// Package der checks that bytes are one element in the Distinguished Encoding
// Rules of X.690, as spec v0.11 §29.10 asks of a CMS signature before it
// looks inside it: definite and minimal lengths, no high tag numbers, no
// constructed form of a type that DER only has primitive, canonical BOOLEAN,
// INTEGER, NULL, OBJECT IDENTIFIER, BIT STRING, UTCTime and GeneralizedTime,
// and no bytes after the element. Go's encoding/asn1 accepts some of what DER
// forbids, and a signature that two implementations read the same must not
// depend on it.
//
// The order of the elements of a SET OF cannot be checked without a schema:
// SetOfSorted does it for the callers that know theirs.
package der

import (
	"bytes"
	"errors"
	"fmt"
	"strings"
	"time"
)

const maxDepth = 32

// Check returns nil when b is exactly one DER element.
func Check(b []byte) error {
	n, err := check(b, 0)
	if err != nil {
		return err
	}
	if n != len(b) {
		return fmt.Errorf("der: %d bytes after the element", len(b)-n)
	}
	return nil
}

// Split returns the content of the DER element b, which must be constructed,
// as the encodings of its children, and the identifier octet of b. It
// assumes Check passed.
func Split(b []byte) (id byte, children [][]byte, err error) {
	if len(b) == 0 || b[0]&0x20 == 0 {
		return 0, nil, errors.New("der: not a constructed element")
	}
	hl, cl, err := header(b)
	if err != nil {
		return 0, nil, err
	}
	rest := b[hl : hl+cl]
	for len(rest) > 0 {
		h, c, err := header(rest)
		if err != nil {
			return 0, nil, err
		}
		children = append(children, rest[:h+c])
		rest = rest[h+c:]
	}
	return b[0], children, nil
}

// Content returns the content octets of the DER element b.
func Content(b []byte) ([]byte, error) {
	hl, cl, err := header(b)
	if err != nil {
		return nil, err
	}
	return b[hl : hl+cl], nil
}

// SetOfSorted reports whether the encodings are in ascending order of their
// bytes, as DER requires of the elements of a SET OF (X.690 11.6). Equal
// elements may repeat, side by side: X.690 does not forbid it, and a time-
// stamping authority may send its certificate twice. Whoever counts the
// elements of a SET OF decides what a repetition means.
func SetOfSorted(elems [][]byte) bool {
	for i := 1; i < len(elems); i++ {
		if bytes.Compare(elems[i-1], elems[i]) > 0 {
			return false
		}
	}
	return true
}

// ParseTime reads a UTCTime or a GeneralizedTime element as DER writes them
// (X.690 11.7 and 11.8): YYMMDDHHMMSSZ, with the years 50 to 99 in the 20th
// century (RFC 5280 4.1.2.5.1), or YYYYMMDDHHMMSS, an optional fraction of
// seconds without a trailing zero, and Z. A date or a time that does not
// exist, a second 60 included, is refused. fraction reports whether a
// GeneralizedTime has one.
func ParseTime(b []byte) (t time.Time, fraction bool, err error) {
	if len(b) < 2 || (b[0] != 0x17 && b[0] != 0x18) {
		return time.Time{}, false, errors.New("der: not a UTCTime or a GeneralizedTime")
	}
	c, err := Content(b)
	if err != nil {
		return time.Time{}, false, err
	}
	return parseTime(b[0], string(c))
}

func parseTime(tag byte, s string) (time.Time, bool, error) {
	bad := errors.New("der: a time that is not in the form of DER")
	if len(s) < 13 || s[len(s)-1] != 'Z' {
		return time.Time{}, false, bad
	}
	body, frac := s[:len(s)-1], ""
	var year int
	switch tag {
	case 0x17:
		if len(body) != 12 || !digits(body) {
			return time.Time{}, false, bad
		}
		year = atoi(body[:2]) + 1900
		if year < 1950 {
			year += 100
		}
		body = body[2:]
	default:
		if dot := strings.IndexByte(body, '.'); dot >= 0 {
			body, frac = body[:dot], body[dot+1:]
			if frac == "" || frac[len(frac)-1] == '0' || !digits(frac) {
				return time.Time{}, false, bad
			}
		}
		if len(body) != 14 || !digits(body) {
			return time.Time{}, false, bad
		}
		year = atoi(body[:4])
		body = body[4:]
	}
	month, day, hour, minute, second := atoi(body[0:2]), atoi(body[2:4]), atoi(body[4:6]), atoi(body[6:8]), atoi(body[8:10])
	if month < 1 || month > 12 || day < 1 || hour > 23 || minute > 59 || second > 59 ||
		day > time.Date(year, time.Month(month)+1, 0, 0, 0, 0, 0, time.UTC).Day() {
		return time.Time{}, false, errors.New("der: a date or a time that does not exist")
	}
	nanos := 0
	for i := 0; i < 9; i++ {
		nanos *= 10
		if i < len(frac) {
			nanos += int(frac[i] - '0')
		}
	}
	return time.Date(year, time.Month(month), day, hour, minute, second, nanos, time.UTC), frac != "", nil
}

func digits(s string) bool {
	for i := 0; i < len(s); i++ {
		if s[i] < '0' || s[i] > '9' {
			return false
		}
	}
	return s != ""
}

func atoi(s string) int {
	n := 0
	for i := 0; i < len(s); i++ {
		n = n*10 + int(s[i]-'0')
	}
	return n
}

