Verify the quoting enclave report body (#18)

Add logic to verify the quoting enclave report body from a quote.

This only contains the logic to verify the quote body.  The other steps
necessary to fulfill the quote verification are listed in the issue #17.

There is an [okteta](https://apps.kde.org/okteta/) definition available at
https://github.com/nick-mobilecoin/okteta-SGXQuote to make it easier for one to
view the `hw_quote.dat` file.  I'm up for suggestions on how to name the current
`hw_quote.dat` file.

untrusted/.gitignore

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+# Generated by Cargo
+# will have compiled files and executables
+/target/
+
+# Remove Cargo.lock from gitignore if creating an executable, leave it for libraries
+# More information here https://doc.rust-lang.org/cargo/guide/cargo-toml-vs-cargo-lock.html
+Cargo.lock
+
+# These are backup files generated by rustfmt
+*.rs.bk

untrusted/Cargo.toml

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+[workspace]
+resolver = "2"
+members = [
+    "quote_verify",
+]

untrusted/quote_verify/Cargo.toml

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+[package]
+name = "mc-sgx-quote-verify"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+pem = "1.0.2"
+sha2 = "0.10.2"
+mbedtls = "0.8.1"
+hex = "0.4.3"

untrusted/quote_verify/src/lib.rs

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+// Copyright (c) 2022 The MobileCoin Foundation
+
+use mbedtls::{alloc::Box as MbedtlsBox, hash::Type as HashType, x509::Certificate};
+use pem::PemError;
+use sha2::{Digest, Sha256};
+
+// THe size of an enclave report (body). Table 5 of
+// https://download.01.org/intel-sgx/latest/dcap-latest/linux/docs/Intel_SGX_ECDSA_QuoteLibReference_DCAP_API.pdf
+const ENCLAVE_REPORT_SIZE: usize = 384;
+
+// Size of one of the components of the ECDSA signature.
+// Table 6 of
+// https://download.01.org/intel-sgx/latest/dcap-latest/linux/docs/Intel_SGX_ECDSA_QuoteLibReference_DCAP_API.pdf
+const SIGNATURE_COMPONENT_SIZE: usize = 32;
+
+// The starting byte of the quote report.  The *QE Report* member of the Quote
+// Signature Data Structure.  Table 4 of
+// https://download.01.org/intel-sgx/latest/dcap-latest/linux/docs/Intel_SGX_ECDSA_QuoteLibReference_DCAP_API.pdf
+// TODO as more of the quote structure is brought in, derive the start by summation
+//  of previous type/member sizes.
+const QUOTING_ENCLAVE_REPORT_START: usize = 0x234;
+
+// The starting byte of the signature for the quote. *QE Report Signature* of
+// the Quote Signature Data Structure. Table 4 of
+// https://download.01.org/intel-sgx/latest/dcap-latest/linux/docs/Intel_SGX_ECDSA_QuoteLibReference_DCAP_API.pdf
+const QUOTING_ENCLAVE_SIGNATURE_START: usize = QUOTING_ENCLAVE_REPORT_START + ENCLAVE_REPORT_SIZE;
+
+// ASN.1 Tag for an integer
+const ASN1_INTEGER: u8 = 2;
+// ASN.1 Tag for a sequence
+const ASN1_SEQUENCE: u8 = 48;
+
+// The byte size of the `type` and `length` fields of the ANS.1
+// type-length-value stream
+const ASN1_TYPE_LENGTH_SIZE: usize = 2;
+
+/// A quote for DCAP attestation
+pub struct Quote {
+    bytes: Vec<u8>,
+}
+
+impl Quote {
+    /// Returns a [Quote] created from the provided `bytes`.
+    ///
+    /// # Arguments
+    ///
+    /// * `bytes` the bytes of the quote as defined in
+    ///     https://download.01.org/intel-sgx/latest/dcap-latest/linux/docs/Intel_SGX_ECDSA_QuoteLibReference_DCAP_API.pdf
+    pub fn from_bytes(bytes: &[u8]) -> Self {
+        Quote {
+            bytes: bytes.to_vec(),
+        }
+    }
+
+    /// Verify the quoting enclave within the report.
+    pub fn verify_quoting_enclave_report(&self) -> Result<(), Error> {
+        let signature = self.get_asn1_signature();
+        let report = self.get_quoting_enclave_report();
+        let hash = Sha256::digest(report);
+        let mut cert = self.get_pck_certificate()?;
+        cert.public_key_mut()
+            .verify(HashType::Sha256, &hash, &signature)
+            .map_err(Error::Signature)
+    }
+
+    /// Returns the PCK certificate that was used for signing the quoting
+    /// enclave report.
+    /// Note: This certificate is assumed to be valid.
