HashAgile: expose the algorithm and the content

This allows to verify an hmac signature emitted by a TPM.

Signed-off-by: Arthur Gautier <baloo@superbaloo.net>

Author: baloo

tss-esapi/src/structures/hash/agile.rs

@@ -6,6 +6,12 @@ use crate::tss2_esys::{TPMT_HA, TPMU_HA};
 use crate::{Error, Result, WrapperErrorKind};
 use std::convert::{TryFrom, TryInto};
 
+#[cfg(all(feature = "abstraction", feature = "rustcrypto"))]
+use {
+    crate::abstraction::AssociatedHashingAlgorithm,
+    digest::{Output, OutputSizeUser},
+};
+
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct HashAgile {
     algorithm: HashingAlgorithm,
@@ -65,3 +71,31 @@ impl TryFrom<TPMT_HA> for HashAgile {
         })
     }
 }
+
+impl HashAgile {
+    /// Return the [`HashingAlgorithm`] of the [`HashAgile`]
+    pub fn hashing_algorithm(&self) -> HashingAlgorithm {
+        self.algorithm
+    }
+
+    #[cfg(all(feature = "abstraction", feature = "rustcrypto"))]
+    #[allow(
+        clippy::unnecessary_fallible_conversions,
+        reason = "GenericArray::From<&[T]> will panic if the payload is not the same length"
+    )]
+    /// Return the hash content of the [`HashAgile`]
+    ///
+    /// # Errors
+    ///
+    /// Return an error if the specified [`AssociatedHashingAlgorithm`] is not the
+    /// one in the [`HashAgile`].
+    pub fn to_output<H: AssociatedHashingAlgorithm + OutputSizeUser>(&self) -> Result<Output<H>> {
+        if self.algorithm == H::TPM_DIGEST {
+            <&Output<H>>::try_from(self.digest.as_bytes())
+                .map_err(|_| Error::local_error(WrapperErrorKind::WrongValueFromTpm))
+                .cloned()
+        } else {
+            Err(Error::local_error(WrapperErrorKind::InvalidParam))
+        }
+    }
+}