security: zeroize pem encoded key

`PemEncodedKey` is now zeroized and no longer contains ASN.1 parsed
data. Because `Vec<simple_asn1::ASN1Block>` owns the underlying bytes
and does not support zeroization, the full ASN.1 parsing has been
replaced by a custom partial ASN.1 implementation. This parsing is
performed only to classify the key type during construction and to
extract the key bytes on access.

This change also allows the removal of the `simple_asn1` dependency.

Author: boxdot

Cargo.toml

@@ -30,7 +30,6 @@ signature = { version = "2.2.0", features = ["std"] }
 
 # For PEM decoding
 pem = { version = "3", optional = true }
-simple_asn1 = { version = "0.6", optional = true }
 
 # "aws_lc_rs" feature
 aws-lc-rs = { version = "1.15.0", optional = true }
@@ -43,6 +42,7 @@ p384 = { version = "0.13.0", optional = true, features = ["ecdsa"] }
 rand = { version = "0.8.5", optional = true, features = ["std"], default-features = false }
 rsa = { version = "0.9.6", optional = true }
 sha2 = { version = "0.10.7", optional = true, features = ["oid"] }
+zeroize = { version = "1.8.2", features = ["derive"] }
 
 [target.'cfg(target_arch = "wasm32")'.dependencies]
 js-sys = "0.3"
@@ -65,7 +65,7 @@ criterion = { version = "0.8", default-features = false }
 
 [features]
 default = ["use_pem"]
-use_pem = ["pem", "simple_asn1"]
+use_pem = ["pem"]
 rust_crypto = ["ed25519-dalek", "hmac", "p256", "p384", "rand", "rsa", "sha2"]
 aws_lc_rs = ["aws-lc-rs"]
 

src/pem/decoder.rs

@@ -1,3 +1,5 @@
+use zeroize::{Zeroize, ZeroizeOnDrop};
+
 use crate::errors::{ErrorKind, Result};
 
 /// Supported PEM files for EC and RSA Public and Private Keys
@@ -40,11 +42,12 @@ enum Classification {
 /// Documentation about these formats is at
 /// PKCS#1: https://tools.ietf.org/html/rfc8017
 /// PKCS#8: https://tools.ietf.org/html/rfc5958
-#[derive(Debug)]
+#[derive(Debug, ZeroizeOnDrop, Zeroize)]
 pub(crate) struct PemEncodedKey {
     content: Vec<u8>,
-    asn1: Vec<simple_asn1::ASN1Block>,
+    #[zeroize(skip)]
     pem_type: PemType,
+    #[zeroize(skip)]
     standard: Standard,
 }
 
@@ -53,22 +56,15 @@ impl PemEncodedKey {
     pub fn new(input: &[u8]) -> Result<PemEncodedKey> {
         match pem::parse(input) {
             Ok(content) => {
-                let asn1_content = match simple_asn1::from_der(content.contents()) {
-                    Ok(asn1) => asn1,
-                    Err(_) => return Err(ErrorKind::InvalidKeyFormat.into()),
-                };
-
                 match content.tag() {
                     // This handles a PKCS#1 RSA Private key
                     "RSA PRIVATE KEY" => Ok(PemEncodedKey {
                         content: content.into_contents(),
-                        asn1: asn1_content,
                         pem_type: PemType::RsaPrivate,
                         standard: Standard::Pkcs1,
                     }),
                     "RSA PUBLIC KEY" => Ok(PemEncodedKey {
                         content: content.into_contents(),
-                        asn1: asn1_content,
                         pem_type: PemType::RsaPublic,
                         standard: Standard::Pkcs1,
                     }),
@@ -79,41 +75,22 @@ impl PemEncodedKey {
 
