Reworking bssl_crypto: Add RSA support

Change-Id: I418b2f459b0b0f9854a6ec94231166cff7ed6946
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/65178
Reviewed-by: Bob Beck <bbe@google.com>

5 files changed, 341 insertions, 0 deletions

diff --git a/rust/bssl-crypto/src/digest.rs b/rust/bssl-crypto/src/digest.rs
index a10b5ab..e5578e5 100644
--- a/rust/bssl-crypto/src/digest.rs
+++ b/rust/bssl-crypto/src/digest.rs
@@ -31,6 +31,7 @@
 //! ```
 
 use crate::{sealed, FfiSlice, ForeignTypeRef};
+use alloc::vec::Vec;
 use bssl_sys;
 
 #[non_exhaustive]
@@ -49,6 +50,9 @@ pub trait Algorithm {
     /// Gets a reference to a message digest algorithm to be used by the HKDF implementation.
     #[doc(hidden)]
     fn get_md(_: sealed::Sealed) -> &'static MdRef;
+
+    /// Hashes a message.
+    fn hash_to_vec(input: &[u8]) -> Vec<u8>;
 }
 
 /// The insecure SHA-1 hash algorithm.
diff --git a/rust/bssl-crypto/src/lib.rs b/rust/bssl-crypto/src/lib.rs
index babb044..9dce7da 100644
--- a/rust/bssl-crypto/src/lib.rs
+++ b/rust/bssl-crypto/src/lib.rs
@@ -54,6 +54,7 @@ pub mod x25519;
 pub mod ec;
 pub mod ecdh;
 pub mod ecdsa;
+pub mod rsa;
 
 mod scoped;
 
diff --git a/rust/bssl-crypto/src/macros.rs b/rust/bssl-crypto/src/macros.rs
index 3473ec6..6ac3d37 100644
--- a/rust/bssl-crypto/src/macros.rs
+++ b/rust/bssl-crypto/src/macros.rs
@@ -31,6 +31,10 @@ macro_rules! unsafe_iuf_algo {
                 // - this always returns a valid pointer to an EVP_MD.
                 unsafe { MdRef::from_ptr(bssl_sys::$evp_md() as *mut _) }
             }
+
+            fn hash_to_vec(input: &[u8]) -> Vec<u8> {
+                Self::hash(input).as_slice().to_vec()
+            }
         }
 
         impl $name {
diff --git a/rust/bssl-crypto/src/rsa.rs b/rust/bssl-crypto/src/rsa.rs
new file mode 100644
index 0000000..ba85535
--- /dev/null
+++ b/rust/bssl-crypto/src/rsa.rs
@@ -0,0 +1,309 @@
+/* Copyright (c) 2024, Google Inc.
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+ * SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+ * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+ * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+//! RSA signatures.
+//!
+//! New protocols should not use RSA, but it's still often found in existing
+//! protocols. This module implements PKCS#1 signatures (the most common type).
+//!
+//! Creating a signature:
+//!
+//! ```
+//! use bssl_crypto::{digest, rsa};
+//! # use bssl_crypto::rsa::TEST_PKCS8_BYTES;
+//!
+//! // Generating an RSA private key is slow, so this examples parses it from
+//! // PKCS#8 DER.
+//! let private_key = rsa::PrivateKey::from_der_private_key_info(TEST_PKCS8_BYTES).unwrap();
+//! let signed_msg = b"hello world";
+//! let sig = private_key.sign_pkcs1::<digest::Sha256>(signed_msg);
+//! ```
+//!
+//! To verify a signature, publish your _public_ key:
+//!
+//! ```
+//! # use bssl_crypto::{rsa};
+//! # use bssl_crypto::rsa::TEST_PKCS8_BYTES;
+//! # let private_key = rsa::PrivateKey::from_der_private_key_info(TEST_PKCS8_BYTES).unwrap();
+//! let public_key_bytes = private_key.as_public().to_der_subject_public_key_info();
+//! ```
+//!
+//! Then verify the signature from above with it:
+//!
