From 76941a609f7f026379d15e0216bb8f051d74650a Mon Sep 17 00:00:00 2001 From: Arvid Norberg Date: Mon, 6 Jan 2014 04:31:56 +0000 Subject: [PATCH] update dht_sec document --- docs/complete_bit_prefixes.png | Bin 0 -> 7795 bytes docs/dht_sec.html | 84 ++++++++++++++++++++++--------- docs/dht_sec.rst | 87 ++++++++++++++++++++++++++------- docs/hash_distribution.png | Bin 0 -> 9035 bytes docs/ip_id_v4.png | Bin 5798 -> 5825 bytes docs/ip_id_v6.png | Bin 6389 -> 6416 bytes docs/ips.py | 56 --------------------- 7 files changed, 130 insertions(+), 97 deletions(-) create mode 100644 docs/complete_bit_prefixes.png create mode 100644 docs/hash_distribution.png delete mode 100644 docs/ips.py diff --git a/docs/complete_bit_prefixes.png b/docs/complete_bit_prefixes.png new file mode 100644 index 0000000000000000000000000000000000000000..3bc73874d9f321c8e4848f6084c41fbc1a3d0371 GIT binary patch literal 7795 zcmZ8`2{@GB`}bfbW5f&w5#}+JwR}-oiZKjE%9^NTYbq@YktGI08SBiDt+Fc>g_5OY z%QCVgk`h@Oq!=3eGR^<-eSg3A{a^2OJSK$=+N2pXJ4ikoEFA-JoX5MSJhO;X7dIi;A{{Q_#1&u!0u0Q z{`WgUM#dKu1p?`g?fDJa7Lf<(`N%`3@*?aOA|rX`kc~+Hne@O{S8DDEFE0=0 zMFPBs5H^K<7t%kHH$t~Fak`=viH1a?(UB(L6B+5z80o=_+?L;jmj^x`ygXlCV27h59YFy?v*?iH#eJLaWU)1RP62f*1wua&IUI8e!#Z5d1c}<0>%kOnZgxg{`U`) z?_Ae3?(J$cDlT)LCjwJ!iC*8Lp(Z9yc*&UV*!ux_hW@R+jb)BYVx%Y)+7$nemI_n8 ztd$C@Jz~qf|DNco4AU;=ovh7iW!7S)n{H)kGK)1|Cg|3-w-MzT39Pr8G{&u~iJ!#c zP(`O#U5fx)3mbGTfGy$5qtxJ8R9K2Bo|$(7#d1{OwuQ-&Qv#klpsDCA=O{L zwug-ziU7{<3la!{`{sd+ZfB$48B~j689jx_L%-qtx$#ATgce&m4ai$#!w1vQ4GgWF|F ziFY2o2AD(!g<5)FMkqdDoo_BWG7m_c6`Q$=Q-v$KUq-v2V-ii}!4H|}qvQwdAlIIb z(x&+0G1o7Go}?oR_{eb7p!{Nbw4{-NUCi4ji81CIvUGp^=R;nDV?lZ(pM$a@CzmEs z{ghYvJI zt#cXqTo3ab!