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 736 <div id="header">
 737 <h1>Git hash function transition</h1>
 738 <span id="revdate">2023-05-17</span>
 739 </div>
 740 <div id="content">
 741 <div class="sect1">
 742 <h2 id="_objective">Objective</h2>
 743 <div class="sectionbody">
 744 <div class="paragraph"><p>Migrate Git from SHA-1 to a stronger hash function.</p></div>
 745 </div>
 746 </div>
 747 <div class="sect1">
 748 <h2 id="_background">Background</h2>
 749 <div class="sectionbody">
 750 <div class="paragraph"><p>At its core, the Git version control system is a content addressable
 751 filesystem. It uses the SHA-1 hash function to name content. For
 752 example, files, directories, and revisions are referred to by hash
 753 values unlike in other traditional version control systems where files
 754 or versions are referred to via sequential numbers. The use of a hash
 755 function to address its content delivers a few advantages:</p></div>
 756 <div class="ulist"><ul>
 757 <li>
 758 <p>
 759 Integrity checking is easy. Bit flips, for example, are easily
 760   detected, as the hash of corrupted content does not match its name.
 761 </p>
 762 </li>
 763 <li>
 764 <p>
 765 Lookup of objects is fast.
 766 </p>
 767 </li>
 768 </ul></div>
 769 <div class="paragraph"><p>Using a cryptographically secure hash function brings additional
 770 advantages:</p></div>
 771 <div class="ulist"><ul>
 772 <li>
 773 <p>
 774 Object names can be signed and third parties can trust the hash to
 775   address the signed object and all objects it references.
 776 </p>
 777 </li>
 778 <li>
 779 <p>
 780 Communication using Git protocol and out of band communication
 781   methods have a short reliable string that can be used to reliably
 782   address stored content.
 783 </p>
 784 </li>
 785 </ul></div>
 786 <div class="paragraph"><p>Over time some flaws in SHA-1 have been discovered by security
 787 researchers. On 23 February 2017 the SHAttered attack
 788 (<a href="https://shattered.io">https://shattered.io</a>) demonstrated a practical SHA-1 hash collision.</p></div>
 789 <div class="paragraph"><p>Git v2.13.0 and later subsequently moved to a hardened SHA-1
 790 implementation by default, which isn&#8217;t vulnerable to the SHAttered
 791 attack, but SHA-1 is still weak.</p></div>
 792 <div class="paragraph"><p>Thus it&#8217;s considered prudent to move past any variant of SHA-1
 793 to a new hash. There&#8217;s no guarantee that future attacks on SHA-1 won&#8217;t
 794 be published in the future, and those attacks may not have viable
 795 mitigations.</p></div>
 796 <div class="paragraph"><p>If SHA-1 and its variants were to be truly broken, Git&#8217;s hash function
 797 could not be considered cryptographically secure any more. This would
 798 impact the communication of hash values because we could not trust
 799 that a given hash value represented the known good version of content
 800 that the speaker intended.</p></div>
 801 <div class="paragraph"><p>SHA-1 still possesses the other properties such as fast object lookup
 802 and safe error checking, but other hash functions are equally suitable
 803 that are believed to be cryptographically secure.</p></div>
 804 </div>
 805 </div>
 806 <div class="sect1">
 807 <h2 id="_choice_of_hash">Choice of Hash</h2>
 808 <div class="sectionbody">
 809 <div class="paragraph"><p>The hash to replace the hardened SHA-1 should be stronger than SHA-1
 810 was: we would like it to be trustworthy and useful in practice for at
 811 least 10 years.</p></div>
 812 <div class="paragraph"><p>Some other relevant properties:</p></div>
 813 <div class="olist arabic"><ol class="arabic">
 814 <li>
 815 <p>
 816 A 256-bit hash (long enough to match common security practice; not
 817    excessively long to hurt performance and disk usage).
 818 </p>
 819 </li>
 820 <li>
 821 <p>
 822 High quality implementations should be widely available (e.g., in
 823    OpenSSL and Apple CommonCrypto).
 824 </p>
 825 </li>
 826 <li>
 827 <p>
 828 The hash function&#8217;s properties should match Git&#8217;s needs (e.g. Git
 829    requires collision and 2nd preimage resistance and does not require
 830    length extension resistance).
 831 </p>
 832 </li>
 833 <li>
 834 <p>
 835 As a tiebreaker, the hash should be fast to compute (fortunately
 836    many contenders are faster than SHA-1).
 837 </p>
 838 </li>
 839 </ol></div>
 840 <div class="paragraph"><p>There were several contenders for a successor hash to SHA-1, including
 841 SHA-256, SHA-512/256, SHA-256x16, K12, and BLAKE2bp-256.</p></div>
 842 <div class="paragraph"><p>In late 2018 the project picked SHA-256 as its successor hash.</p></div>
 843 <div class="paragraph"><p>See 0ed8d8da374 (doc hash-function-transition: pick SHA-256 as
 844 NewHash, 2018-08-04) and numerous mailing list threads at the time,
 845 particularly the one starting at
 846 <a href="https://lore.kernel.org/git/20180609224913.GC38834@genre.crustytoothpaste.net/">https://lore.kernel.org/git/20180609224913.GC38834@genre.crustytoothpaste.net/</a>
 847 for more information.</p></div>
 848 </div>
 849 </div>
 850 <div class="sect1">
 851 <h2 id="_goals">Goals</h2>
 852 <div class="sectionbody">
 853 <div class="olist arabic"><ol class="arabic">
 854 <li>
 855 <p>
 856 The transition to SHA-256 can be done one local repository at a time.
 857 </p>
 858 <div class="olist loweralpha"><ol class="loweralpha">
 859 <li>
 860 <p>
 861 Requiring no action by any other party.
 862 </p>
 863 </li>
 864 <li>
 865 <p>
 866 A SHA-256 repository can communicate with SHA-1 Git servers
 867       (push/fetch).
 868 </p>
 869 </li>
 870 <li>
 871 <p>
 872 Users can use SHA-1 and SHA-256 identifiers for objects
 873       interchangeably (see "Object names on the command line", below).
 874 </p>
 875 </li>
 876 <li>
 877 <p>
 878 New signed objects make use of a stronger hash function than
 879       SHA-1 for their security guarantees.
 880 </p>
 881 </li>
 882 </ol></div>
 883 </li>
 884 <li>
 885 <p>
 886 Allow a complete transition away from SHA-1.
 887 </p>
 888 <div class="olist loweralpha"><ol class="loweralpha">
 889 <li>
 890 <p>
 891 Local metadata for SHA-1 compatibility can be removed from a
 892       repository if compatibility with SHA-1 is no longer needed.
 893 </p>
 894 </li>
 895 </ol></div>
 896 </li>
 897 <li>
 898 <p>
 899 Maintainability throughout the process.
 900 </p>
 901 <div class="olist loweralpha"><ol class="loweralpha">
 902 <li>
 903 <p>
 904 The object format is kept simple and consistent.
 905 </p>
 906 </li>
 907 <li>
 908 <p>
 909 Creation of a generalized repository conversion tool.
 910 </p>
 911 </li>
 912 </ol></div>
 913 </li>
 914 </ol></div>
 915 </div>
 916 </div>
 917 <div class="sect1">
 918 <h2 id="_non_goals">Non-Goals</h2>
 919 <div class="sectionbody">
 920 <div class="olist arabic"><ol class="arabic">
 921 <li>
 922 <p>
 923 Add SHA-256 support to Git protocol. This is valuable and the
 924    logical next step but it is out of scope for this initial design.
