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Vera V1 Specification

Status
Working draft
Author
Gus Narea, Relaycorp

Overview

THIS IS A WORKING DRAFT AT THIS POINT

Initial provisioning done by organisation admins:

  • acme.com MUST have DNSSEC enabled and properly configured.
  • acme.com admin generates an Ed25519 key pair and self-issues an X.509 certificate with it. This is the root Certificate Authority (CA) for all certification paths and all digital signatures under acme.com.
  • TXT record _vera.acme.com contains the Base64-encoded, ASN.1 DER serialisation of the root certificate. This may have to span two records due to 255-character limit.

Authenticating members and bots:

  • acme.com admin issues certificates to members of the organisation or bots that act on behalf of the organisation – collectively known as signers.
    • Each certificate contains a critical extension that specifies the context in which it can be used (e.g., Letro).
    • If the certificate is for a member, the Distinguished Name MUST contain the Vera username (e.g., alice.smith).
  • A signer provisions a certificate by providing an Ed25519 public key to the acme.com CA.

Digital signatures by members and bots:

  • Signers must distribute each content with a Vera signature, which includes:
    • The CMS SignedData signature, embedding the signer’s certificate along with any intermediary CAs. The root CA SHOULD NOT be included.
    • The TXT record _vera.acme.com along with all the intermediary records to verify the TXT record with DNSSEC.
  • Verifiers must verify each content they receive against its respective Vera signature as follows:
    1. Verify the TXT record using DNSSEC.
    2. Verify the authenticity of the content using the SignedData value with the root certificate in the TXT record as they only trusted certificate.
    3. Verify that the signer’s certificate is allowed to use the current service (e.g., Letro).

Validity period

Signature verifiers MUST count the age of a digital signature from the moment the TXT record answer was signed.

Vera service designers MUST specify the maximum Time-to-Live (TTL) for every digital signature in the service. The TTL is counted from the moment the TXT record answer was signed. The TTL MUST be within the following range:

  • 8 hours or more, in order to allow sufficient time for signature-producing apps to renew certificates (and wait for any outages to be resolved).
  • 30 days or less, in order to support Delay-Tolerant Networking.

Vera favours short-lived certificates over long-lived ones, primarily to avoid the use of revocation protocols. Consequently, service designers SHOULD require the shortest TTL that would satisfy their particular requirements.

Signature verifiers MAY require a TTL shorter than that required by the service, but still not shorter than 8 hours. Additionally, signature verifiers MAY allow their end users to specify the shorter TTL.

X.509 certificate

  • Common Name MUST be the domain name in the case of an organisation certificate, the at sign (@) in the case of a bot certificate and the user name in the case of a user certificate.

TXT Record Data

Space-separated fields:

  • Key algorithm: An integer denoting the key algorithm.
    • 0 (RSA 2048).
    • 1 (RSA 3072).
    • 2 (RSA 4096).
  • Key id type: An integer denoting how the key is identified in the following field.
    • 0 (ID): The key id is the key itself (e.g., Ed25519 keys). Reserved for future use.
    • 1 (SHA256): The key id is the SHA-256 digest.
    • 2 (SHA384): The key id is the SHA-384 digest.
    • 3 (SHA512): The key id is the SHA-512 digest.
  • Key id: The Base64 (unpadded) encoding of the public key itself or its digest.
  • TTL override for the DNSSEC chain: A positive integer representing the number of seconds.
  • Service (optional): The OID for the service this chain applies to.

Cryptographic algorithms supported

  • Hashing: SHA-256, SHA-384 and SHA-512.
  • Digital signatures: RSA-PSS with modulus 2048/3072/4096.

DNSSEC Chain Serialisation

Serialise as an answer using the message format from RFC 1035 (Section 4.2), where:

  • Header: contains the ad flag.
  • Question: is _vera.<name>/TXT.
  • Answer: contains the RRset for _vera.<name>/TXT (and respective RRSIGs).
  • Authority: is empty.
  • Additional: contains the rest of the DNSSEC chain, excluding ./DS (which must be provided by the verifier).

Vera Member Id Bundle

ASN.1 SEQUENCE:

  • DNSSEC chain (SEQUENCE).
  • Organisation certificate (SEQUENCE).
  • Member certificate (SEQUENCE).

Vera Signature

ASN.1 SEQUENCE:

  • DNSSEC chain (SEQUENCE).
  • Organisation certificate (SEQUENCE).
  • SignedData (SEQUENCE). Includes member certificate and any intermediate certificates, but not the plaintext (content).
  • Validity period (SEQUENCE).
    • Start date.
    • Expiry date.

Vera Signed Content

ASN.1 SEQUENCE:

  • Content (OCTET STRING).
  • Signature (SEQUENCE).

Security considerations

Reliance on DNSSEC infrastructure

Every DNS zone in a Vera chain is a potential target for cyberattacks, including the root zone. Not to mention that many governments control popular TLDs so, for example, the Libyan government could theoretically issue valid DNSSEC responses for bit.ly.

Homographic and character encoding-based attacks