoteblock p{color:rgba(0,0,0,.85);font-size:1.15rem;line-height:1.75;word-spacing:.1em;letter-spacing:0;font-style:italic;text-align:justify}
.quoteblock blockquote{margin:0;padding:0;border:0}
.quoteblock blockquote::before{content:"\201c";float:left;font-size:2.75em;font-weight:bold;line-height:.6em;margin-left:-.6em;color:#7a2518;text-shadow:0 1px 2px rgba(0,0,0,.1)}
.quoteblock blockquote>.paragraph:last-child p{margin-bottom:0}
.quoteblock .attribution{margin-top:.75em;margin-right:.5ex;text-align:right}
.verseblock{margin:0 1em 1.25em}
.verseblock pre{font-family:"Open Sans","DejaVu Sans",sans-serif;font-size:1.15rem;color:rgba(0,0,0,.85);font-weight:300;text-rendering:optimizeLegibility}
.verseblock pre strong{font-weight:400}
.verseblock .attribution{margin-top:1.25rem;margin-left:.5ex}
.quoteblock .attribution,.verseblock .attribution{font-size:.9375em;line-height:1.45;font-style:italic}
.quoteblock .attribution br,.verseblock .attribution br{display:none}
.quoteblock .attribution cite,.verseblock .attribution cite{display:block;letter-spacing:-.025em;color:rgba(0,0,0,.6)}
.quoteblock.abstract blockquote::before,.quoteblock.excerpt blockquote::before,.quoteblock .quoteblock blockquote::before{display:none}
.quoteblock.abstract blockquote,.quoteblock.abstract p,.quoteblock.excerpt blockquote,.quoteblock.excerpt p,.quoteblock .quoteblock blockquote,.quoteblock .quoteblock p{line-height:1.6;word-spacing:0}
.quoteblock.abstract{margin:0 1em 1.25em;display:block}
.quoteblock.abstract>.title{margin:0 0 .375em;font-size:1.15em;text-align:center}
.quoteblock.excerpt>blockquote,.quoteblock .quoteblock{padding:0 0 .25em 1em;border-left:.25em solid #dddddf}
.quoteblock.excerpt,.quoteblock .quoteblock{margin-left:0}
.quoteblock.excerpt blockquote,.quoteblock.excerpt p,.quoteblock .quoteblock blockquote,.quoteblock .quoteblock p{color:inherit;font-size:1.0625rem}
.quoteblock.excerpt .attribution,.quoteblock .quoteblock .attribution{color:inherit;font-size:.85rem;text-align:left;margin-right:0}
p.tableblock:last-child{margin-bottom:0}
td.tableblock>.content{margin-bottom:1.25em;word-wrap:anywhere}
td.tableblock>.content>:last-child{margin-bottom:-1.25em}
table.tableblock,th.tableblock,td.tableblock{border:0 solid #dedede}
table.grid-all>*>tr>*{border-width:1px}
table.grid-cols>*>tr>*{border-width:0 1px}
table.grid-rows>*>tr>*{border-width:1px 0}
table.frame-all{border-width:1px}
table.frame-ends{border-width:1px 0}
table.frame-sides{border-width:0 1px}
table.frame-none>colgroup+*>:first-child>*,table.frame-sides>colgroup+*>:first-child>*{border-top-width:0}
table.frame-none>:last-child>:last-child>*,table.frame-sides>:last-child>:last-child>*{border-bottom-width:0}
table.frame-none>*>tr>:first-child,table.frame-ends>*>tr>:first-child{border-left-width:0}
table.frame-none>*>tr>:last-child,table.frame-ends>*>tr>:last-child{border-right-width:0}
table.stripes-all>*>tr,table.stripes-odd>*>tr:nth-of-type(odd),table.stripes-even>*>tr:nth-of-type(even),table.stripes-hover>*>tr:hover{background:#f8f8f7}
th.halign-left,td.halign-left{text-align:left}
th.halign-right,td.halign-right{text-align:right}
th.halign-center,td.halign-center{text-align:center}
th.valign-top,td.valign-top{vertical-align:top}
th.valign-bottom,td.valign-bottom{vertical-align:bottom}
th.valign-middle,td.valign-middle{vertical-align:middle}
table thead th,table tfoot th{font-weight:bold}
tbody tr th{background:#f7f8f7}
tbody tr th,tbody tr th p,tfoot tr th,tfoot tr th p{color:rgba(0,0,0,.8);font-weight:bold}
p.tableblock>code:only-child{background:none;padding:0}
p.tableblock{font-size:1em}
ol{margin-left:1.75em}
ul li ol{margin-left:1.5em}
dl dd{margin-left:1.125em}
dl dd:last-child,dl dd:last-child>:last-child{margin-bottom:0}
li p,ul dd,ol dd,.olist .olist,.ulist .ulist,.ulist .olist,.olist .ulist{margin-bottom:.625em}
ul.checklist,ul.none,ol.none,ul.no-bullet,ol.no-bullet,ol.unnumbered,ul.unstyled,ol.unstyled{list-style-type:none}