 925 </p>
 926 </li>
 927 <li>
 928 <p>
 929 Transparently improving the security of existing SHA-1 signed
 930    objects.
 931 </p>
 932 </li>
 933 <li>
 934 <p>
 935 Intermixing objects using multiple hash functions in a single
 936    repository.
 937 </p>
 938 </li>
 939 <li>
 940 <p>
 941 Taking the opportunity to fix other bugs in Git&#8217;s formats and
 942    protocols.
 943 </p>
 944 </li>
 945 <li>
 946 <p>
 947 Shallow clones and fetches into a SHA-256 repository. (This will
 948    change when we add SHA-256 support to Git protocol.)
 949 </p>
 950 </li>
 951 <li>
 952 <p>
 953 Skip fetching some submodules of a project into a SHA-256
 954    repository. (This also depends on SHA-256 support in Git
 955    protocol.)
 956 </p>
 957 </li>
 958 </ol></div>
 959 </div>
 960 </div>
 961 <div class="sect1">
 962 <h2 id="_overview">Overview</h2>
 963 <div class="sectionbody">
 964 <div class="paragraph"><p>We introduce a new repository format extension. Repositories with this
 965 extension enabled use SHA-256 instead of SHA-1 to name their objects.
 966 This affects both object names and object content&#8201;&#8212;&#8201;both the names
 967 of objects and all references to other objects within an object are
 968 switched to the new hash function.</p></div>
 969 <div class="paragraph"><p>SHA-256 repositories cannot be read by older versions of Git.</p></div>
 970 <div class="paragraph"><p>Alongside the packfile, a SHA-256 repository stores a bidirectional
 971 mapping between SHA-256 and SHA-1 object names. The mapping is generated
 972 locally and can be verified using "git fsck". Object lookups use this
 973 mapping to allow naming objects using either their SHA-1 and SHA-256 names
 974 interchangeably.</p></div>
 975 <div class="paragraph"><p>"git cat-file" and "git hash-object" gain options to display an object
 976 in its SHA-1 form and write an object given its SHA-1 form. This
 977 requires all objects referenced by that object to be present in the
 978 object database so that they can be named using the appropriate name
 979 (using the bidirectional hash mapping).</p></div>
 980 <div class="paragraph"><p>Fetches from a SHA-1 based server convert the fetched objects into
 981 SHA-256 form and record the mapping in the bidirectional mapping table
 982 (see below for details). Pushes to a SHA-1 based server convert the
 983 objects being pushed into SHA-1 form so the server does not have to be
 984 aware of the hash function the client is using.</p></div>
 985 </div>
 986 </div>
 987 <div class="sect1">
 988 <h2 id="_detailed_design">Detailed Design</h2>
 989 <div class="sectionbody">
 990 <div class="sect2">
 991 <h3 id="_repository_format_extension">Repository format extension</h3>
 992 <div class="paragraph"><p>A SHA-256 repository uses repository format version <code>1</code> (see
 993 Documentation/technical/repository-version.txt) with extensions
 994 <code>objectFormat</code> and <code>compatObjectFormat</code>:</p></div>
 995 <div class="literalblock">
 996 <div class="content">
 997 <pre><code>[core]
 998         repositoryFormatVersion = 1
 999 [extensions]
1000         objectFormat = sha256
1001         compatObjectFormat = sha1</code></pre>
1002 </div></div>
1003 <div class="paragraph"><p>The combination of setting <code>core.repositoryFormatVersion=1</code> and
1004 populating <code>extensions.*</code> ensures that all versions of Git later than
1005 <code>v0.99.9l</code> will die instead of trying to operate on the SHA-256
1006 repository, instead producing an error message.</p></div>
1007 <div class="literalblock">
1008 <div class="content">
1009 <pre><code># Between v0.99.9l and v2.7.0
1010 $ git status
1011 fatal: Expected git repo version &lt;= 0, found 1
1012 # After v2.7.0
1013 $ git status
1014 fatal: unknown repository extensions found:
1015         objectformat
1016         compatobjectformat</code></pre>
1017 </div></div>
1018 <div class="paragraph"><p>See the "Transition plan" section below for more details on these
1019 repository extensions.</p></div>
1020 </div>
1021 <div class="sect2">
1022 <h3 id="_object_names">Object names</h3>
1023 <div class="paragraph"><p>Objects can be named by their 40 hexadecimal digit SHA-1 name or 64
1024 hexadecimal digit SHA-256 name, plus names derived from those (see
1025 gitrevisions(7)).</p></div>
1026 <div class="paragraph"><p>The SHA-1 name of an object is the SHA-1 of the concatenation of its
1027 type, length, a nul byte, and the object&#8217;s SHA-1 content. This is the
1028 traditional &lt;sha1&gt; used in Git to name objects.</p></div>
1029 <div class="paragraph"><p>The SHA-256 name of an object is the SHA-256 of the concatenation of its
1030 type, length, a nul byte, and the object&#8217;s SHA-256 content.</p></div>
1031 </div>
1032 <div class="sect2">
1033 <h3 id="_object_format">Object format</h3>
1034 <div class="paragraph"><p>The content as a byte sequence of a tag, commit, or tree object named
1035 by SHA-1 and SHA-256 differ because an object named by SHA-256 name refers to
1036 other objects by their SHA-256 names and an object named by SHA-1 name
1037 refers to other objects by their SHA-1 names.</p></div>
1038 <div class="paragraph"><p>The SHA-256 content of an object is the same as its SHA-1 content, except
1039 that objects referenced by the object are named using their SHA-256 names
1040 instead of SHA-1 names. Because a blob object does not refer to any
1041 other object, its SHA-1 content and SHA-256 content are the same.</p></div>
1042 <div class="paragraph"><p>The format allows round-trip conversion between SHA-256 content and
1043 SHA-1 content.</p></div>
1044 </div>
1045 <div class="sect2">
1046 <h3 id="_object_storage">Object storage</h3>
1047 <div class="paragraph"><p>Loose objects use zlib compression and packed objects use the packed
1048 format described in <a href="../gitformat-pack.html">gitformat-pack(5)</a>, just like
1049 today. The content that is compressed and stored uses SHA-256 content
1050 instead of SHA-1 content.</p></div>
1051 </div>
1052 <div class="sect2">
1053 <h3 id="_pack_index">Pack index</h3>
1054 <div class="paragraph"><p>Pack index (.idx) files use a new v3 format that supports multiple
1055 hash functions. They have the following format (all integers are in
1056 network byte order):</p></div>
1057 <div class="ulist"><ul>
1058 <li>
1059 <p>
1060 A header appears at the beginning and consists of the following:
1061 </p>
1062 <div class="ulist"><ul>
1063 <li>
1064 <p>
1065 The 4-byte pack index signature: <em>\377t0c</em>
1066 </p>
1067 </li>
1068 <li>
1069 <p>
1070 4-byte version number: 3
1071 </p>
1072 </li>
1073 <li>
1074 <p>
1075 4-byte length of the header section, including the signature and
1076     version number
1077 </p>
1078 </li>
1079 <li>
1080 <p>
1081 4-byte number of objects contained in the pack
1082 </p>
1083 </li>
1084 <li>
1085 <p>
1086 4-byte number of object formats in this pack index: 2
1087 </p>
1088 </li>
1089 <li>
1090 <p>
1091 For each object format:
1092 </p>
1093 <div class="ulist"><ul>
1094 <li>
1095 <p>
1096 4-byte format identifier (e.g., <em>sha1</em> for SHA-1)
1097 </p>
1098 </li>
1099 <li>
1100 <p>
1101 4-byte length in bytes of shortened object names. This is the
1102       shortest possible length needed to make names in the shortened
1103       object name table unambiguous.
