tools.go
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// Package asn1tools provides tools for managing ASN1 marshaled data.
package asn1tools
import (
"github.com/jcmturner/gofork/encoding/asn1"
)
// MarshalLengthBytes returns the ASN1 encoded bytes for the length 'l'
//
// There are two forms: short (for lengths between 0 and 127), and long definite (for lengths between 0 and 2^1008 -1).
//
// Short form: One octet. Bit 8 has value "0" and bits 7-1 give the length.
//
// Long form: Two to 127 octets. Bit 8 of first octet has value "1" and bits 7-1 give the number of additional length octets. Second and following octets give the length, base 256, most significant digit first.
func MarshalLengthBytes(l int) []byte {
if l <= 127 {
return []byte{byte(l)}
}
var b []byte
p := 1
for i := 1; i < 127; {
b = append([]byte{byte((l % (p * 256)) / p)}, b...)
p = p * 256
l = l - l%p
if l <= 0 {
break
}
}
return append([]byte{byte(128 + len(b))}, b...)
}
// GetLengthFromASN returns the length of a slice of ASN1 encoded bytes from the ASN1 length header it contains.
func GetLengthFromASN(b []byte) int {
if int(b[1]) <= 127 {
return int(b[1])
}
// The bytes that indicate the length
lb := b[2 : 2+int(b[1])-128]
base := 1
l := 0
for i := len(lb) - 1; i >= 0; i-- {
l += int(lb[i]) * base
base = base * 256
}
return l
}
// GetNumberBytesInLengthHeader returns the number of bytes in the ASn1 header that indicate the length.
func GetNumberBytesInLengthHeader(b []byte) int {
if int(b[1]) <= 127 {
return 1
}
// The bytes that indicate the length
return 1 + int(b[1]) - 128
}
// AddASNAppTag adds an ASN1 encoding application tag value to the raw bytes provided.
func AddASNAppTag(b []byte, tag int) []byte {
r := asn1.RawValue{
Class: asn1.ClassApplication,
IsCompound: true,
Tag: tag,
Bytes: b,
}
ab, _ := asn1.Marshal(r)
return ab
}
/*
// The Marshal method of golang's asn1 package does not enable you to define wrapping the output in an application tag.
// This method adds that wrapping tag.
func AddASNAppTag(b []byte, tag int) []byte {
// The ASN1 wrapping consists of 2 bytes:
// 1st byte -> Identifier Octet - Application Tag
// 2nd byte -> The length (this will be the size indicated in the input bytes + 2 for the additional bytes we add here.
// Application Tag:
//| Bit: | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 |
//| Value: | 0 | 1 | 1 | From the RFC spec 4120 |
//| Explanation | Defined by the ASN1 encoding rules for an application tag | A value of 1 indicates a constructed type | The ASN Application tag value |
// Therefore the value of the byte is an integer = ( Application tag value + 96 )
//b = append(MarshalLengthBytes(int(b[1])+2), b...)
b = append(MarshalLengthBytes(len(b)), b...)
b = append([]byte{byte(96 + tag)}, b...)
return b
}
*/