ul.no-bullet,ol.no-bullet,ol.unnumbered{margin-left:.625em}
ul.unstyled,ol.unstyled{margin-left:0}
li>p:empty:only-child::before{content:"";display:inline-block}
ul.checklist>li>p:first-child{margin-left:-1em}
ul.checklist>li>p:first-child>.fa-square-o:first-child,ul.checklist>li>p:first-child>.fa-check-square-o:first-child{width:1.25em;font-size:.8em;position:relative;bottom:.125em}
ul.checklist>li>p:first-child>input[type=checkbox]:first-child{margin-right:.25em}
ul.inline{display:flex;flex-flow:row wrap;list-style:none;margin:0 0 .625em -1.25em}
ul.inline>li{margin-left:1.25em}
.unstyled dl dt{font-weight:400;font-style:normal}
ol.arabic{list-style-type:decimal}
ol.decimal{list-style-type:decimal-leading-zero}
ol.loweralpha{list-style-type:lower-alpha}
ol.upperalpha{list-style-type:upper-alpha}
ol.lowerroman{list-style-type:lower-roman}
ol.upperroman{list-style-type:upper-roman}
ol.lowergreek{list-style-type:lower-greek}
.hdlist>table,.colist>table{border:0;background:none}
.hdlist>table>tbody>tr,.colist>table>tbody>tr{background:none}
td.hdlist1,td.hdlist2{vertical-align:top;padding:0 .625em}
td.hdlist1{font-weight:bold;padding-bottom:1.25em}
td.hdlist2{word-wrap:anywhere}
.literalblock+.colist,.listingblock+.colist{margin-top:-.5em}
.colist td:not([class]):first-child{padding:.4em .75em 0;line-height:1;vertical-align:top}
.colist td:not([class]):first-child img{max-width:none}
.colist td:not([class]):last-child{padding:.25em 0}
.thumb,.th{line-height:0;display:inline-block;border:4px solid #fff;box-shadow:0 0 0 1px #ddd}
.imageblock.left{margin:.25em .625em 1.25em 0}
.imageblock.right{margin:.25em 0 1.25em .625em}
.imageblock>.title{margin-bottom:0}
.imageblock.thumb,.imageblock.th{border-width:6px}
.imageblock.thumb>.title,.imageblock.th>.title{padding:0 .125em}
.image.left,.image.right{margin-top:.25em;margin-bottom:.25em;display:inline-block;line-height:0}
.image.left{margin-right:.625em}
.image.right{margin-left:.625em}
a.image{text-decoration:none;display:inline-block}
a.image object{pointer-events:none}
sup.footnote,sup.footnoteref{font-size:.875em;position:static;vertical-align:super}
sup.footnote a,sup.footnoteref a{text-decoration:none}
sup.footnote a:active,sup.footnoteref a:active{text-decoration:underline}
#footnotes{padding-top:.75em;padding-bottom:.75em;margin-bottom:.625em}
#footnotes hr{width:20%;min-width:6.25em;margin:-.25em 0 .75em;border-width:1px 0 0}
#footnotes .footnote{padding:0 .375em 0 .225em;line-height:1.3334;font-size:.875em;margin-left:1.2em;margin-bottom:.2em}
#footnotes .footnote a:first-of-type{font-weight:bold;text-decoration:none;margin-left:-1.05em}
#footnotes .footnote:last-of-type{margin-bottom:0}
#content #footnotes{margin-top:-.625em;margin-bottom:0;padding:.75em 0}
div.unbreakable{page-break-inside:avoid}
.big{font-size:larger}
.small{font-size:smaller}
.underline{text-decoration:underline}
.overline{text-decoration:overline}
.line-through{text-decoration:line-through}
.aqua{color:#00bfbf}
.aqua-background{background:#00fafa}
.black{color:#000}
.black-background{background:#000}
.blue{color:#0000bf}
.blue-background{background:#0000fa}
.fuchsia{color:#bf00bf}
.fuchsia-background{background:#fa00fa}
.gray{color:#606060}
.gray-background{background:#7d7d7d}
.green{color:#006000}
.green-background{background:#007d00}
.lime{color:#00bf00}
.lime-background{background:#00fa00}
.maroon{color:#600000}
.maroon-background{background:#7d0000}
.navy{color:#000060}
.navy-background{background:#00007d}
.olive{color:#606000}
.olive-background{background:#7d7d00}
.purple{color:#600060}
.purple-background{background:#7d007d}
.red{color:#bf0000}
.red-background{background:#fa0000}
.silver{color:#909090}
.silver-background{background:#bcbcbc}
.teal{color:#006060}
.teal-background{background:#007d7d}
.white{color:#bfbfbf}
.white-background{background:#fafafa}
.yellow{color:#bfbf00}
.yellow-background{background:#fafa00}
