diff --git a/base64.go b/base64.go
index 5bb111c..d2ddf58 100644
--- a/base64.go
+++ b/base64.go
@@ -54,3 +54,25 @@ func MustDecode(b64 string) B64 {
 	}
 	return b
 }
+
+// sB64 is useful for map keys since currently in Go []byte is not allowed to be
+// a map key but string is.  The type really should be []byte and not string,
+// but Go does not yet support this. See https://github.com/golang/go/issues/283
+// and https://github.com/google/go-cmp/issues/67.  SB64 will be deprecated
+// if/when Go supports []byte keys.
+//
+// From https://go.dev/blog/strings >[A] string holds arbitrary bytes. It is not
+// required to hold Unicode text, UTF-8 text, or any other predefined format. As
+// far as the content of a string is concerned, it is exactly equivalent to a
+// slice of bytes.
+type SB64 string
+
+// String implements fmt.Stringer
+func (b SB64) String() string {
+	return B64(b).String()
+}
+
+// GoString implements fmt.GoString
+func (b SB64) GoString() string {
+	return b.String()
+}
diff --git a/base64_test.go b/base64_test.go
index 769598b..0c48a1e 100644
--- a/base64_test.go
+++ b/base64_test.go
@@ -1,8 +1,11 @@
 package coze
 
 import (
+	"bytes"
+	"crypto/rand"
 	"encoding/json"
 	"fmt"
+	"testing"
 )
 
 // B64 of nil is "" while B64 of 0 is "AA".
@@ -127,3 +130,50 @@ func ExampleB64_non_strict_decode() {
 	// invalid character '\n' in string literal
 	// invalid character '\r' in string literal
 }
+
+// ExampleSB64 demonstrates using SB64 as a map key and that fmt prints "RFC
+// 4648 base 64 URI canonical with padding truncated" properly.
+func ExampleSB64() {
+	b := MustDecode("zVzgRU3WFpnrlVJAnI4ZU1Od4Agl5Zd4jIP79oubOW0")
+	b2 := MustDecode("vZIAk8rjcSIKZKokGylCtVoI3DXvFYJn4XNWzf_C_FA")
+
+	lp := make(map[SB64]B64)
+	lp[SB64(b)] = B64(b2)
+
+	fmt.Printf("%s\n", SB64(b))
+	fmt.Printf("%s\n", lp)
+	fmt.Printf("%+v\n", lp)
+	fmt.Printf("%#v\n", lp)
+
+	// Output:
+	// zVzgRU3WFpnrlVJAnI4ZU1Od4Agl5Zd4jIP79oubOW0
+	// map[zVzgRU3WFpnrlVJAnI4ZU1Od4Agl5Zd4jIP79oubOW0:vZIAk8rjcSIKZKokGylCtVoI3DXvFYJn4XNWzf_C_FA]
+	// map[zVzgRU3WFpnrlVJAnI4ZU1Od4Agl5Zd4jIP79oubOW0:vZIAk8rjcSIKZKokGylCtVoI3DXvFYJn4XNWzf_C_FA]
+	// map[coze.SB64]coze.B64{zVzgRU3WFpnrlVJAnI4ZU1Od4Agl5Zd4jIP79oubOW0:vZIAk8rjcSIKZKokGylCtVoI3DXvFYJn4XNWzf_C_FA}
+}
+
+// FuzzCastB64ToString ensures that casting to and from B64 and string does not
+// cause unexpected issues (issues like replacing bytes with the unicode
+// replacement character).
+// https://go.dev/security/fuzz/
+// https://go.dev/doc/tutorial/fuzz
+func FuzzCastB64ToString(f *testing.F) {
+	f.Add(100)
+	f.Fuzz(func(t *testing.T, a int) {
+		var b B64
+		var err error
+		for i := 0; i < a; i++ {
+			b = make([]byte, 32)
+			_, err = rand.Read(b)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			s := string(b)
+			bb := B64(s)
+			if !bytes.Equal(b, bb) {
+				t.Fatalf("Casting to string: %s failed when converting back B64.", s)
+			}
+		}
+	})
+}