+//! ```
+//! # use bssl_crypto::{digest, rsa};
+//! # use bssl_crypto::rsa::TEST_PKCS8_BYTES;
+//! # let private_key = rsa::PrivateKey::from_der_private_key_info(TEST_PKCS8_BYTES).unwrap();
+//! # let signed_msg = b"hello world";
+//! # let mut sig = private_key.sign_pkcs1::<digest::Sha256>(signed_msg);
+//! # let public_key_bytes = private_key.as_public().to_der_subject_public_key_info();
+//! let public_key = rsa::PublicKey::from_der_subject_public_key_info(public_key_bytes.as_ref())
+//!    .unwrap();
+//! assert!(public_key.verify_pkcs1::<digest::Sha256>(signed_msg, sig.as_slice()).is_ok());
+//! sig[0] ^= 1;
+//! assert!(public_key.verify_pkcs1::<digest::Sha256>(signed_msg, sig.as_slice()).is_err());
+//! ```
+
+use crate::{
+    cbb_to_buffer, digest, parse_with_cbs, scoped, sealed, with_output_vec, Buffer, FfiSlice,
+    ForeignTypeRef, InvalidSignatureError,
+};
+use alloc::vec::Vec;
+use core::ptr::null_mut;
+
+/// An RSA public key.
+pub struct PublicKey(*mut bssl_sys::RSA);
+
+impl PublicKey {
+    /// Parse a DER-encoded RSAPublicKey structure (from RFC 8017).
+    pub fn from_der_rsa_public_key(der: &[u8]) -> Option<Self> {
+        Some(PublicKey(parse_with_cbs(
+            der,
+            // Safety: `ptr` is a non-null result from `RSA_parse_public_key` here.
+            |ptr| unsafe { bssl_sys::RSA_free(ptr) },
+            // Safety: cbs is valid per `parse_with_cbs`.
+            |cbs| unsafe { bssl_sys::RSA_parse_public_key(cbs) },
+        )?))
+    }
+
+    /// Serialize to a DER-encoded RSAPublicKey structure (from RFC 8017).
+    pub fn to_der_rsa_public_key(&self) -> Buffer {
+        cbb_to_buffer(/*initial_capacity=*/ 300, |cbb| unsafe {
+            // Safety: `self.0` is valid by construction.
+            assert_eq!(1, bssl_sys::RSA_marshal_public_key(cbb, self.0))
+        })
+    }
+
+    /// Parse a DER-encoded SubjectPublicKeyInfo. This format is found in,
+    /// for example, X.509 certificates.
+    pub fn from_der_subject_public_key_info(spki: &[u8]) -> Option<Self> {
+        let mut pkey = scoped::EvpPkey::from_ptr(parse_with_cbs(
+            spki,
+            // Safety: `pkey` is a non-null result from `EVP_parse_public_key` here.
+            |pkey| unsafe { bssl_sys::EVP_PKEY_free(pkey) },
+            // Safety: cbs is valid per `parse_with_cbs`.
+            |cbs| unsafe { bssl_sys::EVP_parse_public_key(cbs) },
+        )?);
+        let rsa = unsafe { bssl_sys::EVP_PKEY_get1_RSA(pkey.as_ffi_ptr()) };
+        if !rsa.is_null() {
+            // Safety: `EVP_PKEY_get1_RSA` adds a reference so we are not
+            // stealing ownership from `pkey`.
+            Some(PublicKey(rsa))
+        } else {
+            None
+        }
+    }
+
+    /// Serialize to a DER-encoded SubjectPublicKeyInfo. This format is found
+    /// in, for example, X.509 certificates.
+    pub fn to_der_subject_public_key_info(&self) -> Buffer {
+        let mut pkey = scoped::EvpPkey::new();
+        // Safety: this takes a reference to `self.0` and so doesn't steal ownership.
+        assert_eq!(1, unsafe {
+            bssl_sys::EVP_PKEY_set1_RSA(pkey.as_ffi_ptr(), self.0)
+        });
+        cbb_to_buffer(384, |cbb| unsafe {
+            // The arguments are valid so this will only fail if out of memory,
+            // which this crate doesn't handle.