1104 </p>
1105 </li>
1106 <li>
1107 <p>
1108 4-byte integer, recording where tables relating to this format
1109       are stored in this index file, as an offset from the beginning.
1110 </p>
1111 </li>
1112 </ul></div>
1113 </li>
1114 <li>
1115 <p>
1116 4-byte offset to the trailer from the beginning of this file.
1117 </p>
1118 </li>
1119 <li>
1120 <p>
1121 Zero or more additional key/value pairs (4-byte key, 4-byte
1122     value). Only one key is supported: <em>PSRC</em>. See the "Loose objects
1123     and unreachable objects" section for supported values and how this
1124     is used.  All other keys are reserved. Readers must ignore
1125     unrecognized keys.
1126 </p>
1127 </li>
1128 </ul></div>
1129 </li>
1130 <li>
1131 <p>
1132 Zero or more NUL bytes. This can optionally be used to improve the
1133   alignment of the full object name table below.
1134 </p>
1135 </li>
1136 <li>
1137 <p>
1138 Tables for the first object format:
1139 </p>
1140 <div class="ulist"><ul>
1141 <li>
1142 <p>
1143 A sorted table of shortened object names.  These are prefixes of
1144     the names of all objects in this pack file, packed together
1145     without offset values to reduce the cache footprint of the binary
1146     search for a specific object name.
1147 </p>
1148 </li>
1149 <li>
1150 <p>
1151 A table of full object names in pack order. This allows resolving
1152     a reference to "the nth object in the pack file" (from a
1153     reachability bitmap or from the next table of another object
1154     format) to its object name.
1155 </p>
1156 </li>
1157 <li>
1158 <p>
1159 A table of 4-byte values mapping object name order to pack order.
1160     For an object in the table of sorted shortened object names, the
1161     value at the corresponding index in this table is the index in the
1162     previous table for that same object.
1163     This can be used to look up the object in reachability bitmaps or
1164     to look up its name in another object format.
1165 </p>
1166 </li>
1167 <li>
1168 <p>
1169 A table of 4-byte CRC32 values of the packed object data, in the
1170     order that the objects appear in the pack file. This is to allow
1171     compressed data to be copied directly from pack to pack during
1172     repacking without undetected data corruption.
1173 </p>
1174 </li>
1175 <li>
1176 <p>
1177 A table of 4-byte offset values. For an object in the table of
1178     sorted shortened object names, the value at the corresponding
1179     index in this table indicates where that object can be found in
1180     the pack file. These are usually 31-bit pack file offsets, but
1181     large offsets are encoded as an index into the next table with the
1182     most significant bit set.
1183 </p>
1184 </li>
1185 <li>
1186 <p>
1187 A table of 8-byte offset entries (empty for pack files less than
1188     2 GiB). Pack files are organized with heavily used objects toward
1189     the front, so most object references should not need to refer to
1190     this table.
1191 </p>
1192 </li>
1193 </ul></div>
1194 </li>
1195 <li>
1196 <p>
1197 Zero or more NUL bytes.
1198 </p>
1199 </li>
1200 <li>
1201 <p>
1202 Tables for the second object format, with the same layout as above,
1203   up to and not including the table of CRC32 values.
1204 </p>
1205 </li>
1206 <li>
1207 <p>
1208 Zero or more NUL bytes.
1209 </p>
1210 </li>
1211 <li>
1212 <p>
1213 The trailer consists of the following:
1214 </p>
1215 <div class="ulist"><ul>
1216 <li>
1217 <p>
1218 A copy of the 20-byte SHA-256 checksum at the end of the
1219     corresponding packfile.
1220 </p>
1221 </li>
1222 <li>
1223 <p>
1224 20-byte SHA-256 checksum of all of the above.
1225 </p>
1226 </li>
1227 </ul></div>
1228 </li>
1229 </ul></div>
1230 </div>
1231 <div class="sect2">
1232 <h3 id="_loose_object_index">Loose object index</h3>
1233 <div class="paragraph"><p>A new file $GIT_OBJECT_DIR/loose-object-idx contains information about
1234 all loose objects. Its format is</p></div>
1235 <div class="literalblock">
1236 <div class="content">
1237 <pre><code># loose-object-idx
1238 (sha256-name SP sha1-name LF)*</code></pre>
1239 </div></div>
1240 <div class="paragraph"><p>where the object names are in hexadecimal format. The file is not
1241 sorted.</p></div>
1242 <div class="paragraph"><p>The loose object index is protected against concurrent writes by a
1243 lock file $GIT_OBJECT_DIR/loose-object-idx.lock. To add a new loose
1244 object:</p></div>
1245 <div class="olist arabic"><ol class="arabic">
1246 <li>
1247 <p>
1248 Write the loose object to a temporary file, like today.
1249 </p>
1250 </li>
1251 <li>
1252 <p>
1253 Open loose-object-idx.lock with O_CREAT | O_EXCL to acquire the lock.
1254 </p>
1255 </li>
1256 <li>
1257 <p>
1258 Rename the loose object into place.
1259 </p>
1260 </li>
1261 <li>
1262 <p>
1263 Open loose-object-idx with O_APPEND and write the new object
1264 </p>
1265 </li>
1266 <li>
1267 <p>
1268 Unlink loose-object-idx.lock to release the lock.
1269 </p>
1270 </li>
1271 </ol></div>
1272 <div class="paragraph"><p>To remove entries (e.g. in "git pack-refs" or "git-prune"):</p></div>
1273 <div class="olist arabic"><ol class="arabic">
1274 <li>
1275 <p>
1276 Open loose-object-idx.lock with O_CREAT | O_EXCL to acquire the
1277    lock.
