diff --git a/native-certgen/src/certbuilder.rs b/native-certgen/src/certbuilder.rs
new file mode 100644
index 0000000..a3426ed
--- /dev/null
+++ b/native-certgen/src/certbuilder.rs
@@ -0,0 +1,233 @@
+use crate::error::{CertGenError, Result};
+use crate::keybox::ParsedKeybox;
+use crate::types::{CertGenParams, GeneratedKeyPair};
+
+use rcgen::{
+    BasicConstraints, CertificateParams, CustomExtension, DistinguishedName, DnType, IsCa,
+    KeyPair, KeyUsagePurpose, SerialNumber,
+};
+use time::OffsetDateTime;
+
+const ATTESTATION_OID: &[u64] = &[1, 3, 6, 1, 4, 1, 11129, 2, 1, 17];
+
+pub fn build_certificate_chain(
+    key_pair: &GeneratedKeyPair,
+    attestation_ext_der: &[u8],
+    keybox: &ParsedKeybox,
+    params: &CertGenParams,
+) -> Result<Vec<Vec<u8>>> {
+    let issuer_key = KeyPair::try_from(keybox.signing_key_der.as_slice())
+        .map_err(|e| CertGenError::CertBuildFailed(format!("keybox key parse: {e}")))?;
+
+    let issuer_cert = build_issuer_cert(&issuer_key, &keybox.issuer_dn_der)?;
+
+    let subject_key = KeyPair::try_from(key_pair.private_key_pkcs8.as_slice())
+        .map_err(|e| CertGenError::CertBuildFailed(format!("subject key parse: {e}")))?;
+
+    let leaf_params = build_leaf_params(attestation_ext_der, keybox, params)?;
+
+    let leaf_cert = leaf_params
+        .signed_by(&subject_key, &issuer_cert, &issuer_key)
+        .map_err(|e| CertGenError::CertBuildFailed(format!("signing: {e}")))?;
+
+    let mut chain = Vec::with_capacity(1 + keybox.cert_chain_ders.len());
+    chain.push(leaf_cert.der().to_vec());
+    for cert_der in &keybox.cert_chain_ders {
+        chain.push(cert_der.clone());
+    }
+
+    Ok(chain)
+}
+
+fn build_issuer_cert(
+    issuer_key: &KeyPair,
+    issuer_dn_der: &[u8],
+) -> Result<rcgen::Certificate> {
+    let mut issuer_params = CertificateParams::default();
+    issuer_params.distinguished_name = parse_dn_from_der(issuer_dn_der)?;
+    issuer_params.is_ca = IsCa::Ca(BasicConstraints::Unconstrained);
+    // Suppress AKI/SKI generation — we only need this cert as a signing vehicle
+    issuer_params.key_identifier_method = rcgen::KeyIdMethod::PreSpecified(vec![]);
+
+    issuer_params
+        .self_signed(issuer_key)
+        .map_err(|e| CertGenError::CertBuildFailed(format!("issuer self-sign: {e}")))
+}
+
+fn build_leaf_params(
+    attestation_ext_der: &[u8],
+    keybox: &ParsedKeybox,
+    params: &CertGenParams,
+) -> Result<CertificateParams> {
+    let mut cp = CertificateParams::default();
+
+    // Subject DN
+    cp.distinguished_name = if let Some(ref subject_der) = params.cert_subject {
+        parse_dn_from_der(subject_der)?
+    } else {
+        let mut dn = DistinguishedName::new();
+        dn.push(DnType::CommonName, "Android KeyStore Key");
+        dn
+    };
+
+    // Serial number
+    cp.serial_number = if let Some(ref serial_bytes) = params.cert_serial {
+        Some(SerialNumber::from(serial_bytes.clone()))
+    } else {
+        Some(SerialNumber::from(vec![1u8]))
+    };
+
+    // Validity period
+    cp.not_before = timestamp_to_datetime(params.cert_not_before)?;
+    cp.not_after = if params.cert_not_after == -1 {
+        // Fall back to keybox leaf cert's notAfter, or +1 year
+        OffsetDateTime::from_unix_timestamp(keybox.leaf_not_after)
+            .unwrap_or_else(|_| OffsetDateTime::now_utc() + time::Duration::days(365))
+    } else {
+        timestamp_to_datetime(params.cert_not_after)?
