Add support for ECDSA NIST P256 signature credentials

Signed-off-by: Alistair Francis <[email protected]>

README.md

@@ -68,8 +68,11 @@ elf2tab supports adding credentials to the TBF footer of the generated TBF
 files. To add a hash, use one or more of these flags: `--sha256`, `--sha384`,
 `--sha512`.
 
-elf2tab can also sign the TBF with a public/private RSA key pair. To generate
-compatible keys:
+elf2tab can also sign the TBF with a public/private key pairs. elf2tab uses
+ring to do this, a range of commands to generate and prepare keys for ring can
+be found at: https://gist.github.com/briansmith/2ee42439923d8e65a266994d0f70180b
+
+To generate compatible RSA keys:
 
     $ openssl genrsa -aes256 -out tockkey.private.pem 4096
     $ openssl pkcs8 -topk8 -nocrypt -outform der -in tockkey.private.pem -out tockkey.private.pk8
@@ -84,6 +87,16 @@ Example including multiple credentials:
     $ elf2tab --sha256 --sha384 --sha512 --rsa4096-private tockkey.private.pk8 ...
 
 
+To generate compatible ECDSA NIST P256 keys:
+
+Use an existing PEM private key (one that starts with `-----BEGIN PRIVATE KEY-----`)
+
+    $ openssl pkcs8 -in priv_key.pem -topk8 -nocrypt -outform der > p256-private-key.p8
+
+Then pass the keys to elf2tab:
+
+    $ elf2tab --ecdsa-nist-p256-private p256-private-key.p8 ...
+
 elf2tab Details
 ---------------
 

src/cmdline.rs

@@ -214,6 +214,13 @@ pub struct Opt {
         help = "Add an 4096-bit RSA signature credential using this private key"
     )]
     pub rsa4096_private_key: Option<PathBuf>,
+
+    #[arg(
+        long = "ecdsa-nist-p256-private",
+        id = "ecdsa-nist-p256-private-key",
+        help = "Add an ECDSA NIST P256 signature credential using this private key"
+    )]
+    pub ecdsa_nist_p256_private_key: Option<PathBuf>,
 }
 
 mod test {

src/convert.rs

@@ -141,6 +141,7 @@ pub fn elf_to_tbf(
     sha384: bool,
     sha512: bool,
     rsa4096_private_key: Option<PathBuf>,
+    ecdsa_nist_p256_private_key: Option<PathBuf>,
 ) -> io::Result<()> {
     let package_name = package_name.unwrap_or_default();
 
@@ -1115,6 +1116,63 @@ pub fn elf_to_tbf(
         }
     }
 
+    if ecdsa_nist_p256_private_key.is_some() {
+        let private_key_path_str = ecdsa_nist_p256_private_key.unwrap();
+        let private_key_path = Path::new(&private_key_path_str);
+        let private_key_contents = read_rsa_file(private_key_path).unwrap_or_else(|e| {
+            panic!(
+                "Failed to read private key from {:?}: {:?}",
+                private_key_path, e
+            );
+        });
+
+        let rng = rand::SystemRandom::new();
+
+        let key_pair = ring::signature::EcdsaKeyPair::from_pkcs8(
+            &ring::signature::ECDSA_P256_SHA256_FIXED_SIGNING,
+            &private_key_contents,
+        )
+        .unwrap_or_else(|e| {
+            panic!("ECDSA NIST P256 could not be parsed: {:?}", e);
+        });
+
+        let signature = key_pair
+            .sign(&rng, &output[0..tbfheader.binary_end_offset() as usize])
+            .map_err(|e| {
+                panic!("Could not generate ECDSA NIST P256 signature: {:?}", e);
+            })
+            .unwrap();
+
+        let sig_len = signature.as_ref().len();
+
+        // Signature in R, S format is 64 bytes long
+        assert_eq!(sig_len, 64);
+
+        let nist_p256_len = mem::size_of::<header::TbfHeaderTlv>()
+            + mem::size_of::<header::TbfFooterCredentialsType>()
+            + sig_len;
+        let nist_p256_tlv_len = nist_p256_len - mem::size_of::<header::TbfHeaderTlv>();
+
+        let mut credentials = vec![0; sig_len];
+
+        credentials[..sig_len].copy_from_slice(signature.as_ref());
+
+        let nist_p256_credentials = header::TbfFooterCredentials {
+            base: header::TbfHeaderTlv {
+                tipe: header::TbfHeaderTypes::Credentials,
+                length: nist_p256_tlv_len as u16,
+            },
+            format: header::TbfFooterCredentialsType::EcdsaNistP256,
+            data: credentials,
+        };
+
+        output.write_all(nist_p256_credentials.generate().unwrap().get_ref())?;
+        footer_space_remaining -= nist_p256_len;
+        if verbose {
+            println!("Added PKCS#1v1.5 ECDSA NIST P256 signature credential.");
+        }
+    }
+
     let padding_len = footer_space_remaining;
 
     // Need at least space for the base Credentials TLV.

src/header.rs

@@ -34,6 +34,7 @@ pub enum TbfFooterCredentialsType {
     SHA256 = 3,
     SHA384 = 4,
     SHA512 = 5,
+    EcdsaNistP256 = 6,
 }
 
 #[repr(C)]

src/main.rs

@@ -145,6 +145,7 @@ fn main() {
             opt.sha384_enable,
             opt.sha512_enable,
             opt.rsa4096_private_key.clone(),
+            opt.ecdsa_nist_p256_private_key.clone(),
         )
         .unwrap();
         if opt.verbose {