1278 </p>
1279 </li>
1280 <li>
1281 <p>
1282 Write the new content to loose-object-idx.lock.
1283 </p>
1284 </li>
1285 <li>
1286 <p>
1287 Unlink any loose objects being removed.
1288 </p>
1289 </li>
1290 <li>
1291 <p>
1292 Rename to replace loose-object-idx, releasing the lock.
1293 </p>
1294 </li>
1295 </ol></div>
1296 </div>
1297 <div class="sect2">
1298 <h3 id="_translation_table">Translation table</h3>
1299 <div class="paragraph"><p>The index files support a bidirectional mapping between SHA-1 names
1300 and SHA-256 names. The lookup proceeds similarly to ordinary object
1301 lookups. For example, to convert a SHA-1 name to a SHA-256 name:</p></div>
1302 <div class="olist arabic"><ol class="arabic">
1303 <li>
1304 <p>
1305 Look for the object in idx files. If a match is present in the
1306     idx&#8217;s sorted list of truncated SHA-1 names, then:
1307 </p>
1308 <div class="olist loweralpha"><ol class="loweralpha">
1309 <li>
1310 <p>
1311 Read the corresponding entry in the SHA-1 name order to pack
1312        name order mapping.
1313 </p>
1314 </li>
1315 <li>
1316 <p>
1317 Read the corresponding entry in the full SHA-1 name table to
1318        verify we found the right object. If it is, then
1319 </p>
1320 </li>
1321 <li>
1322 <p>
1323 Read the corresponding entry in the full SHA-256 name table.
1324        That is the object&#8217;s SHA-256 name.
1325 </p>
1326 </li>
1327 </ol></div>
1328 </li>
1329 <li>
1330 <p>
1331 Check for a loose object. Read lines from loose-object-idx until
1332     we find a match.
1333 </p>
1334 </li>
1335 </ol></div>
1336 <div class="paragraph"><p>Step (1) takes the same amount of time as an ordinary object lookup:
1337 O(number of packs * log(objects per pack)). Step (2) takes O(number of
1338 loose objects) time. To maintain good performance it will be necessary
1339 to keep the number of loose objects low. See the "Loose objects and
1340 unreachable objects" section below for more details.</p></div>
1341 <div class="paragraph"><p>Since all operations that make new objects (e.g., "git commit") add
1342 the new objects to the corresponding index, this mapping is possible
1343 for all objects in the object store.</p></div>
1344 </div>
1345 <div class="sect2">
1346 <h3 id="_reading_an_object_8217_s_sha_1_content">Reading an object&#8217;s SHA-1 content</h3>
1347 <div class="paragraph"><p>The SHA-1 content of an object can be read by converting all SHA-256 names
1348 of its SHA-256 content references to SHA-1 names using the translation table.</p></div>
1349 </div>
1350 <div class="sect2">
1351 <h3 id="_fetch">Fetch</h3>
1352 <div class="paragraph"><p>Fetching from a SHA-1 based server requires translating between SHA-1
1353 and SHA-256 based representations on the fly.</p></div>
1354 <div class="paragraph"><p>SHA-1s named in the ref advertisement that are present on the client
1355 can be translated to SHA-256 and looked up as local objects using the
1356 translation table.</p></div>
1357 <div class="paragraph"><p>Negotiation proceeds as today. Any "have"s generated locally are
1358 converted to SHA-1 before being sent to the server, and SHA-1s
1359 mentioned by the server are converted to SHA-256 when looking them up
1360 locally.</p></div>
1361 <div class="paragraph"><p>After negotiation, the server sends a packfile containing the
1362 requested objects. We convert the packfile to SHA-256 format using
1363 the following steps:</p></div>
1364 <div class="olist arabic"><ol class="arabic">
1365 <li>
1366 <p>
1367 index-pack: inflate each object in the packfile and compute its
1368    SHA-1. Objects can contain deltas in OBJ_REF_DELTA format against
1369    objects the client has locally. These objects can be looked up
1370    using the translation table and their SHA-1 content read as
1371    described above to resolve the deltas.
1372 </p>
1373 </li>
1374 <li>
1375 <p>
1376 topological sort: starting at the "want"s from the negotiation
1377    phase, walk through objects in the pack and emit a list of them,
1378    excluding blobs, in reverse topologically sorted order, with each
1379    object coming later in the list than all objects it references.
1380    (This list only contains objects reachable from the "wants". If the
1381    pack from the server contained additional extraneous objects, then
1382    they will be discarded.)
1383 </p>
1384 </li>
1385 <li>
1386 <p>
1387 convert to SHA-256: open a new SHA-256 packfile. Read the topologically
1388    sorted list just generated. For each object, inflate its
1389    SHA-1 content, convert to SHA-256 content, and write it to the SHA-256
1390    pack. Record the new SHA-1&#8592;&#8594;SHA-256 mapping entry for use in the idx.
1391 </p>
1392 </li>
1393 <li>
1394 <p>
1395 sort: reorder entries in the new pack to match the order of objects
1396    in the pack the server generated and include blobs. Write a SHA-256 idx
1397    file
1398 </p>
1399 </li>
1400 <li>
1401 <p>
1402 clean up: remove the SHA-1 based pack file, index, and
1403    topologically sorted list obtained from the server in steps 1
1404    and 2.
1405 </p>
1406 </li>
1407 </ol></div>
1408 <div class="paragraph"><p>Step 3 requires every object referenced by the new object to be in the
1409 translation table. This is why the topological sort step is necessary.</p></div>
1410 <div class="paragraph"><p>As an optimization, step 1 could write a file describing what non-blob
1411 objects each object it has inflated from the packfile references. This
1412 makes the topological sort in step 2 possible without inflating the
1413 objects in the packfile for a second time. The objects need to be
1414 inflated again in step 3, for a total of two inflations.</p></div>
1415 <div class="paragraph"><p>Step 4 is probably necessary for good read-time performance. "git
1416 pack-objects" on the server optimizes the pack file for good data
1417 locality (see Documentation/technical/pack-heuristics.txt).</p></div>
1418 <div class="paragraph"><p>Details of this process are likely to change. It will take some
1419 experimenting to get this to perform well.</p></div>
1420 </div>
1421 <div class="sect2">
1422 <h3 id="_push">Push</h3>
1423 <div class="paragraph"><p>Push is simpler than fetch because the objects referenced by the
1424 pushed objects are already in the translation table. The SHA-1 content
1425 of each object being pushed can be read as described in the "Reading
1426 an object&#8217;s SHA-1 content" section to generate the pack written by git
1427 send-pack.</p></div>
1428 </div>
1429 <div class="sect2">
1430 <h3 id="_signed_commits">Signed Commits</h3>
1431 <div class="paragraph"><p>We add a new field "gpgsig-sha256" to the commit object format to allow
1432 signing commits without relying on SHA-1. It is similar to the
1433 existing "gpgsig" field. Its signed payload is the SHA-256 content of the
1434 commit object with any "gpgsig" and "gpgsig-sha256" fields removed.</p></div>
1435 <div class="paragraph"><p>This means commits can be signed</p></div>
1436 <div class="olist arabic"><ol class="arabic">
1437 <li>
1438 <p>
1439 using SHA-1 only, as in existing signed commit objects
1440 </p>
1441 </li>
1442 <li>
1443 <p>
1444 using both SHA-1 and SHA-256, by using both gpgsig-sha256 and gpgsig
1445    fields.
1446 </p>
1447 </li>
1448 <li>
1449 <p>
1450 using only SHA-256, by only using the gpgsig-sha256 field.