+b0FT{6~Z{FnvA$BN>DSIdSaaup5mD~PohlU%pNo%Mv z>GSd69Jf|?w;vXsO$8#z-&wQK^rZN=iT_a8MY*kc0O{d<3_t8ih@`dMNEh=i^C~`m z)gcl1rn2(DKhGOydm*Xox=EW@ONJS-9ihS2l)F%lwYRdw74$Uw?a&ZU3y>t8ZDZXi z?%6dMWK6TB9t$`9M=+k_5cZkLkB5Y#UXHZtf#2+N2p0o$H+)S6Jc7^KR`PxctfmZLlmYs@^c^4xL`22UbpAG+rY!M zQieHY!NODg-iOpvN}aMOgz1jr>$hJeACKjT>>qtK0WHYwA?rg9W}{;Ox(Xc5ps4O| zRY}rZE^KzAFmvrQbA1zr7D+=y#0y+LXb{3~evML_aQfbod#Lm9r9yWVu7j7KQ|gSK zb?aJC^*eQ!fg*ol4youVle43xA!B}GR!0@Cl%Vumh{K+XJ%*>M|5`RVx&+~#XPy~| zNpMchzDz+M#AHLCFiZj*t>}Lz2QXNe{pomH*|>#w6&cti5KfxOrAky3j)X8p+ZQ*g zuSSQXuoPJe@EU`TA?C$nBz10xsZrlMg!K^0T^JW;sFEGfg2q1_oAcP+Y zZUO{|H{Q2f5T?jw{1!xjx@v;D5`;KO=+E3W6a21IC?QEgp2F96BkS6|EWYREjpU_Y z(XK6dz#=~y>gKk55BcKR-R%1&clc%m_am&Q3Ph@aHS^s$;!@a}kt2WdZTVu;7FION zd#fYgy}pW&@+oiiQVIV|bP?c$I)AZ7VPT5uCPJL24jxetI?qa%C`Tpo5$T5HCAUgN zT+~kfHk$JuoZW(jqLQwXBBf3tSHneV8NVPTeIgqK0G12lPai_5JBhp8`z$rt7C_+F z{(N;n>0l=*pOyt~6Y&SqCP1JdIEHIY#zgV>9&ptYxNF=D3SChR&0<-Kd=XUSsxJN* zXhb4R4IPx3S2i=26gJEr1EEL%oKmue`mJwwyu95}cq`c88`#VO>rT&Do_`HHIXccp z?jQO`TXyRw7{X%Td^q+TD7!<>W1gvOlB*Z6v{o~|M5@BXmWfglKM9^1Y#pixO;q0f za!ye*i^Z)P9Pg}-3|hG0Sne%=QwfLu3a_t?cFyBIZkCWR!%sbY2$b~?f=hbwD$b6z zf5>AQ-*{^&vf~5Oj#Q87xV8Qogk@ATDx}bE{YkOXi)`Sckt&>F`-WemnI;?XJ9
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  1. Node ID restriction
  2. bootstrapping
    3. -
  3. enforcement
    4. -
  4. backwards compatibility and transition
    5. -
  5. forward compatibility
    6. +
  6. rationale
    7. +
  7. enforcement
    8. +
  8. backwards compatibility and transition
    9. +
  9. forward compatibility
    10. @@ -112,20 +113,20 @@ distribution of the IDs remoain uniform. This is why CRC32 was chosen as the hash function. See comparisons of hash functions.

