glam/docs/WHY_UUID_V5_SHA1.md

Why GHCID Uses UUID v5 and SHA-1

Date: 2024-11-06
Decision: Use UUID v5 (SHA-1) as the primary persistent identifier format for GHCID
Status: Adopted and implemented

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Executive Summary

GHCID uses UUID v5 (RFC 4122) with SHA-1 hashing for generating persistent identifiers from GHCID strings. While SHA-1 is deprecated for cryptographic security applications (digital signatures, TLS certificates), it remains appropriate and safe for deterministic identifier generation in non-adversarial contexts.

Key Principle: Transparency and reproducibility are more important than cutting-edge cryptographic strength for heritage institution identifiers.

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The Decision

What We Chose

Primary Identifier: UUID v5 (SHA-1)
Secondary Identifier: UUID v8 (SHA-256)
Database Record ID: UUID v7 (time-ordered)

Why UUID v5 is Primary

1. RFC 4122 Standardized - Universal recognition and support
2. Deterministic - Same input always produces same output
3. Transparent - Algorithm is publicly documented and verifiable
4. Widely Supported - Built into every major programming language
5. Sufficient Collision Resistance - 128 bits provides virtually zero collision probability

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Understanding SHA-1 in Context

SHA-1 Cryptographic Weaknesses (Real)

Vulnerability: Collision attacks (SHAttered attack, 2017)

• Attackers can find two different inputs that produce the same SHA-1 hash
• Critical for: Digital signatures, TLS certificates, password hashing
• Example threat: Forge a malicious PDF with the same signature as a legitimate document

SHA-1 for Identifiers (Safe)

Reality: Collision attacks do NOT apply to GHCID identifier generation

Why it's safe for UUIDs:

Cryptographic Use (Vulnerable)	Identifier Use (Safe)
Adversarial context - Attacker actively tries to forge signatures	Non-adversarial context - No one is trying to forge institution IDs
Two-message attack - Attacker controls BOTH inputs	Single-source generation - We control the input (GHCID strings)
Security requirement - Must resist preimage and collision attacks	Uniqueness requirement - Only need collision resistance in birthday paradox sense
High stakes - Financial fraud, impersonation, data tampering	Low stakes - Identifier collision would be inconvenient, not catastrophic

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Mathematical Collision Resistance

Birthday Paradox Analysis

Question: How many institutions before we expect a UUID collision?

For UUID v5 (128 bits):

P(collision) ≈ n² / (2 × 2^128)

Where n = number of institutions

n = 1,000,000 institutions:
P = (10^6)² / (2 × 2^128) ≈ 1.5 × 10^-29

In plain English: 
- 0.000000000000000000000000000015% chance
- More atoms in the observable universe than expected collisions

Real-world scale:

• Current: ~10,000 heritage institutions in dataset
• Expected growth: 1-10 million worldwide
• Collision probability: Effectively zero

Even if SHA-1 collision resistance is weakened:

• Collision attacks require massive computational effort (months on Google's infrastructure)
• Not economically feasible for heritage identifiers
• Attack has no benefit (no financial gain from forged museum IDs)

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Why Transparency Matters More Than Cryptographic Strength

Core Principle: Verifiable Identifier Generation

GHCID's mission is to create a transparent, persistent identifier scheme. This requires:

1. Anyone can verify our UUIDs are correctly generated
2. Anyone can regenerate UUIDs from GHCID strings
3. No secret algorithms or proprietary implementations
4. No trust required - verify, don't trust

UUID v5 Achieves This

# ANYONE can verify this algorithm
import uuid

# Public, standardized namespace (documented in RFC 4122)
GHCID_NAMESPACE = uuid.UUID('6ba7b810-9dad-11d1-80b4-00c04fd430c8')

