mirror of
https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake.git
synced 2025-10-13 20:11:19 -04:00
c9e0dd287f
This package contains a CacheURL function that modifies a URL to be accessed through an AMP cache, and the "AMP armor" data encoding scheme for encoding data into the AMP subset of HTML.
176 lines
5.3 KiB
Go
176 lines
5.3 KiB
Go
package amp
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import (
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"encoding/base64"
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"io"
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)
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// https://amp.dev/boilerplate/
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// https://amp.dev/documentation/guides-and-tutorials/learn/spec/amp-boilerplate/?format=websites
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// https://amp.dev/documentation/guides-and-tutorials/learn/spec/amphtml/?format=websites#the-amp-html-format
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const (
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boilerplateStart = `<!doctype html>
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<html amp>
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<head>
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<meta charset="utf-8">
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<script async src="https://cdn.ampproject.org/v0.js"></script>
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<link rel="canonical" href="#">
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<meta name="viewport" content="width=device-width">
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<style amp-boilerplate>body{-webkit-animation:-amp-start 8s steps(1,end) 0s 1 normal both;-moz-animation:-amp-start 8s steps(1,end) 0s 1 normal both;-ms-animation:-amp-start 8s steps(1,end) 0s 1 normal both;animation:-amp-start 8s steps(1,end) 0s 1 normal both}@-webkit-keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}@-moz-keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}@-ms-keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}@-o-keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}@keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}</style><noscript><style amp-boilerplate>body{-webkit-animation:none;-moz-animation:none;-ms-animation:none;animation:none}</style></noscript>
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</head>
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<body>
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`
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boilerplateEnd = `</body>
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</html>`
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)
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const (
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// We restrict the amount of text may go inside an HTML element, in
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// order to limit the amount a decoder may have to buffer.
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elementSizeLimit = 32 * 1024
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// The payload is conceptually a long base64-encoded string, but we
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// break the string into short chunks separated by whitespace. This is
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// to protect against modification by AMP caches, which reportedly may
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// truncate long words in text:
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// https://bugs.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/25985#note_2592348
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bytesPerChunk = 32
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// We set the number of chunks per element so as to stay under
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// elementSizeLimit. Here, we assume that there is 1 byte of whitespace
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// after each chunk (with an additional whitespace byte at the beginning
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// of the element).
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chunksPerElement = (elementSizeLimit - 1) / (bytesPerChunk + 1)
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)
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// The AMP armor encoder is a chain of a base64 encoder (base64.NewEncoder) and
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// an HTML element encoder (elementEncoder). A top-level encoder (armorEncoder)
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// coordinates these two, and handles prepending and appending the AMP
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// boilerplate. armorEncoder's Write method writes data into the base64 encoder,
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// where it makes its way through the chain.
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// NewArmorEncoder returns a new AMP armor encoder. Anything written to the
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// returned io.WriteCloser will be encoded and written to w. The caller must
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// call Close to flush any partially written data and output the AMP boilerplate
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// trailer.
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func NewArmorEncoder(w io.Writer) (io.WriteCloser, error) {
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// Immediately write the AMP boilerplate header.
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_, err := w.Write([]byte(boilerplateStart))
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if err != nil {
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return nil, err
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}
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element := &elementEncoder{w: w}
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// Write a server–client protocol version indicator, outside the base64
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// layer.
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_, err = element.Write([]byte{'0'})
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if err != nil {
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return nil, err
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}
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base64 := base64.NewEncoder(base64.StdEncoding, element)
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return &armorEncoder{
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w: w,
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element: element,
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base64: base64,
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}, nil
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}
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type armorEncoder struct {
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base64 io.WriteCloser
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element *elementEncoder
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w io.Writer
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}
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func (enc *armorEncoder) Write(p []byte) (int, error) {
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// Write into the chain base64 | element | w.
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return enc.base64.Write(p)
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}
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func (enc *armorEncoder) Close() error {
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// Close the base64 encoder first, to flush out any buffered data and
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// the final padding.
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err := enc.base64.Close()
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if err != nil {
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return err
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}
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// Next, close the element encoder, to close any open elements.
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err = enc.element.Close()
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if err != nil {
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return err
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}
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// Finally, output the AMP boilerplate trailer.
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_, err = enc.w.Write([]byte(boilerplateEnd))
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if err != nil {
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return err
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}
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return nil
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}
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// elementEncoder arranges written data into pre elements, with the text within
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// separated into chunks. It does no HTML encoding, so data written must not
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// contain any bytes that are meaningful in HTML.
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type elementEncoder struct {
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w io.Writer
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chunkCounter int
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elementCounter int
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}
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func (enc *elementEncoder) Write(p []byte) (n int, err error) {
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total := 0
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for len(p) > 0 {
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if enc.elementCounter == 0 && enc.chunkCounter == 0 {
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_, err := enc.w.Write([]byte("<pre>\n"))
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if err != nil {
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return total, err
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}
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}
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n := bytesPerChunk - enc.chunkCounter
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if n > len(p) {
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n = len(p)
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}
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nn, err := enc.w.Write(p[:n])
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if err != nil {
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return total, err
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}
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total += nn
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p = p[n:]
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enc.chunkCounter += n
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if enc.chunkCounter >= bytesPerChunk {
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enc.chunkCounter = 0
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enc.elementCounter += 1
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nn, err = enc.w.Write([]byte("\n"))
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if err != nil {
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return total, err
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}
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total += nn
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}
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if enc.elementCounter >= chunksPerElement {
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enc.elementCounter = 0
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nn, err = enc.w.Write([]byte("</pre>\n"))
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if err != nil {
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return total, err
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}
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total += nn
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}
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}
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return total, nil
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}
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func (enc *elementEncoder) Close() error {
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var err error
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if !(enc.elementCounter == 0 && enc.chunkCounter == 0) {
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if enc.chunkCounter == 0 {
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_, err = enc.w.Write([]byte("</pre>\n"))
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} else {
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_, err = enc.w.Write([]byte("\n</pre>\n"))
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}
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}
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return err
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}
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