diff --git a/challenge.go b/challenge.go
index e8b98fb..d8c8309 100644
--- a/challenge.go
+++ b/challenge.go
@@ -35,7 +35,7 @@ func (c *Challenge) SupportsQOP(qop string) bool {
 
 // ParseChallenge parses the WWW-Authenticate header challenge
 func ParseChallenge(s string) (*Challenge, error) {
-	s, ok := strings.CutPrefix(s, Prefix)
+	s, ok := CutPrefix(s)
 	if !ok {
 		return nil, errors.New("digest: invalid challenge prefix")
 	}
diff --git a/challenge_test.go b/challenge_test.go
index e1a5282..fc1aa28 100644
--- a/challenge_test.go
+++ b/challenge_test.go
@@ -12,6 +12,7 @@ import (
 func TestChallenge(t *testing.T) {
 	tests := []struct {
 		input     string
+		output    string
 		challenge *Challenge
 	}{
 		{
@@ -33,13 +34,25 @@ func TestChallenge(t *testing.T) {
 				QOP:       []string{"auth"},
 			},
 		},
+		{
+			input:  `DIGEST realm="DLI LPC92601002528", nonce="NZAeQHhoCNifFjFa"`,
+			output: `Digest realm="DLI LPC92601002528", nonce="NZAeQHhoCNifFjFa"`,
+			challenge: &Challenge{
+				Realm: "DLI LPC92601002528",
+				Nonce: "NZAeQHhoCNifFjFa",
+			},
+		},
 	}
 	for i, tt := range tests {
 		t.Run(strconv.Itoa(i), func(t *testing.T) {
 			c, err := ParseChallenge(tt.input)
 			assert.NilError(t, err)
 			assert.DeepEqual(t, tt.challenge, c)
-			assert.DeepEqual(t, c.String(), tt.input)
+			output := tt.output
+			if output == "" {
+				output = tt.input
+			}
+			assert.DeepEqual(t, output, c.String())
 		})
 	}
 }
diff --git a/credentials.go b/credentials.go
index 56ff1df..dc86828 100644
--- a/credentials.go
+++ b/credentials.go
@@ -26,7 +26,7 @@ type Credentials struct {
 
 // ParseCredentials parses the Authorization header value into credentials
 func ParseCredentials(s string) (*Credentials, error) {
-	s, ok := strings.CutPrefix(s, Prefix)
+	s, ok := CutPrefix(s)
 	if !ok {
 		return nil, errors.New("digest: invalid credentials prefix")
 	}
diff --git a/digest.go b/digest.go
index 1e6e082..2b87cb4 100644
--- a/digest.go
+++ b/digest.go
@@ -18,7 +18,18 @@ const Prefix = "Digest "
 
 // IsDigest returns true if the header value is a digest auth header
 func IsDigest(header string) bool {
-	return strings.HasPrefix(header, Prefix)
+	if len(header) < len(Prefix) {
+		return false
+	}
+	return strings.EqualFold(header[:len(Prefix)], Prefix)
+}
+
+// CutPrefix removes the digest prefix from the header value
+func CutPrefix(s string) (string, bool) {
+	if !IsDigest(s) {
+		return s, false
+	}
+	return s[len(Prefix):], true
 }
 
 // Options for creating a credentials
diff --git a/digest_test.go b/digest_test.go
index 73222ad..b2ba4bc 100644
--- a/digest_test.go
+++ b/digest_test.go
@@ -70,6 +70,30 @@ func TestDigestMD5(t *testing.T) {
 				Nc:       1,
 			},
 		},
+		{
+			Options{
+				Method:   "REGISTER",
+				URI:      "sip:182.82.132.122",
+				Username: "the-user",
+				Password: "********",
+				Cnonce:   "104adc6bd71f49678798ee646edcaa9a",
+			},
+			&Challenge{
+				Realm: "SipPeer",
+				Nonce: "970a1b42-d8a7-4fce-91a1-4767e9ed561b",
+				QOP:   []string{"auth"},
+			},
+			&Credentials{
+				Username: "the-user",
+				Realm:    "SipPeer",
+				Nonce:    "970a1b42-d8a7-4fce-91a1-4767e9ed561b",
+				URI:      "sip:182.82.132.122",
+				Response: "6cf0981e056709d40c8acc40c87e73c6",
+				Cnonce:   "104adc6bd71f49678798ee646edcaa9a",
+				QOP:      "auth",
+				Nc:       1,
+			},
+		},
 	}
 	for _, tt := range tests {
 		t.Run("", func(t *testing.T) {