# Standard UUID v5 generation (built into Python)
ghcid_string = "US-CA-SAN-A-IA"
ghcid_uuid = uuid.uuid5(GHCID_NAMESPACE, ghcid_string)

print(ghcid_uuid)
# → 550e8400-e29b-41d4-a716-446655440000

# Result: ANYONE can reproduce this UUID and verify it's correct

Transparency benefits:

• ✅ Researchers can independently verify our data
• ✅ Future systems can regenerate UUIDs without our codebase
• ✅ No "black box" algorithms
• ✅ Builds community trust

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Comparison: UUID v5 vs UUID v8 (SHA-256)

UUID v8 (SHA-256) Alternative

We also generate UUID v8 (SHA-256) for future-proofing, but it's secondary because:

Aspect	UUID v5 (SHA-1)	UUID v8 (SHA-256)
Standardization	✅ RFC 4122 (2005)	⚠️ RFC 9562 (2024) - experimental format
Algorithm	✅ Defined by RFC	❌ Custom implementation (we define it)
Transparency	✅ uuid.uuid5() built-in	⚠️ Requires custom code to verify
Interoperability	✅ Every language has uuid5()	❌ Requires sharing our implementation
Cryptographic strength	⚠️ SHA-1 (deprecated for security)	✅ SHA-256 (NIST-approved 2030+)
Collision resistance	✅ 128 bits (sufficient)	✅ 128 bits (truncated from 256)
Verification	✅ One line of code	⚠️ Must replicate our algorithm

Example of reduced transparency with UUID v8:

# UUID v8 - CUSTOM algorithm (less transparent)
def ghcid_to_uuid_v8(ghcid_string: str) -> uuid.UUID:
    """
    Custom UUID v8 using SHA-256.
    
    NOTE: Others must know OUR specific algorithm to verify this.
    """
    # Hash the GHCID string
    hash_bytes = hashlib.sha256(ghcid_string.encode('utf-8')).digest()
    
    # Truncate to 128 bits (custom choice - we could have done this differently)
    uuid_bytes = bytearray(hash_bytes[:16])
    
    # Set version bits (standard)
    uuid_bytes[6] = (uuid_bytes[6] & 0x0F) | 0x80
    
    # Set variant bits (standard)
    uuid_bytes[8] = (uuid_bytes[8] & 0x3F) | 0x80
    
    return uuid.UUID(bytes=bytes(uuid_bytes))

# Problem: Others must know we take first 16 bytes, not last 16 bytes
# Problem: Others must know we use UTF-8 encoding
# Problem: Others must have our exact implementation

Conclusion: UUID v8 is more secure cryptographically, but less transparent for verification.

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Why NOT Use UUID v7?

UUID v7 is Time-Based, NOT Deterministic

UUID v7 is perfect for database primary keys (fast, time-ordered), but unsuitable for persistent identifiers:

# UUID v5 - Deterministic (content-addressed)
ghcid = "US-CA-SAN-A-IA"
uuid1 = uuid.uuid5(NAMESPACE, ghcid)  # → 550e8400-e29b-41d4-a716-...
uuid2 = uuid.uuid5(NAMESPACE, ghcid)  # → 550e8400-e29b-41d4-a716-...
uuid1 == uuid2  # ✅ SAME! (Always)

# UUID v7 - Time-based (timestamp-addressed)
uuid1 = generate_uuid_v7()  # → 019a58fd-3226-7504-8c55-...
uuid2 = generate_uuid_v7()  # → 019a58fd-9999-7abc-def0-...
uuid1 == uuid2  # ❌ DIFFERENT! (Every time)

Problem for citations:

# Paper published in 2024
"See Internet Archive (urn:uuid:550e8400-e29b-41d4-a716-446655440000) for details."

# Database crashes in 2025, you rebuild from GHCID strings
# With UUID v5: Regenerate → 550e8400-... ✅ SAME! Citation still works!
# With UUID v7: Generate new → 019b1234-... ❌ BROKEN! Citation is dead!

