diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index e6be473..d6df4fe 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -16,7 +16,7 @@ jobs:
       fail-fast: false
       matrix:
         features:
-          - name: "default"
+          - name: "default (aws-lc-rs for crypto)"
 
           - name: "no-features"
             cargo-flags: "--no-default-features"
@@ -36,6 +36,9 @@ jobs:
           - name: "native-tls"
             cargo-flags: "--no-default-features --features native-tls"
 
+          - name: "openssl crypto"
+            cargo-flags: "--no-default-features --features hyper-rustls-native-roots,crypto-openssl"
+
     name: CI (${{ matrix.features.name }})
 
     steps:
diff --git a/contract-tests/Cargo.toml b/contract-tests/Cargo.toml
index e7dec85..ba07039 100644
--- a/contract-tests/Cargo.toml
+++ b/contract-tests/Cargo.toml
@@ -23,7 +23,10 @@ reqwest = { version = "0.12.4", features = ["default", "blocking", "json"] }
 async-mutex = "1.4.0"
 
 [features]
-default = ["hyper"]
+default = ["hyper", "crypto-aws-lc-rs"]
+
+crypto-aws-lc-rs = ["launchdarkly-server-sdk/crypto-aws-lc-rs"]
+crypto-openssl = ["launchdarkly-server-sdk/crypto-openssl"]
 
 hyper = [
     "launchdarkly-sdk-transport/hyper",
diff --git a/contract-tests/src/client_entity.rs b/contract-tests/src/client_entity.rs
index 14dffed..ee9036a 100644
--- a/contract-tests/src/client_entity.rs
+++ b/contract-tests/src/client_entity.rs
@@ -15,9 +15,11 @@ use launchdarkly_server_sdk::{
     ServiceEndpointsBuilder, StreamingDataSourceBuilder,
 };
 
+#[cfg(any(feature = "crypto-aws-lc-rs", feature = "crypto-openssl"))]
+use crate::command_params::SecureModeHashResponse;
 use crate::command_params::{
     ContextBuildParams, ContextConvertParams, ContextParam, ContextResponse,
-    MigrationOperationResponse, MigrationVariationResponse, SecureModeHashResponse,
+    MigrationOperationResponse, MigrationVariationResponse,
 };
 use crate::HttpsConnector;
 use crate::{
@@ -221,14 +223,17 @@ impl ClientEntity {
                     ContextResponse::from(Self::context_convert(params)),
                 )))
             }
+            #[cfg(any(feature = "crypto-aws-lc-rs", feature = "crypto-openssl"))]
             "secureModeHash" => {
                 let params = command
                     .secure_mode_hash
                     .ok_or("secureModeHash params should be set")?;
+                let hash = self
+                    .client
+                    .secure_mode_hash(&params.context)
+                    .map_err(|e| e.to_string())?;
                 Ok(Some(CommandResponse::SecureModeHash(
-                    SecureModeHashResponse {
-                        result: self.client.secure_mode_hash(&params.context),
-                    },
+                    SecureModeHashResponse { result: hash },
                 )))
             }
             "migrationVariation" => {
diff --git a/contract-tests/src/command_params.rs b/contract-tests/src/command_params.rs
index 5bbf495..2d19593 100644
--- a/contract-tests/src/command_params.rs
+++ b/contract-tests/src/command_params.rs
@@ -10,6 +10,7 @@ pub enum CommandResponse {
     EvaluateFlag(EvaluateFlagResponse),
     EvaluateAll(EvaluateAllFlagsResponse),
     ContextBuildOrConvert(ContextResponse),
+    #[cfg(any(feature = "crypto-aws-lc-rs", feature = "crypto-openssl"))]
     SecureModeHash(SecureModeHashResponse),
     MigrationVariation(MigrationVariationResponse),
     MigrationOperation(MigrationOperationResponse),
@@ -25,6 +26,7 @@ pub struct CommandParams {
     pub identify_event: Option<IdentifyEventParams>,
     pub context_build: Option<ContextBuildParams>,
     pub context_convert: Option<ContextConvertParams>,
+    #[cfg(any(feature = "crypto-aws-lc-rs", feature = "crypto-openssl"))]
     pub secure_mode_hash: Option<SecureModeHashParams>,
     pub migration_variation: Option<MigrationVariationParams>,
     pub migration_operation: Option<MigrationOperationParams>,
@@ -126,12 +128,14 @@ pub struct ContextConvertParams {
     pub input: String,
 }
 