1451 </p>
1452 </li>
1453 </ol></div>
1454 <div class="paragraph"><p>Old versions of "git verify-commit" can verify the gpgsig signature in
1455 cases (1) and (2) without modifications and view case (3) as an
1456 ordinary unsigned commit.</p></div>
1457 </div>
1458 <div class="sect2">
1459 <h3 id="_signed_tags">Signed Tags</h3>
1460 <div class="paragraph"><p>We add a new field "gpgsig-sha256" to the tag object format to allow
1461 signing tags without relying on SHA-1. Its signed payload is the
1462 SHA-256 content of the tag with its gpgsig-sha256 field and "-----BEGIN PGP
1463 SIGNATURE-----" delimited in-body signature removed.</p></div>
1464 <div class="paragraph"><p>This means tags can be signed</p></div>
1465 <div class="olist arabic"><ol class="arabic">
1466 <li>
1467 <p>
1468 using SHA-1 only, as in existing signed tag objects
1469 </p>
1470 </li>
1471 <li>
1472 <p>
1473 using both SHA-1 and SHA-256, by using gpgsig-sha256 and an in-body
1474    signature.
1475 </p>
1476 </li>
1477 <li>
1478 <p>
1479 using only SHA-256, by only using the gpgsig-sha256 field.
1480 </p>
1481 </li>
1482 </ol></div>
1483 </div>
1484 <div class="sect2">
1485 <h3 id="_mergetag_embedding">Mergetag embedding</h3>
1486 <div class="paragraph"><p>The mergetag field in the SHA-1 content of a commit contains the
1487 SHA-1 content of a tag that was merged by that commit.</p></div>
1488 <div class="paragraph"><p>The mergetag field in the SHA-256 content of the same commit contains the
1489 SHA-256 content of the same tag.</p></div>
1490 </div>
1491 <div class="sect2">
1492 <h3 id="_submodules">Submodules</h3>
1493 <div class="paragraph"><p>To convert recorded submodule pointers, you need to have the converted
1494 submodule repository in place. The translation table of the submodule
1495 can be used to look up the new hash.</p></div>
1496 </div>
1497 <div class="sect2">
1498 <h3 id="_loose_objects_and_unreachable_objects">Loose objects and unreachable objects</h3>
1499 <div class="paragraph"><p>Fast lookups in the loose-object-idx require that the number of loose
1500 objects not grow too high.</p></div>
1501 <div class="paragraph"><p>"git gc --auto" currently waits for there to be 6700 loose objects
1502 present before consolidating them into a packfile. We will need to
1503 measure to find a more appropriate threshold for it to use.</p></div>
1504 <div class="paragraph"><p>"git gc --auto" currently waits for there to be 50 packs present
1505 before combining packfiles. Packing loose objects more aggressively
1506 may cause the number of pack files to grow too quickly. This can be
1507 mitigated by using a strategy similar to Martin Fick&#8217;s exponential
1508 rolling garbage collection script:
1509 <a href="https://gerrit-review.googlesource.com/c/gerrit/+/35215">https://gerrit-review.googlesource.com/c/gerrit/+/35215</a></p></div>
1510 <div class="paragraph"><p>"git gc" currently expels any unreachable objects it encounters in
1511 pack files to loose objects in an attempt to prevent a race when
1512 pruning them (in case another process is simultaneously writing a new
1513 object that refers to the about-to-be-deleted object). This leads to
1514 an explosion in the number of loose objects present and disk space
1515 usage due to the objects in delta form being replaced with independent
1516 loose objects.  Worse, the race is still present for loose objects.</p></div>
1517 <div class="paragraph"><p>Instead, "git gc" will need to move unreachable objects to a new
1518 packfile marked as UNREACHABLE_GARBAGE (using the PSRC field; see
1519 below). To avoid the race when writing new objects referring to an
1520 about-to-be-deleted object, code paths that write new objects will
1521 need to copy any objects from UNREACHABLE_GARBAGE packs that they
1522 refer to new, non-UNREACHABLE_GARBAGE packs (or loose objects).
1523 UNREACHABLE_GARBAGE are then safe to delete if their creation time (as
1524 indicated by the file&#8217;s mtime) is long enough ago.</p></div>
1525 <div class="paragraph"><p>To avoid a proliferation of UNREACHABLE_GARBAGE packs, they can be
1526 combined under certain circumstances. If "gc.garbageTtl" is set to
1527 greater than one day, then packs created within a single calendar day,
1528 UTC, can be coalesced together. The resulting packfile would have an
1529 mtime before midnight on that day, so this makes the effective maximum
1530 ttl the garbageTtl + 1 day. If "gc.garbageTtl" is less than one day,
1531 then we divide the calendar day into intervals one-third of that ttl
1532 in duration. Packs created within the same interval can be coalesced
1533 together. The resulting packfile would have an mtime before the end of
1534 the interval, so this makes the effective maximum ttl equal to the
1535 garbageTtl * 4/3.</p></div>
1536 <div class="paragraph"><p>This rule comes from Thirumala Reddy Mutchukota&#8217;s JGit change
1537 <a href="https://git.eclipse.org/r/90465">https://git.eclipse.org/r/90465</a>.</p></div>
1538 <div class="paragraph"><p>The UNREACHABLE_GARBAGE setting goes in the PSRC field of the pack
1539 index. More generally, that field indicates where a pack came from:</p></div>
1540 <div class="ulist"><ul>
1541 <li>
1542 <p>
1543 1 (PACK_SOURCE_RECEIVE) for a pack received over the network
1544 </p>
1545 </li>
1546 <li>
1547 <p>
1548 2 (PACK_SOURCE_AUTO) for a pack created by a lightweight
1549    "gc --auto" operation
1550 </p>
1551 </li>
1552 <li>
1553 <p>
1554 3 (PACK_SOURCE_GC) for a pack created by a full gc
1555 </p>
1556 </li>
1557 <li>
1558 <p>
1559 4 (PACK_SOURCE_UNREACHABLE_GARBAGE) for potential garbage
1560    discovered by gc
1561 </p>
1562 </li>
1563 <li>
1564 <p>
1565 5 (PACK_SOURCE_INSERT) for locally created objects that were
1566    written directly to a pack file, e.g. from "git add ."
1567 </p>
1568 </li>
1569 </ul></div>
1570 <div class="paragraph"><p>This information can be useful for debugging and for "gc --auto" to
1571 make appropriate choices about which packs to coalesce.</p></div>
1572 </div>
1573 </div>
1574 </div>
1575 <div class="sect1">
1576 <h2 id="_caveats">Caveats</h2>
1577 <div class="sectionbody">
1578 <div class="sect2">
1579 <h3 id="_invalid_objects">Invalid objects</h3>
1580 <div class="paragraph"><p>The conversion from SHA-1 content to SHA-256 content retains any
1581 brokenness in the original object (e.g., tree entry modes encoded with
1582 leading 0, tree objects whose paths are not sorted correctly, and
1583 commit objects without an author or committer). This is a deliberate
1584 feature of the design to allow the conversion to round-trip.</p></div>
1585 <div class="paragraph"><p>More profoundly broken objects (e.g., a commit with a truncated "tree"
1586 header line) cannot be converted but were not usable by current Git
1587 anyway.</p></div>
1588 </div>
1589 <div class="sect2">
1590 <h3 id="_shallow_clone_and_submodules">Shallow clone and submodules</h3>
1591 <div class="paragraph"><p>Because it requires all referenced objects to be available in the
1592 locally generated translation table, this design does not support
1593 shallow clone or unfetched submodules. Protocol improvements might
1594 allow lifting this restriction.</p></div>
1595 </div>
1596 <div class="sect2">
1597 <h3 id="_alternates">Alternates</h3>
1598 <div class="paragraph"><p>For the same reason, a SHA-256 repository cannot borrow objects from a
1599 SHA-1 repository using objects/info/alternates or
1600 $GIT_ALTERNATE_OBJECT_REPOSITORIES.</p></div>
1601 </div>
1602 <div class="sect2">
1603 <h3 id="_git_notes">git notes</h3>
1604 <div class="paragraph"><p>The "git notes" tool annotates objects using their SHA-1 name as key.