                     // This handles PKCS#8 certificates and public & private keys
                     tag @ "PRIVATE KEY" | tag @ "PUBLIC KEY" | tag @ "CERTIFICATE" => {
-                        match classify_pem(&asn1_content) {
-                            Some(c) => {
-                                let is_private = tag == "PRIVATE KEY";
-                                let pem_type = match c {
-                                    Classification::Ec => {
-                                        if is_private {
-                                            PemType::EcPrivate
-                                        } else {
-                                            PemType::EcPublic
-                                        }
-                                    }
-                                    Classification::Ed => {
-                                        if is_private {
-                                            PemType::EdPrivate
-                                        } else {
-                                            PemType::EdPublic
-                                        }
-                                    }
-                                    Classification::Rsa => {
-                                        if is_private {
-                                            PemType::RsaPrivate
-                                        } else {
-                                            PemType::RsaPublic
-                                        }
-                                    }
-                                };
-                                Ok(PemEncodedKey {
-                                    content: content.into_contents(),
-                                    asn1: asn1_content,
-                                    pem_type,
-                                    standard: Standard::Pkcs8,
-                                })
-                            }
-                            None => Err(ErrorKind::InvalidKeyFormat.into()),
-                        }
+                        let is_private = tag == "PRIVATE KEY";
+                        let pem_type = match classify_der(content.contents())
+                            .ok_or(ErrorKind::InvalidKeyFormat)?
+                        {
+                            Classification::Ec if is_private => PemType::EcPrivate,
+                            Classification::Ec => PemType::EcPublic,
+                            Classification::Ed if is_private => PemType::EdPrivate,
+                            Classification::Ed => PemType::EdPublic,
+                            Classification::Rsa if is_private => PemType::RsaPrivate,
+                            Classification::Rsa => PemType::RsaPublic,
+                        };
+                        Ok(PemEncodedKey {
+                            content: content.into_contents(),
+                            pem_type,
+                            standard: Standard::Pkcs8,
+                        })
                     }
 
                     // Unknown/unsupported type
@@ -140,7 +117,8 @@ impl PemEncodedKey {
         match self.standard {
             Standard::Pkcs1 => Err(ErrorKind::InvalidKeyFormat.into()),
             Standard::Pkcs8 => match self.pem_type {
-                PemType::EcPublic => extract_first_bitstring(&self.asn1),
+                PemType::EcPublic => extract_first_bitstring_der(&self.content)
+                    .ok_or_else(|| ErrorKind::InvalidKeyFormat.into()),
                 _ => Err(ErrorKind::InvalidKeyFormat.into()),
             },
         }
@@ -162,7 +140,8 @@ impl PemEncodedKey {
         match self.standard {
             Standard::Pkcs1 => Err(ErrorKind::InvalidKeyFormat.into()),
             Standard::Pkcs8 => match self.pem_type {
-                PemType::EdPublic => extract_first_bitstring(&self.asn1),
+                PemType::EdPublic => extract_first_bitstring_der(&self.content)
+                    .ok_or_else(|| ErrorKind::InvalidKeyFormat.into()),
                 _ => Err(ErrorKind::InvalidKeyFormat.into()),
             },
         }
@@ -173,68 +152,175 @@ impl PemEncodedKey {
         match self.standard {
             Standard::Pkcs1 => Ok(self.content.as_slice()),
             Standard::Pkcs8 => match self.pem_type {
-                PemType::RsaPrivate => extract_first_bitstring(&self.asn1),
-                PemType::RsaPublic => extract_first_bitstring(&self.asn1),
+                PemType::RsaPrivate | PemType::RsaPublic => {
+                    extract_first_bitstring_der(&self.content)
+                        .ok_or_else(|| ErrorKind::InvalidKeyFormat.into())
+                }
                 _ => Err(ErrorKind::InvalidKeyFormat.into()),
             },
         }
     }
 }
 
