309 lines
8.6 KiB
Go
309 lines
8.6 KiB
Go
// Copyright 2017 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package cryptobyte
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import (
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"errors"
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"fmt"
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)
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// A Builder builds byte strings from fixed-length and length-prefixed values.
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// Builders either allocate space as needed, or are ‘fixed’, which means that
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// they write into a given buffer and produce an error if it's exhausted.
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//
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// The zero value is a usable Builder that allocates space as needed.
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//
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// Simple values are marshaled and appended to a Builder using methods on the
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// Builder. Length-prefixed values are marshaled by providing a
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// BuilderContinuation, which is a function that writes the inner contents of
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// the value to a given Builder. See the documentation for BuilderContinuation
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// for details.
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type Builder struct {
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err error
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result []byte
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fixedSize bool
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child *Builder
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offset int
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pendingLenLen int
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pendingIsASN1 bool
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inContinuation *bool
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}
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// NewBuilder creates a Builder that appends its output to the given buffer.
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// Like append(), the slice will be reallocated if its capacity is exceeded.
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// Use Bytes to get the final buffer.
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func NewBuilder(buffer []byte) *Builder {
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return &Builder{
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result: buffer,
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}
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}
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// NewFixedBuilder creates a Builder that appends its output into the given
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// buffer. This builder does not reallocate the output buffer. Writes that
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// would exceed the buffer's capacity are treated as an error.
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func NewFixedBuilder(buffer []byte) *Builder {
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return &Builder{
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result: buffer,
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fixedSize: true,
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}
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}
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// Bytes returns the bytes written by the builder or an error if one has
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// occurred during during building.
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func (b *Builder) Bytes() ([]byte, error) {
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if b.err != nil {
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return nil, b.err
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}
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return b.result[b.offset:], nil
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}
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// BytesOrPanic returns the bytes written by the builder or panics if an error
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// has occurred during building.
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func (b *Builder) BytesOrPanic() []byte {
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if b.err != nil {
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panic(b.err)
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}
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return b.result[b.offset:]
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}
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// AddUint8 appends an 8-bit value to the byte string.
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func (b *Builder) AddUint8(v uint8) {
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b.add(byte(v))
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}
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// AddUint16 appends a big-endian, 16-bit value to the byte string.
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func (b *Builder) AddUint16(v uint16) {
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b.add(byte(v>>8), byte(v))
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}
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// AddUint24 appends a big-endian, 24-bit value to the byte string. The highest
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// byte of the 32-bit input value is silently truncated.
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func (b *Builder) AddUint24(v uint32) {
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b.add(byte(v>>16), byte(v>>8), byte(v))
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}
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// AddUint32 appends a big-endian, 32-bit value to the byte string.
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func (b *Builder) AddUint32(v uint32) {
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b.add(byte(v>>24), byte(v>>16), byte(v>>8), byte(v))
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}
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// AddBytes appends a sequence of bytes to the byte string.
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func (b *Builder) AddBytes(v []byte) {
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b.add(v...)
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}
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// BuilderContinuation is continuation-passing interface for building
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// length-prefixed byte sequences. Builder methods for length-prefixed
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// sequences (AddUint8LengthPrefixed etc) will invoke the BuilderContinuation
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// supplied to them. The child builder passed to the continuation can be used
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// to build the content of the length-prefixed sequence. For example:
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//
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// parent := cryptobyte.NewBuilder()
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// parent.AddUint8LengthPrefixed(func (child *Builder) {
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// child.AddUint8(42)
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// child.AddUint8LengthPrefixed(func (grandchild *Builder) {
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// grandchild.AddUint8(5)
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// })
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// })
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//
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// It is an error to write more bytes to the child than allowed by the reserved
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// length prefix. After the continuation returns, the child must be considered
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// invalid, i.e. users must not store any copies or references of the child
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// that outlive the continuation.
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//
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// If the continuation panics with a value of type BuildError then the inner
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// error will be returned as the error from Bytes. If the child panics
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// otherwise then Bytes will repanic with the same value.
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type BuilderContinuation func(child *Builder)
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// BuildError wraps an error. If a BuilderContinuation panics with this value,
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// the panic will be recovered and the inner error will be returned from
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// Builder.Bytes.
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type BuildError struct {
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Err error
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}
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// AddUint8LengthPrefixed adds a 8-bit length-prefixed byte sequence.
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func (b *Builder) AddUint8LengthPrefixed(f BuilderContinuation) {
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b.addLengthPrefixed(1, false, f)
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}
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// AddUint16LengthPrefixed adds a big-endian, 16-bit length-prefixed byte sequence.
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func (b *Builder) AddUint16LengthPrefixed(f BuilderContinuation) {
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b.addLengthPrefixed(2, false, f)
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}
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// AddUint24LengthPrefixed adds a big-endian, 24-bit length-prefixed byte sequence.