1605 This design does not describe a way to migrate notes trees to use
1606 SHA-256 names. That migration is expected to happen separately (for
1607 example using a file at the root of the notes tree to describe which
1608 hash it uses).</p></div>
1609 </div>
1610 <div class="sect2">
1611 <h3 id="_server_side_cost">Server-side cost</h3>
1612 <div class="paragraph"><p>Until Git protocol gains SHA-256 support, using SHA-256 based storage
1613 on public-facing Git servers is strongly discouraged. Once Git
1614 protocol gains SHA-256 support, SHA-256 based servers are likely not
1615 to support SHA-1 compatibility, to avoid what may be a very expensive
1616 hash re-encode during clone and to encourage peers to modernize.</p></div>
1617 <div class="paragraph"><p>The design described here allows fetches by SHA-1 clients of a
1618 personal SHA-256 repository because it&#8217;s not much more difficult than
1619 allowing pushes from that repository. This support needs to be guarded
1620 by a configuration option&#8201;&#8212;&#8201;servers like git.kernel.org that serve a
1621 large number of clients would not be expected to bear that cost.</p></div>
1622 </div>
1623 <div class="sect2">
1624 <h3 id="_meaning_of_signatures">Meaning of signatures</h3>
1625 <div class="paragraph"><p>The signed payload for signed commits and tags does not explicitly
1626 name the hash used to identify objects. If some day Git adopts a new
1627 hash function with the same length as the current SHA-1 (40
1628 hexadecimal digit) or SHA-256 (64 hexadecimal digit) objects then the
1629 intent behind the PGP signed payload in an object signature is
1630 unclear:</p></div>
1631 <div class="literalblock">
1632 <div class="content">
1633 <pre><code>object e7e07d5a4fcc2a203d9873968ad3e6bd4d7419d7
1634 type commit
1635 tag v2.12.0
1636 tagger Junio C Hamano &lt;gitster@pobox.com&gt; 1487962205 -0800</code></pre>
1637 </div></div>
1638 <div class="literalblock">
1639 <div class="content">
1640 <pre><code>Git 2.12</code></pre>
1641 </div></div>
1642 <div class="paragraph"><p>Does this mean Git v2.12.0 is the commit with SHA-1 name
1643 e7e07d5a4fcc2a203d9873968ad3e6bd4d7419d7 or the commit with
1644 new-40-digit-hash-name e7e07d5a4fcc2a203d9873968ad3e6bd4d7419d7?</p></div>
1645 <div class="paragraph"><p>Fortunately SHA-256 and SHA-1 have different lengths. If Git starts
1646 using another hash with the same length to name objects, then it will
1647 need to change the format of signed payloads using that hash to
1648 address this issue.</p></div>
1649 </div>
1650 <div class="sect2">
1651 <h3 id="_object_names_on_the_command_line">Object names on the command line</h3>
1652 <div class="paragraph"><p>To support the transition (see Transition plan below), this design
1653 supports four different modes of operation:</p></div>
1654 <div class="olist arabic"><ol class="arabic">
1655 <li>
1656 <p>
1657 ("dark launch") Treat object names input by the user as SHA-1 and
1658     convert any object names written to output to SHA-1, but store
1659     objects using SHA-256.  This allows users to test the code with no
1660     visible behavior change except for performance.  This allows
1661     running even tests that assume the SHA-1 hash function, to
1662     sanity-check the behavior of the new mode.
1663 </p>
1664 </li>
1665 <li>
1666 <p>
1667 ("early transition") Allow both SHA-1 and SHA-256 object names in
1668     input. Any object names written to output use SHA-1. This allows
1669     users to continue to make use of SHA-1 to communicate with peers
1670     (e.g. by email) that have not migrated yet and prepares for mode 3.
1671 </p>
1672 </li>
1673 <li>
1674 <p>
1675 ("late transition") Allow both SHA-1 and SHA-256 object names in
1676     input. Any object names written to output use SHA-256. In this
1677     mode, users are using a more secure object naming method by
1678     default.  The disruption is minimal as long as most of their peers
1679     are in mode 2 or mode 3.
1680 </p>
1681 </li>
1682 <li>
1683 <p>
1684 ("post-transition") Treat object names input by the user as
1685     SHA-256 and write output using SHA-256. This is safer than mode 3
1686     because there is less risk that input is incorrectly interpreted
1687     using the wrong hash function.
1688 </p>
1689 </li>
1690 </ol></div>
1691 <div class="paragraph"><p>The mode is specified in configuration.</p></div>
1692 <div class="paragraph"><p>The user can also explicitly specify which format to use for a
1693 particular revision specifier and for output, overriding the mode. For
1694 example:</p></div>
1695 <div class="literalblock">
1696 <div class="content">
1697 <pre><code>git --output-format=sha1 log abac87a^{sha1}..f787cac^{sha256}</code></pre>
1698 </div></div>
1699 </div>
1700 </div>
1701 </div>
1702 <div class="sect1">
1703 <h2 id="_transition_plan">Transition plan</h2>
1704 <div class="sectionbody">
1705 <div class="paragraph"><p>Some initial steps can be implemented independently of one another:</p></div>
1706 <div class="ulist"><ul>
1707 <li>
1708 <p>
1709 adding a hash function API (vtable)
1710 </p>
1711 </li>
1712 <li>
1713 <p>
1714 teaching fsck to tolerate the gpgsig-sha256 field
1715 </p>
1716 </li>
1717 <li>
1718 <p>
1719 excluding gpgsig-* from the fields copied by "git commit --amend"
1720 </p>
1721 </li>
1722 <li>
1723 <p>
1724 annotating tests that depend on SHA-1 values with a SHA1 test
1725   prerequisite
1726 </p>
1727 </li>
1728 <li>
1729 <p>
1730 using "struct object_id", GIT_MAX_RAWSZ, and GIT_MAX_HEXSZ
1731   consistently instead of "unsigned char *" and the hardcoded
1732   constants 20 and 40.