    The expression to calculate a valid ID prefix (from an IPv4 address) is:

    -crc32((ip & 0x01071f7f) .. r)
+crc32((ip & 0x030f3fff) .. r)

    And for an IPv6 address (ip is the high 64 bits of the address):

    -crc32((ip & 0x000103070f1f3f7f) ..  r)
+crc32((ip & 0x0103070f1f3f7fff) ..  r)

    r is a random number in the range [0, 7]. The resulting integer, representing the masked IP address is supposed to be big-endian before hashed. The ".." means concatenation.

    The details of implementing this is to evaluate the expression, store the result in a big endian 64 bit integer and hash those 8 bytes with CRC32.

    -

    The first 4 bytes of the node ID used in the DHT MUST match the first 4 -bytes in the resulting hash. The last byte of the hash MUST match the -random number (r) used to generate the hash.

    +

    The first (most significant) 21 bits of the node ID used in the DHT MUST +match the first 21 bits of the resulting hash. The last byte of the hash MUST +match the random number (r) used to generate the hash.

    ip_id_v4.png ip_id_v6.png

    Example code code for calculating a valid node ID:

    @@ -134,39 +135,40 @@ uint8_t* ip; // our external IPv4 or IPv6 address (network byte order) int num_octets; // the number of octets to consider in ip (4 or 8) uint8_t node_id[20]; // resulting node ID -uint8_t v4mask[] = { 0x01, 0x07, 0x1f, 0x7f }; -uint8_t v6mask[] = { 0x00, 0x01, 0x03, 0x07, 0x0f, 0x1f, 0x3f, 0x7f }; -uint8_t* mask = num_octets == 4 ? v4_mask : v8_mask; +uint8_t v4_mask[] = { 0x03, 0x0f, 0x3f, 0xff }; +uint8_t v6_mask[] = { 0x01, 0x03, 0x07, 0x0f, 0x1f, 0x3f, 0x7f, 0xff }; +uint8_t* mask = num_octets == 4 ? v4_mask : v6_mask; for (int i = 0; i < num_octets; ++i) ip[i] &= mask[i]; -uint32_t rand = rand() & 0xff; +uint32_t rand = std::rand() & 0xff; uint8_t r = rand & 0x7; -uint32_t crc = crc32(0, NULL, 0); +uint32_t crc = crc32(0, nullptr, 0); crc = crc32(crc, ip, num_octets); crc = crc32(crc, &r, 1); +// only take the top 21 bits from crc node_id[0] = (crc >> 24) & 0xff; node_id[1] = (crc >> 16) & 0xff; -node_id[2] = (crc >> 8) & 0xff; -node_id[3] = crc & 0xff; -for (int i = 4; i < 19; ++i) node_id[i] = std::rand(); +node_id[2] = ((crc >> 8) & 0xf8) | (std::rand() & 0x7); +for (int i = 3; i < 19; ++i) node_id[i] = std::rand(); node_id[19] = rand;

    test vectors:

     IP           rand  example node ID
 ============ ===== ==========================================
-124.31.75.21   1   1712f6c7 0c5d6a4ec8a88e4c6ab4c28b95eee4 01
-21.75.31.124  86   946406c1 4e7a08645677bbd1cfe7d8f956d532 56
-65.23.51.170  22   fefd9220 bc8f112a3d426c84764f8c2a1150e6 16
-84.124.73.14  65   af1546dd 1bb1fe518101ceef99462b947a01ff 41
-43.213.53.83  90   a9e920bf 5b7c4be0237986d5243b87aa6d5130 5a
+124.31.75.21   1   d2a6df f10c5d6a4ec8a88e4c6ab4c28b95eee4 01
+21.75.31.124  86   48cb19 c14e7a08645677bbd1cfe7d8f956d532 56
+65.23.51.170  22   fd334a 20bc8f112a3d426c84764f8c2a1150e6 16
+84.124.73.14  65   6aa169 dd1bb1fe518101ceef99462b947a01ff 41
+43.213.53.83  90   eb6434 bf5b7c4be0237986d5243b87aa6d5130 5a

    The bold parts of the node ID are the important parts. The rest are -random numbers.

    +random numbers. The last bold number of each row has only its most significant +bit pulled from the CRC function. The lower 3 bits are random.

    bootstrapping

    @@ -187,6 +189,44 @@ not the node has a correct understanding of its external IP or not. This could be done by voting, or only restart the DHT once at least a certain number of nodes, from separate searches, tells you your node ID is incorrect.

    +
    +

    rationale

    +

    The choice of using CRC32 instead of a more traditional cryptographic hash +function is justified primarily of these reasons:

    +
      +
    1. it is a fast function
      2. +
    2. produces well distributed results
      3. +
    3. there is no need for the hash function to be one-way (the input set is +so small that any hash function could be reversed).
      4. +
    +

    There are primarily two tests run on SHA-1 and CRC32 to establish the +distribution of results. The first one is the number of bits in the output +set that contain every possible combination of bits. The CRC function +has a longer such prefix in its output than SHA-1. This means nodes will still +have well uniformly distributed IDs, even when IP addresses in use are not +uniformly distributed.

    +

    The following graph illustrate a few different hash functions with regard +to this property.

    +complete_bit_prefixes.png +

    This test takes into account IP addresses that are not globally routable, i.e. +reserved for local networks, multicast and other things. It also takes into +account that some /8 blocks are not in use by end-users and exremely unlikely +to ever run a DHT node. This makes the results likely to be very similar to +what we would see in the wild.

    +

    These results indicate that CRC32 provides the best uniformity in the results +in terms of bit prefixes where all possibilities are represented, and that +no more than 21 bits should be used from the result. If more than 21 bits +were to be used, there would be certain node IDs that would be impossible to +have, which would make routing sub-optimal.

    +

    The second test is more of a sanity test for the uniform distribution property. +The target space (32 bit interger) is divided up into 1000 buckets. Every valid +IP and r input is run through the algorithm and the result is put in the +bucket it falls in. The expectation is that each bucket has roughly an equal +number of results falling into it. The following graph shows the resulting +histogram, comparing SHA-1 and CRC32.

    +hash_distribution.png +

    The source code for these tests can be found here.