+    fn get_pck_certificate(&self) -> Result<MbedtlsBox<Certificate>, Error> {
+        //TODO this should only be looking at the `Certification Data`
+        // of the `QE Certification Data`, Table 9 of
+        // https://download.01.org/intel-sgx/latest/dcap-latest/linux/docs/Intel_SGX_ECDSA_QuoteLibReference_DCAP_API.pdf
+        // However the pem crate walks over the other data nicely for initial development
+        let pem = pem::parse(self.bytes.as_slice())?;
+        Certificate::from_pem(&pem.contents).map_err(Error::Certificate)
+    }
+
+    /// Returns the quoting enclave report from the overall quote.
+    fn get_quoting_enclave_report(&self) -> &[u8] {
+        &self.bytes
+            [QUOTING_ENCLAVE_REPORT_START..QUOTING_ENCLAVE_REPORT_START + ENCLAVE_REPORT_SIZE]
+    }
+
+    /// Returns the ASN.1 version of the signature.
+    /// mbedtls wants an ASN.1 version of the signature, while the raw quote
+    /// format has only the r and s values of the ECDSA signature
+    fn get_asn1_signature(&self) -> Vec<u8> {
+        let mut start = QUOTING_ENCLAVE_SIGNATURE_START;
+        let r = &self.bytes[start..start + SIGNATURE_COMPONENT_SIZE];
+        start += SIGNATURE_COMPONENT_SIZE;
+        let s = &self.bytes[start..start + SIGNATURE_COMPONENT_SIZE];
+
+        let sequence_length = ASN1_TYPE_LENGTH_SIZE + r.len() + ASN1_TYPE_LENGTH_SIZE + s.len();
+        let mut signature = vec![ASN1_SEQUENCE, sequence_length as u8];
+        for component in [r, s] {
+            signature.extend([ASN1_INTEGER, component.len() as u8]);
+            signature.extend(component);
+        }
+        signature
+    }
+}
+
+#[derive(Debug, PartialEq)]
+pub enum Error {
+    /// Unable to load the Certificate with mbedtls
+    Certificate(mbedtls::Error),
+    /// Failure to parse the Pem files from the quote data
+    PemParsing(PemError),
+    /// Failure to verify a Signature
+    Signature(mbedtls::Error),
+}
+
+impl From<PemError> for Error {
+    fn from(src: PemError) -> Self {
+        Self::PemParsing(src)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const HW_QUOTE: &[u8] = include_bytes!("../tests/data/hw_quote.dat");
+    #[test]
+    fn verify_valid_quote_report() {
+        let quote = Quote::from_bytes(HW_QUOTE);
+        assert!(quote.verify_quoting_enclave_report().is_ok());
+    }
+
+    #[test]
+    fn invalid_quote_report() {
+        let mut bad_quote = HW_QUOTE.to_vec();
+        bad_quote[QUOTING_ENCLAVE_REPORT_START + 1] = 0;
+        let quote = Quote::from_bytes(&bad_quote);
+        assert_eq!(
+            quote.verify_quoting_enclave_report(),
+            Err(Error::Signature(mbedtls::Error::EcpVerifyFailed))
+        );
+    }
+
+    #[test]
+    fn failure_to_parse_pem_certificates() {
+        // TODO Once more of the quote parsing logic comes in remove hard coded
+        //  value of 0x41c, based on current quote data file.
+        let quote = Quote::from_bytes(&HW_QUOTE[..0x41c]);
+        assert_eq!(
+            quote.verify_quoting_enclave_report(),
+            Err(Error::PemParsing(PemError::MalformedFraming))
+        );
+    }
+
+    #[test]
+    fn failure_to_load_certificate() {
+        let mut bad_cert = HW_QUOTE.to_vec();
+        // TODO Once more of the quote parsing logic comes in remove hard coded
+        //  value of 0x440, based on current quote data file.
+        bad_cert[0x440] = 0;
+        let quote = Quote::from_bytes(&bad_cert);
+
+        // Since the pem is utilized for parsing the certificates out of the
+        // quote data, it fails before `Certificate::from_pem()` gets a chance
+        // to fail
+        assert!(matches!(
+            quote.verify_quoting_enclave_report(),
+            Err(Error::PemParsing(_))
+        ));
+    }
+}

untrusted/quote_verify/tests/data/README.md

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+Data files to help with testing quote verifications
+
+* hw_quote.dat:
+  A valid DCAP quote from an SGX platform. This quote was made using the QuoteGenerationSample from
+  the [SGXDataCenterAttestationPrimitives](https://github.com/intel/SGXDataCenterAttestationPrimitives) repository.  
+