1733 </p>
1734 </li>
1735 <li>
1736 <p>
1737 introducing index v3
1738 </p>
1739 </li>
1740 <li>
1741 <p>
1742 adding support for the PSRC field and safer object pruning
1743 </p>
1744 </li>
1745 </ul></div>
1746 <div class="paragraph"><p>The first user-visible change is the introduction of the objectFormat
1747 extension (without compatObjectFormat). This requires:</p></div>
1748 <div class="ulist"><ul>
1749 <li>
1750 <p>
1751 teaching fsck about this mode of operation
1752 </p>
1753 </li>
1754 <li>
1755 <p>
1756 using the hash function API (vtable) when computing object names
1757 </p>
1758 </li>
1759 <li>
1760 <p>
1761 signing objects and verifying signatures
1762 </p>
1763 </li>
1764 <li>
1765 <p>
1766 rejecting attempts to fetch from or push to an incompatible
1767   repository
1768 </p>
1769 </li>
1770 </ul></div>
1771 <div class="paragraph"><p>Next comes introduction of compatObjectFormat:</p></div>
1772 <div class="ulist"><ul>
1773 <li>
1774 <p>
1775 implementing the loose-object-idx
1776 </p>
1777 </li>
1778 <li>
1779 <p>
1780 translating object names between object formats
1781 </p>
1782 </li>
1783 <li>
1784 <p>
1785 translating object content between object formats
1786 </p>
1787 </li>
1788 <li>
1789 <p>
1790 generating and verifying signatures in the compat format
1791 </p>
1792 </li>
1793 <li>
1794 <p>
1795 adding appropriate index entries when adding a new object to the
1796   object store
1797 </p>
1798 </li>
1799 <li>
1800 <p>
1801 --output-format option
1802 </p>
1803 </li>
1804 <li>
1805 <p>
1806 </p>
1807 </li>
1808 <li>
1809 <p>
1810 configuration to specify default input and output format (see
1811   "Object names on the command line" above)
1812 </p>
1813 </li>
1814 </ul></div>
1815 <div class="paragraph"><p>The next step is supporting fetches and pushes to SHA-1 repositories:</p></div>
1816 <div class="ulist"><ul>
1817 <li>
1818 <p>
1819 allow pushes to a repository using the compat format
1820 </p>
1821 </li>
1822 <li>
1823 <p>
1824 generate a topologically sorted list of the SHA-1 names of fetched
1825   objects
1826 </p>
1827 </li>
1828 <li>
1829 <p>
1830 convert the fetched packfile to SHA-256 format and generate an idx
1831   file
1832 </p>
1833 </li>
1834 <li>
1835 <p>
1836 re-sort to match the order of objects in the fetched packfile
1837 </p>
1838 </li>
1839 </ul></div>
1840 <div class="paragraph"><p>The infrastructure supporting fetch also allows converting an existing
1841 repository. In converted repositories and new clones, end users can
1842 gain support for the new hash function without any visible change in
1843 behavior (see "dark launch" in the "Object names on the command line"
1844 section). In particular this allows users to verify SHA-256 signatures
1845 on objects in the repository, and it should ensure the transition code
1846 is stable in production in preparation for using it more widely.</p></div>
1847 <div class="paragraph"><p>Over time projects would encourage their users to adopt the "early
1848 transition" and then "late transition" modes to take advantage of the
1849 new, more futureproof SHA-256 object names.</p></div>
1850 <div class="paragraph"><p>When objectFormat and compatObjectFormat are both set, commands
1851 generating signatures would generate both SHA-1 and SHA-256 signatures
1852 by default to support both new and old users.</p></div>
1853 <div class="paragraph"><p>In projects using SHA-256 heavily, users could be encouraged to adopt
1854 the "post-transition" mode to avoid accidentally making implicit use
1855 of SHA-1 object names.</p></div>
1856 <div class="paragraph"><p>Once a critical mass of users have upgraded to a version of Git that
1857 can verify SHA-256 signatures and have converted their existing
1858 repositories to support verifying them, we can add support for a
1859 setting to generate only SHA-256 signatures. This is expected to be at
1860 least a year later.</p></div>
1861 <div class="paragraph"><p>That is also a good moment to advertise the ability to convert
1862 repositories to use SHA-256 only, stripping out all SHA-1 related
1863 metadata. This improves performance by eliminating translation
1864 overhead and security by avoiding the possibility of accidentally
1865 relying on the safety of SHA-1.</p></div>
1866 <div class="paragraph"><p>Updating Git&#8217;s protocols to allow a server to specify which hash
1867 functions it supports is also an important part of this transition. It
1868 is not discussed in detail in this document but this transition plan
1869 assumes it happens. :)</p></div>
1870 </div>
1871 </div>
1872 <div class="sect1">
1873 <h2 id="_alternatives_considered">Alternatives considered</h2>
1874 <div class="sectionbody">
1875 <div class="sect2">
1876 <h3 id="_upgrading_everyone_working_on_a_particular_project_on_a_flag_day">Upgrading everyone working on a particular project on a flag day</h3>
1877 <div class="paragraph"><p>Projects like the Linux kernel are large and complex enough that
1878 flipping the switch for all projects based on the repository at once
1879 is infeasible.</p></div>
1880 <div class="paragraph"><p>Not only would all developers and server operators supporting
1881 developers have to switch on the same flag day, but supporting tooling
1882 (continuous integration, code review, bug trackers, etc) would have to
1883 be adapted as well. This also makes it difficult to get early feedback
1884 from some project participants testing before it is time for mass
1885 adoption.</p></div>
1886 </div>
1887 <div class="sect2">
1888 <h3 id="_using_hash_functions_in_parallel">Using hash functions in parallel</h3>
1889 <div class="paragraph"><p>(e.g. <a href="https://lore.kernel.org/git/22708.8913.864049.452252@chiark.greenend.org.uk/">https://lore.kernel.org/git/22708.8913.864049.452252@chiark.greenend.org.uk/</a> )
1890 Objects newly created would be addressed by the new hash, but inside
1891 such an object (e.g. commit) it is still possible to address objects
1892 using the old hash function.</p></div>
1893 <div class="ulist"><ul>
1894 <li>
1895 <p>
1896 You cannot trust its history (needed for bisectability) in the
1897   future without further work
1898 </p>
1899 </li>
1900 <li>
1901 <p>
1902 Maintenance burden as the number of supported hash functions grows
1903   (they will never go away, so they accumulate). In this proposal, by
1904   comparison, converted objects lose all references to SHA-1.
1905 </p>
1906 </li>
1907 </ul></div>
1908 </div>
1909 <div class="sect2">
1910 <h3 id="_signed_objects_with_multiple_hashes">Signed objects with multiple hashes</h3>
1911 <div class="paragraph"><p>Instead of introducing the gpgsig-sha256 field in commit and tag objects
1912 for SHA-256 content based signatures, an earlier version of this design
1913 added "hash sha256 &lt;SHA-256 name&gt;" fields to strengthen the existing
1914 SHA-1 content based signatures.</p></div>
1915 <div class="paragraph"><p>In other words, a single signature was used to attest to the object
1916 content using both hash functions. This had some advantages:</p></div>
1917 <div class="ulist"><ul>
1918 <li>
1919 <p>
1920 Using one signature instead of two speeds up the signing process.
1921 </p>
1922 </li>
1923 <li>
1924 <p>
1925 Having one signed payload with both hashes allows the signer to
1926   attest to the SHA-1 name and SHA-256 name referring to the same object.