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func (b *Builder) AddUint24LengthPrefixed(f BuilderContinuation) {
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b.addLengthPrefixed(3, false, f)
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}
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// AddUint32LengthPrefixed adds a big-endian, 32-bit length-prefixed byte sequence.
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func (b *Builder) AddUint32LengthPrefixed(f BuilderContinuation) {
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b.addLengthPrefixed(4, false, f)
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}
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func (b *Builder) callContinuation(f BuilderContinuation, arg *Builder) {
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if !*b.inContinuation {
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*b.inContinuation = true
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defer func() {
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*b.inContinuation = false
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r := recover()
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if r == nil {
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return
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}
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if buildError, ok := r.(BuildError); ok {
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b.err = buildError.Err
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} else {
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panic(r)
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}
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}()
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}
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f(arg)
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}
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func (b *Builder) addLengthPrefixed(lenLen int, isASN1 bool, f BuilderContinuation) {
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// Subsequent writes can be ignored if the builder has encountered an error.
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if b.err != nil {
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return
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}
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offset := len(b.result)
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b.add(make([]byte, lenLen)...)
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if b.inContinuation == nil {
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b.inContinuation = new(bool)
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}
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b.child = &Builder{
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result: b.result,
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fixedSize: b.fixedSize,
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offset: offset,
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pendingLenLen: lenLen,
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pendingIsASN1: isASN1,
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inContinuation: b.inContinuation,
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}
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b.callContinuation(f, b.child)
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b.flushChild()
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if b.child != nil {
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panic("cryptobyte: internal error")
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}
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}
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func (b *Builder) flushChild() {
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if b.child == nil {
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return
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}
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b.child.flushChild()
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child := b.child
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b.child = nil
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if child.err != nil {
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b.err = child.err
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return
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}
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length := len(child.result) - child.pendingLenLen - child.offset
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if length < 0 {
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panic("cryptobyte: internal error") // result unexpectedly shrunk
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}
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if child.pendingIsASN1 {
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// For ASN.1, we reserved a single byte for the length. If that turned out
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// to be incorrect, we have to move the contents along in order to make
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// space.
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if child.pendingLenLen != 1 {
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panic("cryptobyte: internal error")
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}
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var lenLen, lenByte uint8
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if int64(length) > 0xfffffffe {
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b.err = errors.New("pending ASN.1 child too long")
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return
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} else if length > 0xffffff {
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lenLen = 5
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lenByte = 0x80 | 4
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} else if length > 0xffff {
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lenLen = 4
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lenByte = 0x80 | 3
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} else if length > 0xff {
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lenLen = 3
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lenByte = 0x80 | 2
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} else if length > 0x7f {
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lenLen = 2
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lenByte = 0x80 | 1
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} else {
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lenLen = 1
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lenByte = uint8(length)
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length = 0
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}
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// Insert the initial length byte, make space for successive length bytes,
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// and adjust the offset.
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child.result[child.offset] = lenByte
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extraBytes := int(lenLen - 1)
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if extraBytes != 0 {
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child.add(make([]byte, extraBytes)...)
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childStart := child.offset + child.pendingLenLen
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copy(child.result[childStart+extraBytes:], child.result[childStart:])
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}
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child.offset++
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child.pendingLenLen = extraBytes
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}
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l := length
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for i := child.pendingLenLen - 1; i >= 0; i-- {
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child.result[child.offset+i] = uint8(l)
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l >>= 8
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}
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if l != 0 {
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b.err = fmt.Errorf("cryptobyte: pending child length %d exceeds %d-byte length prefix", length, child.pendingLenLen)
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return
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}
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if !b.fixedSize {
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b.result = child.result // In case child reallocated result.
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}
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}
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func (b *Builder) add(bytes ...byte) {
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if b.err != nil {
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return
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}
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if b.child != nil {
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panic("attempted write while child is pending")
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}
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if len(b.result)+len(bytes) < len(bytes) {
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b.err = errors.New("cryptobyte: length overflow")
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}
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if b.fixedSize && len(b.result)+len(bytes) > cap(b.result) {
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b.err = errors.New("cryptobyte: Builder is exceeding its fixed-size buffer")
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return
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}
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b.result = append(b.result, bytes...)
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}
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// A MarshalingValue marshals itself into a Builder.
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type MarshalingValue interface {
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// Marshal is called by Builder.AddValue. It receives a pointer to a builder
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// to marshal itself into. It may return an error that occurred during
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// marshaling, such as unset or invalid values.
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Marshal(b *Builder) error
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}
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// AddValue calls Marshal on v, passing a pointer to the builder to append to.
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// If Marshal returns an error, it is set on the Builder so that subsequent
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// appends don't have an effect.
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func (b *Builder) AddValue(v MarshalingValue) {
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err := v.Marshal(b)
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if err != nil {
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b.err = err
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}
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}
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