    +

    enforcement

    Once enforced, write tokens from peers whose node ID does not match its external diff --git a/docs/dht_sec.rst b/docs/dht_sec.rst index 221d11cab..6984341f7 100644 --- a/docs/dht_sec.rst +++ b/docs/dht_sec.rst @@ -71,11 +71,11 @@ __ http://blog.libtorrent.org/2012/12/dht-security/ The expression to calculate a valid ID prefix (from an IPv4 address) is:: - crc32((ip & 0x01071f7f) .. r) + crc32((ip & 0x030f3fff) .. r) And for an IPv6 address (``ip`` is the high 64 bits of the address):: - crc32((ip & 0x000103070f1f3f7f) .. r) + crc32((ip & 0x0103070f1f3f7fff) .. r) ``r`` is a random number in the range [0, 7]. The resulting integer, representing the masked IP address is supposed to be big-endian before @@ -84,9 +84,9 @@ hashed. The ".." means concatenation. The details of implementing this is to evaluate the expression, store the result in a big endian 64 bit integer and hash those 8 bytes with CRC32. -The first 4 bytes of the node ID used in the DHT MUST match the first 4 -bytes in the resulting hash. The last byte of the hash MUST match the -random number (``r``) used to generate the hash. +The first (most significant) 21 bits of the node ID used in the DHT MUST +match the first 21 bits of the resulting hash. The last byte of the hash MUST +match the random number (``r``) used to generate the hash. .. image:: ip_id_v4.png .. image:: ip_id_v6.png @@ -97,25 +97,25 @@ Example code code for calculating a valid node ID:: int num_octets; // the number of octets to consider in ip (4 or 8) uint8_t node_id[20]; // resulting node ID - uint8_t v4mask[] = { 0x01, 0x07, 0x1f, 0x7f }; - uint8_t v6mask[] = { 0x00, 0x01, 0x03, 0x07, 0x0f, 0x1f, 0x3f, 0x7f }; - uint8_t* mask = num_octets == 4 ? v4_mask : v8_mask; + uint8_t v4_mask[] = { 0x03, 0x0f, 0x3f, 0xff }; + uint8_t v6_mask[] = { 0x01, 0x03, 0x07, 0x0f, 0x1f, 0x3f, 0x7f, 0xff }; + uint8_t* mask = num_octets == 4 ? v4_mask : v6_mask; for (int i = 0; i < num_octets; ++i) ip[i] &= mask[i]; - uint32_t rand = rand() & 0xff; + uint32_t rand = std::rand() & 0xff; uint8_t r = rand & 0x7; - uint32_t crc = crc32(0, NULL, 0); + uint32_t crc = crc32(0, nullptr, 0); crc = crc32(crc, ip, num_octets); crc = crc32(crc, &r, 1); + // only take the top 21 bits from crc node_id[0] = (crc >> 24) & 0xff; node_id[1] = (crc >> 16) & 0xff; - node_id[2] = (crc >> 8) & 0xff; - node_id[3] = crc & 0xff; - for (int i = 4; i < 19; ++i) node_id[i] = std::rand(); + node_id[2] = ((crc >> 8) & 0xf8) | (std::rand() & 0x7); + for (int i = 3; i < 19; ++i) node_id[i] = std::rand(); node_id[19] = rand; test vectors: @@ -124,14 +124,15 @@ test vectors: IP rand example node ID ============ ===== ========================================== - 124.31.75.21 1 **1712f6c7** 0c5d6a4ec8a88e4c6ab4c28b95eee4 **01** - 21.75.31.124 86 **946406c1** 4e7a08645677bbd1cfe7d8f956d532 **56** - 65.23.51.170 22 **fefd9220** bc8f112a3d426c84764f8c2a1150e6 **16** - 84.124.73.14 65 **af1546dd** 1bb1fe518101ceef99462b947a01ff **41** - 43.213.53.83 90 **a9e920bf** 5b7c4be0237986d5243b87aa6d5130 **5a** + 124.31.75.21 1 **d2a6df** f10c5d6a4ec8a88e4c6ab4c28b95eee4 **01** + 21.75.31.124 86 **48cb19** c14e7a08645677bbd1cfe7d8f956d532 **56** + 65.23.51.170 22 **fd334a** 20bc8f112a3d426c84764f8c2a1150e6 **16** + 84.124.73.14 65 **6aa169** dd1bb1fe518101ceef99462b947a01ff **41** + 43.213.53.83 90 **eb6434** bf5b7c4be0237986d5243b87aa6d5130 **5a** The bold parts of the node ID are the important parts. The rest are -random numbers. +random numbers. The last bold number of each row has only its most significant +bit pulled from the CRC function. The lower 3 bits