der: add PemWriter and 1-pass EncodePem (#618)

Adds a `PemWriter` type which impls the `Writer` trait and can perform
on-the-fly PEM encoding.

Author: tarcieri

der/src/encode.rs

@@ -3,15 +3,18 @@
 use crate::{Encoder, Header, Length, Result, Tagged, Writer};
 
 #[cfg(feature = "alloc")]
-use {crate::ErrorKind, alloc::vec::Vec, core::iter};
+use {alloc::vec::Vec, core::iter};
 
 #[cfg(feature = "pem")]
 use {
+    crate::PemWriter,
     alloc::string::String,
     pem_rfc7468::{self as pem, LineEnding, PemLabel},
-    zeroize::Zeroizing,
 };
 
+#[cfg(any(feature = "alloc", feature = "pem"))]
+use crate::ErrorKind;
+
 #[cfg(doc)]
 use crate::Tag;
 
@@ -95,10 +98,30 @@ pub trait EncodePem: Encode + PemLabel {
 #[cfg(feature = "pem")]
 #[cfg_attr(docsrs, doc(cfg(feature = "pem")))]
 impl<T: Encode + PemLabel> EncodePem for T {
+    #[allow(clippy::integer_arithmetic)]
     fn to_pem(&self, line_ending: LineEnding) -> Result<String> {
-        // TODO(tarcieri): support for encoding directly from PEM (instead of two-pass)
-        let der = Zeroizing::new(self.to_vec()?);
-        Ok(pem::encode_string(Self::PEM_LABEL, line_ending, &der)?)
+        // TODO(tarcieri): checked arithmetic, maybe extract this into `base64ct::base64_len`?
+        let der_len = usize::try_from(self.encoded_len()?)?;
+        let mut base64_len = (((der_len * 4) / 3) + 3) & !3;
+
+        // Add the length of the line endings which will be inserted when
+        // encoded Base64 is line wrapped
+        // TODO(tarcieri): factor this logic into `pem-rfc7468`
+        base64_len += base64_len
+            .saturating_sub(1)
+            .checked_div(64)
+            .and_then(|len| len.checked_add(line_ending.len()))
+            .ok_or(ErrorKind::Overflow)?;
+
+        let pem_len = pem::encapsulated_len(Self::PEM_LABEL, line_ending, base64_len)?;
+
+        let mut buf = vec![0u8; pem_len];
+        let mut writer = PemWriter::new(Self::PEM_LABEL, line_ending, &mut buf)?;
+        self.encode(&mut writer)?;
+
+        let actual_len = writer.finish()?;
+        buf.truncate(actual_len);
+        Ok(String::from_utf8(buf)?)
     }
 }
 

der/src/error.rs

@@ -118,6 +118,13 @@ impl From<Utf8Error> for Error {
     }
 }
 
+#[cfg(feature = "alloc")]
+impl From<alloc::string::FromUtf8Error> for Error {
+    fn from(err: alloc::string::FromUtf8Error) -> Error {
+        ErrorKind::Utf8(err.utf8_error()).into()
+    }
+}
+
 #[cfg(feature = "oid")]
 impl From<const_oid::Error> for Error {
     fn from(_: const_oid::Error) -> Error {
@@ -150,6 +157,7 @@ impl From<time::error::ComponentRange> for Error {
         ErrorKind::DateTime.into()
     }
 }
+
 /// Error type.
 #[derive(Copy, Clone, Debug, Eq, PartialEq)]
 #[non_exhaustive]

der/src/lib.rs

@@ -338,7 +338,8 @@
 //! [`Utf8String`]: asn1::Utf8String
 
 #[cfg(feature = "alloc")]
-#[cfg_attr(test, macro_use)]
+#[allow(unused_imports)]
+#[macro_use]
 extern crate alloc;
 #[cfg(feature = "std")]
 extern crate std;
@@ -396,7 +397,7 @@ pub use der_derive::{Choice, Enumerated, Newtype, Sequence, ValueOrd};
 #[cfg(feature = "pem")]
 #[cfg_attr(docsrs, doc(cfg(feature = "pem")))]
 pub use {
-    crate::{decode::DecodePem, encode::EncodePem},
+    crate::{decode::DecodePem, encode::EncodePem, writer::PemWriter},
     pem_rfc7468 as pem,
 };
 

der/src/writer.rs

@@ -2,6 +2,9 @@
 
 use crate::Result;
 
+#[cfg(feature = "pem")]
+use crate::pem;
+
 #[cfg(feature = "std")]
 use std::io;
 
@@ -16,6 +19,48 @@ pub trait Writer {
     }
 }
 
+/// `Writer` type which outputs PEM-encoded data.
+#[cfg(feature = "pem")]
+#[cfg_attr(docsrs, doc(cfg(feature = "pem")))]
+pub struct PemWriter<'o>(pem::Encoder<'static, 'o>);
+
+#[cfg(feature = "pem")]
+#[cfg_attr(docsrs, doc(cfg(feature = "pem")))]
+impl<'o> PemWriter<'o> {
+    /// Create a new PEM writer which outputs into the provided buffer.
+    ///
+    /// Uses the default 64-character line wrapping.
+    pub fn new(
+        type_label: &'static str,
+        line_ending: pem::LineEnding,
+        out: &'o mut [u8],
+    ) -> Result<Self> {
+        Ok(Self(pem::Encoder::new(type_label, line_ending, out)?))
+    }
+
+    /// Get the PEM label which will be used in the encapsulation boundaries
+    /// for this document.
+    pub fn type_label(&self) -> &'static str {
+        self.0.type_label()
+    }
+
+    /// Finish encoding PEM, writing the post-encapsulation boundary.
+    ///
+    /// On success, returns the total number of bytes written to the output buffer.
+    pub fn finish(self) -> Result<usize> {
+        Ok(self.0.finish()?)
+    }
+}
+
+#[cfg(feature = "pem")]
+#[cfg_attr(docsrs, doc(cfg(feature = "pem")))]
+impl Writer for PemWriter<'_> {
+    fn write(&mut self, slice: &[u8]) -> Result<()> {
+        self.0.encode(slice)?;
+        Ok(())
+    }
+}
+
 #[cfg(feature = "std")]
 #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
 impl<W: io::Write> Writer for W {