+#[cfg(any(feature = "crypto-aws-lc-rs", feature = "crypto-openssl"))]
 #[derive(Deserialize, Debug)]
 #[serde(rename_all = "camelCase")]
 pub struct SecureModeHashParams {
     pub context: Context,
 }
 
+#[cfg(any(feature = "crypto-aws-lc-rs", feature = "crypto-openssl"))]
 #[derive(Serialize, Debug, Clone)]
 #[serde(rename_all = "camelCase")]
 pub struct SecureModeHashResponse {
diff --git a/contract-tests/src/main.rs b/contract-tests/src/main.rs
index 2d00e03..480b2ad 100644
--- a/contract-tests/src/main.rs
+++ b/contract-tests/src/main.rs
@@ -103,6 +103,7 @@ async fn status() -> impl Responder {
             "tags".to_string(),
             "service-endpoints".to_string(),
             "context-type".to_string(),
+            #[cfg(any(feature = "crypto-aws-lc-rs", feature = "crypto-openssl"))]
             "secure-mode-hash".to_string(),
             "inline-context-all".to_string(),
             "anonymous-redaction".to_string(),
diff --git a/launchdarkly-server-sdk/Cargo.toml b/launchdarkly-server-sdk/Cargo.toml
index 89e8996..e21934c 100644
--- a/launchdarkly-server-sdk/Cargo.toml
+++ b/launchdarkly-server-sdk/Cargo.toml
@@ -39,7 +39,8 @@ bytes = "1.11"
 bitflags = "2.4"
 rand = "0.9"
 flate2 = { version = "1.0.35", optional = true }
-aws-lc-rs = "1.14.1"
+aws-lc-rs = { version = "1.14.1", optional = true }
+openssl = { version = "0.10.75", optional = true }
 
 [dev-dependencies]
 maplit = "1.0.1"
@@ -54,7 +55,10 @@ reqwest = { version = "0.12.4", features = ["json"] }
 testing_logger = "0.1.1"
 
 [features]
-default = ["hyper-rustls-native-roots"]
+default = ["hyper-rustls-native-roots", "crypto-aws-lc-rs"]
+
+crypto-aws-lc-rs = ["dep:aws-lc-rs"]
+crypto-openssl = ["dep:openssl"]
 
 hyper = [
     "launchdarkly-sdk-transport/hyper",
diff --git a/launchdarkly-server-sdk/src/client.rs b/launchdarkly-server-sdk/src/client.rs
index 39af0d8..c9b5a3d 100644
--- a/launchdarkly-server-sdk/src/client.rs
+++ b/launchdarkly-server-sdk/src/client.rs
@@ -156,6 +156,10 @@ pub struct Client {
     started: AtomicBool,
     offline: bool,
     daemon_mode: bool,
+    #[cfg_attr(
+        not(any(feature = "crypto-openssl", feature = "crypto-aws-lc-rs")),
+        allow(dead_code)
+    )]
     sdk_key: String,
     shutdown_broadcast: broadcast::Sender<()>,
     runtime: RwLock<Option<Runtime>>,
@@ -585,15 +589,39 @@ impl Client {
             .try_map(|val| val.as_json(), default, eval::Error::WrongType)
     }
 