span.icon>.fa{cursor:default}
a span.icon>.fa{cursor:inherit}
.admonitionblock td.icon [class^="fa icon-"]{font-size:2.5em;text-shadow:1px 1px 2px rgba(0,0,0,.5);cursor:default}
.admonitionblock td.icon .icon-note::before{content:"\f05a";color:#19407c}
.admonitionblock td.icon .icon-tip::before{content:"\f0eb";text-shadow:1px 1px 2px rgba(155,155,0,.8);color:#111}
.admonitionblock td.icon .icon-warning::before{content:"\f071";color:#bf6900}
.admonitionblock td.icon .icon-caution::before{content:"\f06d";color:#bf3400}
.admonitionblock td.icon .icon-important::before{content:"\f06a";color:#bf0000}
.conum[data-value]{display:inline-block;color:#fff!important;background:rgba(0,0,0,.8);border-radius:50%;text-align:center;font-size:.75em;width:1.67em;height:1.67em;line-height:1.67em;font-family:"Open Sans","DejaVu Sans",sans-serif;font-style:normal;font-weight:bold}
.conum[data-value] *{color:#fff!important}
.conum[data-value]+b{display:none}
.conum[data-value]::after{content:attr(data-value)}
pre .conum[data-value]{position:relative;top:-.125em}
b.conum *{color:inherit!important}
.conum:not([data-value]):empty{display:none}
dt,th.tableblock,td.content,div.footnote{text-rendering:optimizeLegibility}
h1,h2,p,td.content,span.alt,summary{letter-spacing:-.01em}
p strong,td.content strong,div.footnote strong{letter-spacing:-.005em}
p,blockquote,dt,td.content,td.hdlist1,span.alt,summary{font-size:1.0625rem}
p{margin-bottom:1.25rem}
.sidebarblock p,.sidebarblock dt,.sidebarblock td.content,p.tableblock{font-size:1em}
.exampleblock>.content{background:#fffef7;border-color:#e0e0dc;box-shadow:0 1px 4px #e0e0dc}
.print-only{display:none!important}
@page{margin:1.25cm .75cm}
@media print{*{box-shadow:none!important;text-shadow:none!important}
html{font-size:80%}
a{color:inherit!important;text-decoration:underline!important}
a.bare,a[href^="#"],a[href^="mailto:"]{text-decoration:none!important}
a[href^="http:"]:not(.bare)::after,a[href^="https:"]:not(.bare)::after{content:"(" attr(href) ")";display:inline-block;font-size:.875em;padding-left:.25em}
abbr[title]{border-bottom:1px dotted}
abbr[title]::after{content:" (" attr(title) ")"}
pre,blockquote,tr,img,object,svg{page-break-inside:avoid}
thead{display:table-header-group}
svg{max-width:100%}
p,blockquote,dt,td.content{font-size:1em;orphans:3;widows:3}
h2,h3,#toctitle,.sidebarblock>.content>.title{page-break-after:avoid}
#header,#content,#footnotes,#footer{max-width:none}
#toc,.sidebarblock,.exampleblock>.content{background:none!important}
#toc{border-bottom:1px solid #dddddf!important;padding-bottom:0!important}
body.book #header{text-align:center}
body.book #header>h1:first-child{border:0!important;margin:2.5em 0 1em}
body.book #header .details{border:0!important;display:block;padding:0!important}
body.book #header .details span:first-child{margin-left:0!important}
body.book #header .details br{display:block}
body.book #header .details br+span::before{content:none!important}
body.book #toc{border:0!important;text-align:left!important;padding:0!important;margin:0!important}
body.book #toc,body.book #preamble,body.book h1.sect0,body.book .sect1>h2{page-break-before:always}
.listingblock code[data-lang]::before{display:block}
#footer{padding:0 .9375em}
.hide-on-print{display:none!important}
.print-only{display:block!important}
.hide-for-print{display:none!important}
.show-for-print{display:inherit!important}}
@media amzn-kf8,print{#header>h1:first-child{margin-top:1.25rem}
.sect1{padding:0!important}
.sect1+.sect1{border:0}
#footer{background:none}
#footer-text{color:rgba(0,0,0,.6);font-size:.9em}}
@media amzn-kf8{#header,#content,#footnotes,#footer{padding:0}}
</style>
</head>
<body class="article">
<div id="header">
</div>
<div id="content">
<div class="sect1">
<h2 id="blockchain">The Blockchain</h2>
<div class="sectionbody">
<div class="paragraph">
<p>The blockchain is the history of every confirmed Bitcoin transaction.
