identity: add pkg for random id generation

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

identity/randomid.go

@@ -0,0 +1,53 @@
+package identity
+
+import (
+	cryptorand "crypto/rand"
+	"fmt"
+	"io"
+	"math/big"
+)
+
+var (
+	// idReader is used for random id generation. This declaration allows us to
+	// replace it for testing.
+	idReader = cryptorand.Reader
+)
+
+// parameters for random identifier generation. We can tweak this when there is
+// time for further analysis.
+const (
+	randomIDEntropyBytes = 17
+	randomIDBase         = 36
+
+	// To ensure that all identifiers are fixed length, we make sure they
+	// get padded out or truncated to 25 characters.
+	//
+	// For academics,  f5lxx1zz5pnorynqglhzmsp33  == 2^128 - 1. This value
+	// was calculated from floor(log(2^128-1, 36)) + 1.
+	//
+	// While 128 bits is the largest whole-byte size that fits into 25
+	// base-36 characters, we generate an extra byte of entropy to fill
+	// in the high bits, which would otherwise be 0. This gives us a more
+	// even distribution of the first character.
+	//
+	// See http://mathworld.wolfram.com/NumberLength.html for more information.
+	maxRandomIDLength = 25
+)
+
+// NewID generates a new identifier for use where random identifiers with low
+// collision probability are required.
+//
+// With the parameters in this package, the generated identifier will provide
+// ~129 bits of entropy encoded with base36. Leading padding is added if the
+// string is less 25 bytes. We do not intend to maintain this interface, so
+// identifiers should be treated opaquely.
+func NewID() string {
+	var p [randomIDEntropyBytes]byte
+
+	if _, err := io.ReadFull(idReader, p[:]); err != nil {
+		panic(fmt.Errorf("failed to read random bytes: %v", err))
+	}
+
+	p[0] |= 0x80 // set high bit to avoid the need for padding
+	return (&big.Int{}).SetBytes(p[:]).Text(randomIDBase)[1 : maxRandomIDLength+1]
+}