+    #[cfg(any(feature = "crypto-aws-lc-rs", feature = "crypto-openssl"))]
     /// Generates the secure mode hash value for a context.
     ///
     /// For more information, see the Reference Guide:
     /// <https://docs.launchdarkly.com/sdk/features/secure-mode#rust>.
-    pub fn secure_mode_hash(&self, context: &Context) -> String {
-        let key = aws_lc_rs::hmac::Key::new(aws_lc_rs::hmac::HMAC_SHA256, self.sdk_key.as_bytes());
-        let tag = aws_lc_rs::hmac::sign(&key, context.canonical_key().as_bytes());
-
-        data_encoding::HEXLOWER.encode(tag.as_ref())
+    pub fn secure_mode_hash(&self, context: &Context) -> Result<String, String> {
+        #[cfg(feature = "crypto-aws-lc-rs")]
+        {
+            let key =
+                aws_lc_rs::hmac::Key::new(aws_lc_rs::hmac::HMAC_SHA256, self.sdk_key.as_bytes());
+            let tag = aws_lc_rs::hmac::sign(&key, context.canonical_key().as_bytes());
+
+            Ok(data_encoding::HEXLOWER.encode(tag.as_ref()))
+        }
+        #[cfg(feature = "crypto-openssl")]
+        {
+            use openssl::hash::MessageDigest;
+            use openssl::pkey::PKey;
+            use openssl::sign::Signer;
+
+            let key = PKey::hmac(self.sdk_key.as_bytes())
+                .map_err(|e| format!("Failed to create HMAC key: {e}"))?;
+            let mut signer = Signer::new(MessageDigest::sha256(), &key)
+                .map_err(|e| format!("Failed to create signer: {e}"))?;
+            signer
+                .update(context.canonical_key().as_bytes())
+                .map_err(|e| format!("Failed to update signer: {e}"))?;
+            let hmac = signer
+                .sign_to_vec()
+                .map_err(|e| format!("Failed to sign: {e}"))?;
+
+            Ok(data_encoding::HEXLOWER.encode(&hmac))
+        }
     }
 
     /// Returns an object that encapsulates the state of all feature flags for a given context. This
@@ -1830,6 +1858,7 @@ mod tests {
     }
 
     #[test]
+    #[cfg(any(feature = "crypto-aws-lc-rs", feature = "crypto-openssl"))]
     fn secure_mode_hash() {
         let config = ConfigBuilder::new("secret")
             .offline(true)
@@ -1841,12 +1870,15 @@ mod tests {
             .expect("Failed to create context");
 
         assert_eq!(
-            client.secure_mode_hash(&context),
+            client
+                .secure_mode_hash(&context)
+                .expect("Hash should be computed"),
             "aa747c502a898200f9e4fa21bac68136f886a0e27aec70ba06daf2e2a5cb5597"
         );
     }
 
     #[test]
+    #[cfg(any(feature = "crypto-aws-lc-rs", feature = "crypto-openssl"))]
     fn secure_mode_hash_with_multi_kind() {
         let config = ConfigBuilder::new("secret")
             .offline(true)
@@ -1869,7 +1901,9 @@ mod tests {
             .expect("failed to build multi-context");
 
         assert_eq!(
-            client.secure_mode_hash(&context),
+            client
+                .secure_mode_hash(&context)
+                .expect("Hash should be computed"),
             "5687e6383b920582ed50c2a96c98a115f1b6aad85a60579d761d9b8797415163"
         );
     }
@@ -2740,4 +2774,21 @@ mod tests {
     fn make_mocked_client() -> (Client, Receiver<OutputEvent>) {
         make_mocked_client_with_delay(0, false, false)
     }
+
+    #[test]
+    fn client_builds_successfully() {
+        let config = ConfigBuilder::new("sdk-key")
+            .offline(true)
+            .build()
+            .expect("config should build");
+
+        let client = Client::build(config).expect("client should build successfully");
+
+        assert!(
+            !client.started.load(Ordering::SeqCst),
+            "client should not be started yet"
+        );
+        assert!(client.offline, "client should be in offline mode");
+        assert_eq!(client.sdk_key, "sdk-key", "sdk_key should match");
+    }
 }