4.2: Publishing Commitments for Privacy

Amira and Ben have a contract that ensures that SisterSpaces will always be able to use Amira's work. Maybe they're comfortable publishing that full contract, but maybe they prefer to keep details to themselves, to maximize the privacy and protection of both parties. This section explains how they could do so.

🧠 Related Concepts. After completing this tutorial, explore Data Minimization and Elision Cryptography.

Objectives of this Section

After working through this section, a developer will be able to:

Store hashed records of a contract as a commitment.
Create herd privacy through the publication of numerous hashes.

Supporting objectives include the ability to:

Understand what a commitment means.
Understand what herd privacy is.

Amira's Story: Preserving Privacy

Amira has signed a CLA to allow her work to be used in SisterSpaces' SecureAuth Library. Most open-source repositories would publish CLAs like that as part of their repos. But Ben is very aware of privacy concerns, as many of the women who use his services come from abusive backgrounds. He extends those privacy concerns to the people working with him.

Because she is standing behind her BRadvoc8 identity, Amira doesn't have the same concerns as people who are working with SisterSpaces without the benefit of a pseudonymous identity. In fact, she's eager to reveal her connection to them, to improve the trust for her identity. But that doesn't mean she wants everyone to know the entire contents of the contract she signed, or at the least doesn't want the whole huge text incorporated into her XID.

How can licensing rights be ensured without endangering any of the participants? The answer is commitments.

The Power of Commitments

We've seen commitments previously, in §2.2. Their use there was simple: we created a claim; we entirely elided that claim; and then we used the hash of the claim as proof that the claim existed (and had existed for some period of time).

At the time we said that the commitment could be published, but we didn't really say how. This chapter explores that more:

To start with, we offer two major methods to create a commitment: shasum a plain file (which we saw in §4.1) and digest an envelope (which we use here, and which currently defaults to sha-256).

Hashing Method	App	Section
SHASum a File	`shasum`	§4.1
Digest an Envelope	`envelope`	§4.2

Each of these digests can also be published in a variety of ways. Ben incorporated a shasum into an envelope in the last section. Here we're going to see a digest incorporated into an envelope and also discuss how digests can be published in plain (.md) files.

Publication Method	Section
Incorporate into a XID	§4.1, §4.2
Publish as a File	§4.2
Publish in a Long List	§4.2

The Power of Herd Privacy

As noted above, one of the methods of publishing commitments is as part of a long list. Doing so can create herd privacy. When a long list of commitments is created, each one blends in with the others. An observer simply sees a long list of commitments: they can't easily determine which one belongs to the human rights worker, which to a student, and which to a retiree. The crowd provides cover.

Contributors	Observer's Challenge
1	Trivial to identify
10	Difficult to identify
50	Needle in haystack
500	Effectively anonymous

Even if an attacker discovered a flaw in the methodology that Ben uses to make commitments, they wouldn't know which hash to try and attack to prove that BRadvoc8 (or anyone else) is involved. The specifics are lost among the crowd.

Part 0: Verify Dependencies

Before you get started, you should (as usual) check your envelope-cli version:

envelope --version

│ bc-envelope-cli 0.34.1

Then, reload your XID. We're also going to be using Amira's attestation key to create a new edge about her work with SisterSpaces, which will also depend on the CLA she signed.

XID=$(cat envelopes/BRadvoc8-xid-private-4-01.envelope)
XID_ID=$(envelope xid id $XID)
PASSWORD="your-password-from-previous-tutorials"
ATTESTATION_PRVKEYS=$(cat envelopes/key-attestation-private-2-01.ur)
SIGNED_ACCEPTED_CLA=$(cat envelopes/cla-bradvoc8-accepted-4-01.envelope)

Part I: Incorporating a Contract into a XID

Amira is thrilled to talk about her work with SisterSpaces. That's because Ben's signature on Amira's contract is another step forward in the progressive trust of BRadvoc8 as a reliable security expert.

On the other hand, she does have some privacy concerns. She knows that Ben only releases the full CLAs if they're legally necessary to prove ownership of SisterSpaces code. She wants to respect his privacy and also doesn't want to clutter up in XID with the full contract.

