Backend Phase B (1/2): password auth + sessions

Argon2id password hashing (PHC format, self-encoded/decoded without an
external lib) with constant-time verification, UserStore (create/get by
email and id) and SessionStore (opaque crypto/rand tokens, SHA-256 hashed
in DB, create/lookup/revoke, last_seen_at bump on lookup).

HTTP layer: Register/Login/Logout handlers + RequireAuth middleware.
Login uses a dummy-hash path so unknown-email and wrong-password yield the
same timing/shape, narrowing user enumeration. Tokens accepted via Bearer
header (native clients) or session cookie (HttpOnly, SameSite=Lax).

Routes wired in api.go: POST /auth/register, /auth/login, /auth/logout.
Verified with go test, go vet and an end-to-end smoke test against a real
PostgreSQL container (register/login/logout/duplicate/short-pw/wrong-pw).

OIDC (Phase B part 2) follows next; the issueSession helper is reused.

backend/internal/auth/password.go

@@ -0,0 +1,137 @@
+// Package auth implements password hashing, session management and (later) OIDC
+// verification for the mitbringsl backend.
+package auth
+
+import (
+	"crypto/rand"
+	"crypto/subtle"
+	"encoding/base64"
+	"errors"
+	"fmt"
+	"strconv"
+	"strings"
+
+	"golang.org/x/crypto/argon2"
+)
+
+// Argon2Params holds the cost parameters for the Argon2id key derivation.
+// Defaults follow the RFC-9106 "first recommended" option for memory-hard
+// single-instance hashing (m=64MiB, t=3, p=4). They are deliberately not
+// configurable via env to avoid accidental downgrades; change via code instead.
+type Argon2Params struct {
+	Memory      uint32 // in KiB
+	Iterations  uint32
+	Parallelism uint8
+	SaltLength  uint32 // bytes
+	KeyLength   uint32 // bytes
+}
+
+// DefaultArgon2Params are tuned for a backend with ~256MiB headroom per login.
+// Memory = 64 * 1024 KiB = 64 MiB.
+var DefaultArgon2Params = Argon2Params{
+	Memory:      64 * 1024,
+	Iterations:  3,
+	Parallelism: 4,
+	SaltLength:  16,
+	KeyLength:   32,
+}
+
+// ErrInvalidHash is returned when a stored password hash is not a valid PHC string.
+var ErrInvalidHash = errors.New("invalid password hash")
+
+// HashPassword derives an Argon2id hash from the password and encodes it in the
+// PHC string format: $argon2id$v=19$m=<m>,t=<t>,p=<p>$<salt_b64>$<hash_b64>.
+// The returned string is safe to store verbatim in the users.password_hash column.
+func HashPassword(password string, p Argon2Params) (string, error) {
+	salt := make([]byte, p.SaltLength)
+	if _, err := rand.Read(salt); err != nil {
+		return "", fmt.Errorf("generate salt: %w", err)
+	}
+	key := argon2.IDKey([]byte(password), salt, p.Iterations, p.Memory, p.Parallelism, p.KeyLength)
+	return encodePHC(p, salt, key), nil
+}
+
+// VerifyPassword compares a password against a stored PHC-format hash in constant
+// time. It returns a non-nil error if the hash is malformed or the password does
+// not match.
+func VerifyPassword(password, encoded string) error {
+	p, salt, hash, err := decodePHC(encoded)
+	if err != nil {
+		return err
+	}
+	otherKey := argon2.IDKey([]byte(password), salt, p.Iterations, p.Memory, p.Parallelism, uint32(len(hash)))
+	if subtle.ConstantTimeCompare(hash, otherKey) != 1 {
+		return errors.New("password does not match")
+	}
+	return nil
+}
+
+// --- PHC encoding helpers ----------------------------------------------------
+
+const phcAlg = "argon2id"
+
+func encodePHC(p Argon2Params, salt, key []byte) string {
+	b64 := base64.RawStdEncoding.EncodeToString
+	return fmt.Sprintf("$%s$v=%d$m=%d,t=%d,p=%d$%s$%s",
+		phcAlg, argon2.Version, p.Memory, p.Iterations, p.Parallelism,
+		b64(salt), b64(key))
+}
+
+func decodePHC(encoded string) (Argon2Params, []byte, []byte, error) {
+	parts := strings.Split(encoded, "$")
+	// Expected: ["", "argon2id", "v=19", "m=..,t=..,p=..", "<salt>", "<hash>"]
+	if len(parts) != 6 || parts[1] != phcAlg {
+		return Argon2Params{}, nil, nil, fmt.Errorf("%w: wrong format", ErrInvalidHash)
+	}
+
+	var version int
+	if _, err := fmt.Sscanf(parts[2], "v=%d", &version); err != nil {
+		return Argon2Params{}, nil, nil, fmt.Errorf("%w: parse version: %v", ErrInvalidHash, err)
+	}
+	if version != argon2.Version {
+		return Argon2Params{}, nil, nil, fmt.Errorf("%w: unsupported version %d", ErrInvalidHash, version)
+	}
+
+	p, err := parseParams(parts[3])
+	if err != nil {
+		return Argon2Params{}, nil, nil, err
+	}
+
+	salt, err := base64.RawStdEncoding.DecodeString(parts[4])
+	if err != nil {
+		return Argon2Params{}, nil, nil, fmt.Errorf("%w: decode salt: %v", ErrInvalidHash, err)
+	}
+	key, err := base64.RawStdEncoding.DecodeString(parts[5])
+	if err != nil {
+		return Argon2Params{}, nil, nil, fmt.Errorf("%w: decode key: %v", ErrInvalidHash, err)
+	}
+	p.KeyLength = uint32(len(key))
+	p.SaltLength = uint32(len(salt))
+	return p, salt, key, nil
+}
+
+func parseParams(s string) (Argon2Params, error) {
+	var p Argon2Params
+	for _, kv := range strings.Split(s, ",") {
+		k, v, ok := strings.Cut(kv, "=")
+		if !ok {
+			return Argon2Params{}, fmt.Errorf("%w: malformed param %q", ErrInvalidHash, kv)
+		}
+		n, err := strconv.ParseUint(v, 10, 32)
+		if err != nil {
+			return Argon2Params{}, fmt.Errorf("%w: parse %s: %v", ErrInvalidHash, k, err)
+		}
+		switch k {
+		case "m":
+			p.Memory = uint32(n)
+		case "t":
+			p.Iterations = uint32(n)
+		case "p":
+			p.Parallelism = uint8(n)
+		}
+	}
+	if p.Memory == 0 || p.Iterations == 0 || p.Parallelism == 0 {
+		return Argon2Params{}, fmt.Errorf("%w: missing cost parameter", ErrInvalidHash)
+	}
+	return p, nil
+}

