dkimHeader.go

package dkim

import (
	"bytes"
	"fmt"
	"net/mail"
	"net/textproto"
	"strconv"
	"strings"
	"time"
)

type DKIMHeader struct {
	// Version  This tag defines the version of DKIM
	// specification that applies to the signature record.
	// tag v
	Version string

	// The algorithm used to generate the signature..
	// Verifiers MUST support "rsa-sha1" and "rsa-sha256";
	// Signers SHOULD sign using "rsa-sha256".
	// tag a
	Algorithm string

	// The signature data (base64).
	// Whitespace is ignored in this value and MUST be
	// ignored when reassembling the original signature.
	// In particular, the signing process can safely insert
	// FWS in this value in arbitrary places to conform to line-length
	// limits.
	// tag b
	SignatureData string

	// The hash of the canonicalized body part of the message as
	// limited by the "l=" tag (base64; REQUIRED).
	// Whitespace is ignored in this value and MUST be ignored when reassembling the original
	// signature.  In particular, the signing process can safely insert
	// FWS in this value in arbitrary places to conform to line-length
	// limits.
	// tag bh
	BodyHash string

	// Message canonicalization (plain-text; OPTIONAL, default is
	//"simple/simple").  This tag informs the Verifier of the type of
	// canonicalization used to prepare the message for signing.  It
	// consists of two names separated by a "slash" (%d47) character,
	// corresponding to the header and body canonicalization algorithms,
	// respectively.  These algorithms are described in Section 3.4.  If
	// only one algorithm is named, that algorithm is used for the header
	// and "simple" is used for the body.  For example, "c=relaxed" is
	// treated the same as "c=relaxed/simple".
	// tag c
	MessageCanonicalization string

	// The SDID claiming responsibility for an introduction of a message
	//  into the mail stream (plain-text; REQUIRED).  Hence, the SDID
	//  value is used to form the query for the public key.  The SDID MUST
	// correspond to a valid DNS name under which the DKIM key record is
	// published.  The conventions and semantics used by a Signer to
	// create and use a specific SDID are outside the scope of this
	// specification, as is any use of those conventions and semantics.
	// When presented with a signature that does not meet these
	// requirements, Verifiers MUST consider the signature invalid.
	// Internationalized domain names MUST be encoded as A-labels, as
	// described in Section 2.3 of [RFC5890].
	// tag d
	Domain string

	// Signed header fields (plain-text, but see description; REQUIRED).
	// A colon-separated list of header field names that identify the
	// header fields presented to the signing algorithm.  The field MUST
	// contain the complete list of header fields in the order presented
	// to the signing algorithm.  The field MAY contain names of header
	// fields that do not exist when signed; nonexistent header fields do
	// not contribute to the signature computation (that is, they are
	// treated as the null input, including the header field name, the
	// separating colon, the header field value, and any CRLF
	// terminator).  The field MAY contain multiple instances of a header
	// field name, meaning multiple occurrences of the corresponding
	// header field are included in the header hash.  The field MUST NOT
	// include the DKIM-Signature header field that is being created or
	// verified but may include others.  Folding whitespace (FWS) MAY be
	// included on either side of the colon separator.  Header field
	// names MUST be compared against actual header field names in a
	// case-insensitive manner.  This list MUST NOT be empty.  See
	// Section 5.4 for a discussion of choosing header fields to sign and
	// Section 5.4.2 for requirements when signing multiple instances of
	// a single field.
	// tag h
	Headers []string

	// The Agent or User Identifier (AUID) on behalf of which the SDID is
	// taking responsibility (dkim-quoted-printable; OPTIONAL, default is
	// an empty local-part followed by an "@" followed by the domain from
	// the "d=" tag).
	// The syntax is a standard email address where the local-part MAY be
	// omitted.  The domain part of the address MUST be the same as, or a
	// subdomain of, the value of the "d=" tag.
	// Internationalized domain names MUST be encoded as A-labels, as
	// described in Section 2.3 of [RFC5890].
	// tag i
	AUID string