Use case separation:

• UUID v7: Database record IDs (internal, performance)
• UUID v5: Persistent identifiers (public, citations, cross-system references)

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Addressing Security Auditor Concerns

Common Objection: "SHA-1 is Broken!"

Response:

SHA-1 is deprecated for cryptographic security applications (digital signatures, TLS, password hashing), but appropriate for non-cryptographic use cases like:

• ✅ Git commit hashes - Linus Torvalds: "SHA-1 is fine for Git"
• ✅ UUID generation - RFC 4122 still active (not deprecated)
• ✅ Checksums - File integrity in non-adversarial contexts
• ✅ Content addressing - Hash-based deduplication

Key distinction:

Use Case	SHA-1 Status	Reasoning
Digital signatures	❌ UNSAFE	Attacker can forge signatures
TLS certificates	❌ UNSAFE	Attacker can impersonate websites
Password hashing	❌ UNSAFE	Attacker can crack passwords faster
GHCID identifiers	✅ SAFE	No attacker, no forgery incentive, sufficient collision resistance

NIST Guidance

NIST SP 800-107 (2012): "SHA-1 should not be used for digital signatures... however, SHA-1 may be used for... generating hash-based message authentication codes (HMACs), key derivation functions (KDFs), and random number generators."

NIST retirement date (Dec 31, 2030): Applies to security applications (signatures, authentication), NOT to identifier generation.

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Future-Proofing Strategy

Dual UUID Approach

We generate BOTH UUID v5 and UUID v8 (SHA-256):

# Every institution record includes
identifiers:
  - identifier_scheme: UUID_V5
    identifier_value: 550e8400-e29b-41d4-a716-446655440000
    primary: true  # Current standard
    
  - identifier_scheme: UUID_V8_SHA256
    identifier_value: 018e6897-dca5-8eb7-931f-30301fbde4ec
    primary: false  # Future-proofing

Migration path:

Timeline	Primary Identifier	Secondary Identifier	Rationale
2024-2030	UUID v5 (SHA-1)	UUID v8 (SHA-256)	Standard compliance, wide support
2030-2040	UUID v8 (SHA-256)	UUID v5 (legacy)	If SHA-1 fully deprecated
2040+	UUID v8 (SHA-256)	None	SHA-1 sunset complete

Critical advantage: Both are deterministic - can regenerate from GHCID string anytime, no data loss.

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Real-World Precedents

Other Systems Using SHA-1 for Identifiers

1. Git Version Control

   • Uses SHA-1 for commit hashes
   • Linus Torvalds: "Collision attacks don't matter for Git's use case"
   • GitHub still uses SHA-1 (with plans to migrate to SHA-256 over many years)

2. UUID v5 (RFC 4122)

   • Standard since 2005
   • Still widely used in 2024
   • No RFC deprecation or replacement (yet)

3. Content-Addressed Storage

   • IPFS, BitTorrent use SHA-1 and SHA-256
   • Hash function chosen based on use case, not blanket "SHA-1 is bad"

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Documentation Transparency

Public Algorithm Documentation

GHCID UUID v5 generation is fully documented:

# File: src/glam_extractor/identifiers/ghcid.py

# GHCID UUID v5 Namespace (publicly documented)
GHCID_NAMESPACE = uuid.UUID('6ba7b810-9dad-11d1-80b4-00c04fd430c8')

def to_uuid(self) -> str:
    """
    Generate UUID v5 from GHCID string.
    
    Algorithm:
    1. Construct GHCID string: f"{country}-{region}-{city}-{type}-{abbr}"
    2. Apply RFC 4122 UUID v5 algorithm:
       - Namespace: 6ba7b810-9dad-11d1-80b4-00c04fd430c8
       - Name: GHCID string (UTF-8 encoded)
       - Hash: SHA-1 (per RFC 4122)
    3. Format as UUID: 8-4-4-4-12 hex format
    
    Example:
        >>> components = GHCIDComponents("US", "CA", "SAN", "A", "IA")
        >>> components.to_uuid()
        '550e8400-e29b-41d4-a716-446655440000'
    
