fix(rand): return errors for invalid random string inputs

pkg/util/rand/rand.go

@@ -14,7 +14,16 @@ func String(n int) (string, error) {
 }
 
 // StringFromCharset generates, from a given charset, a cryptographically-secure pseudo-random string of a given length.
+//
+// Returns an error when n is negative, or when n > 0 and charset is empty.
+// n == 0 returns "" with no error regardless of the charset.
 func StringFromCharset(n int, charset string) (string, error) {
+	if n < 0 {
+		return "", fmt.Errorf("rand: requested length %d is negative", n)
+	}
+	if n > 0 && len(charset) == 0 {
+		return "", fmt.Errorf("rand: cannot generate %d-character string from empty charset", n)
+	}
 	b := make([]byte, n)
 	maxIdx := big.NewInt(int64(len(charset)))
 	for i := 0; i < n; i++ {

pkg/util/rand/rand_test.go

@@ -0,0 +1,83 @@
+package rand
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestString_ValidLengths(t *testing.T) {
+	tests := []struct {
+		name string
+		n    int
+	}{
+		{"zero length", 0},
+		{"single char", 1},
+		{"medium length", 24},
+		{"PKCE-style length", 43},
+		{"long", 128},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := String(tt.n)
+			require.NoError(t, err)
+			assert.Len(t, got, tt.n)
+			for i := 0; i < len(got); i++ {
+				assert.True(t, strings.IndexByte(letterBytes, got[i]) >= 0,
+					"byte %q at position %d not in default letter charset", got[i], i)
+			}
+		})
+	}
+}
+
+func TestString_NegativeLengthReturnsError(t *testing.T) {
+	got, err := String(-1)
+	require.Error(t, err)
+	assert.Equal(t, "", got)
+}
+
+func TestStringFromCharset_ValidInputs(t *testing.T) {
+	tests := []struct {
+		name    string
+		n       int
+		charset string
+	}{
+		{"empty length, empty charset", 0, ""},
+		{"empty length, non-empty charset", 0, "abc"},
+		{"single byte from single-byte charset", 1, "x"},
+		{"PKCE verifier charset", 43, "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-._~"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := StringFromCharset(tt.n, tt.charset)
+			require.NoError(t, err)
+			assert.Len(t, got, tt.n)
+			for i := 0; i < len(got); i++ {
+				assert.True(t, strings.IndexByte(tt.charset, got[i]) >= 0,
+					"byte %q at position %d not in charset %q", got[i], i, tt.charset)
+			}
+		})
+	}
+}
+
+func TestStringFromCharset_InvalidInputsReturnError(t *testing.T) {
+	tests := []struct {
+		name    string
+		n       int
+		charset string
+	}{
+		{"negative length, non-empty charset", -1, "abc"},
+		{"negative length, empty charset", -1, ""},
+		{"positive length, empty charset", 1, ""},
+		{"large positive length, empty charset", 100, ""},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := StringFromCharset(tt.n, tt.charset)
+			require.Error(t, err)
+			assert.Equal(t, "", got)
+		})
+	}
+}