// header returns the length of the identifier and length octets of the
// element at the start of b, and the length of its content, which must fit
// in b.
func header(b []byte) (headerLen, contentLen int, err error) {
	if len(b) < 2 {
		return 0, 0, errors.New("der: truncated element")
	}
	if b[0]&0x1f == 0x1f {
		return 0, 0, errors.New("der: a tag number of 31 or more")
	}
	l := b[1]
	switch {
	case l < 0x80:
		headerLen, contentLen = 2, int(l)
	case l == 0x80:
		return 0, 0, errors.New("der: an indefinite length")
	case l == 0xff:
		return 0, 0, errors.New("der: a length of 0xff")
	default:
		n := int(l & 0x7f)
		if n > 4 || len(b) < 2+n {
			return 0, 0, errors.New("der: a length that does not fit")
		}
		if b[2] == 0 {
			return 0, 0, errors.New("der: a length with a leading zero")
		}
		v := 0
		for _, c := range b[2 : 2+n] {
			v = v<<8 | int(c)
		}
		if v < 0x80 {
			return 0, 0, errors.New("der: a long form for a length under 128")
		}
		headerLen, contentLen = 2+n, v
	}
	if contentLen < 0 || contentLen > len(b)-headerLen {
		return 0, 0, errors.New("der: an element longer than its container")
	}
	return headerLen, contentLen, nil
}

// check validates the element at the start of b and returns its length.
func check(b []byte, depth int) (int, error) {
	if depth > maxDepth {
		return 0, errors.New("der: nested too deep")
	}
	hl, cl, err := header(b)
	if err != nil {
		return 0, err
	}
	content := b[hl : hl+cl]
	id := b[0]
	class, constructed, tag := id>>6, id&0x20 != 0, id&0x1f
	if constructed {
		if class == 0 && tag != 16 && tag != 17 {
			return 0, fmt.Errorf("der: constructed form of the universal type %d", tag)
		}
		for len(content) > 0 {
			n, err := check(content, depth+1)
			if err != nil {
				return 0, err
			}
			content = content[n:]
		}
		return hl + cl, nil
	}
	if class == 0 {
		if err := checkPrimitive(tag, content); err != nil {
			return 0, err
		}
	}
	return hl + cl, nil
}

func checkPrimitive(tag byte, c []byte) error {
	switch tag {
	case 1: // BOOLEAN
		if len(c) != 1 || (c[0] != 0 && c[0] != 0xff) {
			return errors.New("der: a BOOLEAN that is not 00 or FF")
		}
	case 2, 10: // INTEGER, ENUMERATED
		if len(c) == 0 {
			return errors.New("der: an empty INTEGER")
		}
		if len(c) > 1 && (c[0] == 0 && c[1]&0x80 == 0 || c[0] == 0xff && c[1]&0x80 != 0) {
			return errors.New("der: an INTEGER that is not minimal")
		}
	case 3: // BIT STRING
		if len(c) == 0 || c[0] > 7 || len(c) == 1 && c[0] != 0 {
			return errors.New("der: a malformed BIT STRING")
		}
		if len(c) > 1 && c[0] != 0 && c[len(c)-1]&(1<<c[0]-1) != 0 {
			return errors.New("der: a BIT STRING with unused bits that are not zero")
		}
	case 5: // NULL
		if len(c) != 0 {
			return errors.New("der: a NULL with content")
		}
	case 6: // OBJECT IDENTIFIER
		if len(c) == 0 || c[len(c)-1]&0x80 != 0 {
			return errors.New("der: a malformed OBJECT IDENTIFIER")
		}
		start := true
		for _, x := range c {
			if start && x == 0x80 {
				return errors.New("der: an OBJECT IDENTIFIER with a leading 0x80 in a subidentifier")
			}
			start = x&0x80 == 0
		}
	case 23, 24: // UTCTime, GeneralizedTime
		if _, _, err := parseTime(tag, string(c)); err != nil {
			return err
		}
	case 4, 12, 19, 20, 22, 26, 28, 30: // OCTET STRING and the string types of X.509
	case 16, 17:
		return fmt.Errorf("der: the universal type %d in primitive form", tag)
	default:
		// 0 is the end of contents of BER, and the rest are types that no
		// certificate, signature or token of the profile has.
		return fmt.Errorf("der: the universal type %d, which the profile does not use", tag)
	}
	return nil
}