A commitment of the contract offers a compromise that will meet her needs to show off the connection but still protect Ben's privacy and her own XID's accessability.

Step 1: Create a Contract Edge

To record the contract, Amira will create a new self-attestation edge that she is working on SisterSpaces.

As with any other edge, she first needs to define the three core elements of the edge: isA, source, and target:

ISA="foaf:Project"
SOURCE_XID_ID=$XID_ID
TARGET_XID_ID=$XID_ID

To create the commitment of the contract she will use a similar methodology to that of §2.2: she creates a digest of the CLA envelope.

DIGEST_CLA=$(envelope digest "$SIGNED_ACCEPTED_CLA")

However, there's a difference: in §2.2, we elided the envelope, leaving only the digest. How we published that commitment was hand-waved. Here, we're going to offer a first method to do so: by incorporating the digest into a claim that will be added to an envelope as an edge. If anyone asks for more info they can ask BRadvoc8 for the full contract, and she can decide whether it's appropriate to give it out.

The commitment to the contract, as with the rest of the details of the claim, are placed as a target subenvelope:

PROJECT_TARGET=$(envelope subject type ur $TARGET_XID_ID)
PROJECT_TARGET=$(envelope assertion add pred-obj string $ISA string "SisterSpaces" "$PROJECT_TARGET")
PROJECT_TARGET=$(envelope assertion add pred-obj known verifiableAt string "https://github.com/SisterSpaces/SecureAuth/CLAs/README.md" "$PROJECT_TARGET")
PROJECT_TARGET=$(envelope assertion add pred-obj string "claDigest" digest "$DIGEST_CLA" "$PROJECT_TARGET")

With those details, you can now put together the contract edge:

EDGE=$(envelope subject type string "project-sister-spaces-secureauth")
EDGE=$(envelope assertion add pred-obj known isA string "$ISA" "$EDGE")
EDGE=$(envelope assertion add pred-obj known source ur "$SOURCE_XID_ID" "$EDGE")
EDGE=$(envelope assertion add pred-obj known target envelope "$PROJECT_TARGET" "$EDGE")

echo "SisterSpaces edge details:"
envelope format "$EDGE"

| SisterSpaces edge details:
| 
| "project-sister-spaces-secureauth" [
|     'isA': "foaf:Project"
|     'source': XID(5f1c3d9e)
|     'target': XID(5f1c3d9e) [
|         "claDigest": Digest(cb26376e)
|         "foaf:Project": "SisterSpaces"
|         'verifiableAt': "https://github.com/SisterSpaces/SecureAuth/CLAs/README.md"
|     ]
| ]

This is a relatively simple edge, but that's because the CLA commitment stands in for the entire CLA!

Step 2: Publish Your Contract Edge

You can now take the several steps to attach this edge and publish the result.

First, incorporate the edge:

WRAPPED_EDGE=$(envelope subject type wrapped "$EDGE")
SIGNED_EDGE=$(envelope sign --signer "$ATTESTATION_PRVKEYS" "$WRAPPED_EDGE")

XID_WITH_CONTRACT_EDGE=$(envelope xid edge add \
    --verify inception \
    $SIGNED_EDGE $XID_WITH_CONTRACT_KEY)

Second, advance the provenance mark, sign your envelope, and produce a public version:

XID_WITH_CONTRACT_EDGE=$(envelope xid provenance next \
    --password "$PASSWORD" \
    --sign inception \
    --private encrypt \
    --generator encrypt \
    --encrypt-password "$PASSWORD" \
    "$XID_WITH_CONTRACT_EDGE")
PUBLIC_XID_WITH_CONTRACT_EDGE=$(envelope xid export --private elide --generator elide "$XID_WITH_CONTRACT_EDGE")

Third, store everything:

echo "$PUBLIC_XID_WITH_CONTRACT_EDGE" > envelopes/BRadvoc8-xid-public-4-02.envelope
echo "$XID_WITH_CONTRACT_EDGE" > envelopes/BRadvoc8-xid-private-4-02.envelope

You've demonstrated how Amira highlights her new connection to SisterSpaces while preserving privacy, but that's just half the story.

