Security audit: Fix 8 vulnerabilities in authentication, crypto, and input validation

[Copilot]

Security audit identified 8 vulnerabilities across gRPC connections, cryptographic practices, input validation, and error handling.

Critical Fixes

Insecure gRPC connections

• Added SecureConnection() with TLS 1.2+ for production use
• Changed DefaultConnection() to return error instead of log.Fatalf()
• Existing function now deprecated with security warning

Weak cryptographic hash

• Replaced SHA-1 with SHA-256 in test models (hash collision risk)

SSRF vulnerability

• Added URL validation restricting to HTTP/HTTPS schemes
• Applied to NewUploadFromURL(), CopyFromURL(), and getRemoteFile()

Panic on invalid input (DoS vector)

• HashID() and HashCID() now return (ID, error) instead of panicking
• Added ValidateEntityState() helper for pre-validation

Path traversal

• Added path sanitization using filepath.Clean() with .. detection
• Applied to NewUpload() and NewUploadFromURL()

HTTP client security

• Created secureHTTPClient() with 30s timeout and connection limits
• Replaced all http.DefaultClient usage

API Changes

// Before
conn := keystone.DefaultConnection(host, port, vendorID, appID, token)
obj := keystone.NewUpload("path/file.txt", proto.ObjectType_Standard)
id := keystone.HashID("user-input")

// After
conn, err := keystone.SecureConnection(host, port, vendorID, appID, token)
obj, err := keystone.NewUpload("path/file.txt", proto.ObjectType_Standard)
id, err := keystone.HashID("user-input")

New Error Variables

• ErrHashIDContainsInvalidChar
• ErrInvalidEntityState
• ErrInvalidURL
• ErrUnsupportedScheme
• ErrInvalidPath

Documentation

• SECURITY.md - Security guidelines and best practices
• MIGRATION.md - Migration guide with code examples
• SECURITY_AUDIT_SUMMARY.md - Complete audit findings

Compliance Impact

Addresses OWASP A01:2021 (Broken Access Control), A02:2021 (Cryptographic Failures), CWE-918 (SSRF), CWE-22 (Path Traversal), CWE-327 (Weak Crypto).

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