diff --git a/attestation-agent/kbs_protocol/src/client/rcar_client.rs b/attestation-agent/kbs_protocol/src/client/rcar_client.rs
index e82196bc0..18f3a054f 100644
--- a/attestation-agent/kbs_protocol/src/client/rcar_client.rs
+++ b/attestation-agent/kbs_protocol/src/client/rcar_client.rs
@@ -56,6 +56,26 @@ async fn get_request_extra_params() -> serde_json::Value {
     extra_params
 }
 
+async fn get_hash_algorithm(extra_params: serde_json::Value) -> Result<HashAlgorithm> {
+    let algorithm = match extra_params.get(SELECTED_HASH_ALGORITHM_JSON_KEY) {
+        Some(selected_hash_algorithm) => {
+            let name = selected_hash_algorithm
+                .as_str()
+                .ok_or(Error::UnexpectedJSONDataType(
+                    "string".into(),
+                    selected_hash_algorithm.to_string(),
+                ))?
+                .to_lowercase();
+
+            name.parse::<HashAlgorithm>()
+                .map_err(|_| Error::InvalidHashAlgorithm(name))?
+        }
+        None => DEFAULT_HASH_ALGORITHM,
+    };
+
+    Ok(algorithm)
+}
+
 async fn build_request(tee: Tee) -> Request {
     let extra_params = get_request_extra_params().await;
 
@@ -168,19 +188,7 @@ impl KbsClient<Box<dyn EvidenceProvider>> {
 
         let extra_params = challenge.extra_params;
 
-        let algorithm = match extra_params.get(SELECTED_HASH_ALGORITHM_JSON_KEY) {
-            Some(selected_hash_algorithm) => {
-                // Note the blank string which will be handled as an error when parsed.
-                let name = selected_hash_algorithm
-                    .as_str()
-                    .unwrap_or("")
-                    .to_lowercase();
-
-                name.parse::<HashAlgorithm>()
-                    .map_err(|_| Error::InvalidHashAlgorithm(name))?
-            }
-            None => DEFAULT_HASH_ALGORITHM,
-        };
+        let algorithm = get_hash_algorithm(extra_params).await?;
 
         let tee_pubkey = self.tee_key.export_pubkey()?;
         let runtime_data = json!({
@@ -350,16 +358,19 @@ impl KbsClientCapabilities for KbsClient<Box<dyn EvidenceProvider>> {
 #[cfg(test)]
 mod test {
     use crypto::HashAlgorithm;
+    use rstest::rstest;
+    use serde_json::{json, Value};
     use std::{env, path::PathBuf, time::Duration};
     use testcontainers::{clients, images::generic::GenericImage};
     use tokio::fs;
 
     use crate::{
-        evidence_provider::NativeEvidenceProvider, KbsClientBuilder, KbsClientCapabilities,
+        evidence_provider::NativeEvidenceProvider, Error, KbsClientBuilder, KbsClientCapabilities,
     };
 
