109 lines
2.8 KiB
Go
109 lines
2.8 KiB
Go
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// Copyright 2011-2014 Dmitry Chestnykh. All rights reserved.
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// Use of this source code is governed by a MIT-style
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// license that can be found in the LICENSE file.
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package captcha
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import (
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"crypto/hmac"
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"crypto/rand"
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"crypto/sha256"
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"io"
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)
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// idLen is a length of captcha id string.
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// (20 bytes of 62-letter alphabet give ~119 bits.)
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const idLen = 20
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// idChars are characters allowed in captcha id.
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var idChars = []byte("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789")
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// rngKey is a secret key used to deterministically derive seeds for
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// PRNGs used in image and audio. Generated once during initialization.
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var rngKey [32]byte
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func init() {
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if _, err := io.ReadFull(rand.Reader, rngKey[:]); err != nil {
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panic("captcha: error reading random source: " + err.Error())
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}
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}
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// Purposes for seed derivation. The goal is to make deterministic PRNG produce
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// different outputs for images and audio by using different derived seeds.
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const (
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imageSeedPurpose = 0x01
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audioSeedPurpose = 0x02
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)
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// deriveSeed returns a 16-byte PRNG seed from rngKey, purpose, id and digits.
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// Same purpose, id and digits will result in the same derived seed for this
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// instance of running application.
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//
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// out = HMAC(rngKey, purpose || id || 0x00 || digits) (cut to 16 bytes)
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//
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func deriveSeed(purpose byte, id string, digits []byte) (out [16]byte) {
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var buf [sha256.Size]byte
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h := hmac.New(sha256.New, rngKey[:])
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h.Write([]byte{purpose})
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io.WriteString(h, id)
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h.Write([]byte{0})
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h.Write(digits)
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sum := h.Sum(buf[:0])
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copy(out[:], sum)
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return
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}
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// RandomDigits returns a byte slice of the given length containing
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// pseudorandom numbers in range 0-9. The slice can be used as a captcha
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// solution.
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func RandomDigits(length int) []byte {
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return randomBytesMod(length, 10)
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}
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// randomBytes returns a byte slice of the given length read from CSPRNG.
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func randomBytes(length int) (b []byte) {
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b = make([]byte, length)
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if _, err := io.ReadFull(rand.Reader, b); err != nil {
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panic("captcha: error reading random source: " + err.Error())
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}
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return
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}
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// randomBytesMod returns a byte slice of the given length, where each byte is
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// a random number modulo mod.
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func randomBytesMod(length int, mod byte) (b []byte) {
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if length == 0 {
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return nil
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}
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if mod == 0 {
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panic("captcha: bad mod argument for randomBytesMod")
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}
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maxrb := 255 - byte(256%int(mod))
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b = make([]byte, length)
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i := 0
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for {
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r := randomBytes(length + (length / 4))
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for _, c := range r {
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if c > maxrb {
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// Skip this number to avoid modulo bias.
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continue
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}
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b[i] = c % mod
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i++
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if i == length {
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return
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}
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}
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}
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}
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// randomId returns a new random id string.
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func randomId() string {
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b := randomBytesMod(idLen, byte(len(idChars)))
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for i, c := range b {
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b[i] = idChars[c]
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}
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return string(b)
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}
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