gitea-mcp-forward-auth/internal/auth/auth.go

package auth

import (
	"crypto/sha256"
	"crypto/subtle"
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"strings"
)

const EnvTokens = "AUTH_PROXY_TOKENS"

var (
	ErrMissingAuthHeader  = errors.New("missing authorization header")
	ErrInvalidAuthHeader  = errors.New("invalid authorization header format")
	ErrEmptyTokenSet      = errors.New("no tokens configured")
	ErrTokenDirUnreadable = errors.New("unable to read token directory")
)

type Validator struct {
	tokenDigests [][sha256.Size]byte
}

func NewValidator(tokens []string) (*Validator, error) {
	if len(tokens) == 0 {
		return nil, ErrEmptyTokenSet
	}

	unique := make(map[[sha256.Size]byte]struct{}, len(tokens))
	digests := make([][sha256.Size]byte, 0, len(tokens))
	for _, token := range tokens {
		t := strings.TrimSpace(token)
		if t == "" {
			continue
		}
		digest := sha256.Sum256([]byte(t))
		if _, exists := unique[digest]; exists {
			continue
		}
		unique[digest] = struct{}{}
		digests = append(digests, digest)
	}

	if len(digests) == 0 {
		return nil, ErrEmptyTokenSet
	}

	return &Validator{tokenDigests: digests}, nil
}

func (v *Validator) IsValid(token string) bool {
	token = strings.TrimSpace(token)
	if token == "" || v == nil {
		return false
	}

	provided := sha256.Sum256([]byte(token))
	matched := 0
	for _, allowed := range v.tokenDigests {
		matched |= subtle.ConstantTimeCompare(provided[:], allowed[:])
	}

	return matched == 1
}

func (v *Validator) TokenCount() int {
	if v == nil {
		return 0
	}
	return len(v.tokenDigests)
}

func ParseBearerToken(authHeader string) (string, error) {
	authHeader = strings.TrimSpace(authHeader)
	if authHeader == "" {
		return "", ErrMissingAuthHeader
	}

	parts := strings.Fields(authHeader)
	if len(parts) != 2 || !strings.EqualFold(parts[0], "Bearer") || strings.TrimSpace(parts[1]) == "" {
		return "", ErrInvalidAuthHeader
	}

	return parts[1], nil
}

func LoadTokens(tokensDir, tokensCSV string) ([]string, error) {
	tokens := make([]string, 0)

	dirTokens, err := readTokensFromDir(tokensDir)
	if err != nil {
		return nil, err
	}
	tokens = append(tokens, dirTokens...)
	tokens = append(tokens, parseTokensCSV(tokensCSV)...)

	cleaned := make([]string, 0, len(tokens))
	for _, token := range tokens {
		t := strings.TrimSpace(token)
		if t != "" {
			cleaned = append(cleaned, t)
		}
	}

	if len(cleaned) == 0 {
		return nil, ErrEmptyTokenSet
	}

	return cleaned, nil
}

func readTokensFromDir(tokensDir string) ([]string, error) {
	if strings.TrimSpace(tokensDir) == "" {
		return nil, nil
	}

	entries, err := os.ReadDir(tokensDir)
	if err != nil {
		return nil, fmt.Errorf("%w %q: %v", ErrTokenDirUnreadable, tokensDir, err)
	}

	tokens := make([]string, 0, len(entries))
	for _, entry := range entries {
		if entry.IsDir() {
			continue
		}
		path := filepath.Join(tokensDir, entry.Name())
		content, err := os.ReadFile(path)
		if err != nil {
			return nil, fmt.Errorf("read token file %q: %w", path, err)
		}
		tokens = append(tokens, strings.TrimSpace(string(content)))
	}

	return tokens, nil
}

func parseTokensCSV(tokensCSV string) []string {
	if strings.TrimSpace(tokensCSV) == "" {
		return nil
	}
	parts := strings.Split(tokensCSV, ",")
	tokens := make([]string, 0, len(parts))
	for _, part := range parts {
		t := strings.TrimSpace(part)
		if t != "" {
			tokens = append(tokens, t)
		}
	}
	return tokens
}

func TokenFingerprint(token string) string {
	token = strings.TrimSpace(token)
	if token == "" {
		return ""
	}
	sum := sha256.Sum256([]byte(token))
	return fmt.Sprintf("sha256:%x", sum[:6])
}