It’s what allows every full node to independently determine what keys and
scripts control which bitcoins. In this chapter, we’ll look at the
structure of the blockchain and see how it uses cryptographic
commitments and other clever tricks to make every part of it easy for
full nodes (and sometimes lightweight clients) to validate.</p>
</div>
<div class="paragraph">
<p>The blockchain data structure is
an ordered, back-linked list of blocks of transactions. The blockchain
can be stored as a flat file or in a simple database.
Blocks are linked "back," each referring to the previous block in the
chain. The blockchain is often visualized
as a vertical stack, with blocks layered on top of each other and the
first block serving as the foundation of the stack. The visualization of
blocks stacked on top of each other results in the use of terms such as
"height" to refer to the distance from the first block, and "top" or
"tip" to refer to the most recently added block.</p>
</div>
<div class="paragraph">
<p>Each block
within the blockchain is identified by a hash, generated using the
SHA256 cryptographic hash algorithm on the header of the block. Each
block also commits to the previous block, known as the <em>parent</em> block,
through the "previous block hash" field in the block header.
The sequence of hashes linking each block to its parent creates a chain
going back all the way to the first block ever created, known as the
<em>genesis block</em>.</p>
</div>
<div class="paragraph">
<p>Although a block has just one parent, it can have multiple
children. Each of the children commits to the same parent block.
Multiple children arise during a blockchain "fork," a temporary
situation that can occur when different blocks are discovered almost
simultaneously by different miners (see <a href="#forks">[forks]</a>). Eventually only one
child block becomes part of the blockchain accepted by all full nodes, and the "fork" is resolved.</p>
</div>
<div class="paragraph">
<p>The "previous block hash" field is inside the block header and thereby
affects the <em>current</em> block’s hash.
Any change to a parent block
requires a child block’s hash to change, which requires a change in the
pointer of the grandchild, which in turn changes the grandchild, and so
on. This sequence ensures that, once a block has many generations
following it, it cannot be changed without forcing a recalculation of
all subsequent blocks. Because such a recalculation would require
enormous computation (and therefore energy consumption), the existence
of a long chain of blocks makes the blockchain’s deep history impractical to change,
which is a key feature of Bitcoin’s security.</p>
</div>
<div class="paragraph">
<p>One way to think about the blockchain is like layers in a geological
formation, or glacier core sample. The surface layers might change with
the seasons, or even be blown away before they have time to settle. But
once you go a few inches deep, geological layers become more and more
stable. By the time you look a few hundred feet down, you are looking at
a snapshot of the past that has remained undisturbed for millions of
years. In the blockchain, the most recent few blocks might be revised if
there is a chain reorganization due to a fork. The top six blocks are
like a few inches of topsoil. But once you go more deeply into the
blockchain, beyond six blocks, blocks are less and less likely to
change. After 100 blocks back there is so much stability that
the coinbase transaction—​the transaction containing the reward in
bitcoin for creating a new block—​can be spent.