	// Body length count (plain-text unsigned decimal integer; OPTIONAL,
	// default is entire body).  This tag informs the Verifier of the
	// number of octets in the body of the email after canonicalization
	// included in the cryptographic hash, starting from 0 immediately
	// following the CRLF preceding the body.  This value MUST NOT be
	// larger than the actual number of octets in the canonicalized
	// message body.  See further discussion in Section 8.2.
	// tag l
	BodyLength uint

	// A colon-separated list of query methods used to retrieve the
	// public key (plain-text; OPTIONAL, default is "dns/txt").  Each
	// query method is of the form "type[/options]", where the syntax and
	// semantics of the options depend on the type and specified options.
	// If there are multiple query mechanisms listed, the choice of query
	// mechanism MUST NOT change the interpretation of the signature.
	// Implementations MUST use the recognized query mechanisms in the
	// order presented.  Unrecognized query mechanisms MUST be ignored.
	// Currently, the only valid value is "dns/txt", which defines the
	// DNS TXT resource record (RR) lookup algorithm described elsewhere
	// in this document.  The only option defined for the "dns" query
	// type is "txt", which MUST be included.  Verifiers and Signers MUST
	// support "dns/txt".
	// tag q
	QueryMethods []string

	// The selector subdividing the namespace for the "d=" (domain) tag
	// (plain-text; REQUIRED).
	// Internationalized selector names MUST be encoded as A-labels, as
	// described in Section 2.3 of [RFC5890].
	// tag s
	Selector string

	// Signature Timestamp (plain-text unsigned decimal integer;
	// RECOMMENDED, default is an unknown creation time).  The time that
	// this signature was created.  The format is the number of seconds
	// since 00:00:00 on January 1, 1970 in the UTC time zone.  The value
	// is expressed as an unsigned integer in decimal ASCII.  This value
	// is not constrained to fit into a 31- or 32-bit integer.
	// Implementations SHOULD be prepared to handle values up to at least
	// 10^12 (until approximately AD 200,000; this fits into 40 bits).
	// To avoid denial-of-service attacks, implementations MAY consider
	// any value longer than 12 digits to be infinite.  Leap seconds are
	// not counted.  Implementations MAY ignore signatures that have a
	// timestamp in the future.
	// tag t
	SignatureTimestamp time.Time

	// Signature Expiration (plain-text unsigned decimal integer;
	// RECOMMENDED, default is no expiration).  The format is the same as
	// in the "t=" tag, represented as an absolute date, not as a time
	// delta from the signing timestamp.  The value is expressed as an
	// unsigned integer in decimal ASCII, with the same constraints on
	// the value in the "t=" tag.  Signatures MAY be considered invalid
	// if the verification time at the Verifier is past the expiration
	// date.  The verification time should be the time that the message
	// was first received at the administrative domain of the Verifier if
	// that time is reliably available; otherwise, the current time
	// should be used.  The value of the "x=" tag MUST be greater than
	// the value of the "t=" tag if both are present.
	//tag x
	SignatureExpiration time.Time

	// Copied header fields (dkim-quoted-printable, but see description;
	// OPTIONAL, default is null).  A vertical-bar-separated list of
	// selected header fields present when the message was signed,
	// including both the field name and value.  It is not required to
	// include all header fields present at the time of signing.  This
	// field need not contain the same header fields listed in the "h="
	// tag.  The header field text itself must encode the vertical bar
	// ("|", %x7C) character (i.e., vertical bars in the "z=" text are
	// meta-characters, and any actual vertical bar characters in a
	// copied header field must be encoded).  Note that all whitespace
	// must be encoded, including whitespace between the colon and the
	// header field value.  After encoding, FWS MAY be added at arbitrary
	// locations in order to avoid excessively long lines; such
	// whitespace is NOT part of the value of the header field and MUST
	// be removed before decoding.
	// The header fields referenced by the "h=" tag refer to the fields
	// in the [RFC5322] header of the message, not to any copied fields
	// in the "z=" tag.  Copied header field values are for diagnostic
	// use.
	// tag z
	CopiedHeaderFields []string