backend/internal/auth/password_test.go

@@ -0,0 +1,67 @@
+package auth
+
+import (
+	"strings"
+	"testing"
+)
+
+// fastParams keeps the test fast while still exercising the full PHC path.
+var fastParams = Argon2Params{Memory: 4 * 1024, Iterations: 1, Parallelism: 1, SaltLength: 16, KeyLength: 32}
+
+func TestHashPassword_PHCFormat(t *testing.T) {
+	h, err := HashPassword("hunter2", fastParams)
+	if err != nil {
+		t.Fatalf("HashPassword: %v", err)
+	}
+	if !strings.HasPrefix(h, "$argon2id$v=19$") {
+		t.Fatalf("unexpected PHC prefix: %s", h)
+	}
+	if strings.Count(h, "$") != 5 {
+		t.Fatalf("expected 5 '$' separators, got %q", h)
+	}
+}
+
+func TestVerifyPassword_RoundTrip(t *testing.T) {
+	pw := "correct horse battery staple"
+	h, err := HashPassword(pw, fastParams)
+	if err != nil {
+		t.Fatalf("HashPassword: %v", err)
+	}
+	if err := VerifyPassword(pw, h); err != nil {
+		t.Fatalf("VerifyPassword correct: %v", err)
+	}
+	if err := VerifyPassword("wrong", h); err == nil {
+		t.Fatal("VerifyPassword wrong: expected error, got nil")
+	}
+}
+
+func TestVerifyPassword_Malformed(t *testing.T) {
+	cases := []string{
+		"",
+		"not-a-hash",
+		"$argon2id$v=19$m=1024,t=1,p=1$AAAA$BB",
+		"$argon2i$v=19$m=1024,t=1,p=1$AAAA$BBBB",
+		"$argon2id$v=99$m=1024,t=1,p=1$AAAA$BBBB",
+		"$argon2id$v=19$m=0,t=1,p=1$AAAA$BBBB",
+	}
+	for _, c := range cases {
+		if err := VerifyPassword("x", c); err == nil {
+			t.Fatalf("VerifyPassword(%q): expected error", c)
+		}
+	}
+}
+
+func TestHashPassword_DifferentSalts(t *testing.T) {
+	a, _ := HashPassword("same", fastParams)
+	b, _ := HashPassword("same", fastParams)
+	if a == b {
+		t.Fatal("two hashes of the same password should differ due to random salt")
+	}
+	// both must still verify against the original password
+	if err := VerifyPassword("same", a); err != nil {
+		t.Fatalf("verify a: %v", err)
+	}
+	if err := VerifyPassword("same", b); err != nil {
+		t.Fatalf("verify b: %v", err)
+	}
+}