     use crate::client::rcar_client::{
-        build_request, get_request_extra_params, KBS_PROTOCOL_VERSION,
+        build_request, get_hash_algorithm, get_request_extra_params, Result,
+        DEFAULT_HASH_ALGORITHM, KBS_PROTOCOL_VERSION, SELECTED_HASH_ALGORITHM_JSON_KEY,
         SUPPORTED_HASH_ALGORITHMS_JSON_KEY,
     };
     use kbs_types::Tee;
@@ -502,4 +513,63 @@ mod test {
             assert_eq!(request.extra_params, expected_extra_params);
         }
     }
+
+    #[tokio::test]
+    #[rstest]
+    #[case(json!({}), Ok(DEFAULT_HASH_ALGORITHM))]
+    #[rstest]
+    #[case(json!({SELECTED_HASH_ALGORITHM_JSON_KEY: ""}), Err(Error::InvalidHashAlgorithm("".into())))]
+    #[rstest]
+    #[case(json!({SELECTED_HASH_ALGORITHM_JSON_KEY: "foo"}), Err(Error::InvalidHashAlgorithm("foo".into())))]
+    #[rstest]
+    #[case(json!({SELECTED_HASH_ALGORITHM_JSON_KEY: "sha256"}), Ok(HashAlgorithm::Sha256))]
+    #[rstest]
+    #[case(json!({SELECTED_HASH_ALGORITHM_JSON_KEY: "SHA256"}), Ok(HashAlgorithm::Sha256))]
+    #[rstest]
+    #[case(json!({SELECTED_HASH_ALGORITHM_JSON_KEY: "sha384"}), Ok(HashAlgorithm::Sha384))]
+    #[rstest]
+    #[case(json!({SELECTED_HASH_ALGORITHM_JSON_KEY: "SHA384"}), Ok(HashAlgorithm::Sha384))]
+    #[rstest]
+    #[case(json!({SELECTED_HASH_ALGORITHM_JSON_KEY: "sha512"}), Ok(HashAlgorithm::Sha512))]
+    #[rstest]
+    #[case(json!({SELECTED_HASH_ALGORITHM_JSON_KEY: "SHA512"}), Ok(HashAlgorithm::Sha512))]
+    #[rstest]
+    #[case(json!({SELECTED_HASH_ALGORITHM_JSON_KEY: []}), Err(Error::UnexpectedJSONDataType("string".into(), "[]".into())))]
+    #[rstest]
+    #[case(json!({SELECTED_HASH_ALGORITHM_JSON_KEY: {}}), Err(Error::UnexpectedJSONDataType("string".into(), "{}".into())))]
+    #[rstest]
+    #[case(json!({SELECTED_HASH_ALGORITHM_JSON_KEY: true}), Err(Error::UnexpectedJSONDataType("string".into(), "true".into())))]
+    #[rstest]
+    #[case(json!({SELECTED_HASH_ALGORITHM_JSON_KEY: 99999}), Err(Error::UnexpectedJSONDataType("string".into(), "99999".into())))]
+    #[rstest]
+    #[case(json!({SELECTED_HASH_ALGORITHM_JSON_KEY: 3.141}), Err(Error::UnexpectedJSONDataType("string".into(), "3.141".into())))]
+    async fn test_get_hash_algorithm(
+        #[case] extra_params: Value,
+        #[case] expected_result: Result<HashAlgorithm>,
+    ) {
+        let msg =
+            format!("test: extra_params: {extra_params:?}, expected result: {expected_result:?}");
+
+        let actual_result = get_hash_algorithm(extra_params).await;
+
+        let msg = format!("{msg}, actual result: {actual_result:?}");
+
+        if std::env::var("DEBUG").is_ok() {
+            println!("DEBUG: {}", msg);
+        }
+
+        if expected_result.is_err() {
+            let expected_result_msg = format!("{expected_result:?}");
+            let actual_result_msg = format!("{actual_result:?}");
+
+            assert_eq!(expected_result_msg, actual_result_msg, "{msg:?}");
+
+            return;
+        }
+
+        let expected_hash_algorithm = expected_result.unwrap();
+        let actual_hash_algorithm = actual_result.unwrap();
+
+        assert_eq!(expected_hash_algorithm, actual_hash_algorithm, "{msg:?}");
+    }
 }
diff --git a/attestation-agent/kbs_protocol/src/error.rs b/attestation-agent/kbs_protocol/src/error.rs
index 97d95ccb6..5a75d59cb 100644
--- a/attestation-agent/kbs_protocol/src/error.rs
+++ b/attestation-agent/kbs_protocol/src/error.rs
@@ -47,4 +47,7 @@ pub enum Error {
 
     #[error("invalid hash algorithm: {0}")]
     InvalidHashAlgorithm(String),
+
+    #[error("unexpected JSON data type: expected {0}, got {1}")]
+    UnexpectedJSONDataType(String, String),
 }