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|
package pkcs7
import (
"bytes"
"errors"
)
// var encodeIndent = 0
type asn1Object interface {
EncodeTo(writer *bytes.Buffer) error
}
type asn1Structured struct {
tagBytes []byte
content []asn1Object
}
func (s asn1Structured) EncodeTo(out *bytes.Buffer) error {
//fmt.Printf("%s--> tag: % X\n", strings.Repeat("| ", encodeIndent), s.tagBytes)
//encodeIndent++
inner := new(bytes.Buffer)
for _, obj := range s.content {
err := obj.EncodeTo(inner)
if err != nil {
return err
}
}
//encodeIndent--
out.Write(s.tagBytes)
encodeLength(out, inner.Len())
out.Write(inner.Bytes())
return nil
}
type asn1Primitive struct {
tagBytes []byte
length int
content []byte
}
func (p asn1Primitive) EncodeTo(out *bytes.Buffer) error {
_, err := out.Write(p.tagBytes)
if err != nil {
return err
}
if err = encodeLength(out, p.length); err != nil {
return err
}
//fmt.Printf("%s--> tag: % X length: %d\n", strings.Repeat("| ", encodeIndent), p.tagBytes, p.length)
//fmt.Printf("%s--> content length: %d\n", strings.Repeat("| ", encodeIndent), len(p.content))
out.Write(p.content)
return nil
}
func ber2der(ber []byte) ([]byte, error) {
if len(ber) == 0 {
return nil, errors.New("ber2der: input ber is empty")
}
//fmt.Printf("--> ber2der: Transcoding %d bytes\n", len(ber))
out := new(bytes.Buffer)
obj, _, err := readObject(ber, 0)
if err != nil {
return nil, err
}
obj.EncodeTo(out)
// if offset < len(ber) {
// return nil, fmt.Errorf("ber2der: Content longer than expected. Got %d, expected %d", offset, len(ber))
//}
return out.Bytes(), nil
}
// encodes lengths that are longer than 127 into string of bytes
func marshalLongLength(out *bytes.Buffer, i int) (err error) {
n := lengthLength(i)
for ; n > 0; n-- {
err = out.WriteByte(byte(i >> uint((n-1)*8)))
if err != nil {
return
}
}
return nil
}
// computes the byte length of an encoded length value
func lengthLength(i int) (numBytes int) {
numBytes = 1
for i > 255 {
numBytes++
i >>= 8
}
return
}
// encodes the length in DER format
// If the length fits in 7 bits, the value is encoded directly.
//
// Otherwise, the number of bytes to encode the length is first determined.
// This number is likely to be 4 or less for a 32bit length. This number is
// added to 0x80. The length is encoded in big endian encoding follow after
//
// Examples:
// length | byte 1 | bytes n
// 0 | 0x00 | -
// 120 | 0x78 | -
// 200 | 0x81 | 0xC8
// 500 | 0x82 | 0x01 0xF4
//
func encodeLength(out *bytes.Buffer, length int) (err error) {
if length >= 128 {
l := lengthLength(length)
err = out.WriteByte(0x80 | byte(l))
if err != nil {
return
}
err = marshalLongLength(out, length)
if err != nil {
return
}
} else {
err = out.WriteByte(byte(length))
if err != nil {
return
}
}
return
}
func readObject(ber []byte, offset int) (asn1Object, int, error) {
//fmt.Printf("\n====> Starting readObject at offset: %d\n\n", offset)
tagStart := offset
b := ber[offset]
offset++
tag := b & 0x1F // last 5 bits
if tag == 0x1F {
tag = 0
for ber[offset] >= 0x80 {
tag = tag*128 + ber[offset] - 0x80
offset++
}
tag = tag*128 + ber[offset] - 0x80
offset++
}
tagEnd := offset
kind := b & 0x20
/*
if kind == 0 {
fmt.Print("--> Primitive\n")
} else {
fmt.Print("--> Constructed\n")
}
*/
// read length
var length int
l := ber[offset]
offset++
indefinite := false
if l > 0x80 {
numberOfBytes := (int)(l & 0x7F)
if numberOfBytes > 4 { // int is only guaranteed to be 32bit
return nil, 0, errors.New("ber2der: BER tag length too long")
}
if numberOfBytes == 4 && (int)(ber[offset]) > 0x7F {
return nil, 0, errors.New("ber2der: BER tag length is negative")
}
if 0x0 == (int)(ber[offset]) {
return nil, 0, errors.New("ber2der: BER tag length has leading zero")
}
//fmt.Printf("--> (compute length) indicator byte: %x\n", l)
//fmt.Printf("--> (compute length) length bytes: % X\n", ber[offset:offset+numberOfBytes])
for i := 0; i < numberOfBytes; i++ {
length = length*256 + (int)(ber[offset])
offset++
}
} else if l == 0x80 {
indefinite = true
} else {
length = (int)(l)
}
//fmt.Printf("--> length : %d\n", length)
contentEnd := offset + length
if contentEnd > len(ber) {
return nil, 0, errors.New("ber2der: BER tag length is more than available data")
}
//fmt.Printf("--> content start : %d\n", offset)
//fmt.Printf("--> content end : %d\n", contentEnd)
//fmt.Printf("--> content : % X\n", ber[offset:contentEnd])
var obj asn1Object
if indefinite && kind == 0 {
return nil, 0, errors.New("ber2der: Indefinite form tag must have constructed encoding")
}
if kind == 0 {
obj = asn1Primitive{
tagBytes: ber[tagStart:tagEnd],
length: length,
content: ber[offset:contentEnd],
}
} else {
var subObjects []asn1Object
for (offset < contentEnd) || indefinite {
var subObj asn1Object
var err error
subObj, offset, err = readObject(ber, offset)
if err != nil {
return nil, 0, err
}
subObjects = append(subObjects, subObj)
if indefinite {
terminated, err := isIndefiniteTermination(ber, offset)
if err != nil {
return nil, 0, err
}
if terminated {
break
}
}
}
obj = asn1Structured{
tagBytes: ber[tagStart:tagEnd],
content: subObjects,
}
}
// Apply indefinite form length with 0x0000 terminator.
if indefinite {
contentEnd = offset + 2
}
return obj, contentEnd, nil
}
func isIndefiniteTermination(ber []byte, offset int) (bool, error) {
if len(ber) - offset < 2 {
return false, errors.New("ber2der: Invalid BER format")
}
return bytes.Index(ber[offset:], []byte{0x0, 0x0}) == 0, nil
}
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