Secret Sharing Pads
Secret sharing pads are a simple, computer-free way to securely share passwords with trusted friends and family for use in emergencies.
When you encrypt your passwords using secret sharing pads, the encrypted password is split amongst several printed pads using a technique called secret sharing. On their own the pads cannot be decrypted but, when any two pads are used together, the secret information can be decrypted using just a pen and paper. (Take a look at a sample Secret Sharing Pad.)
Try it out
You can generate your own secret sharing pads online:
The software can also be downloaded and used offline:
Finally, you can obtain the source code and build it yourself on GitHub:
Why use secret sharing pads?
Secret sharing pads have several advantages over other low-technology password sharing methods:
Unlike sharing sealed envelopes containing your passwords:
- Secret sharing pads do not burden the holder with the responsibility of storing unencrypted sensitive information.
- Two pad holders are needed to decrypt the passwords eliminating the temptation for any individual to try and reveal the passwords.
- If you stop trusting a person holding a secret sharing pad, you can render their pad useless by instructing the other pad holders to destroy their pads.
Unlike using a solicitor to hold a copy of your passwords:
- In the event of an emergency, the pads may be decrypted immediately without any delays while the request is verified.
- Secret sharing pads are completely free to use.
Secret sharing pads also have advantages over more high-tech methods:
- Unlike password management services' emergency access schemes:
- Secret sharing pads do not depend on a third party which can shutdown or terminate your account unexpectedly.
- Decryption of a secret sharing pad requires no technical knowledge, special software, or in fact any computers at all. Each pad includes a complete set of decryption instructions requiring only a pen and paper.
- The decision of when decrypt a secret sharing pad is made by the judgement of the people you trust, not by an automated system.
When shouldn't you use a secret sharing pad?
Despite their benefits, secret sharing pads are not be the right solution for everybody:
- Secret sharing pads require you to completely trust the people you share them with. There are no legal or technical barriers to pairs of pad holders colluding and acting in bad faith.
- There is no way to know when or if your secret sharing pads have been used.
- New secret sharing pads must be created and distributed every time a password is added or changed. For most people, access to just a small number of accounts (e.g. email), will be sufficient to unlock access to other services.
- Secret sharing pads make the assumption that several pads will not be stolen from your trusted pad holders in a series of coordinated robberies.
- Secret sharing pads could be legally seized from your trusted pad holders against your will. If this is a concern, secret sharing pads are not for you.
How does it work?
Secret sharing pads generated by this software use trivial 2-of-N secret sharing. This scheme essentially uses a one-time pad for every pairwise combination of pads to encrypt the secrets and so is uncrackable, if slightly space inefficient.
For every secret and every pairing of pads, the following encryption procedure is followed:
- For a secret L characters long, L random numbers in the range 0-999 are chosen and printed on one pad.
- For each character, c, in the secret and random number, r, in the random
sequence another number is generated as
mod(1000 + ascii(c) - r, 1000). This sequence of numbers is printed on the second pad.
The length of the secret may be optionally obscured by first adding a random number of null padding bytes to the end.
Decryption proceeds by taking matching pairs of characters from each pad, summing them modulo 1000, and then decoding the ASCII character value.
This scheme has a few useful properties:
- The encrypted secrets can be displayed as a series of 3-digit decimal numbers.
- The modulo operation, in decimal arithmetic, is simply a case of discarding all but the last three digits.
- The restriction to ASCII characters makes providing a number-to-character lookup table trivial (sorry, non English-password-using folks).
Why use this software?
The encryption procedure is sufficiently simple that it can be performed using
just a spreadsheet or even dice, pen and paper. In fact, only a tiny part of
the software in this repository is related to the encryption process
Aside from convenience, the major selling point of using this software to create secret sharing pads (vs rolling your own with a spreadsheet) is the care that has gone into the design of the printed pads. In particular, the instructions provided on each pad have been refined so that they are both clear and precise enough for a non-technical person to follow them unambiguously. You can try these out on the example secret sharing pads.
What's with the logo?
The logo is an abstract representation of a loop of string wrapped around three padlocks. The string can be completely freed by opening any two padlocks.