add RandomHexString for unguessable identifiers and tokens

RandomHexString returns the requested number of bytes from crypto/rand as
lowercase hexadecimal, so callers reaching for a token do not end up using the
predictable math/rand instead. Comes with a doc comment and table-driven tests.
(closes #13)

Model: opus-5

random.go

@@ -0,0 +1,28 @@
+package util
+
+import (
+	"crypto/rand"
+	"encoding/hex"
+	"errors"
+)
+
+// RandomHexString returns byteLength random bytes from the operating system's
+// random source, written as lowercase hexadecimal, so the returned string is
+// twice as long as byteLength. The bytes come from crypto/rand, which means the
+// result is fit for session tokens, temporary filenames and anything else a
+// stranger should not be able to guess.
+//
+// An error comes back only for a negative length or if the random source
+// itself fails, which does not happen on a working system.
+func RandomHexString(byteLength int) (string, error) {
+	if byteLength < 0 {
+		return "", errors.New("byte length cannot be negative")
+	}
+
+	buffer := make([]byte, byteLength)
+	if _, err := rand.Read(buffer); err != nil {
+		return "", err
+	}
+
+	return hex.EncodeToString(buffer), nil
+}

random_test.go

@@ -0,0 +1,57 @@
+package util
+
+import (
+	"encoding/hex"
+	"testing"
+)
+
+func TestRandomHexStringLength(t *testing.T) {
+	tests := []struct {
+		name           string
+		byteLength     int
+		expectedLength int
+	}{
+		{"no bytes at all", 0, 0},
+		{"one byte", 1, 2},
+		{"eight bytes", 8, 16},
+		{"sixteen bytes", 16, 32},
+		{"thirty-two bytes", 32, 64},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			got, err := RandomHexString(test.byteLength)
+			if err != nil {
+				t.Fatalf("did not expect an error, got %v", err)
+			}
+			if len(got) != test.expectedLength {
+				t.Errorf("expected a string of %d characters, got %d (%q)", test.expectedLength, len(got), got)
+			}
+			if _, err := hex.DecodeString(got); err != nil {
+				t.Errorf("expected valid hexadecimal, got %q: %v", got, err)
+			}
+		})
+	}
+}
+
+func TestRandomHexStringRejectsNegativeLength(t *testing.T) {
+	got, err := RandomHexString(-1)
+	if err == nil {
+		t.Errorf("expected an error for a negative length, got %q", got)
+	}
+}
+
+func TestRandomHexStringDiffersBetweenCalls(t *testing.T) {
+	seen := make(map[string]struct{})
+
+	for i := 0; i < 100; i++ {
+		value, err := RandomHexString(16)
+		if err != nil {
+			t.Fatalf("did not expect an error, got %v", err)
+		}
+		if _, repeated := seen[value]; repeated {
+			t.Fatalf("got the same string twice: %q", value)
+		}
+		seen[value] = struct{}{}
+	}
+}