+const TAG_BIT_STRING: u8 = 0x03;
+const TAG_OCTET_STRING: u8 = 0x04;
+const TAG_OID: u8 = 0x06;
+const TAG_SEQUENCE: u8 = 0x30;
+
 // This really just finds and returns the first bitstring or octet string
 // Which is the x coordinate for EC public keys
 // And the DER contents of an RSA key
 // Though PKCS#11 keys shouldn't have anything else.
 // It will get confusing with certificates.
-fn extract_first_bitstring(asn1: &[simple_asn1::ASN1Block]) -> Result<&[u8]> {
-    for asn1_entry in asn1.iter() {
-        match asn1_entry {
-            simple_asn1::ASN1Block::Sequence(_, entries) => {
-                if let Ok(result) = extract_first_bitstring(entries) {
-                    return Ok(result);
+fn extract_first_bitstring_der(bytes: &[u8]) -> Option<&[u8]> {
+    let mut stack = vec![bytes];
+
+    while let Some(bytes) = stack.pop() {
+        let Some((tag, value, rest)) = read_tlv(bytes) else {
+            continue; // Skip invalid TLV
+        };
+
+        if !rest.is_empty() {
+            stack.push(rest);
+        }
+
+        match tag {
+            TAG_BIT_STRING => {
+                if value.is_empty() {
+                    return None; // Missing padding length
+                } else if value[0] != 0 {
+                    return None; // Padding length must be zero for cryptographic keys
                 }
+                return Some(&value[1..]);
             }
-            simple_asn1::ASN1Block::BitString(_, _, value) => {
-                return Ok(value.as_ref());
-            }
-            simple_asn1::ASN1Block::OctetString(_, value) => {
-                return Ok(value.as_ref());
+            TAG_OCTET_STRING => return Some(value),
+            TAG_SEQUENCE => {
+                stack.push(value);
             }
-            _ => (),
+            _ => {}
         }
     }
 
