mdw@git.distorted.org.uk Git - distorted-keys/blob - README

   1 distorted.org.uk KEY MANAGEMENT
   2
   3 The various files are organized into subdirectories as follows.
   4
   5 infra/                  Infrastructure keys used to keep this system going.
   6 recov/
   7
   8 File extensions used are as follows.
   9
  10 .pub            Seccure public key.  (See description of Seccure data
  11                 formats below.)
  12
  13 .recov          Seccure ciphertext of key
  14
  15
  16
  17 recov.pub       `seccure' public key for recovery
  18
  19 krb5-master     Kerberos master password
  20 bkp-LABEL       LUKS keyfile for backup volume LABEL
  21 disk-HOST       LUKS keyfile for HOST's disk
  22
  23 keys/
  24 |- keeper/
  25 |  '- KEEPER/
  26 |     |- meta
  27 |     '- I.pub
  28 |- key/
  29 |  '- ???
  30 '- recov/
  31    '- RECOV/
  32       |- keepers
  33       |- current@
  34       '- I/
  35          |- pub
  36          |- KEEPER.param
  37          |- KEEPER.I.share
  38          '- SECRET.recov
  39
  40
  41 * Reference
  42
  43 ** Asymmetric cryptography
  44
  45 I've used B. Poettering's Seccure package for my asymmetric
  46 cryptography.  It's been in Debian for a fair while and seems sane. If
  47 you're interested in what it does, I wrote my own implementation in
  48 Python.  It seems pretty sensible, actually.  It uses ECIES with AES
  49 in counter mode, and SHA256-HMAC for asymmetric encryption, and a
  50 variant of ECDSA with SHA512 for signatures.
  51
  52 Seccure wants to read a single line of stuff as a passphrase.  I use
  53 this rune to generate a public key.
  54
  55         dd if=/dev/random of=master bs=1 count=512 |
  56                 openssl sha384 -binary >priv
  57
  58 To derive the public key, I say this:
  59
  60         openssl base64 -in priv | seccure-key -q -F/dev/stdin -cp256 >pub
  61
  62 For encryption, I use a 128-bit MAC.  For decryption, you need this rune.
  63
  64         openssl base64 -in priv |
  65                 seccure-decrypt -q -F/dev/stdin -m128 ciphertext
  66
  67 ** Secret sharing
  68
  69 I've written my own tool for doing Shamir secret sharing.  The
  70 underlying machinery is compatible with Daniel Silverstone's `gfshare'
  71 program and my Catacomb library's secret sharing.  My `shamir' program
  72 has a number of important differences:
  73
  74   * it produces output as plain text files which can be transported
  75     easily and so on;
  76
  77   * it includes metadata, such as the number of shares, the threshold,
  78     and a hash of the final secret, along with the share data;
  79
  80   * it stores the share index with the share data too, rather than
  81     encoding it in the file name where it's likely to be lost; and
  82
  83   * it doesn't choose random share indices when issuing shares,
  84     because that's pointless.
  85
  86 The `shamir issue' command writes one line for each share that it
  87 produces.  I use this rune to split them into separate files.
  88
  89         shamir issue 3/5 master |
  90                 sed 's/^.*;i=\([^;]*\);/\1 &/' |
  91                 while read i share; do
  92                   echo $share >master.$i
  93                 done
  94
  95 You can recover the original secret by feeding shares, one per line,
  96 into `shamir recover'.  All of the parameters are in the share data,
  97 so you don't need to know any of them.  (I used the defaults anyway,
  98 since I carefully chose them to match what I wanted.)
  99
 100 A share line has the following format:
 101
 102         shamir-share:KEY=VALUE;KEY=VALUE;...
 103
 104 where the following keys are defined (they must appear in this order):
 105
 106   * n = total number of shares issued;
 107   * t = threshold (i.e., number of shares needed for recovery);
 108   * f = hash function name (an OpenSSL name, e.g., `sha256');
 109   * h = base-64 encoded hash of the secret (using hash function `f');
 110   * i = index of this share (starting from 0); and
 111   * y = base-64 share data.
 112
 113 You can turn such a file of such lines into files suitable for
 114 `gfcombine' like this:
 115
 116         sed 's/^.*;i=\(.*\);y=\(.*\)$/\1 \2/' |
 117         while read i sh; do
 118           ix=$(printf %03d $((i + 1)))
 119           echo $sh | openssl base64 -d >tmp/share.$ix
 120         done