Replace SHA-1 with SHA-256 for X.509 SubjectKeyIdentifier computation

[G-Gobi]

Summary

Fixes #39

• Removes the deprecated crypto/sha1 import and bigIntHash(*big.Int) method, which hashed only the RSA key modulus (privateKey.N) using SHA-1.
• Introduces computeSubjectKeyId(*rsa.PublicKey) implementing RFC 7093 Method 4: SHA-256 of the full DER-encoded SubjectPublicKeyInfo structure via x509.MarshalPKIXPublicKey.
• All 52 existing unit tests pass without modification.

Why this matters

SHA-1 is deprecated by NIST SP 800-131A Rev 2 and is explicitly disallowed under FIPS 140-2/3 compliance requirements. Any deployment of config-server in a FIPS-enforcing environment (e.g. US Government, regulated industries) will fail cert generation at runtime with the current implementation.

What changed and why

	Before	After
Hash algorithm	SHA-1	SHA-256
Hash input	privateKey.N (modulus only)	Full SubjectPublicKeyInfo DER
Output size	20 bytes	32 bytes
RFC alignment	Non-standard (missing exponent + algorithm OID)	RFC 7093 Method 4

The previous implementation was doubly incorrect: beyond using SHA-1, it only hashed the RSA modulus N, omitting the public exponent E and the algorithm identifier. Two keys with the same modulus but different exponents could produce the same SubjectKeyId. The new implementation hashes the complete SubjectPublicKeyInfo as defined in RFC 5280 / RFC 7093.

Impact on existing deployments

• Existing certificates in the database are not affected — only newly generated certs are changed.
• Chain validation is unaffected — TLS chain verification uses cryptographic signatures, not SKI/AKI values.
• Mixed environments (new certs signed by old SHA-1-SKI CAs): the AKI on the new cert is copied directly from the signing CA's SubjectKeyId field, so it correctly reflects the CA's original 20-byte value. RFC 5280 does not require AKI and SKI to be the same size.

Test plan

• go build ./types/... compiles cleanly
• go test ./types/... 52/52 specs pass
• Verified SubjectKeyId is non-nil on generated certs
• Verified self-signed CA SubjectKeyId == AuthorityKeyId
• Verified leaf cert AuthorityKeyId == signingCA.SubjectKeyId

References

• NIST SP 800-131A Rev 2 - SHA-1 deprecation
• RFC 5280 Section 4.2.1.2 - SubjectKeyIdentifier extension
• RFC 7093 - Additional Methods for Generating Key Identifiers

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: G-Gobi / name: Gobi Ganesan (85fa45a)

[Copilot]

Pull request overview

This PR updates X.509 certificate generation to compute SubjectKeyId using SHA-256 over the full DER-encoded SubjectPublicKeyInfo (via x509.MarshalPKIXPublicKey), replacing the previous SHA-1 hash over only the RSA modulus.

Changes:

• Replaced SHA-1–based SubjectKeyId generation with SHA-256 over DER-encoded SubjectPublicKeyInfo.
• Removed the old modulus-only hashing helper and added a dedicated computeSubjectKeyId helper with error handling.

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