-    Err(ErrorKind::InvalidEcdsaKey.into())
+    None
 }
 
 /// Find whether this is EC, RSA, or Ed
-fn classify_pem(asn1: &[simple_asn1::ASN1Block]) -> Option<Classification> {
-    // These should be constant but the macro requires
-    // #![feature(const_vec_new)]
-    let ec_public_key_oid = simple_asn1::oid!(1, 2, 840, 10_045, 2, 1);
-    let rsa_public_key_oid = simple_asn1::oid!(1, 2, 840, 113_549, 1, 1, 1);
-    let ed25519_oid = simple_asn1::oid!(1, 3, 101, 112);
-
-    for asn1_entry in asn1.iter() {
-        match asn1_entry {
-            simple_asn1::ASN1Block::Sequence(_, entries) => {
-                if let Some(classification) = classify_pem(entries) {
-                    return Some(classification);
-                }
-            }
-            simple_asn1::ASN1Block::ObjectIdentifier(_, oid) => {
-                if oid == ec_public_key_oid {
-                    return Some(Classification::Ec);
-                }
-                if oid == rsa_public_key_oid {
-                    return Some(Classification::Rsa);
-                }
-                if oid == ed25519_oid {
-                    return Some(Classification::Ed);
-                }
+fn classify_der(bytes: &[u8]) -> Option<Classification> {
+    const EC_PUBLIC_KEY_OID: &[u8] = &[0x2A, 0x86, 0x48, 0xCE, 0x3D, 0x02, 0x01]; // 1.2.840.10045.2.1
+    const RSA_PUBLIC_KEY_OID: &[u8] = &[0x2A, 0x86, 0x48, 0x86, 0xF7, 0x0D, 0x01, 0x01, 0x01]; // 1.2.840.113549.1.1.1
+    const ED25519_OID: &[u8] = &[0x2B, 0x65, 0x70]; // 1.3.101.112
+
+    let mut stack = vec![bytes];
+
+    while let Some(bytes) = stack.pop() {
+        let Some((tag, value, rest)) = read_tlv(bytes) else {
+            continue; // Skip invalid TLV
+        };
+
+        if !rest.is_empty() {
+            stack.push(rest);
+        }
+
+        if tag == TAG_OID {
+            match value {
+                EC_PUBLIC_KEY_OID => return Some(Classification::Ec),
+                RSA_PUBLIC_KEY_OID => return Some(Classification::Rsa),
+                ED25519_OID => return Some(Classification::Ed),
+                _ => {}
             }
-            _ => {}
+        } else if tag == TAG_SEQUENCE {
+            stack.push(value);
         }
     }
+
     None
 }
+
+/// Returns `Some((tag, value, rest))` or `None` if the TLV is invalid.
+fn read_tlv(mut bytes: &[u8]) -> Option<(u8, &[u8], &[u8])> {
+    if bytes.len() < 2 {
+        return None;
+    }
+
+    let tag = bytes[0];
+    let len = bytes[1];
+    bytes = &bytes[2..];
+
+    let len = if len < 0x80 {
+        len as usize
+    } else {
+        let len_len = (len & 0x7f) as usize;
+        if len_len == 0 {
+            return None; // Indefinite length
+        } else if size_of::<usize>() < len_len {
+            return None; // Too long; prevents usize overflow
+        } else if bytes.len() < len_len {
+            return None; // Not enough bytes
+        }
+        let (len_bytes, rest) = bytes.split_at(len_len);
+        bytes = rest;
+        len_bytes.iter().fold(0, |acc, &x| acc * 256 + x as usize)
+    };
+
+    if bytes.len() < len {
+        return None; // Not enough bytes
+    }
+
+    let (value, rest) = bytes.split_at(len);
+    Some((tag, value, rest))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn classify_ec_key() {
+        let pem = pem::parse(include_bytes!("../../tests/ecdsa/public_ecdsa_key.pem")).unwrap();
+        assert_eq!(classify_der(pem.contents()), Some(Classification::Ec));
+    }
+
+    #[test]
+    fn classify_rsa_key() {
+        let pem = pem::parse(include_bytes!("../../tests/rsa/public_rsa_key_pkcs8.pem")).unwrap();
+        assert_eq!(classify_der(pem.contents()), Some(Classification::Rsa));
+    }
+
+    #[test]
+    fn classify_ed25519_key() {
+        let pem = pem::parse(include_bytes!("../../tests/eddsa/public_ed25519_key.pem")).unwrap();
+        assert_eq!(classify_der(pem.contents()), Some(Classification::Ed));
+    }
+
+    #[test]
+    fn ec_public_key_extraction() {
+        let key =
+            PemEncodedKey::new(include_bytes!("../../tests/ecdsa/public_ecdsa_key.pem")).unwrap();
+        let bytes = key.as_ec_public_key().unwrap();
+        assert_eq!(bytes[0], 0x04); // uncompressed point
+        assert_eq!(bytes.len(), 65); // 1 + 32 + 32 for P-256
+    }
+
+    #[test]
+    fn ed_public_key_extraction() {
+        let key =
+            PemEncodedKey::new(include_bytes!("../../tests/eddsa/public_ed25519_key.pem")).unwrap();
+        let bytes = key.as_ed_public_key().unwrap();
+        assert_eq!(bytes.len(), 32);
+    }
+
+    #[test]
+    fn rsa_pkcs8_key_extraction() {
+        let key =
+            PemEncodedKey::new(include_bytes!("../../tests/rsa/public_rsa_key_pkcs8.pem")).unwrap();
+        let bytes = key.as_rsa_key().unwrap();
+        assert_eq!(bytes[0], 0x30); // SEQUENCE
+    }
+    #[test]
+    fn rsa_pkcs1_key() {
+        let key = PemEncodedKey::new(include_bytes!("../../tests/rsa/private_rsa_key_pkcs1.pem"))
+            .unwrap();
+        let bytes = key.as_rsa_key().unwrap();
+        assert_eq!(bytes[0], 0x30); // SEQUENCE
+    }
+}