1927 </p>
1928 </li>
1929 <li>
1930 <p>
1931 All users consume the same signature. Broken signatures are likely
1932   to be detected quickly using current versions of git.
1933 </p>
1934 </li>
1935 </ul></div>
1936 <div class="paragraph"><p>However, it also came with disadvantages:</p></div>
1937 <div class="ulist"><ul>
1938 <li>
1939 <p>
1940 Verifying a signed object requires access to the SHA-1 names of all
1941   objects it references, even after the transition is complete and
1942   translation table is no longer needed for anything else. To support
1943   this, the design added fields such as "hash sha1 tree &lt;SHA-1 name&gt;"
1944   and "hash sha1 parent &lt;SHA-1 name&gt;" to the SHA-256 content of a signed
1945   commit, complicating the conversion process.
1946 </p>
1947 </li>
1948 <li>
1949 <p>
1950 Allowing signed objects without a SHA-1 (for after the transition is
1951   complete) complicated the design further, requiring a "nohash sha1"
1952   field to suppress including "hash sha1" fields in the SHA-256 content
1953   and signed payload.
1954 </p>
1955 </li>
1956 </ul></div>
1957 </div>
1958 <div class="sect2">
1959 <h3 id="_lazily_populated_translation_table">Lazily populated translation table</h3>
1960 <div class="paragraph"><p>Some of the work of building the translation table could be deferred to
1961 push time, but that would significantly complicate and slow down pushes.
1962 Calculating the SHA-1 name at object creation time at the same time it is
1963 being streamed to disk and having its SHA-256 name calculated should be
1964 an acceptable cost.</p></div>
1965 </div>
1966 </div>
1967 </div>
1968 <div class="sect1">
1969 <h2 id="_document_history">Document History</h2>
1970 <div class="sectionbody">
1971 <div class="paragraph"><p>2017-03-03
1972 <a href="mailto:bmwill@google.com">bmwill@google.com</a>, <a href="mailto:jonathantanmy@google.com">jonathantanmy@google.com</a>, <a href="mailto:jrnieder@gmail.com">jrnieder@gmail.com</a>,
1973 <a href="mailto:sbeller@google.com">sbeller@google.com</a></p></div>
1974 <div class="ulist"><ul>
1975 <li>
1976 <p>
1977 Initial version sent to <a href="https://lore.kernel.org/git/20170304011251.GA26789@aiede.mtv.corp.google.com">https://lore.kernel.org/git/20170304011251.GA26789@aiede.mtv.corp.google.com</a>
1978 </p>
1979 </li>
1980 </ul></div>
1981 <div class="paragraph"><p>2017-03-03 <a href="mailto:jrnieder@gmail.com">jrnieder@gmail.com</a>
1982 Incorporated suggestions from jonathantanmy and sbeller:</p></div>
1983 <div class="ulist"><ul>
1984 <li>
1985 <p>
1986 Describe purpose of signed objects with each hash type
1987 </p>
1988 </li>
1989 <li>
1990 <p>
1991 Redefine signed object verification using object content under the
1992   first hash function
1993 </p>
1994 </li>
1995 </ul></div>
1996 <div class="paragraph"><p>2017-03-06 <a href="mailto:jrnieder@gmail.com">jrnieder@gmail.com</a></p></div>
1997 <div class="ulist"><ul>
1998 <li>
1999 <p>
2000 Use SHA3-256 instead of SHA2 (thanks, Linus and brian m. carlson).[1][2]
2001 </p>
2002 </li>
2003 <li>
2004 <p>
2005 Make SHA3-based signatures a separate field, avoiding the need for
2006   "hash" and "nohash" fields (thanks to peff[3]).
2007 </p>
2008 </li>
2009 <li>
2010 <p>
2011 Add a sorting phase to fetch (thanks to Junio for noticing the need
2012   for this).
2013 </p>
2014 </li>
2015 <li>
2016 <p>
2017 Omit blobs from the topological sort during fetch (thanks to peff).
2018 </p>
2019 </li>
2020 <li>
2021 <p>
2022 Discuss alternates, git notes, and git servers in the caveats
2023   section (thanks to Junio Hamano, brian m. carlson[4], and Shawn
2024   Pearce).
2025 </p>
2026 </li>
2027 <li>
2028 <p>
2029 Clarify language throughout (thanks to various commenters,
2030   especially Junio).
2031 </p>
2032 </li>
2033 </ul></div>
2034 <div class="paragraph"><p>2017-09-27 <a href="mailto:jrnieder@gmail.com">jrnieder@gmail.com</a>, <a href="mailto:sbeller@google.com">sbeller@google.com</a></p></div>
2035 <div class="ulist"><ul>
2036 <li>
2037 <p>
2038 Use placeholder NewHash instead of SHA3-256
2039 </p>
2040 </li>
2041 <li>
2042 <p>
2043 Describe criteria for picking a hash function.
2044 </p>
2045 </li>
2046 <li>
2047 <p>
2048 Include a transition plan (thanks especially to Brandon Williams
2049   for fleshing these ideas out)
2050 </p>
2051 </li>
2052 <li>
2053 <p>
2054 Define the translation table (thanks, Shawn Pearce[5], Jonathan
2055   Tan, and Masaya Suzuki)
2056 </p>
2057 </li>
2058 <li>
2059 <p>
2060 Avoid loose object overhead by packing more aggressively in
2061   "git gc --auto"
2062 </p>
2063 </li>
2064 </ul></div>
2065 <div class="paragraph"><p>Later history:</p></div>
2066 <div class="ulist"><ul>
2067 <li>
2068 <p>
2069 See the history of this file in git.git for the history of subsequent
2070   edits. This document history is no longer being maintained as it
2071   would now be superfluous to the commit log
2072 </p>
2073 </li>
2074 </ul></div>
2075 <div class="paragraph"><p>References:</p></div>
2076 <div class="literalblock">
2077 <div class="content">
2078 <pre><code>[1] https://lore.kernel.org/git/CA+55aFzJtejiCjV0e43+9oR3QuJK2PiFiLQemytoLpyJWe6P9w@mail.gmail.com/
2079 [2] https://lore.kernel.org/git/CA+55aFz+gkAsDZ24zmePQuEs1XPS9BP_s8O7Q4wQ7LV7X5-oDA@mail.gmail.com/
2080 [3] https://lore.kernel.org/git/20170306084353.nrns455dvkdsfgo5@sigill.intra.peff.net/
2081 [4] https://lore.kernel.org/git/20170304224936.rqqtkdvfjgyezsht@genre.crustytoothpaste.net
2082 [5] https://lore.kernel.org/git/CAJo=hJtoX9=AyLHHpUJS7fueV9ciZ_MNpnEPHUz8Whui6g9F0A@mail.gmail.com/</code></pre>
2083 </div></div>
2084 </div>
2085 </div>
2086 </div>
2087 <div id="footnotes"><hr /></div>
2088 <div id="footer">
2089 <div id="footer-text">
2090 Last updated
2091  2023-01-30 14:44:53 PST
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