XID Version Comparison

You've now created a sixth XID edition of Amira's XID. Here's another overview of what each version contains

XID Version	New Content	Created In
seq 0	👤 Identity	§1.3+§1.4
seq 1	🔑 Attestation Key	§2.1
seq 2	🗣️ GitHub Edge	§3.1
seq 3	🗣️ Endorsement Edge	§3.3
seq 4	🔑 Contract Key	§4.1
seq 5	📄 Contract Commitment	§4.2

Part II: Incorporating a Contract into a Commitment List

Ben also needs to publish the CLA, to show everyone that his software is safe to use, because he has contracts with all the authors. But, Ben has 50 different people who have worked on SisterSpaces and signed CLAs. He doesn't want all of that in his XID (or even in a XID he created for SisterSpaces). Nor does he want to just publish those CLAs separately, because of the privacy concerns.

His answer: a commitment list

Step 3: Compute the Digests

Ben maintains a directory on his local encrypted drive of all of the CLAs for SisterSpaces. Whenever he adds a CLA he runs a script that regenerates a README.

cd private-cla-directory

echo "# CLA Commitment List" > README.md
echo "" >> README.md
echo "This is a list of commitments for CLAs guaranteeing rights to work done for SisterSpaces. CLAs are published as envelope-cli digest hashes to preserve privacy. Original CLAs are held by the Project Manager (currently Ben) and can be privately produced if necessary. This list is a living example of the power of herd privacy." >> README.md
echo "" >> README.md

for cla in *.envelope
do
  envelope digest $(cat $cla) >> README.md
done

This produces a file like the following:

# CLA Commitment List

This is a list of commitments for CLAs guaranteeing rights to work
done for SisterSpaces. CLAs are published as envelope-cli digest
hashes to preserve privacy. Original CLAs are held by the Project
Manager (currently Ben) and can be privately produced if
necessary.This list is a living example of the power of herd privacy.

ur:digest/hdcxnydmgooejsgwjywfnbaxotlgaotibdbzynroutlebgditsfrroreisihptytndmtlkhyidfs
ur:digest/hdcxcaoldkfpisoesfoxdnatamwyytasdwsbuomnkicxlbaavehsfzksmdinrogachhnsolbpahg
ur:digest/hdcxvwpkrfmokeonsalojefseovliertiykoimzcjkdwkswktiisdloskngokoiakistwnwlhfot
ur:digest/hdcxzewtfzfxrdndktdpeocwnnttbdoloxeyskyninhtpazehsgsynfpfwgrlbbgdskosefddktp
ur:digest/hdcxlkdtieyafsbkhdftbdgmztfxmoflgmpmlghladmkutfymundfmdrnbaeatspctclynkgsnen
ur:digest/hdcxhgfdesknwdrdmusgbspsbdgmjpflfwdafhhybgytdwcklskohpbtdkeskgrfvwaolycesnyn
ur:digest/hdcxayptgsktmssbhdsnfpetnsrppmsttlahlrtnaeyndloyvawsdmcxdesokgeomknnwfjklpnl
ur:digest/hdcxcaoldkfpisoesfoxdnatamwyytasdwsbuomnkicxlbaavehsfzksmdinrogachhnsolbpahg
ur:digest/hdcxvwpkrfmokeonsalojefseovliertiykoimzcjkdwkswktiisdloskngokoiakistwnwlhfot
ur:digest/hdcxzewtfzfxrdndktdpeocwnnttbdoloxeyskyninhtpazehsgsynfpfwgrlbbgdskosefddktp
ur:digest/hdcxlkdtieyafsbkhdftbdgmztfxmoflgmpmlghladmkutfymundfmdrnbaeatspctclynkgsnen
ur:digest/hdcxhgfdesknwdrdmusgbspsbdgmjpflfwdafhhybgytdwcklskohpbtdkeskgrfvwaolycesnyn
ur:digest/hdcxayptgsktmssbhdsnfpetnsrppmsttlahlrtnaeyndloyvawsdmcxdesokgeomknnwfjklpnl
ur:digest/hdcxinmerocyjzgrmhskpshyrfsnuthhpsjoswbyiyttstldaxytlsiefdcfytdebyhhgwzevwtl
ur:digest/hdcxsbdsemjtwlrtvwfdetmwenlujndlsbethljzkpnbmsgmchaajolsgekbpypmtijevolukglt
ur:digest/hdcxswjzwzjzzopawmieytwpjtgwlsoyfrhlntsghldsrnpstswsknehwsrltpeyonfriouooykp
ur:digest/hdcxhsimvekidtktvdvtahbwwdongttiidykimmwhntpadihasjsssmhynstqdrykijesoskvslg
ur:digest/hdcxwyflkitaplesgamojnjohhimjlfyqdlyuorsjyhprdrycmtoahgwvtfzlgamzcenbnnsfwhl
ur:digest/hdcxhlesrpmsfhkbfdcsdszmbemwtlotjynbqdcxmuisbsldktjsinzostwekbtohhsegapyiome
ur:digest/hdcxchrdpkuraaeedpmynscxihbttdhtmdrdndrtvsmdzetonyeosstngmtidaguzefngmmkjltk
ur:digest/hdcxftcmvyetfycslpwnlfswsngsamsndnaekizsmtwyglkepaplnbceltretbamoemtwpsedtrn
ur:digest/hdcxwyflkitaplesgamojnjohhimjlfyqdlyuorsjyhprdrycmtoahgwvtfzlgamzcenbnnsfwhl
ur:digest/hdcxmhmslyjlgrsespryvtfsaxceadyknntnveglrnfrbeossrknmodidahptygsrfcemerowzge
ur:digest/hdcxyarhehpdfyaynndassgtfxmwwpoydpglhtlkuylfnbhlrntybaynwnzokiosswctrsencmim