	// HeaderMailFromDomain store the raw email address of the header Mail From
	// used for verifying in case of multiple DKIM header (we will prioritise
	// header with d = mail from domain)
	//HeaderMailFromDomain string

	// RawForsign represents the raw part (without canonicalization) of the header
	// used for computint sig in verify process
	rawForSign string
}

// NewDkimHeaderBySigOptions return a new DkimHeader initioalized with sigOptions value
func newDkimHeaderBySigOptions(options SigOptions) *DKIMHeader {
	h := new(DKIMHeader)
	h.Version = "1"
	h.Algorithm = options.Algo
	h.MessageCanonicalization = options.Canonicalization
	h.Domain = options.Domain
	h.Headers = options.Headers
	h.AUID = options.Auid
	h.BodyLength = options.BodyLength
	h.QueryMethods = options.QueryMethods
	h.Selector = options.Selector
	if options.AddSignatureTimestamp {
		h.SignatureTimestamp = time.Now()
	}
	if options.SignatureExpireIn > 0 {
		h.SignatureExpiration = time.Now().Add(time.Duration(options.SignatureExpireIn) * time.Second)
	}
	h.CopiedHeaderFields = options.CopiedHeaderFields
	return h
}

// newDkimHeaderFromEmail return a new DKIMHeader by parsing an email
// Note: according to RFC 6376 an email can have multiple DKIM Header
// in this case we return the last inserted or the last with d== mail from
func newDkimHeaderFromEmail(email []byte) (*DKIMHeader, error) {
	m, err := mail.ReadMessage(bytes.NewReader(email))
	if err != nil {
		return nil, err
	}

	// DKIM header ?
	if len(m.Header[textproto.CanonicalMIMEHeaderKey("DKIM-Signature")]) == 0 {
		return nil, ErrDkimHeaderNotFound
	}

	// Get mail from domain
	mailFromDomain := ""
	mailfrom, err := mail.ParseAddress(m.Header.Get(textproto.CanonicalMIMEHeaderKey("From")))
	if err != nil {
		if err.Error() != "mail: no address" {
			return nil, err
		}
	} else {
		t := strings.SplitAfter(mailfrom.Address, "@")
		if len(t) > 1 {
			mailFromDomain = strings.ToLower(t[1])
		}
	}

	// get raw dkim header
	// we can't use m.header because header key will be converted with textproto.CanonicalMIMEHeaderKey
	// ie if key in header is not DKIM-Signature but Dkim-Signature or DKIM-signature ot... other
	// combination of case, verify will fail.
	rawHeaders, _, err := getHeadersBody(email)
	if err != nil {
		return nil, ErrBadMailFormat
	}
	rawHeadersList, err := getHeadersList(rawHeaders)
	if err != nil {
		return nil, err
	}
	dkHeaders := []string{}
	for h := rawHeadersList.Front(); h != nil; h = h.Next() {
		if strings.HasPrefix(strings.ToLower(h.Value.(string)), "dkim-signature") {
			dkHeaders = append(dkHeaders, h.Value.(string))
		}
	}

	var keep *DKIMHeader
	var keepErr error
	//for _, dk := range m.Header[textproto.CanonicalMIMEHeaderKey("DKIM-Signature")] {
	for _, h := range dkHeaders {
		parsed, err := parseDkHeader(h)
		// if malformed dkim header try next
		if err != nil {
			keepErr = err
			continue
		}
		// Keep first dkim headers
		if keep == nil {
			keep = parsed
		}
		// if d flag == domain keep this header and return
		if mailFromDomain == parsed.Domain {
			return parsed, nil
		}
	}
	if keep == nil {
		return nil, keepErr
	}
	return keep, nil
}

