diff --git a/digest/algorithm.go b/digest/algorithm.go
index f44b50db..b71314e6 100644
--- a/digest/algorithm.go
+++ b/digest/algorithm.go
@@ -72,6 +72,10 @@ func (a *Algorithm) Set(value string) error {
 		*a = Algorithm(value)
 	}
 
+	if !a.Available() {
+		return ErrDigestUnsupported
+	}
+
 	return nil
 }
 
diff --git a/digest/algorithm_test.go b/digest/algorithm_test.go
new file mode 100644
index 00000000..12db5d13
--- /dev/null
+++ b/digest/algorithm_test.go
@@ -0,0 +1,100 @@
+package digest
+
+import (
+	"bytes"
+	"crypto/rand"
+	_ "crypto/sha256"
+	_ "crypto/sha512"
+	"flag"
+	"fmt"
+	"strings"
+	"testing"
+)
+
+func TestFlagInterface(t *testing.T) {
+	var (
+		alg     Algorithm
+		flagSet flag.FlagSet
+	)
+
+	flagSet.Var(&alg, "algorithm", "set the digest algorithm")
+	for _, testcase := range []struct {
+		Name     string
+		Args     []string
+		Err      error
+		Expected Algorithm
+	}{
+		{
+			Name: "Invalid",
+			Args: []string{"-algorithm", "bean"},
+			Err:  ErrDigestUnsupported,
+		},
+		{
+			Name:     "Default",
+			Args:     []string{"unrelated"},
+			Expected: "sha256",
+		},
+		{
+			Name:     "Other",
+			Args:     []string{"-algorithm", "sha512"},
+			Expected: "sha512",
+		},
+	} {
+		t.Run(testcase.Name, func(t *testing.T) {
+			alg = Canonical
+			if err := flagSet.Parse(testcase.Args); err != testcase.Err {
+				if testcase.Err == nil {
+					t.Fatal("unexpected error", err)
+				}
+
+				// check that flag package returns correct error
+				if !strings.Contains(err.Error(), testcase.Err.Error()) {
+					t.Fatalf("unexpected error: %v != %v", err, testcase.Err)
+				}
+				return
+			}
+
+			if alg != testcase.Expected {
+				t.Fatalf("unexpected algorithm: %v != %v", alg, testcase.Expected)
+			}
+		})
+	}
+}
+
+func TestFroms(t *testing.T) {
+	p := make([]byte, 1<<20)
+	rand.Read(p)
+
+	for alg := range algorithms {
+		h := alg.Hash()
+		h.Write(p)
+		expected := Digest(fmt.Sprintf("%s:%x", alg, h.Sum(nil)))
+		readerDgst, err := alg.FromReader(bytes.NewReader(p))
+		if err != nil {
+			t.Fatalf("error calculating hash from reader: %v", err)
+		}
+
+		dgsts := []Digest{
+			alg.FromBytes(p),
+			alg.FromString(string(p)),
+			readerDgst,
+		}
+
+		if alg == Canonical {
+			readerDgst, err := FromReader(bytes.NewReader(p))
+			if err != nil {
+				t.Fatalf("error calculating hash from reader: %v", err)
+			}
+
+			dgsts = append(dgsts,
+				FromBytes(p),
+				FromString(string(p)),
+				readerDgst)
+		}
+		for _, dgst := range dgsts {
+			if dgst != expected {
+				t.Fatalf("unexpected digest %v != %v", dgst, expected)
+			}
+		}
+	}
+}
diff --git a/digest/digest.go b/digest/digest.go
index 761f2e54..a9ab8e51 100644
--- a/digest/digest.go
+++ b/digest/digest.go
@@ -94,17 +94,10 @@ func FromString(s string) Digest {
 func (d Digest) Validate() error {
 	s := string(d)
 
-	if !DigestRegexpAnchored.MatchString(s) {
-		return ErrDigestInvalidFormat
-	}
-
 	i := strings.Index(s, ":")
-	if i < 0 {
-		return ErrDigestInvalidFormat
-	}
 
-	// case: "sha256:" with no hex.
-	if i+1 == len(s) {
+	// validate i then run through regexp
+	if i < 0 || i+1 == len(s) || !DigestRegexpAnchored.MatchString(s) {
 		return ErrDigestInvalidFormat
 	}
 
diff --git a/digest/digest_test.go b/digest/digest_test.go
index 62158add..d85c476f 100644
--- a/digest/digest_test.go
+++ b/digest/digest_test.go
@@ -1,8 +1,6 @@
 package digest
 
 import (
-	_ "crypto/sha256"
-	_ "crypto/sha512"
 	"testing"
 )
 
@@ -28,6 +26,11 @@ func TestParseDigest(t *testing.T) {
 			input: "sha256:",
 			err:   ErrDigestInvalidFormat,
 		},
+		{
+			// empty hex
+			input: ":",
+			err:   ErrDigestInvalidFormat,
+		},
 		{
 			// just hex
 			input: "d41d8cd98f00b204e9800998ecf8427e",
@@ -80,5 +83,10 @@ func TestParseDigest(t *testing.T) {
 		if newParsed != digest {
 			t.Fatalf("expected equal: %q != %q", newParsed, digest)
 		}
+
+		newFromHex := NewDigestFromHex(newParsed.Algorithm().String(), newParsed.Hex())
+		if newFromHex != digest {
+			t.Fatalf("%v != %v", newFromHex, digest)
+		}
 	}
 }