+            assert_eq!(1, bssl_sys::EVP_marshal_public_key(cbb, pkey.as_ffi_ptr()));
+        })
+    }
+
+    /// Verify that `signature` is a valid signature of a digest of
+    /// `signed_msg`, by this public key. The digest of the message will be
+    /// computed with the specified hash function.
+    pub fn verify_pkcs1<Hash: digest::Algorithm>(
+        &self,
+        signed_msg: &[u8],
+        signature: &[u8],
+    ) -> Result<(), InvalidSignatureError> {
+        let digest = Hash::hash_to_vec(signed_msg);
+        // Safety: `get_md` always returns a valid pointer.
+        let hash_nid = unsafe { bssl_sys::EVP_MD_nid(Hash::get_md(sealed::Sealed).as_ptr()) };
+        let result = unsafe {
+            // Safety: all buffers are valid and `self.0` is valid by construction.
+            bssl_sys::RSA_verify(
+                hash_nid,
+                digest.as_slice().as_ffi_ptr(),
+                digest.len(),
+                signature.as_ffi_ptr(),
+                signature.len(),
+                self.0,
+            )
+        };
+        if result == 1 {
+            Ok(())
+        } else {
+            Err(InvalidSignatureError)
+        }
+    }
+}
+
+impl Drop for PublicKey {
+    fn drop(&mut self) {
+        // Safety: this object owns `self.0`.
+        unsafe { bssl_sys::RSA_free(self.0) }
+    }
+}
+
+/// The set of supported RSA key sizes for key generation.
+#[allow(missing_docs)]
+pub enum KeySize {
+    Rsa2048 = 2048,
+    Rsa3072 = 3072,
+    Rsa4096 = 4096,
+}
+
+/// An RSA private key.
+pub struct PrivateKey(*mut bssl_sys::RSA);
+
+impl PrivateKey {
+    /// Generate a fresh RSA private key of the given size.
+    pub fn generate(size: KeySize) -> Self {
+        let e = scoped::Bignum::from_u64(bssl_sys::RSA_F4 as u64);
+        let ptr = unsafe { bssl_sys::RSA_new() };
+        assert!(!ptr.is_null());
+
+        let result = unsafe {
+            // Safety: `rsa` and `e` are valid and initialized, just above.
+            bssl_sys::RSA_generate_key_ex(ptr, size as core::ffi::c_int, e.as_ffi_ptr(), null_mut())
+        };
+        assert_eq!(1, result);
+        // Safety: this function owns `ptr` and thus can move ownership here.
+        Self(ptr)
+    }
+
+    /// Parse a DER-encoded RSAPrivateKey structure (from RFC 8017).
+    pub fn from_der_rsa_private_key(der: &[u8]) -> Option<Self> {
+        Some(PrivateKey(parse_with_cbs(
+            der,
+            // Safety: `ptr` is a non-null result from `RSA_parse_private_key` here.
+            |ptr| unsafe { bssl_sys::RSA_free(ptr) },
+            // Safety: `cbs` is valid per `parse_with_cbs`.
+            |cbs| unsafe { bssl_sys::RSA_parse_private_key(cbs) },
+        )?))
+    }
+
+    /// Serialize to a DER-encoded RSAPrivateKey structure (from RFC 8017).
+    pub fn to_der_rsa_private_key(&self) -> Buffer {
+        cbb_to_buffer(/*initial_capacity=*/ 512, |cbb| unsafe {
+            // Safety: `self.0` is valid by construction.
+            assert_eq!(1, bssl_sys::RSA_marshal_private_key(cbb, self.0))
+        })
+    }
+
+    /// Parse a DER-encrypted PrivateKeyInfo struct (from RFC 5208). This is often called "PKCS#8 format".
+    pub fn from_der_private_key_info(der: &[u8]) -> Option<Self> {
+        let mut pkey = scoped::EvpPkey::from_ptr(parse_with_cbs(
+            der,
+            // Safety: `ptr` is a non-null result from `EVP_parse_private_key` here.
+            |pkey| unsafe { bssl_sys::EVP_PKEY_free(pkey) },
+            // Safety: `cbs` is valid per `parse_with_cbs`.
