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+---
+dip: 330
+title: Self-Describing Hash Format for Attestation ReportData
+status: Draft
+author: Hang Yin <hangyin@phala.network>
+created: 2025-09-13
+---
+
+## Abstract
+
+This DIP specifies a compact, self-describing hash string format that fits within the strict 64-byte limit of Intel DCAP’s `reportdata` field. The format includes a purpose prefix and algorithm identifier to improve readability, domain separation, and verifiability:
+
+```
+<purpose>:<algo>:<base64url-hash>
+```
+
+The format is directly designed for remote attestation use cases. Other applications may adopt it, but they are out of scope for this proposal.
+
+## Motivation
+
+Intel DCAP’s remote attestation `reportdata` field provides exactly 64 bytes for application-defined data. Existing hash formats either:
+
+* Overflow the field (e.g. Docker’s `sha256:<hex>` representation),
+* Omit algorithm metadata (bare digests), or
+* Lack a way to indicate purpose/domain.
+
+To solve this, we propose a representation that:
+
+1. Explicitly encodes the **algorithm** (`sha-256`),
+2. Uses **base64url encoding** for compactness,
+3. Adds a **purpose prefix** for human readability and domain separation,
+4. Always fits within the 64-byte `reportdata` field (for SHA-256).
+
+This enables straightforward claim verification in remote attestation workflows without extra metadata.
+
+## Specification
+
+The format is:
+
+```
+<purpose>:<algo>:<base64url-hash>
+```
+
+* **purpose**: Short identifier (≤10 characters recommended). Examples: `attest`, `claim`.
+* **algo**: Lowercase algorithm identifier with optional dash, e.g. `sha-256`.
+* **base64url-hash**: Digest of the preimage, encoded in URL-safe Base64 without padding (`=`).
+
+Separators: two colons (`:`).
+
+### Size Constraints
+
+For SHA-256:
+
+* Digest (base64url, no padding): 43 characters
+* Algorithm (`sha-256`): 7 characters
+* Purpose: up to 10 characters (recommended)
+* Separators: 2 characters
+
+**Total: 62 characters**, under the 64-byte maximum.
+
+Longer digests (e.g. SHA-512, 86 chars in base64url) will not fit. Therefore, **SHA-256 is the normative algorithm** for attestation `reportdata`.
+
+### Example
+
+```
+attest:sha-256:7zYbTVHDz2A3sfmGdPZLhv17Qrs5X7s1U6B7YyBR9nM
+```
+
+* `attest`: purpose
+* `sha-256`: algorithm
+* `7zYb...9nM`: base64url digest
+
+## Rationale
+
+* **Purpose prefix** improves readability and domain separation (e.g. differentiating attestation claims vs. config hashes).
+* **Algorithm field** avoids ambiguity and enables algorithm agility.
+* **Base64url encoding** is compact (43 chars vs. 64 hex chars for SHA-256) and URL-safe.
+* **Colon separator** is widely used (e.g. Docker digests, URIs) and compatible with base64 encoding, and algorithm names containing hyphens.
+
+## Implementation Notes
+
+This format can reuse existing parsing and serialization libraries:
+
+* **ni:// (RFC 6920)** already uses `algo;base64url-digest`. Libraries exist in Python (`rfc6920`), Go, Ruby, Perl.
+* **SRI strings** (`algo-base64digest`) are supported by the Node.js `ssri` package and browsers.
+
+By simply converting to/from `<algo>:<base64url-hash>`, developers can leverage these mature libraries for digest verification and serialization, minimizing new code.
+
+## Backwards Compatibility
+
+This is a new format, scoped for Intel DCAP remote attestation. No existing systems are broken. Other contexts may adopt it later, but that is not addressed in this DIP.
+
+## Security Considerations
+
+* Security strength derives from the algorithm (SHA-256 recommended).
+* Purpose is metadata only and not cryptographically bound unless explicitly included in the preimage.
+* Implementations must ensure canonical encoding: lowercase algorithm names, no padding, and URL-safe characters only.
+
+## Test Vectors
+
+| Preimage                    | Purpose | Algorithm | Base64url Digest                              | Full String                                                  |
+| --------------------------- | ------- | --------- | --------------------------------------------- | ------------------------------------------------------------ |
+| `"hello"` (0x68656c6c6f)    | attest  | sha-256   | `LPJNul-wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ` | `attest:sha-256:LPJNul-wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ` |
+| `"dstack"` (0x64737461636b) | claim   | sha-256   | `FPBeUtMvvR54kqX8DSmVNzKiaLvtGPpqna9xvv6l6ws` | `claim:sha-256:FPBeUtMvvR54kqX8DSmVNzKiaLvtGPpqna9xvv6l6ws`  |
+
+> All digests shown are base64url-encoded SHA-256 values without padding.
+
+## References
+
+* [Intel SGX DCAP Attestation Architecture](https://www.intel.com/content/www/us/en/developer/articles/technical/intel-software-guard-extensions-data-center-attestation-primitives.html)
+* [RFC 6920: Naming Things with Hashes (ni://)](https://www.rfc-editor.org/rfc/rfc6920)
+* [W3C Subresource Integrity (SRI)](https://www.w3.org/TR/SRI/)
+* [Docker/OCI Image Manifest Specification](https://github.com/opencontainers/image-spec)
+
+## Copyright
+
+Copyright and related rights waived via CC0.