// parseDkHeader parse raw dkim header
func parseDkHeader(header string) (dkh *DKIMHeader, err error) {
	dkh = new(DKIMHeader)

	keyVal := strings.SplitN(header, ":", 2)

	t := strings.LastIndex(header, "b=")
	if t == -1 {
		return nil, ErrDkimHeaderBTagNotFound
	}
	dkh.rawForSign = header[0 : t+2]
	p := strings.IndexByte(header[t:], ';')
	if p != -1 {
		dkh.rawForSign = dkh.rawForSign + header[t+p:]
	}

	// Mandatory
	mandatoryFlags := make(map[string]bool, 7) //(b'v', b'a', b'b', b'bh', b'd', b'h', b's')
	mandatoryFlags["v"] = false
	mandatoryFlags["a"] = false
	mandatoryFlags["b"] = false
	mandatoryFlags["bh"] = false
	mandatoryFlags["d"] = false
	mandatoryFlags["h"] = false
	mandatoryFlags["s"] = false

	// default values
	dkh.MessageCanonicalization = "simple/simple"
	dkh.QueryMethods = []string{"dns/txt"}

	// unfold && clean
	val := removeFWS(keyVal[1])
	val = strings.Replace(val, " ", "", -1)

	fs := strings.Split(val, ";")
	for _, f := range fs {
		if f == "" {
			continue
		}
		flagData := strings.SplitN(f, "=", 2)

		// https://github.com/toorop/go-dkim/issues/2
		// if flag is not in the form key=value (eg doesn't have "=")
		if len(flagData) != 2 {
			return nil, ErrDkimHeaderBadFormat
		}
		flag := strings.ToLower(strings.TrimSpace(flagData[0]))
		data := strings.TrimSpace(flagData[1])
		switch flag {
		case "v":
			if data != "1" {
				return nil, ErrDkimVersionNotsupported
			}
			dkh.Version = data
			mandatoryFlags["v"] = true
		case "a":
			dkh.Algorithm = strings.ToLower(data)
			if dkh.Algorithm != "rsa-sha1" && dkh.Algorithm != "rsa-sha256" {
				return nil, ErrSignBadAlgo
			}
			mandatoryFlags["a"] = true
		case "b":
			//dkh.SignatureData = removeFWS(data)
			// remove all space
			dkh.SignatureData = strings.Replace(removeFWS(data), " ", "", -1)
			if len(dkh.SignatureData) != 0 {
				mandatoryFlags["b"] = true
			}
		case "bh":
			dkh.BodyHash = removeFWS(data)
			if len(dkh.BodyHash) != 0 {
				mandatoryFlags["bh"] = true
			}
		case "d":
			dkh.Domain = strings.ToLower(data)
			if len(dkh.Domain) != 0 {
				mandatoryFlags["d"] = true
			}
		case "h":
			data = strings.ToLower(data)
			dkh.Headers = strings.Split(data, ":")
			if len(dkh.Headers) != 0 {
				mandatoryFlags["h"] = true
			}
			fromFound := false
			for _, h := range dkh.Headers {
				if h == "from" {
					fromFound = true
				}
			}
			if !fromFound {
				return nil, ErrDkimHeaderNoFromInHTag
			}
		case "s":
			dkh.Selector = strings.ToLower(data)
			if len(dkh.Selector) != 0 {
				mandatoryFlags["s"] = true
			}
		case "c":
			dkh.MessageCanonicalization, err = validateCanonicalization(strings.ToLower(data))
			if err != nil {
				return nil, err
			}
		case "i":
			if data != "" {
				if !strings.HasSuffix(data, dkh.Domain) {
					return nil, ErrDkimHeaderDomainMismatch
				}
				dkh.AUID = data
			}
		case "l":
			ui, err := strconv.ParseUint(data, 10, 32)
			if err != nil {
				return nil, err
			}
			dkh.BodyLength = uint(ui)
		case "q":
			dkh.QueryMethods = strings.Split(data, ":")
			if len(dkh.QueryMethods) == 0 || strings.ToLower(dkh.QueryMethods[0]) != "dns/txt" {
				return nil, errQueryMethodNotsupported
			}
		case "t":
			ts, err := strconv.ParseInt(data, 10, 64)
			if err != nil {
				return nil, err
			}
			dkh.SignatureTimestamp = time.Unix(ts, 0)

		case "x":
			ts, err := strconv.ParseInt(data, 10, 64)
			if err != nil {
				return nil, err
			}
			dkh.SignatureExpiration = time.Unix(ts, 0)
		case "z":
			dkh.CopiedHeaderFields = strings.Split(data, "|")
		}
	}

