Adding upstream version 1.0.1.

Signed-off-by: Daniel Baumann <daniel@debian.org>

random.go

@@ -0,0 +1,109 @@
+// Copyright 2011-2014 Dmitry Chestnykh. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package captcha
+
+import (
+	"crypto/hmac"
+	"crypto/rand"
+	"crypto/sha256"
+	"io"
+)
+
+// idLen is a length of captcha id string.
+// (20 bytes of 62-letter alphabet give ~119 bits.)
+const idLen = 20
+
+// idChars are characters allowed in captcha id.
+var idChars = []byte("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789")
+
+// rngKey is a secret key used to deterministically derive seeds for
+// PRNGs used in image. Generated once during initialization.
+var rngKey [32]byte
+
+func init() {
+	if _, err := io.ReadFull(rand.Reader, rngKey[:]); err != nil {
+		panic("captcha: error reading random source: " + err.Error())
+	}
+}
+
+// Purposes for seed derivation. The goal is to make deterministic PRNG produce
+// different outputs for images and audio by using different derived seeds.
+const (
+	imageSeedPurpose = 0x01
+	audioSeedPurpose = 0x02
+)
+
+// deriveSeed returns a 16-byte PRNG seed from rngKey, purpose, id and digits.
+// Same purpose, id and digits will result in the same derived seed for this
+// instance of running application.
+//
+//	out = HMAC(rngKey, purpose || id || 0x00 || digits)
+func deriveSeed(purpose byte, id string, digits []byte) (out [32]byte) {
+	var buf [sha256.Size]byte
+
+	h := hmac.New(sha256.New, rngKey[:])
+	h.Write([]byte{purpose})
+	h.Write([]byte(id))
+	h.Write([]byte{0})
+	h.Write(digits)
+
+	sum := h.Sum(buf[:0])
+	copy(out[:], sum)
+	return
+}
+
+// RandomDigits returns a byte slice of the given length containing
+// pseudorandom numbers in range 0-9. The slice can be used as a captcha
+// solution.
+func RandomDigits(length int) []byte {
+	return randomBytesMod(length, 10)
+}
+
+// randomBytes returns a byte slice of the given length read from CSPRNG.
+func randomBytes(length int) (b []byte) {
+	b = make([]byte, length)
+	if _, err := io.ReadFull(rand.Reader, b); err != nil {
+		panic("captcha: error reading random source: " + err.Error())
+	}
+	return
+}
+
+// randomBytesMod returns a byte slice of the given length, where each byte is
+// a random number modulo mod.
+func randomBytesMod(length int, mod byte) (b []byte) {
+	if length == 0 {
+		return nil
+	}
+	if mod == 0 {
+		panic("captcha: bad mod argument for randomBytesMod")
+	}
+
+	maxrb := 255 - byte(256%int(mod))
+	b = make([]byte, length)
+	i := 0
+	for {
+		r := randomBytes(length + (length / 4))
+		for _, c := range r {
+			if c > maxrb {
+				// Skip this number to avoid modulo bias.
+				continue
+			}
+			b[i] = c % mod
+			i++
+			if i == length {
+				return
+			}
+		}
+	}
+}
+
+// randomID returns a new random id string.
+func randomID() string {
+	b := randomBytesMod(idLen, byte(len(idChars)))
+	for i, c := range b {
+		b[i] = idChars[c]
+	}
+	return string(b)
+}