+            |cbs| unsafe { bssl_sys::EVP_parse_private_key(cbs) },
+        )?);
+        // Safety: `pkey` is valid and was created just above.
+        let rsa = unsafe { bssl_sys::EVP_PKEY_get1_RSA(pkey.as_ffi_ptr()) };
+        if rsa.is_null() {
+            return None;
+        }
+        Some(Self(rsa))
+    }
+
+    /// Serialize to a DER-encrypted PrivateKeyInfo struct (from RFC 5208). This is often called "PKCS#8 format".
+    pub fn to_der_private_key_info(&self) -> Buffer {
+        let mut pkey = scoped::EvpPkey::new();
+        assert_eq!(1, unsafe {
+            // Safety: `pkey` was just constructed. This takes a reference and
+            // so doesn't steal ownership from `self`.
+            bssl_sys::EVP_PKEY_set1_RSA(pkey.as_ffi_ptr(), self.0)
+        });
+        unsafe {
+            cbb_to_buffer(/*initial_capacity=*/ 384, |cbb| {
+                // Safety: `pkey` is valid and owned by this function.
+                assert_eq!(1, bssl_sys::EVP_marshal_private_key(cbb, pkey.as_ffi_ptr()));
+            })
+        }
+    }
+
+    /// Compute the signature of the digest of `to_be_signed` with PKCS#1 using
+    /// this private key. The specified hash function is used to compute the
+    //  digest.
+    pub fn sign_pkcs1<Hash: digest::Algorithm>(&self, to_be_signed: &[u8]) -> Vec<u8> {
+        let digest = Hash::hash_to_vec(to_be_signed);
+        // Safety: `get_md` always returns a valid pointer.
+        let hash_nid = unsafe { bssl_sys::EVP_MD_nid(Hash::get_md(sealed::Sealed).as_ptr()) };
+        let max_output = unsafe { bssl_sys::RSA_size(self.0) } as usize;
+
+        unsafe {
+            with_output_vec(max_output, |out_buf| {
+                let mut out_len: core::ffi::c_uint = 0;
+                // Safety: `out_buf` points to at least `RSA_size` bytes, as
+                // required. `self.0` is valid by construction.
+                let result = bssl_sys::RSA_sign(
+                    hash_nid,
+                    digest.as_slice().as_ffi_ptr(),
+                    digest.len(),
+                    out_buf,
+                    &mut out_len,
+                    self.0,
+                );
+                // `RSA_sign` should always be successful unless it's out of
+                // memory, which this crate doesn't handle.
+                assert_eq!(1, result);
+                let out_len = out_len as usize;
+                assert!(out_len <= max_output);
+                // Safety: `out_len` bytes have been written.
+                out_len
+            })
+        }
+    }
+
+    /// Return the public key corresponding to this private key.
+    pub fn as_public(&self) -> PublicKey {
+        // Safety: `self.0` is valid by construction and `RSA_up_ref` means
+        // we we can pass an ownership reference to `PublicKey`.
+        unsafe { bssl_sys::RSA_up_ref(self.0) };
+        PublicKey(self.0)
+    }
+}
+
+impl Drop for PrivateKey {
+    fn drop(&mut self) {
+        // Safety: `self.0` is always owned by this struct.
+        unsafe { bssl_sys::RSA_free(self.0) }
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    use crate::digest;
+
+    #[test]
+    fn sign_and_verify() {
+        let key = PrivateKey::from_der_private_key_info(TEST_PKCS8_BYTES).unwrap();
+        let signed_msg = b"hello world";
+        let sig = key.sign_pkcs1::<digest::Sha256>(signed_msg);
+        assert!(key
+            .as_public()
+            .verify_pkcs1::<digest::Sha256>(signed_msg, &sig)
+            .is_ok());
+    }
+}
+
+// RSA generation is slow, but serialized keys are large. In order to use this
+// in doctests, it's included here and public, but undocumented.