	// All mandatory flags are in ?
	for _, p := range mandatoryFlags {
		if !p {
			return nil, ErrDkimHeaderMissingRequiredTag
		}
	}

	// default for i/AUID
	if dkh.AUID == "" {
		dkh.AUID = "@" + dkh.Domain
	}

	// defaut for query method
	if len(dkh.QueryMethods) == 0 {
		dkh.QueryMethods = []string{"dns/text"}
	}

	return dkh, nil
}

// GetHeaderBase return base header for signers
// Todo: some refactoring needed...
func (d *DKIMHeader) getHeaderBaseForSigning(bodyHash string) string {
	h := "DKIM-Signature: v=" + d.Version + "; a=" + d.Algorithm + "; q=" + strings.Join(d.QueryMethods, ":") + "; c=" + d.MessageCanonicalization + ";" + CRLF + TAB
	subh := "s=" + d.Selector + ";"
	if len(subh)+len(d.Domain)+4 > MaxHeaderLineLength {
		h += subh + FWS
		subh = ""
	}
	subh += " d=" + d.Domain + ";"

	// AUID
	if len(d.AUID) != 0 {
		if len(subh)+len(d.AUID)+4 > MaxHeaderLineLength {
			h += subh + FWS
			subh = ""
		}
		subh += " i=" + d.AUID + ";"
	}

	/*h := "DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=tmail.io; i=@tmail.io;" + FWS
	subh := "q=dns/txt; s=test;"*/

	// signature timestamp
	if !d.SignatureTimestamp.IsZero() {
		ts := d.SignatureTimestamp.Unix()
		if len(subh)+14 > MaxHeaderLineLength {
			h += subh + FWS
			subh = ""
		}
		subh += " t=" + fmt.Sprintf("%d", ts) + ";"
	}
	if len(subh)+len(d.Domain)+4 > MaxHeaderLineLength {
		h += subh + FWS
		subh = ""
	}

	// Expiration
	if !d.SignatureExpiration.IsZero() {
		ts := d.SignatureExpiration.Unix()
		if len(subh)+14 > MaxHeaderLineLength {
			h += subh + FWS
			subh = ""
		}
		subh += " x=" + fmt.Sprintf("%d", ts) + ";"
	}

	// body length
	if d.BodyLength != 0 {
		bodyLengthStr := fmt.Sprintf("%d", d.BodyLength)
		if len(subh)+len(bodyLengthStr)+4 > MaxHeaderLineLength {
			h += subh + FWS
			subh = ""
		}
		subh += " l=" + bodyLengthStr + ";"
	}

	// Headers
	if len(subh)+len(d.Headers)+4 > MaxHeaderLineLength {
		h += subh + FWS
		subh = ""
	}
	subh += " h="
	for _, header := range d.Headers {
		if len(subh)+len(header)+1 > MaxHeaderLineLength {
			h += subh + FWS
			subh = ""
		}
		subh += header + ":"
	}
	subh = subh[:len(subh)-1] + ";"

	// BodyHash
	if len(subh)+5+len(bodyHash) > MaxHeaderLineLength {
		h += subh + FWS
		subh = ""
	} else {
		subh += " "
	}
	subh += "bh="
	l := len(subh)
	for _, c := range bodyHash {
		subh += string(c)
		l++
		if l >= MaxHeaderLineLength {
			h += subh + FWS
			subh = ""
			l = 0
		}
	}
	h += subh + ";" + FWS + "b="
	return h
}