diff --git a/Cargo.toml b/Cargo.toml
index 862e2e7..44e0989 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -41,6 +41,10 @@ arbitrary = ["dep:arbitrary", "std"]
 get-info-full = []
 # enables support for implementing the large-blobs extension, see src/sizes.rs
 large-blobs = []
+# Size bumps in src/sizes.rs for ML-DSA packed attestation.
+mldsa44 = []
+mldsa65 = []
+mldsa87 = []
 third-party-payment = []
 
 log-all = []
diff --git a/src/ctap2.rs b/src/ctap2.rs
index 3a07a0e..b145210 100644
--- a/src/ctap2.rs
+++ b/src/ctap2.rs
@@ -292,8 +292,8 @@ impl<'de> Deserialize<'de> for AttestationStatement {
                 use serde::de::Error as _;
 
                 let mut alg: Option<i32> = None;
-                let mut sig: Option<Bytes<ASN1_SIGNATURE_LENGTH>> = None;
-                let mut x5c: Option<Vec<Bytes<1024>, 1>> = None;
+                let mut sig: Option<Bytes<MAX_PACKED_SIG_LENGTH>> = None;
+                let mut x5c: Option<Vec<Bytes<MAX_X5C_CERT_LENGTH>, 1>> = None;
                 let mut has_values = false;
                 while let Some(key) = map.next_key::<&str>()? {
                     has_values = true;
@@ -366,9 +366,9 @@ pub struct NoneAttestationStatement {}
 #[cfg_attr(feature = "platform-serde", derive(Deserialize))]
 pub struct PackedAttestationStatement {
     pub alg: i32,
-    pub sig: Bytes<ASN1_SIGNATURE_LENGTH>,
+    pub sig: Bytes<MAX_PACKED_SIG_LENGTH>,
     #[serde(skip_serializing_if = "Option::is_none")]
-    pub x5c: Option<Vec<Bytes<1024>, 1>>,
+    pub x5c: Option<Vec<Bytes<MAX_X5C_CERT_LENGTH>, 1>>,
 }
 
 #[derive(Clone, Debug, Default, Eq, PartialEq)]
@@ -561,7 +561,14 @@ pub trait Authenticator {
         Ok(response)
     }
 
-    fn get_next_assertion(&mut self) -> Result<get_assertion::Response>;
+    fn get_next_assertion_into(&mut self, response: &mut get_assertion::Response) -> Result<()>;
+
+    fn get_next_assertion(&mut self) -> Result<get_assertion::Response> {
+        let mut response = get_assertion::Response::empty();
+        self.get_next_assertion_into(&mut response)?;
+        Ok(response)
+    }
+
     fn reset(&mut self) -> Result<()>;
     fn client_pin(&mut self, request: &client_pin::Request) -> Result<client_pin::Response>;
     fn credential_management(
@@ -709,10 +716,13 @@ pub trait Authenticator {
 
     #[inline(never)]
     fn dispatch_get_next_assertion(&mut self, response: &mut Response) -> Result<()> {
-        *response = Response::GetNextAssertion(self.get_next_assertion().inspect_err(|_e| {
+        *response = Response::GetNextAssertion(get_assertion::Response::empty());
+        let Response::GetNextAssertion(inner) = response else {
+            unreachable!()
+        };
+        self.get_next_assertion_into(inner).inspect_err(|_e| {
             debug!("error: {:?}", _e);
-        })?);
-        Ok(())
+        })
     }
 
     #[inline(never)]
diff --git a/src/ctap2/get_assertion.rs b/src/ctap2/get_assertion.rs
index 9e139c0..f9befbf 100644
--- a/src/ctap2/get_assertion.rs
+++ b/src/ctap2/get_assertion.rs
@@ -135,7 +135,7 @@ pub struct Request<'a> {
 pub struct Response {
     pub credential: PublicKeyCredentialDescriptor,
     pub auth_data: Bytes<AUTHENTICATOR_DATA_LENGTH>,
-    pub signature: Bytes<ASN1_SIGNATURE_LENGTH>,
+    pub signature: Bytes<MAX_PACKED_SIG_LENGTH>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub user: Option<PublicKeyCredentialUserEntity>,
     #[serde(skip_serializing_if = "Option::is_none")]
@@ -158,7 +158,7 @@ pub struct Response {
 pub struct ResponseBuilder {
     pub credential: PublicKeyCredentialDescriptor,
     pub auth_data: Bytes<AUTHENTICATOR_DATA_LENGTH>,
-    pub signature: Bytes<ASN1_SIGNATURE_LENGTH>,
+    pub signature: Bytes<MAX_PACKED_SIG_LENGTH>,
 }
 
