Santa's signatures part 4 -- Saving Santa some signing: Using delegations to distribute data

2021-08-12 · Posted by: Marina Moore · Categories: Informational · Comments

In parts 1-3, we discussed how very bright children can use signatures, verification, and namespacing to ensure that their Christmas cards actually came from Santa. But Santa is busy managing his Christmas empire, and so he needs to have his elves sign Christmas cards on his behalf. He could create a stamp of his signature and distribute this to his elves, but if any of the many copies of this stamp are lost or stolen, anyone in possession of it would have the full signing authority of Santa. So he needs a way to indicate that his elves are signing on his behalf without actually giving up control over his signature. Even a magical holiday icon cannot distribute every elf’s driver’s licence to every child. So what he needs is a simple way to indicate when he is passing his signing responsibility on to an elf.

Santa can achieve this through the use of delegations. As the delegator, Santa passes some of his responsibilities to another party (the delegatee), which in this case is an elf. In terms of package signing, a delegation is a statement that serves as proof that someone else has been authorized to sign. When using cryptographic signatures, a delegation would include information about the public key of the delegatee and the scope of their trust, signed by the delegator. A user that trusted the delegator can then verify the delegation using a trusted public key, they can then use the public key indicated in the delegation to verify the package.

Delegations may be revoked at any time by the delegator by replacing the delegation. So if an elf intern works in Santa’s workshop for a winter, Santa can delegate a set of cards to them when they join. At the end of the winter intern season, Santa can then release a new set of delegations that excludes the intern. Anyone verifying signatures after this new delegation is created will no longer trust the intern to sign on behalf of Santa.

For example, a delegation may contain the following information, wrapped in a cryptographic signature by a trusted party. (adapted from the TUF specification):

"delegations": {
      "keys": {
        "f761033eb880143c52358d941d987ca5577675090e2215e856ba0099bc0ce4f6": {
          "keytype": "ed25519",
          "scheme": "ed25519",
          "keyval": {
            "public": "b6e40fb71a6041212a3d84331336ecaa1f48a0c523f80ccc762a034c727606fa"
"roles": [
          "keyids": [
          "name": "project",
          "paths": [

This metadata indicates that the public key starting with “b6e4” is trusted to sign “project/file3.txt.”

Namespaces separate a collection of items by name so that one can delegate authority for some, but not all items. In the above example, the delegator took advantage of namespaces to delegate authority only for “project/file3.txt,” and not for other files in the project. As another example, a Linux developer could delegate the “ls” utility to a developer without giving this individual permission to sign “chmod”.

Santa can take advantage of namespaces so that a rogue elf has limited ability to forge Santa’s signature. He can designate an elf for each type of toy that will accompany the Christmas cards, and that elf is only allowed to sign cards for children who receive that type of toy. This combination of delegations and namespacing allows for fine-grained control over who is trusted to sign and what they are trusted to sign.

Namespacing provides some protection in the event of a stolen cryptographic key, as the delegatee’s key is limited in scope. For example, a particularly careless elf might lose a signing key going home from the North Pole bar. If the key is picked up by the Grinch, he would only be able to use it for children delegated to that elf. Further, once the sheepish elf reports the stolen key to Santa, Santa could re-issue a delegation that does not include the stolen key.

Delegations do not have to stop at one level. Say the elf responsible for vehicle-related toy Christmas cards finds herself in trouble this year as there are too many cards for her alone to sign. She decides to further delegate some of these cards through the North Pole bureaucracy. To do so, she signs a new delegation that includes four of her direct reports who will split up the card signing. She uses namespacing to give each of them control over one type of toy vehicle (trains, cars, planes, or boats) and signs this delegation with her own signature. She distributes her signed delegation alongside each of the cards, so that the addressed child can verify the chain of delegations.

Now, Santa can safely delegate Christmas card signing to his elves without sharing his signing key or giving away too much power. And, a child verifying their Christmas card can start with a trusted key for Santa, use this to verify a delegation to the responsible elf, check for further delegations, and then compare the signature on their Christmas card to that elf’s signature.

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