are random. bootstrapping ------------- @@ -156,6 +157,54 @@ not the node has a correct understanding of its external IP or not. This could be done by voting, or only restart the DHT once at least a certain number of nodes, from separate searches, tells you your node ID is incorrect. +rationale +--------- + +The choice of using CRC32 instead of a more traditional cryptographic hash +function is justified primarily of these reasons: + +1. it is a fast function +2. produces well distributed results +3. there is no need for the hash function to be one-way (the input set is + so small that any hash function could be reversed). + +There are primarily two tests run on SHA-1 and CRC32 to establish the +distribution of results. The first one is the number of bits in the output +set that contain every possible combination of bits. The CRC function +has a longer such prefix in its output than SHA-1. This means nodes will still +have well uniformly distributed IDs, even when IP addresses in use are not +uniformly distributed. + +The following graph illustrate a few different hash functions with regard +to this property. + +.. image:: complete_bit_prefixes.png + +This test takes into account IP addresses that are not globally routable, i.e. +reserved for local networks, multicast and other things. It also takes into +account that some /8 blocks are not in use by end-users and exremely unlikely +to ever run a DHT node. This makes the results likely to be very similar to +what we would see in the wild. + +These results indicate that CRC32 provides the best uniformity in the results +in terms of bit prefixes where all possibilities are represented, and that +no more than 21 bits should be used from the result. If more than 21 bits +were to be used, there would be certain node IDs that would be impossible to +have, which would make routing sub-optimal. + +The second test is more of a sanity test for the uniform distribution property. +The target space (32 bit interger) is divided up into 1000 buckets. Every valid +IP and ``r`` input is run through the algorithm and the result is put in the +bucket it falls in. The expectation is that each bucket has roughly an equal +number of results falling into it. The following graph shows the resulting +histogram, comparing SHA-1 and CRC32. + +.. image:: hash_distribution.png + +The source code for these tests can be found here_. + +.. _here: https://github.com/arvidn/hash_complete_prefix + enforcement ----------- diff --git a/docs/hash_distribution.png b/docs/hash_distribution.png new file mode 100644 index 0000000000000000000000000000000000000000..7bc1be79e5adb6fd1c53182f67b2e971573404e7 GIT binary patch literal 9035 zcmdsd2T&93w|CS;0w^p+cu~rNNH5Ywq%211D4-zHd;@|M=~b!=N&uBmCDIfSL=dD% zljcj1fD{#JQpJGMJ4ns9`j-Fw{&()oow+mL%>6Q(Z1$O_oadb1IcN8GHtM3G)?tRD z3@8-pu#Ps)7==0jqEP!*u?LWr<&0`w*C73Z)T-qJ_~yX`h~oi6KKZ#@-|2jvhTKCnslMU|?@= z9~v4;B9Y3<${HIR+uPeGCMJ+27G-6L^TZeZlZ%%Z$7rN#IP)V50;s)d6Xg3hCrdtq1LgtTD~(WJ?ZB z*{{yJcjX^-cVqXzXGibTekmFIVBwi*=Xvz)hNxgaACFP{7u!Xn^6-#@N1%^^;ED+$ zePG^?CpmQln(tmQ>%+;a-$_|elh~`Am@QL?!e(dyVYw1pw6>u07lXK;=car%2|+<) zPSkNRUpi5VgV5N*QXjgyYkYnBk@jRBu&G#Frc=DVdK%vO?WZ*3DpRNi7(~$nXz+~< 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"octet-wise modulus" with lines, \ - "ip_id_v4.dat" using 1:3 title "hash of IP" with lines - -set output "ip_id_v6.png" -set title "Number of possible node IDs" -set xlabel "bits controlled in IPv6" -plot "ip_id_v6.dat" using 1:2 title "octet-wise modulus" with lines, \ - "ip_id_v6.dat" using 1:3 title "hash of IP" with lines -''') -f.close() -os.system('gnuplot ip_id.gnuplot') -