+#[doc(hidden)]
+pub const TEST_PKCS8_BYTES: &[u8] = b"\x30\x82\x04\xbd\x02\x01\x00\x30\x0d\x06\x09\x2a\x86\x48\x86\xf7\x0d\x01\x01\x01\x05\x00\x04\x82\x04\xa7\x30\x82\x04\xa3\x02\x01\x00\x02\x82\x01\x01\x00\x98\x3f\xf5\xc4\x89\xb7\x6f\x12\xc8\xb5\x82\xaa\x98\xe6\x75\x39\xc4\x44\x46\xa0\x45\x62\x42\x43\x21\x81\xa0\x53\x17\x47\xb3\xdc\xfc\x3b\x76\x03\xd6\xd4\xce\x5e\x9d\x22\xe5\xa3\x59\xa2\x47\x0c\xe4\x82\x33\x7a\x21\xa5\x61\x2a\x77\xa2\x6b\xfa\xa3\x45\x41\x50\xc2\xf7\x0d\xe1\xa6\x3a\x83\x5b\xe6\xb8\x1f\x24\x1e\x24\x89\xf8\x8d\xde\x5f\xf1\x50\x27\x0f\x2b\xbe\x58\xaa\x64\x67\xef\x22\x57\x1e\xf4\x3f\x2e\xba\x4b\x2f\xc3\x5e\x67\xcc\xc3\xf6\xdd\x6b\x31\x58\xb9\xbd\x7b\xf9\x23\xac\xf2\xa9\xb6\x8f\x88\x75\x0f\x73\xdf\xd2\x14\xaa\x41\x28\x5c\x9a\xd6\xc4\xab\x6f\xb0\x53\xb9\x0a\x2c\xfb\x56\x6e\x56\x94\xaa\x1a\x25\x29\x3b\x01\x0c\x7e\x44\x1b\xe1\x76\x12\x73\xc4\x16\x62\x64\x3d\xe6\xf7\x9f\x69\x3f\xc9\x3b\x75\xd6\x80\xee\x87\x68\x83\xde\x2d\x18\xe4\x26\xdd\x1a\x02\xd8\xd2\x1d\xb6\xf1\x71\xf5\x63\x62\x0c\xd7\x35\x21\xc6\x75\xb4\xd5\x0f\x89\x08\x17\x13\x24\x07\xc2\x7c\x73\xe2\x17\x00\x12\x8a\xc9\x39\xdb\xf0\xc8\x6f\x1f\xf7\x99\xed\x8c\x67\x9c\xf2\x30\x5c\xd0\xd0\x0d\xc1\x15\x07\xa3\x1d\xf5\xd4\x92\x82\xfd\x9c\x5a\x11\x69\x3b\x02\x03\x01\x00\x01\x02\x82\x01\x00\x44\xe1\x5a\xfd\x8a\x18\xd5\x45\xb8\x4c\x76\x4b\x5c\x55\x97\x5f\x85\x2e\x26\x8d\xc8\x16\x46\x48\x3c\xd6\x7a\x84\x5d\x19\xf1\x83\xdf\x11\xbf\xb8\xc8\xef\x0a\x56\xbf\xdc\xd3\xeb\xed\x57\x7f\xb1\x93\x88\x5c\x65\xba\xe7\x29\x68\x9f\x2b\x7a\x92\xb0\x5f\x5a\xc7\x81\x0d\x68\xd8\x57\xee\x4d\x13\xbc\xf4\x3c\x12\x89\x18\x9a\xdb\x3a\xc4\x0a\xc0\x10\x35\x3b\xa5\xdc\xbe\x1c\x88\xc4\x84\xea\x12\x64\x4c\xb8\x71\x19\x93\x7e\x8e\x73\x1d\x9f\x04\x61\xa1\x97\x27\x82\x2e\xb6\x4d\x6a\x4f\xfb\xa4\xe5\xa7\x54\x94\xb5\xf1\x41\xc8\xa4\x3d\xa1\xe6\x4a\xf0\xdb\xbb\xc2\x91\x26\x9a\x0f\xbf\xdd\x57\x1e\x83\x5c\x9a\x7b\x28\x53\x1d\x2d\x44\x91\x1f\x02\x81\x7b\x6f\xb5\xf7\x48\x7d\xa0\x12\x22\xdb\xbf\xd9\x04\x17\xe4\x97\xf2\xac\x32