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