Step 4: Publish a Commitment List

Once Ben generates his README.md file, he distributes it to the CLA directories of all his project repos. The long list of commitments guarantees herd privacy, but simultaneously promises that the CLAs are available if they're needed to adjudicate a legal matter.

Step 5: Check a Commitment

If that legal adjudication was ever required, Ben could supply the appropriate CLA, for example Amira's, and the recipient could derive a digest from it:

BR_CLA_DIGEST=$(envelope digest $(cat cla-bradvoc8-accepted-4-01.envelope))

Afterward, they could verify that the digest had been in the published README file:

if grep -q $BR_CLA_DIGEST README.md; then
  echo "✅ CLA commitment was in README.md"
else
  echo "❌ CLA was not properly commited to"
fi

| ✅ CLA commitment was in README.md

Obviously, having the required CLA is the important part, but the fact that Ben previously published it improves his own level of trust, because when he has to reveal a CLA, he also proves that he stands behind what he says in his commitments.

Summary: Publishing Commitments for Privacy

In §2.2, you learned how to make commitments by creating an elided envelope. This section introduced a second method, where you just store the digest and also proposed two methods for publishing that commitment:

As part of a claim in an envelope.
As part of a long list protected by herd privacy.

These methodologies extend the power of commitments, providing more ways than ever to improve the trust level of a pseudonymous identifier.

Additional Files

Envelopes: The envelopes directory contains numerous data created in this section, the most important of which are Amira's sixth private and public XIDs as well as Ben's README.

Scripts: The scripts directory contains 04_2_Publishing_for_Privacy.md, which runs through all the commands in this section. From the command line, git clone https://github.com/BlockchainCommons/XID-Quickstart.git, then cd XID-Quickstart, then bash scripts/04_2_Publishing_for_Privacy.md to test it.

Exercises

Commit to the contents of a file using shasum.
Commit to the contents of an envelope using the digest command.
Create an edge that records a commitment.
Dump a large set of commitments into a file and see how that helps to hide each individual commitment.

What's Next

Though we've completed Amira's first arc, there's one open question: has her XID gotten too big? §4.3: Creating New Views talks about how to clean it up.

Appendix I: Key Terminology

Commitment List: A list of commitments, also offered with commentary or identification.

Herd Privacy: A method for protecting the privacy of any individual content by "hiding" it amidst a set of similarly looking content.