backend/internal/auth/pgcode.go

@@ -0,0 +1,14 @@
+package auth
+
+import (
+	"errors"
+
+	"github.com/jackc/pgx/v5/pgconn"
+)
+
+// isUniqueViolation reports whether err is a PostgreSQL unique_violation (SQLSTATE 23505),
+// e.g. from a duplicate INSERT on a UNIQUE column.
+func isUniqueViolation(err error) bool {
+	var pgErr *pgconn.PgError
+	return errors.As(err, &pgErr) && pgErr.Code == "23505"
+}

backend/internal/auth/session.go

@@ -0,0 +1,117 @@
+package auth
+
+import (
+	"context"
+	"crypto/rand"
+	"crypto/sha256"
+	"encoding/base64"
+	"errors"
+	"fmt"
+	"time"
+
+	"github.com/google/uuid"
+	"github.com/jackc/pgx/v5"
+	"github.com/jackc/pgx/v5/pgxpool"
+)
+
+// Session is the application-side view of a row in the sessions table.
+type Session struct {
+	ID        uuid.UUID
+	UserID    uuid.UUID
+	ExpiresAt time.Time
+	RevokedAt *time.Time
+}
+
+// ErrSessionNotFound is returned when no (valid, non-expired, non-revoked)
+// session matches the given token.
+var ErrSessionNotFound = errors.New("session not found")
+
+// SessionConfig controls token generation and persistence.
+type SessionConfig struct {
+	TokenBytes int           // entropy of the raw token before base64url encoding
+	TTL        time.Duration // validity window from creation
+}
+
+// SessionStore wraps database access for the sessions table and handles opaque
+// token generation. Only the SHA-256 hash of a token is ever stored.
+type SessionStore struct {
+	pool *pgxpool.Pool
+	cfg  SessionConfig
+}
+
+// NewSessionStore constructs a SessionStore. tokenBytes must be >= 16.
+func NewSessionStore(pool *pgxpool.Pool, cfg SessionConfig) *SessionStore {
+	if cfg.TokenBytes < 16 {
+		cfg.TokenBytes = 32
+	}
+	return &SessionStore{pool: pool, cfg: cfg}
+}
+
+// Create issues a new session for the user and returns the raw token (to send to
+// the client exactly once) together with the persisted Session row.
+func (s *SessionStore) Create(ctx context.Context, userID uuid.UUID, userAgent, ip string) (token string, sess Session, err error) {
+	raw := make([]byte, s.cfg.TokenBytes)
+	if _, err = rand.Read(raw); err != nil {
+		return "", Session{}, fmt.Errorf("generate session token: %w", err)
+	}
+	token = base64.RawURLEncoding.EncodeToString(raw)
+	tokenHash := hashToken(token)
+	expiresAt := time.Now().Add(s.cfg.TTL)
+
+	const q = `
+		INSERT INTO sessions (user_id, token_hash, expires_at, user_agent, ip)
+		VALUES ($1, $2, $3, NULLIF($4, ''), NULLIF($5, ''))
+		RETURNING id, user_id, expires_at, revoked_at`
+	err = s.pool.QueryRow(ctx, q, userID, tokenHash, expiresAt, userAgent, ip).
+		Scan(&sess.ID, &sess.UserID, &sess.ExpiresAt, &sess.RevokedAt)
+	if err != nil {
+		return "", Session{}, fmt.Errorf("insert session: %w", err)
+	}
+	return token, sess, nil
+}
+
+// Lookup returns the active session for a raw token. It rejects expired and
+// revoked sessions and updates last_seen_at (best-effort, errors logged by caller).
+func (s *SessionStore) Lookup(ctx context.Context, token string) (Session, error) {
+	if token == "" {
+		return Session{}, ErrSessionNotFound
+	}
+	const q = `
+		UPDATE sessions
+		   SET last_seen_at = now()
+		 WHERE token_hash = $1
+		   AND revoked_at IS NULL
+		   AND expires_at > now()
+		 RETURNING id, user_id, expires_at, revoked_at`
+	var sess Session
+	err := s.pool.QueryRow(ctx, q, hashToken(token)).Scan(
+		&sess.ID, &sess.UserID, &sess.ExpiresAt, &sess.RevokedAt)
+	if err != nil {
+		if errors.Is(err, pgx.ErrNoRows) {
+			return Session{}, ErrSessionNotFound
+		}
+		return Session{}, fmt.Errorf("lookup session: %w", err)
+	}
+	return sess, nil
+}
+
+// Revoke marks the session matching token as revoked. Missing tokens are a no-op
+// so logout stays idempotent.
+func (s *SessionStore) Revoke(ctx context.Context, token string) error {
+	if token == "" {
+		return nil
+	}
+	const q = `UPDATE sessions SET revoked_at = now() WHERE token_hash = $1 AND revoked_at IS NULL`
+	_, err := s.pool.Exec(ctx, q, hashToken(token))
+	if err != nil {
+		return fmt.Errorf("revoke session: %w", err)
+	}
+	return nil
+}
+
+// hashToken returns the lowercase hex SHA-256 digest of a raw token. The hash is
+// what we store; the raw token never touches the database.
+func hashToken(token string) string {
+	sum := sha256.Sum256([]byte(token))
+	return base64.RawURLEncoding.EncodeToString(sum[:])
+}