While the
protocol always allows a chain to be undone by a longer chain and while
the possibility of any block being reversed always exists, the
probability of such an event decreases as time passes until it becomes
infinitesimal.</p>
</div>
<div class="sect2">
<h3 id="_structure_of_a_block">Structure of a Block</h3>
<div class="paragraph">
<p>A block is a container data structure that aggregates
transactions for inclusion in the blockchain. The
block is made of a header, containing metadata, followed by a long list
of transactions that make up the bulk of its size. The block header is
80 bytes, whereas the total size of all transactions in a block can be
up to about 4,000,000 bytes. A complete block,
with all transactions, can therefore be almost 50,000 times larger than the block
header. <a href="#block_structure1">[block_structure1]</a> describes how Bitcoin Core stores the structure of a block.</p>
</div>
<table id="block_structure1">
<caption>The structure of a block</caption>
<thead>
<tr>
<th>Size</th>
<th>Field</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td><p>4 bytes</p></td>
<td><p>Block Size</p></td>
<td><p>The size of the block, in bytes, following this field</p></td>
</tr>
<tr>
<td><p>80 bytes</p></td>
<td><p>Block Header</p></td>
<td><p>Several fields form the block header</p></td>
</tr>
<tr>
<td><p>1–3 bytes (compactSize)</p></td>
<td><p>Transaction Counter</p></td>
<td><p>How many transactions follow</p></td>
</tr>
<tr>
<td><p>Variable</p></td>
<td><p>Transactions</p></td>
<td><p>The transactions recorded in this block</p></td>
</tr>
</tbody>
</table>
</div>
<div class="sect2">
<h3 id="block_header">Block Header</h3>
<div class="paragraph">
<p>The block header consists of
block metadata as shown in <a href="#block_header_structure_ch09">[block_header_structure_ch09]</a>.</p>
</div>
<table id="block_header_structure_ch09">
<caption>The structure of the block header</caption>
<thead>
<tr>
<th>Size</th>
<th>Field</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td><p>4 bytes</p></td>
<td><p>Version</p></td>
<td><p>Originally a version field; its use has evolved over time</p></td>
</tr>
<tr>
<td><p>32 bytes</p></td>
<td><p>Previous Block Hash</p></td>
<td><p>A hash of the previous (parent) block in the chain</p></td>
</tr>
<tr>
<td><p>32 bytes</p></td>
<td><p>Merkle Root</p></td>
<td><p>The root hash of the merkle tree of this block’s transactions</p></td>
</tr>
<tr>
<td><p>4 bytes</p></td>
<td><p>Timestamp</p></td>
<td><p>The approximate creation time of this block (Unix epoch time)</p></td>
</tr>
<tr>
<td><p>4 bytes</p></td>
<td><p>Target</p></td>
<td><p>A compact encoding of the proof-of-work target for this block</p></td>
</tr>
<tr>
<td><p>4 bytes</p></td>
<td><p>Nonce</p></td>
<td><p>Arbitrary data used for the proof-of-work algorithm</p></td>
</tr>
</tbody>
</table>
<div class="paragraph">
<p>The nonce, target, and timestamp are used in the mining
process and will be discussed in more detail in <a href="#mining">[mining]</a>.</p>
</div>
</div>
<div class="sect2">
<h3 id="block_hash">Block Identifiers: Block Header Hash and Block Height</h3>
<div class="paragraph">
<p>The primary identifier of a block
is its cryptographic hash, a commitment made by hashing the
block header twice through the SHA256 algorithm. The resulting 32-byte
hash is called the <em>block hash</em> but is more accurately the <em>block header
hash</em>, <span class="keep-together">because only the block header is
used to compute it. For example,</span>
000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f is
the block hash of the first block on Bitcoin’s blockchain. The block hash
identifies a block uniquely and unambiguously and can be independently
derived by any node by simply hashing the block header.</p>
</div>
<div class="paragraph">
<p>Note that the block hash is not actually included inside the block’s
data structure.
Instead, the block’s hash is computed by each node as the
block is received from the network. The block hash might be stored in a
separate database table as part of the block’s metadata, to facilitate
indexing and faster retrieval of blocks from disk.</p>
</div>
<div class="paragraph">
<p>A second way to identify a block is by its position in the blockchain,
called the <span class="keep-together"><em>block height</em>. The
genesis block is at block height 0 (zero) and is the</span>
<span class="keep-together">same block that was previously
referenced by the following block hash</span>
000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f. A
block can thus be identified in two ways: by referencing the block hash
or by referencing the block height. Each subsequent block added "on top"
of that first block is one position "higher" in the blockchain, like
boxes stacked one on top of the other. The block height 800,000 was
reached during the writing of this book in mid-2023, meaning there were
800,000 blocks stacked on top of the first block created in January
2009.</p>
</div>
<div class="paragraph">
<p>Unlike the block hash, the block height is not a unique identifier.