\xf8\x70\xfa\x75\xe3\x5a\xb0\xef\x1f\x2d\x24\xb9\x26\x83\x33\xe7\x3c\x3c\xfb\x0b\xd8\x70\x33\x76\xb1\x1c\x1d\x38\x06\x0a\xdb\xbd\xd2\x34\x5e\xe6\xb1\x6f\x5d\x8f\x18\xac\x94\xd2\x0d\xee\x39\x0b\xa3\xb4\xcf\xf1\xe1\x91\x30\xcb\xce\xa5\x2f\xa9\xcc\x4f\xee\xe4\xdd\xee\x8a\x77\x0e\xd1\xbd\xcc\xb0\x11\x55\x15\x5e\x99\xf1\x02\x81\x81\x00\xd1\x75\x33\xe4\x31\xc2\xfc\x09\x6c\xf6\x04\x97\xc7\xa3\xb1\x88\x36\x26\xd8\x4e\x86\x2d\xb8\x99\x68\x97\xd8\x0b\xc6\xc3\xe7\x58\x49\xc3\x41\xcd\xcd\x33\x09\xa0\x90\xb2\x77\xfa\xa3\xb6\x71\x09\x33\x43\x0a\x6a\xd8\xc3\x36\xaf\xa9\x11\x54\x64\x77\x82\xf4\xf1\xe0\x12\x5a\xb8\x9f\x5a\x04\xb3\x29\xd4\xc6\xba\x4c\xdc\x04\x97\xfb\xb6\x7e\x1b\x89\x09\x0c\x8a\xb8\x6c\x9f\x2b\x91\x0d\x34\x18\x39\xf3\x38\xf9\xe6\xed\x29\x48\x30\xe4\x3c\x09\x15\x33\xe0\xb8\x2f\xd8\xfa\xf2\x6d\x1f\xf1\xee\x02\xc2\xb4\xf9\xf4\x63\x4b\xa5\x02\x81\x81\x00\xba\x14\x89\xff\x65\xb5\xe6\x52\x45\x23\x37\x5e\x0c\x62\xde\xe9\x7f\xa9\x05\xee\x28\x0d\x91\xb1\x99\xd6\x8b\xf8\x58\x50\x8b\xb1\xee\x57\xbd\x2b\x7b\xf0\x25\x03\xeb\xbc\x87\x73\xc8\xbf\x57\x16\xda\x49\x7a\x79\x82\x25\x99\x46\x9c\xb3\xd2\xd5\xb0\xae\xec\xeb\xbc\xd2\x4b\xae\xd0\x0a\x54\xcd\xad\x44\x90\x74\x79\xa2\x34\x73\x8a\x3a\x6c\x0b\x13\x20\x5d\xa4\xcc\x7b\xb4\x64\xcf\x61\x6e\xdf\xc1\x8c\xd4\x84\x22\xf1\x19\x32\x6d\xf1\x6f\xe1\x1e\xa5\xf6\x20\x6c\xc6\xa8\x9c\x4d\x8d\x59\xdf\x90\x71\x67\x1a\x48\xa3\x4b\x5f\x02\x81\x81\x00\x97\x4d\x8f\x7f\x7e\x86\xb8\x23\x62\xe7\x50\x28\x07\xd9\x72\x4b\xcf\xba\x3d\xb4\x73\x6e\xa1\x93\x87\x9f\x70\x3c\x09\x87\xc8\x1c\xd9\xa3\xc7\x6c\x0f\x97\x97\x93\xba\x12\x81\x62\xb7\x51\xf9\xd3\x48\x89\x5c\x04\x14\xb2\xe7\x54\xfa\xce\xfe\xe4\x58\x04\x6c\x46\x30\xb3\x71\x7f\x3d\xf4\xfb\xc2\x24\x2c\x84\xa5\x5d\x11\xed\xeb\x8f\xb3\xa2\xe2\xe7\x19\x77\x4a\xd9\xaf\xf5\x46\xb6\x50\x10\x5a\x93\xb9\xe3\x65\x79\xef\xc5\x4b\x55\xad\xf8\xc4\x22\xe1\xc7\xa9\xa5\x3e\x9a\xff\xf5\xde\x06\x98\x04\xbc\x7b\x98\xb7\x75\xe6\xd5\x02\x81\x80\x0a\x7f\x38\x1d\xa9\x2e\x2e\xb4\xfb\x63\x76\x2f\x1f\x01\xc0\xd3\x69\x39\x2e\xb5\x75\x9a\xf6\x5a\x0f\x74\x93\xe6\xc9\x8c\x99\xa4\xca\xee\x36\x24\xaa\xd4\x2c\x32\x61\x6c\xfc\x33\x22\xe2\xf0\x55\xc0\xb0\x9e\x71\x16\x4f\x6a\xab\x1a\x11\xe6\xd5\xd9\x26\xb5\x04\xc3\x5d\x15\x99\xe1\xf0\x83\x42\x2b\x01\x10\x29\x11\xe7\x7d\x8f\xfa\xff\x3a\xb3\x11\x3c\x25\x2c\x33\xc0\xd2\xb7\x51\x1f\x8c\xf2\xa0\x67\x82\x61\x85\xdb\x15\xf1\xcb\x53\xf0\x5c\xc1\xae\xd9\x08\x91\x3a\x4f\xae\xa9\x8d\x4c\xc1\x98\xd3\x5c\xde\x95\xb4\x68\x7f\x02\x81\x80\x7d\x3e\x6b\x2c\x16\xe8\x17\x2c\x27\x9c\xc5\xc5\xfb\x30\x1a\xf7\x32\x53\x93\xfe\xc1\xa0\x5d\xac\x7d\x6f\xba\x1b\x56\x7e\x34\xf6\xa7\x91\x1f\x39\x84\x1c\x94\x58\x13\xe2\xb9\xec\xb6\x24\xfe\x76\x35\x1b\xcc\x4f\x8e\x0d\x88\x5b\x5a\x6f\xb6\xa2\x0b\xc3\xb6\x98\x2d\xca\xce\xce\x26\xb4\x36\x37\x42\xa4\xc0\xa9\x85\x57\x4b\x6b\xc2\xed\x14\x96\xe5\xbc\x2b\x83\x32\xe9\x83\x24\x7f\x85\x74\x09\x3c\xfa\x45\xfd\x21\xeb\xd8\xa3\x02\xd2\x70\x0a\x9a\x9d\x7d\xe4\x39\xc4\x59\xc8\x16\x6f\xce\xd5\x1d\xea\x91\x4d\x12\x78\xc3\x30";
diff --git a/rust/bssl-crypto/src/scoped.rs b/rust/bssl-crypto/src/scoped.rs
index fd889a5..391a0c6 100644
--- a/rust/bssl-crypto/src/scoped.rs
+++ b/rust/bssl-crypto/src/scoped.rs
@@ -15,6 +15,8 @@
 
 //! Helpers to ensure that some temporary objects are always freed.
 
+use crate::initialized_struct;
+
 /// A scoped `EC_KEY`.
 pub struct EvpPkey(*mut bssl_sys::EVP_PKEY);
 
@@ -64,3 +66,24 @@ impl Drop for EcKey {
         unsafe { bssl_sys::EC_KEY_free(self.0) }
     }
 }
+
+/// A scoped `BIGNUM`.
+pub struct Bignum(bssl_sys::BIGNUM);
+
+impl Bignum {
+    pub fn from_u64(value: u64) -> Self {
+        let mut ret = Bignum(unsafe { initialized_struct(|ptr| bssl_sys::BN_init(ptr)) });
+        assert_eq!(1, unsafe { bssl_sys::BN_set_u64(&mut ret.0, value) });
+        ret
+    }
+
+    pub unsafe fn as_ffi_ptr(&self) -> *const bssl_sys::BIGNUM {
+        &self.0
+    }
+}
+
+impl Drop for Bignum {
+    fn drop(&mut self) {
+        unsafe { bssl_sys::BN_free(&mut self.0) }
+    }
+}