Split private key in order to share among users

[Andrew Gallagher]

> On 20 Dec 2020, at 09:19, Alexander Kriegisch <alexander at kriegisch.name> wrote:
> 
> The original PGP used to have this feature around 20 years ago already,
> maybe some people remember. In the list archive I found two threads,
> both several years old, asking about this feature in GnuPG, but there
> were no conclusive answers, only workaround suggestions like to split
> the binary or ASCII key file or print the password and share parts of
> the passwords, neither of which satisfy the original requirements
> covered by the original PGP functionality. Example:
> 
> I split a private key file with PGP into these shares:
>  -- User A gets a piece of key worth 2 shares.
>  -- User B gets a piece of key worth 2 shares.
>  -- User C gets a piece of key worth 1 share.
>  -- User D gets a piece of key worth 1 share.
>  -- User E gets a piece of key worth 1 share.
>  -- User F gets a piece of key worth 1 share.
> 
> I define that at least 5 shares are necessary to re-assemble a valid
> decryption key, i.e. we need for example
>  -- A + B + one other user
>  -- C + D + E + either A or B
> for decryption.
> 

You’re referring to Shamir’s secret sharing scheme, for which several implementations exist. If you are using Linux, it should be as simple as installing the “ssss” package. 

A