Although a single block will always have a specific and invariant block
height, the reverse is not true—the block height does not always
identify a single block. Two or more blocks might have the same block
height, competing for the same position in the blockchain. This scenario
is discussed in detail in the section <a href="#forks">[forks]</a>. In early blocks, the block height was
also not a part of the block’s data structure; it was not stored within
the block. Each node dynamically identified a block’s position (height)
in the blockchain when it was received from the Bitcoin network. A
later protocol change (BIP34) began including the block height in the
coinbase transaction, although its purpose was to ensure each block had
a different coinbase transaction. Nodes still need to dynamically
identify a block’s height in order to validate the coinbase field. The
block height might also be stored as metadata in an indexed database
table for faster retrieval.</p>
</div>
<div class="admonitionblock tip">
<table>
<tr>
<td class="icon">
<div class="title">Tip</div>
</td>
<td class="content">
<div class="paragraph">
<p>A block’s <em>block hash</em> always identifies a single block uniquely. A
block also always has a specific <em>block height</em>. However, it is not
always the case that a specific block height identifies a single
block. Rather, two or more blocks might compete for a single position in
the blockchain.</p>
</div>
</td>
</tr>
</table>
</div>
</div>
<div class="sect2">
<h3 id="_the_genesis_block">The Genesis Block</h3>
<div class="paragraph">
<p>The first block in the blockchain is called the <em>genesis block</em>
and was created in 2009. It is the common ancestor of all the blocks in
the blockchain, meaning that if you start at any block and follow the
chain backward in time, you will eventually arrive at the genesis block.</p>
</div>
<div class="paragraph">
<p>Every node always starts with a blockchain of at least one block because
the genesis block is statically encoded within Bitcoin Core,
such that it cannot be altered. Every node always "knows" the
genesis block’s hash and structure, the fixed time it was created, and
even the single transaction within. Thus, every node has the starting
point for the blockchain, a secure "root" from which to build a trusted
blockchain.</p>
</div>
<div class="paragraph">
<p>See the statically encoded genesis block inside the Bitcoin Core client
in <a href="https://oreil.ly/FqPW5"><em>chainparams.cpp</em></a>.</p>
</div>
<div class="paragraph">
<p>The following identifier hash belongs to the genesis block:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f</pre>
</div>
</div>
<div class="paragraph">
<p>You can search for that block hash in almost any block explorer website, such
as <em>blockstream.info</em>, and you will find a page describing the contents
of this block, with a URL containing that hash:</p>
</div>
<div class="quoteblock">
<blockquote>
<div class="paragraph">
<p><a href="https://blockstream.info/block/000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f" class="bare">https://blockstream.info/block/000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f</a></p>
</div>
</blockquote>
</div>
<div class="paragraph">
<p>Alternatively, you can get the block using Bitcoin Core on the command line:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>$ bitcoin-cli getblock \
000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f</pre>
</div>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code class="language-json" data-lang="json">{
"hash": "000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f",
"confirmations": 790496,
"height": 0,
"version": 1,
"versionHex": "00000001",
"merkleroot": "4a5e1e4baab89f3a32518a88c3[...]76673e2cc77ab2127b7afdeda33b",
"time": 1231006505,
"mediantime": 1231006505,
"nonce": 2083236893,
"bits": "1d00ffff",
"difficulty": 1,
"chainwork": "[...]000000000000000000000000000000000000000000000100010001",
"nTx": 1,
"nextblockhash": "00000000839a8e6886ab5951d7[...]fc90947ee320161bbf18eb6048",
"strippedsize": 285,
"size": 285,
"weight": 1140,
"tx": [
"4a5e1e4baab89f3a32518a88c31bc87f618f76673e2cc77ab2127b7afdeda33b"
]
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>The genesis block contains a message within it. The coinbase
transaction input contains the text "The Times 03/Jan/2009 Chancellor on
brink of second bailout for banks." This message was intended to offer
proof of the earliest date this block could have been created, by referencing the
headline of the British newspaper <em>The Times</em>. It also serves as a
tongue-in-cheek reminder of the importance of an independent monetary
system, with Bitcoin’s launch occurring at the same time as an
unprecedented worldwide monetary crisis. The message was embedded in the
first block by Satoshi Nakamoto, Bitcoin’s creator.</p>
</div>
</div>
<div class="sect2">
<h3 id="_linking_blocks_in_the_blockchain">Linking Blocks in the Blockchain</h3>
<div class="paragraph">
<p>Bitcoin full nodes validate every
block in the blockchain after the genesis block. Their local view of
the blockchain is constantly updated as new blocks are found and used to
extend the chain. As a node receives incoming blocks from the network,
it will validate these blocks and then link them to its view of the existing
blockchain. To establish a link, a node will examine the incoming block
header and look for the "previous block hash."</p>
</div>
<div class="paragraph less_space pagebreak-before">
<p>Let’s assume, for example, that a node has 277,314 blocks in the local
copy of the blockchain. The last block the node knows about is block
277,314, with a block header hash of:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>00000000000000027e7ba6fe7bad39faf3b5a83daed765f05f7d1b71a1632249</pre>
</div>
</div>
<div class="paragraph">
<p>The Bitcoin node then receives a new block from the network, which it
parses as follows:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code class="language-json" data-lang="json">{
"size" : 43560,
"version" : 2,
"previousblockhash" :
"00000000000000027e7ba6fe7bad39faf3b5a83daed765f05f7d1b71a1632249",
"merkleroot" :
"5e049f4030e0ab2debb92378f53c0a6e09548aea083f3ab25e1d94ea1155e29d",
"time" : 1388185038,
"difficulty" : 1180923195.25802612,
"nonce" : 4215469401,
"tx" : [
"257e7497fb8bc68421eb2c7b699dbab234831600e7352f0d9e6522c7cf3f6c77",
"[... many more transactions omitted ...]",
"05cfd38f6ae6aa83674cc99e4d75a1458c165b7ab84725eda41d018a09176634"
]
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>Looking at this new block, the node finds the previousblockhash field,
which contains the hash of its parent block. It is a hash known to the
node, that of the last block on the chain at height 277,314. Therefore,
this new block is a child of the last block on the chain and extends the
existing blockchain. The node adds this new block to the end of the
chain, making the blockchain longer with a new height of 277,315.