+    };
+
+    // Suppress AKI/SKI — Android attestation leaf certs don't include these
+    cp.key_identifier_method = rcgen::KeyIdMethod::PreSpecified(vec![]);
+
+    // KeyUsage from purposes
+    cp.key_usages = map_key_usages(&params.purposes);
+
+    // Attestation extension (non-critical)
+    let mut attest_ext = CustomExtension::from_oid_content(ATTESTATION_OID, attestation_ext_der.to_vec());
+    attest_ext.set_criticality(false);
+    cp.custom_extensions.push(attest_ext);
+
+    Ok(cp)
+}
+
+fn map_key_usages(purposes: &[i32]) -> Vec<KeyUsagePurpose> {
+    let mut usages = Vec::new();
+    for &purpose in purposes {
+        match purpose {
+            2 | 3 => {
+                // SIGN or VERIFY -> digitalSignature
+                if !usages.contains(&KeyUsagePurpose::DigitalSignature) {
+                    usages.push(KeyUsagePurpose::DigitalSignature);
+                }
+            }
+            0 => {
+                // ENCRYPT -> keyEncipherment
+                if !usages.contains(&KeyUsagePurpose::KeyEncipherment) {
+                    usages.push(KeyUsagePurpose::KeyEncipherment);
+                }
+            }
+            1 => {
+                // DECRYPT -> dataEncipherment
+                if !usages.contains(&KeyUsagePurpose::DataEncipherment) {
+                    usages.push(KeyUsagePurpose::DataEncipherment);
+                }
+            }
+            5 => {
+                // WRAP_KEY -> keyEncipherment
+                if !usages.contains(&KeyUsagePurpose::KeyEncipherment) {
+                    usages.push(KeyUsagePurpose::KeyEncipherment);
+                }
+            }
+            6 => {
+                // AGREE_KEY -> keyAgreement
+                if !usages.contains(&KeyUsagePurpose::KeyAgreement) {
+                    usages.push(KeyUsagePurpose::KeyAgreement);
+                }
+            }
+            7 => {
+                // ATTEST_KEY -> keyCertSign
+                if !usages.contains(&KeyUsagePurpose::KeyCertSign) {
+                    usages.push(KeyUsagePurpose::KeyCertSign);
+                }
+            }
+            _ => {}
+        }
+    }
+    usages
+}
+
+fn timestamp_to_datetime(ts: i64) -> Result<OffsetDateTime> {
+    if ts == -1 {
+        return Ok(OffsetDateTime::now_utc());
+    }
+    // Params use milliseconds for validity timestamps
+    OffsetDateTime::from_unix_timestamp(ts / 1000)
+        .map_err(|e| CertGenError::CertBuildFailed(format!("invalid timestamp {ts}: {e}")))
+}
+
+// Parse a DER-encoded X.500 Name into rcgen DistinguishedName.
+// We only extract the CN (most common for Android keystore certs).
+// If parsing fails, fall back to empty DN.
+fn parse_dn_from_der(der: &[u8]) -> Result<DistinguishedName> {
+    use x509_cert::name::Name;
+    use der::{Decode, Encode};
+
+    let name = Name::from_der(der)
+        .map_err(|e| CertGenError::CertBuildFailed(format!("DN parse: {e}")))?;
+
+    let mut dn = DistinguishedName::new();
+
+    for rdn in name.0.iter() {
+        for atv in rdn.0.iter() {
+            let oid_str = atv.oid.to_string();
+            // Map common OIDs to rcgen DnType
+            let dn_type = match oid_str.as_str() {
+                "2.5.4.3" => DnType::CommonName,
+                "2.5.4.6" => DnType::CountryName,
+                "2.5.4.7" => DnType::LocalityName,
+                "2.5.4.8" => DnType::StateOrProvinceName,
+                "2.5.4.10" => DnType::OrganizationName,
+                "2.5.4.11" => DnType::OrganizationalUnitName,
+                other => DnType::CustomDnType(
+                    other.split('.').filter_map(|s| s.parse().ok()).collect(),
+                ),
+            };
+
+            // Extract the string value from the AttributeValue (ANY type)
+            // The value is DER-encoded; try to read it as UTF8String or PrintableString
+            let value_bytes = atv.value.to_der()
+                .map_err(|e| CertGenError::CertBuildFailed(format!("DN value encode: {e}")))?;
+            let value_str = extract_string_from_der_any(&value_bytes);
+            dn.push(dn_type, value_str);
+        }
+    }
+
+    Ok(dn)
+}
+
+// Extract a string from a DER-encoded ASN.1 string type (UTF8String, PrintableString, etc.)
+fn extract_string_from_der_any(der: &[u8]) -> String {
+    if der.len() < 2 {
+        return String::new();
+    }
+    // Tag byte at [0], length at [1..], then content
+    let tag = der[0];
+    let (content_len, header_len) = if der[1] < 0x80 {
+        (der[1] as usize, 2)
+    } else {
+        let num = (der[1] & 0x7f) as usize;
+        if num == 0 || 2 + num > der.len() {
+            return String::new();
+        }
+        let mut len = 0usize;
+        for i in 0..num {
+            len = (len << 8) | der[2 + i] as usize;
+        }
+        (len, 2 + num)
+    };
+
+    let end = header_len + content_len;
+    if end > der.len() {
+        return String::new();
+    }
+    let content = &der[header_len..end];
+
+    match tag {
+        0x0C | 0x13 | 0x16 => {
+            // UTF8String (0x0C), PrintableString (0x13), IA5String (0x16)
+            String::from_utf8_lossy(content).into_owned()
+        }
+        _ => String::from_utf8_lossy(content).into_owned(),
+    }
+}
diff --git a/native-certgen/src/lib.rs b/native-certgen/src/lib.rs
index 9bbc550..147ae6e 100644
--- a/native-certgen/src/lib.rs
+++ b/native-certgen/src/lib.rs
@@ -3,4 +3,5 @@ mod types;
 mod keygen;
 pub mod keybox;
 pub mod attestation;
+pub mod certbuilder;
 pub mod logging;