diff --git a/commands/blob/codec.go b/commands/blob/codec.go
index febaebc..36453c7 100644
--- a/commands/blob/codec.go
+++ b/commands/blob/codec.go
@@ -25,16 +25,22 @@ var (
 	errBothVariants = errors.New("InvalidUnion", "union holds both variants")
 )
 
-// SpecFromBlob returns the spec of a blob whose digest is known. Its digest
-// code is the digest's own; a digest that is not a multihash has code 0.
-func SpecFromBlob(b Blob) BlobSpec {
-	s := BlobSpec{digest: b.Digest, size: b.Size, valid: true}
-	if d, err := multihash.Decode(b.Digest); err == nil {
+// SpecFromDigest returns the spec of a blob of size bytes whose digest is
+// known. Its digest code is the digest's own; a digest that is not a multihash
+// has code 0.
+func SpecFromDigest(digest multihash.Multihash, size uint64) BlobSpec {
+	s := BlobSpec{digest: digest, size: size, valid: true}
+	if d, err := multihash.Decode(digest); err == nil {
 		s.code = d.Code
 	}
 	return s
 }
 
+// SpecFromBlob returns the spec of a blob whose digest is known.
+func SpecFromBlob(b Blob) BlobSpec {
+	return SpecFromDigest(b.Digest, b.Size)
+}
+
 // SpecFromDigestCode returns the spec of a blob of size bytes whose digest is
 // to be computed with the multihash function code.
 func SpecFromDigestCode(code, size uint64) BlobSpec {
diff --git a/commands/blob/spec_test.go b/commands/blob/spec_test.go
index 629aded..e617f29 100644
--- a/commands/blob/spec_test.go
+++ b/commands/blob/spec_test.go
@@ -56,7 +56,8 @@ func TestBlobSpecRoundTrip(t *testing.T) {
 
 func TestBlobSpecAccessors(t *testing.T) {
 	digest := testutil.RandomMultihash(t)
-	byDigest := blob.SpecFromBlob(blob.Blob{Digest: digest, Size: 7})
+	byDigest := blob.SpecFromDigest(digest, 7)
+	require.Equal(t, blob.SpecFromBlob(blob.Blob{Digest: digest, Size: 7}), byDigest, "a Blob specifies the same blob")
 	got, ok := byDigest.Digest()
 	require.True(t, ok)
 	require.Equal(t, digest, got)
diff --git a/commands/blob/types.go b/commands/blob/types.go
index 5a439b9..112c2cd 100644
--- a/commands/blob/types.go
+++ b/commands/blob/types.go
@@ -21,10 +21,10 @@ type Blob struct {
 // compute it as the data streams. A spec naming a digest has the digest's
 // code too.
 //
-// Construct one with [SpecFromBlob] or [SpecFromDigestCode]. Encoding the zero
-// value fails, as does decoding a spec that names both a digest and a digest
-// code, or neither. The codec is hand-written (codec.go), over the generated
-// codec of [BlobSpecModel].
+// Construct one with [SpecFromBlob], [SpecFromDigest] or [SpecFromDigestCode].
+// Encoding the zero value fails, as does decoding a spec that names both a
+// digest and a digest code, or neither. The codec is hand-written (codec.go),
+// over the generated codec of [BlobSpecModel].
 type BlobSpec struct {
 	digest multihash.Multihash
 	code   uint64