<a href="#chain_of_blocks">Blocks linked in a chain by each referencing the previous block header hash.</a> shows the chain of three blocks, linked by
references in the previousblockhash field.</p>
</div>
<div id="chain_of_blocks" class="imageblock">
<div class="content">
<img src="images/mbc3_1101.png" alt="mbc3 1101">
</div>
<div class="title">Figure 1. Blocks linked in a chain by each referencing the previous block header hash.</div>
</div>
</div>
<div class="sect2">
<h3 id="merkle_trees">Merkle Trees</h3>
<div class="paragraph">
<p>Each block in the Bitcoin blockchain contains
a summary of all the transactions in the block using a <em>merkle tree</em>.</p>
</div>
<div class="paragraph">
<p>A <em>merkle tree</em>, also known
as a <em>binary hash tree</em>, is a data structure used for efficiently
summarizing and verifying the integrity of large sets of data. Merkle
trees are binary trees containing cryptographic hashes. The term "tree"
is used in computer science to describe a branching data structure, but
these trees are usually displayed upside down with the "root" at the top
and the "leaves" at the bottom of a diagram, as you will see in the
examples that follow.</p>
</div>
<div class="paragraph">
<p>Merkle trees are used in Bitcoin to summarize all the transactions in a
block, producing an overall commitment to the entire set of
transactions and permitting a very efficient process to verify whether a
transaction is included in a block. A merkle tree is constructed by
recursively hashing pairs of elements until there is only one hash, called
the <em>root</em>, or <em>merkle root</em>. The cryptographic hash algorithm used in
Bitcoin’s merkle trees is SHA256 applied twice, also known as
double-SHA256.</p>
</div>
<div class="paragraph">
<p>When N data elements are hashed and summarized in a merkle tree, you can
check to see if any one data element is included in the tree with
about log~2~(N) calculations, making this a very efficient data
structure.</p>
</div>
<div class="paragraph">
<p>The merkle tree is constructed bottom-up. In the following example, we
start with four transactions, A, B, C, and D, which form the <em>leaves</em> of
the merkle tree, as shown in <a href="#simple_merkle">Calculating the nodes in a merkle tree.</a>. The transactions are not
stored in the merkle tree; rather, their data is hashed and the
resulting hash is stored in each leaf node as H<sub>A</sub>, H<sub>B</sub>, H<sub>C</sub>, and
H<sub>D</sub>:</p>
</div>
<pre data-type="codelisting">
H<sub>A</sub> = SHA256(SHA256(Transaction A))
</pre>
<div class="paragraph">
<p>Consecutive pairs of leaf nodes are then summarized in a parent node by
concatenating the two hashes and hashing them together. For example, to
construct the parent node H<sub>AB</sub>, the two 32-byte hashes of the children
are concatenated to create a 64-byte string. That string is then
double-hashed to produce the parent node’s hash:</p>
</div>
<pre data-type="codelisting">
H<sub>AB</sub> = SHA256(SHA256(H<sub>A</sub> || H<sub>B</sub>))
</pre>
<div class="paragraph">
<p>The process continues until there is only one node at the top, the node
known as the merkle root. That 32-byte hash is stored in the block
header and summarizes all the data in all four transactions.
<a href="#simple_merkle">Calculating the nodes in a merkle tree.</a> shows how the root is calculated by pair-wise hashes
of the nodes.</p>
</div>
<div id="simple_merkle" class="imageblock">
<div class="content">
<img src="images/mbc3_1102.png" alt="merkle_tree">
</div>
<div class="title">Figure 2. Calculating the nodes in a merkle tree.</div>
</div>
<div class="paragraph">
<p>Because the merkle tree is a binary tree, it needs
an even number of leaf nodes. If there are an odd number of transactions
to summarize, the last transaction hash will be duplicated to create an
even number of leaf nodes, also known as a <em>balanced tree</em>. This is
shown in <a href="#merkle_tree_odd">Duplicating one data element achieves an even number of data elements.</a>, where transaction C is duplicated.
