x509-cert: adds a convenience helper to generate serials

This follows the CABF ballot 164 recommendation to have serials use 64
bits from a CSPRNG.

This also provides the user with the option to prefix its values (for
use of an instance id).

x509-cert/Cargo.toml

@@ -17,6 +17,7 @@ rust-version = "1.65"
 [dependencies]
 const-oid = { version = "0.9.3", features = ["db"] }
 der = { version = "0.7.6", features = ["alloc", "derive", "flagset", "oid"] }
+generic-array = { version = "0.14.7", default-features = false }
 spki = { version = "0.7.2", features = ["alloc"] }
 
 # optional dependencies

x509-cert/src/serial_number.rs

@@ -7,6 +7,20 @@ use der::{
     DecodeValue, EncodeValue, ErrorKind, FixedTag, Header, Length, Reader, Result, Tag, ValueOrd,
     Writer,
 };
+#[cfg(feature = "builder")]
+use {
+    core::ops::Add,
+    generic_array::{
+        typenum::{
+            consts::{U20, U8},
+            marker_traits::Unsigned,
+            type_operators::{Max, Min},
+            uint::UTerm,
+        },
+        ArrayLength, GenericArray,
+    },
+    signature::rand_core::CryptoRngCore,
+};
 
 use crate::certificate::{Profile, Rfc5280};
 
@@ -65,6 +79,48 @@ impl<P: Profile> SerialNumber<P> {
     pub fn as_bytes(&self) -> &[u8] {
         self.inner.as_bytes()
     }
+
+    /// Generates a random serial number from RNG.
+    ///
+    /// This follows the recommendation the CAB forum [ballot 164] and uses a minimum of 64 bits
+    /// of output from the CSPRNG.
+    ///
+    /// [ballot 164]: https://cabforum.org/2016/03/31/ballot-164/
+    #[cfg(feature = "builder")]
+    pub fn generate<N>(rng: &mut impl CryptoRngCore) -> Result<Self>
+    where
+        N: Unsigned + ArrayLength<u8>,
+        N: Min<U8>,  // Rand minimum is 64 bits
+        N: Max<U20>, // Max length is 20 bytes
+    {
+        Self::generate_with_prefix::<UTerm, N>(GenericArray::default(), rng)
+    }
+
+    /// Generates a random serial number from RNG. Include a prefix value.
+    ///
+    /// This follows the recommendation the CAB forum [ballot 164] and uses a minimum of 64 bits
+    /// of output from the CSPRNG.
+    ///
+    /// [ballot 164]: https://cabforum.org/2016/03/31/ballot-164/
+    #[cfg(feature = "builder")]
+    pub fn generate_with_prefix<Prefix, N>(
+        prefix: GenericArray<u8, Prefix>,
+        rng: &mut impl CryptoRngCore,
+    ) -> Result<Self>
+    where
+        N: Unsigned,
+        Prefix: Unsigned + ArrayLength<u8>,
+        Prefix: Add<N>,
+        <Prefix as Add<N>>::Output: ArrayLength<u8>,
+        N: Min<U8>,                           // Rand minimum is 64 bits
+        <Prefix as Add<N>>::Output: Max<U20>, // Max length is 20 bytes
+    {
+        let mut buf = GenericArray::<_, <Prefix as Add<N>>::Output>::default();
+        buf[..Prefix::USIZE].copy_from_slice(&prefix);
+        rng.fill_bytes(&mut buf[Prefix::USIZE..]);
+
+        Self::new(&buf)
+    }
 }
 
 impl<P: Profile> EncodeValue for SerialNumber<P> {
@@ -151,6 +207,9 @@ impl<'a, P: Profile> arbitrary::Arbitrary<'a> for SerialNumber<P> {
 mod tests {
     use alloc::string::ToString;
 
+    #[cfg(feature = "builder")]
+    use generic_array::{arr, typenum::consts::U17};
+
     use super::*;
 
     #[test]
@@ -193,4 +252,22 @@ mod tests {
             assert_eq!(sn.to_string(), "01")
         }
     }
+
+    #[cfg(feature = "builder")]
+    #[test]
+    fn serial_number_generate() {
+        let sn = SerialNumber::<Rfc5280>::generate::<U17>(&mut rand::thread_rng()).unwrap();
+
+        // Underlying storage uses signed int for compatibility reasons,
+        // we may need to prefix the value with 0x00 to make it an unsigned.
+        // in which case the length is going to be 18.
+        assert!(matches!(sn.as_bytes().len(), 17..=18));
+
+        let sn = SerialNumber::<Rfc5280>::generate_with_prefix::<_, U17>(
+            arr![u8; 1,2,3],
+            &mut rand::thread_rng(),
+        )
+        .unwrap();
+        assert_eq!(sn.as_bytes().len(), 20);
+    }
 }