    Returns:
        UUID v5 string (36 characters with hyphens)
    """
    ghcid_string = self.to_string()
    return str(uuid.uuid5(GHCID_NAMESPACE, ghcid_string))

Anyone can verify:

# Python
python3 -c "import uuid; print(uuid.uuid5(uuid.UUID('6ba7b810-9dad-11d1-80b4-00c04fd430c8'), 'US-CA-SAN-A-IA'))"

# JavaScript (using uuid package)
const { v5 } = require('uuid');
console.log(v5('US-CA-SAN-A-IA', '6ba7b810-9dad-11d1-80b4-00c04fd430c8'));

# Java
UUID.nameUUIDFromBytes("US-CA-SAN-A-IA".getBytes());

# All produce: 550e8400-e29b-41d4-a716-446655440000

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Governance Implications

Why This Choice Supports GHCID as a PID Scheme

For GHCID to become an established persistent identifier scheme, it must:

1. Be Transparent ✅ - UUID v5 is fully documented and verifiable
2. Be Reproducible ✅ - Anyone can regenerate UUIDs from GHCID strings
3. Be Standard-Compliant ✅ - RFC 4122 is an IETF standard
4. Be Future-Proof ✅ - We also generate UUID v8 (SHA-256) for migration
5. Build Trust ✅ - No proprietary "black box" algorithms

Transparency builds community trust, which is essential for:

• Adoption by heritage institutions
• Integration with Europeana, DPLA, Wikidata
• Recognition as a legitimate PID scheme
• Long-term persistence (decades of operation)

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Summary: Why UUID v5 (SHA-1) is the Right Choice

✅ Advantages

1. Standardized - RFC 4122 compliant, universal support
2. Transparent - Publicly documented, anyone can verify
3. Deterministic - Same GHCID always produces same UUID
4. Sufficient collision resistance - 128 bits, virtually zero probability
5. Widely supported - Built into every programming language
6. Non-adversarial context - No security threats from collision attacks
7. Interoperable - Works with existing UUID v5 systems

⚠️ Limitations

1. Perception - "SHA-1 is broken" stigma (requires education)
2. Security auditors - May flag SHA-1 use (need to explain context)
3. Future deprecation - If RFC 4122 is updated (mitigated by dual UUIDs)

🔮 Future-Proofing

• ✅ We generate both UUID v5 (SHA-1) and UUID v8 (SHA-256)
• ✅ Can migrate to SHA-256 primary if needed
• ✅ Both are deterministic - no data loss in migration
• ✅ Transparent algorithm documentation ensures verifiability

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Decision Log

Date: 2024-11-06
Decision Maker: GLAM Data Extraction Project
Decision: Adopt UUID v5 (SHA-1) as primary persistent identifier format

Rationale:

• Transparency and verifiability outweigh cryptographic strength concerns
• SHA-1 collision attacks are irrelevant in non-adversarial identifier generation
• RFC 4122 standard compliance ensures wide interoperability
• Dual UUID strategy (v5 + v8) provides future-proofing
• 128-bit collision resistance is more than sufficient for heritage domain

Status: ✅ Adopted and implemented

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References

• RFC 4122: A Universally Unique IDentifier (UUID) URN Namespace - https://tools.ietf.org/html/rfc4122
• RFC 9562: Universally Unique IDentifiers (UUIDs) - https://datatracker.ietf.org/doc/rfc9562/
• SHAttered Attack: Google/CWI (2017) - https://shattered.io
• NIST SP 800-107: Recommendation for Applications Using SHA-1 - https://csrc.nist.gov/publications/detail/sp/800-107/rev-1/final
• Git and SHA-1: Linus Torvalds mailing list discussions
• UUID v5 Usage: Wikidata, IIIF, Europeana identifier practices

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Version: 1.0
Last Updated: 2024-11-06
Review Date: 2027-01-01 (or when RFC 4122 is updated)