Similarly, if there are an odd number of hashes to process at any level,
the last hash is duplicated.</p>
</div>
<div id="merkle_tree_odd" class="imageblock">
<div class="content">
<img src="images/mbc3_1103.png" alt="merkle_tree_odd">
</div>
<div class="title">Figure 3. Duplicating one data element achieves an even number of data elements.</div>
</div>
<div class="sidebarblock">
<div class="content">
<div class="title">A Design Flaw in Bitcoin’s Merkle Tree</div>
<div class="paragraph">
<p>An extended comment in Bitcoin Core’s source code, reproduced here with slight revisions, describes a
significant problem in the design of Bitcoin’s duplication of odd
elements in its merkle tree:</p>
</div>
<div class="quoteblock">
<blockquote>
<div class="paragraph">
<p>WARNING! If you’re reading this because you’re learning about crypto
and/or designing a new system that will use merkle trees, keep in mind
that the following merkle tree algorithm has a serious flaw related to
duplicate txids, resulting in a vulnerability (CVE-2012-2459).</p>
</div>
<div class="paragraph">
<p>The reason is that if the number of hashes in the list at a given level
is odd, the last one is duplicated before computing the next level (which
is unusual in merkle trees). This results in certain sequences of
transactions leading to the same merkle root. For example, the two
trees in <a href="#cve_tree">Two Bitcoin-style merkle trees with the same root but a different number of leaves.</a>:</p>
</div>
<div id="cve_tree" class="imageblock width-90">
<div class="content">
<img src="images/mbc3_1104.png" alt="Two Bitcoin-style merkle trees with the same root but a different number of leaves">
</div>
<div class="title">Figure 4. Two Bitcoin-style merkle trees with the same root but a different number of leaves.</div>
</div>
<div class="paragraph">
<p>The transaction lists [1,2,3,4,5,6] and [1,2,3,4,5,6,5,6] (where 5 and
6 are repeated) result in the same root hash A (because the hash of both
of (F) and (F,F) is C).</p>
</div>
<div class="paragraph">
<p>The vulnerability results from being able to send a block with such a
transaction list, with the same merkle root, and the same block hash as
the original without duplication, resulting in failed validation. If the
receiving node proceeds to mark that block as permanently invalid
however, it will fail to accept further unmodified (and thus potentially
valid) versions of the same block. We defend against this by detecting
the case where we would hash two identical hashes at the end of the list
together, and treating that identically to the block having an invalid
merkle root. Assuming no double-SHA256 collisions, this will detect all
known ways of changing the transactions without affecting the merkle
root.</p>
</div>
<p data-type="attribution">Bitcoin Core <em>src/consensus/merkle.cpp</em></p>
</blockquote>
</div>
</div>
</div>
<div class="paragraph less_space pagebreak-before">
<p>The same method for constructing a tree from four transactions can be
generalized to construct trees of any size. In Bitcoin it is common to
have several thousand transactions in a single
block, which are summarized in exactly the same way, producing just 32
bytes of data as the single merkle root. In <a href="#merkle_tree_large">A merkle tree summarizing many data elements.</a>, you
will see a tree built from 16 transactions. Note that although the root
looks bigger than the leaf nodes in the diagram, it is the exact same
size, just 32 bytes. Whether there is one transaction or ten
thousand transactions in the block, the merkle root always summarizes
them into 32 bytes.</p>
</div>
<div class="paragraph">
<p>To prove that a specific transaction is
included in a block, a node only needs to produce approximately log~2~(N) 32-byte
hashes, constituting an <em>authentication path</em> or <em>merkle path</em>
connecting the specific transaction to the root of the tree. This is
especially important as the number of transactions increases because
the base-2 logarithm of the number of transactions increases much more
slowly. This allows Bitcoin nodes to efficiently produce paths of 10 or
12 hashes (320–384 bytes), which can provide proof of a single
transaction out of more than a thousand transactions in a multimegabyte
block.</p>
</div>
<div id="merkle_tree_large" class="imageblock">
<div class="content">
<img src="images/mbc3_1105.png" alt="merkle_tree_large">
</div>
<div class="title">Figure 5. A merkle tree summarizing many data elements.</div>
</div>
<div class="paragraph">
<p>In <a href="#merkle_tree_path">A merkle path used to prove inclusion of a data element.</a>, a node can prove that a transaction K is
included in the block by producing a merkle path that is only four