What is the safest physical to store a private key only to be accessed after death? Assume that my friend would like to encrypt his Bitcoin private key(s) and safely store them in a secret location. This located would only be identified to heirs after death in his will. Should he put the key:


In a safety deposit box
Buried in a weatherrpoof container somewhere on my property?
In the possession of my attorney?
Somewhere else


Please answer the same question above for my encryption key which I would of course store separately.

As an alternative would a mult-sig solution be helpful? If so how would you design it?
If you are comfortable with a command line, I have used ssss-split and ssss-combine for sharding encryption keys in a n-of-m scheme.  These utilities use Shamir's Secret Sharing Scheme to safely break up secrets for later reassembly.

For example, you could create a 4-of-7 scheme, where you would hide shards in 7 different locations (personal safe, safety deposit box, trusted lawyer, etc...).  Any 4 shards could be used to re-create the original secret.  It has the property where only having 3 shards gives an attacker no more chance of cracking the secret than if they had 0 shards.

One benefit to this scheme is that up to 3 shards could be lost, and the original secret could still be recovered.  The other benefit is that up to 3 shards could be stolen, and the attacker still doesn't have enough to obtain the secret.

One important thing to note, however, is that you still need to make sure that the collection of private keys is not lost, even if it is encrypted.  BIP32 wallets are a great place to start (so you only need to shard a single key), but otherwise make sure that you have redundant copies of your encrypted wallet file.  The decryption key won't be any use if you lose the data that it decrypts.  Again, avoid that trouble by using a BIP32 wallet.