 impl ResponseBuilder {
diff --git a/src/sizes.rs b/src/sizes.rs
index eb2c321..cc02e81 100644
--- a/src/sizes.rs
+++ b/src/sizes.rs
@@ -1,9 +1,45 @@
-pub const AUTHENTICATOR_DATA_LENGTH: usize = 676;
+// Largest enabled ML-DSA parameter set: public key / signature bytes (FIPS 204).
+const MLDSA: bool = cfg!(any(
+    feature = "mldsa44",
+    feature = "mldsa65",
+    feature = "mldsa87"
+));
+const MLDSA_PK: usize = if cfg!(feature = "mldsa87") {
+    2592
+} else if cfg!(feature = "mldsa65") {
+    1952
+} else if cfg!(feature = "mldsa44") {
+    1312
+} else {
+    0
+};
+const MLDSA_SIG: usize = if cfg!(feature = "mldsa87") {
+    4627
+} else if cfg!(feature = "mldsa65") {
+    3309
+} else if cfg!(feature = "mldsa44") {
+    2420
+} else {
+    0
+};
+
+// authData holds the key in a COSE_Key map, plus AAGUID and credId.
+pub const AUTHENTICATOR_DATA_LENGTH: usize = if MLDSA { MLDSA_PK + 736 } else { 676 };
 // pub const AUTHENTICATOR_DATA_LENGTH_BYTES: usize = 512;
 
 pub const ASN1_SIGNATURE_LENGTH: usize = 77;
 // pub const ASN1_SIGNATURE_LENGTH_BYTES: usize = 72;
 
+// P-256 fits ASN1_SIGNATURE_LENGTH; ML-DSA needs the raw signature.
+pub const MAX_PACKED_SIG_LENGTH: usize = if MLDSA {
+    MLDSA_SIG + 12
+} else {
+    ASN1_SIGNATURE_LENGTH
+};
+
+// One x5c entry, bounded by what trussed's Reply.der carries.
+pub const MAX_X5C_CERT_LENGTH: usize = if MLDSA { 2048 } else { 1024 };
+
 pub const COSE_KEY_LENGTH: usize = 256;
 // pub const COSE_KEY_LENGTH_BYTES: usize = 256;
 
diff --git a/src/webauthn.rs b/src/webauthn.rs
index 59a908c..0c26598 100644
--- a/src/webauthn.rs
+++ b/src/webauthn.rs
@@ -142,15 +142,49 @@ impl PublicKeyCredentialUserEntity {
 pub enum KnownPublicKeyCredentialParameters {
     ES256,
     EdDSA,
+    #[cfg(feature = "mldsa44")]
+    MLDSA44,
+    #[cfg(feature = "mldsa65")]
+    MLDSA65,
+    #[cfg(feature = "mldsa87")]
+    MLDSA87,
 }
 
 impl KnownPublicKeyCredentialParameters {
-    pub const ALL: [Self; COUNT_KNOWN_ALGS] = [Self::ES256, Self::EdDSA];
+    pub const ALL: [Self; COUNT_KNOWN_ALGS] = {
+        let mut all = [Self::ES256; COUNT_KNOWN_ALGS];
+        let mut i = 1;
+        all[i] = Self::EdDSA;
+        i += 1;
+        #[cfg(feature = "mldsa44")]
+        {
+            all[i] = Self::MLDSA44;
+            i += 1;
+        }
+        #[cfg(feature = "mldsa65")]
+        {
+            all[i] = Self::MLDSA65;
+            i += 1;
+        }
+        #[cfg(feature = "mldsa87")]
+        {
+            all[i] = Self::MLDSA87;
+            i += 1;
+        }
+        let _ = i;
+        all
+    };
 
     pub fn alg(&self) -> i32 {
         match self {
             Self::ES256 => ES256,
             Self::EdDSA => ED_DSA,
+            #[cfg(feature = "mldsa44")]
+            Self::MLDSA44 => ML_DSA_44,
+            #[cfg(feature = "mldsa65")]
+            Self::MLDSA65 => ML_DSA_65,
+            #[cfg(feature = "mldsa87")]
+            Self::MLDSA87 => ML_DSA_87,
         }
     }
 }
@@ -173,8 +207,20 @@ pub enum UnknownPKCredentialParam {
 const ES256: i32 = -7;
 /// EdDSA
 const ED_DSA: i32 = -8;
-
-pub const COUNT_KNOWN_ALGS: usize = 2;
+/// ML-DSA-44 (FIPS 204, NIST level 2)
+#[cfg(feature = "mldsa44")]
+pub const ML_DSA_44: i32 = -48;
+/// ML-DSA-65 (FIPS 204, NIST level 3)
+#[cfg(feature = "mldsa65")]
+pub const ML_DSA_65: i32 = -49;
+/// ML-DSA-87 (FIPS 204, NIST level 5)
+#[cfg(feature = "mldsa87")]
+pub const ML_DSA_87: i32 = -50;
+
+pub const COUNT_KNOWN_ALGS: usize = 2
+    + cfg!(feature = "mldsa44") as usize
+    + cfg!(feature = "mldsa65") as usize
+    + cfg!(feature = "mldsa87") as usize;
 
 impl TryFrom<PublicKeyCredentialParameters> for KnownPublicKeyCredentialParameters {
     type Error = UnknownPKCredentialParam;
@@ -186,6 +232,12 @@ impl TryFrom<PublicKeyCredentialParameters> for KnownPublicKeyCredentialParamete
             match value.alg {
                 ES256 => Ok(Self::ES256),
                 ED_DSA => Ok(Self::EdDSA),
+                #[cfg(feature = "mldsa44")]
+                ML_DSA_44 => Ok(Self::MLDSA44),
+                #[cfg(feature = "mldsa65")]
+                ML_DSA_65 => Ok(Self::MLDSA65),
+                #[cfg(feature = "mldsa87")]
+                ML_DSA_87 => Ok(Self::MLDSA87),
                 _ => Err(UnknownPKCredentialParam::UnknownAlg),
             }
         }