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177 lines
4.1 KiB
HTML
<body text="#000000" link="#0000ff" bgcolor="#ffffff"><center><table width="80%">
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<tr><td><h1>ssss</h1>
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<h2>Split and Combine Secrets using Shamir's Secret Sharing Scheme.</h2>
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<h2>Synopsis</h2>
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<b>
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ssss-split -t <em>threshold</em> -n <em>shares</em> [-w <em>token</em>]
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[-s <em>level</em>] [-x] [-q] [-Q] [-D] [-v]<br>
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ssss-combine -t <em>threshold</em> [-x] [-q] [-Q] [-D] [-v]<br>
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</b>
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<h2>Description</h2>
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<p>ssss is an implementation of Shamir's Secret Sharing Scheme. The
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program suite does both: the generation of shares for a known secret,
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and the reconstruction of a secret using user-provided shares.</p>
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<h2>Commands</h2>
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<p><b>ssss-split</b>: prompt the user for a secret and generate a set of
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corresponding shares.</p>
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<p><b>ssss-combine</b>: read in a set of shares and reconstruct
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the secret.</p>
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<h2>Options</h2>
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<p><b>-t <em>threshold</em></b></p>
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<p>Specify the number of
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shares necessary to reconstruct the secret.</p>
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<p><b>-n <em>shares</em></b></p>
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<p>Specify the number of shares to be generated.</p>
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<p><b>-w <em>token</em></b></p>
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<p>Text token to name shares in order to avoid confusion in case one
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utilizes secret sharing to protect several independent secrets. The
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generated shares are prefixed by these tokens.</p>
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<p><b>-s <em>level</em></b></p>
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<p>Enforce the scheme's security level (in bits). This option
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implies an upper bound for the length of the shared secret
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(shorter secrets are padded). Only multiples of 8 in the range
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from 8 to 1024 are allowed. If this option is ommitted (or the
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value given is 0) the security level is chosen automatically
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depending on the secret's length. The security level directly
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determines the length of the shares.</p>
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<p><b>-x</b></p>
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<p>Hex mode: use hexadecimal digits in place of ASCII characters for
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I/O. This is useful if one wants to protect binary data, like
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block cipher keys.</p>
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<p><b>-q</b></p>
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<p>Quiet mode: disable all unnecessary output. Useful in scripts.
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</p>
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<p><b>-Q</b></p>
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<p>Extra quiet mode: like <b>-q</b>, but also suppress
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warnings.</p>
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<p><b>-D</b></p>
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<p>Disable the diffusion layer added in version 0.2. This option
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is needed when shares are combined that where generated with
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ssss version 0.1.</p>
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<p><b>-v</b></p>
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<p>Print version information.</p>
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<h2>Example</h2>
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<p>
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In case you want to protect your login password with a set of ten
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shares in such a way that any three of them can reconstruct the
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password, you simply run the command
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</p>
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<p>
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ssss-split -t 3 -n 10 -w passwd
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</p>
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<p>
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To reconstruct the password pass three of the generated shares
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(in any order) to
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</p>
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<p>
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ssss-combine -t 3
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</p>
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<h2>Notes</h2>
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<p>
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To protect a secret larger than 1024 bits a hybrid technique has to be
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applied: encrypt the secret with a block cipher and apply secret
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sharing to just the key. Among others openssl and gpg can do the
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encryption part:
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</p>
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<p>
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openssl bf -e < file.plain > file.encrypted
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</p>
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<p>
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gpg -c < file.plain > file.encrypted
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</p>
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<h2>Security</h2>
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<p>
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<b>ssss</b> tries to lock its virtual address space into RAM for
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privacy reasons. But this may fail for two reasons: either the current uid
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doesn't permit page locking, or the RLIMIT_MEMLOCK is set too
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low. After printing a warning message <b>ssss</b> will run even without
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obtaining the desired mlock.
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</p>
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<h2>Author</h2>
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This software (v0.5) was written in 2006 by B. Poettering
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(ssss AT point-at-infinity.org). Find the newest version of
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ssss on the project's homepage: <a href = "http://point-at-infinity.org/ssss/">http://point-at-infinity.org/ssss/</a>.
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<h2>Further reading</h2>
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<a href = "http://en.wikipedia.org/wiki/Secret_sharing">http://en.wikipedia.org/wiki/Secret_sharing</a>
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</td></tr></table></center>
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</body>
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