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