backend/internal/auth/user.go

@@ -0,0 +1,95 @@
+package auth
+
+import (
+	"context"
+	"errors"
+	"fmt"
+
+	"github.com/google/uuid"
+	"github.com/jackc/pgx/v5"
+	"github.com/jackc/pgx/v5/pgxpool"
+)
+
+// User is the application-side view of a row in the users table.
+type User struct {
+	ID           uuid.UUID
+	Email        string
+	PasswordHash *string // nil for OIDC-only accounts
+	OIDCSubject  *string
+	OIDCIssuer   *string
+	DisplayName  *string
+}
+
+// ErrUserNotFound is returned when no user matches the query.
+var ErrUserNotFound = errors.New("user not found")
+
+// UserStore wraps database access for the users table.
+type UserStore struct {
+	pool *pgxpool.Pool
+}
+
+// NewUserStore constructs a UserStore backed by the given pool.
+func NewUserStore(pool *pgxpool.Pool) *UserStore {
+	return &UserStore{pool: pool}
+}
+
+// CreateUser inserts a new user with a password hash (email/password accounts).
+// It returns the created user. Email uniqueness violations surface as ErrEmailTaken.
+var ErrEmailTaken = errors.New("email already registered")
+
+func (s *UserStore) CreateUser(ctx context.Context, email, passwordHash, displayName string) (User, error) {
+	const q = `
+		INSERT INTO users (email, password_hash, display_name)
+		VALUES ($1, $2, NULLIF($3, ''))
+		RETURNING id, email, password_hash, oidc_subject, oidc_issuer, display_name`
+	var u User
+	var dn *string
+	err := s.pool.QueryRow(ctx, q, email, passwordHash, displayName).
+		Scan(&u.ID, &u.Email, &u.PasswordHash, &u.OIDCSubject, &u.OIDCIssuer, &dn)
+	if err != nil {
+		if isUniqueViolation(err) {
+			return User{}, fmt.Errorf("%w: %s", ErrEmailTaken, email)
+		}
+		return User{}, fmt.Errorf("create user: %w", err)
+	}
+	u.DisplayName = dn
+	return u, nil
+}
+
+// GetUserByEmail loads a user by its (case-sensitive) email address.
+func (s *UserStore) GetUserByEmail(ctx context.Context, email string) (User, error) {
+	const q = `
+		SELECT id, email, password_hash, oidc_subject, oidc_issuer, display_name
+		FROM users WHERE email = $1`
+	var u User
+	var dn *string
+	err := s.pool.QueryRow(ctx, q, email).
+		Scan(&u.ID, &u.Email, &u.PasswordHash, &u.OIDCSubject, &u.OIDCIssuer, &dn)
+	if err != nil {
+		if errors.Is(err, pgx.ErrNoRows) {
+			return User{}, ErrUserNotFound
+		}
+		return User{}, fmt.Errorf("get user by email: %w", err)
+	}
+	u.DisplayName = dn
+	return u, nil
+}
+
+// GetUserByID loads a user by its primary key.
+func (s *UserStore) GetUserByID(ctx context.Context, id uuid.UUID) (User, error) {
+	const q = `
+		SELECT id, email, password_hash, oidc_subject, oidc_issuer, display_name
+		FROM users WHERE id = $1`
+	var u User
+	var dn *string
+	err := s.pool.QueryRow(ctx, q, id).
+		Scan(&u.ID, &u.Email, &u.PasswordHash, &u.OIDCSubject, &u.OIDCIssuer, &dn)
+	if err != nil {
+		if errors.Is(err, pgx.ErrNoRows) {
+			return User{}, ErrUserNotFound
+		}
+		return User{}, fmt.Errorf("get user by id: %w", err)
+	}
+	u.DisplayName = dn
+	return u, nil
+}