diff --git a/README.md b/README.md
index 0711d42..16838ed 100644
--- a/README.md
+++ b/README.md
@@ -1,3 +1,5 @@
-# jwt4d
+# JWT4D
 
-JSON Web Token library to generate, parse, sign, and verify JWTs.
\ No newline at end of file
+A JSON Web Token library to generate, parse, sign, and verify JWTs.
+
+Currently, only the "HS256" algorithm is supported.
diff --git a/source/jwt4d/model.d b/source/jwt4d/model.d
index d63a62d..0797027 100644
--- a/source/jwt4d/model.d
+++ b/source/jwt4d/model.d
@@ -1,9 +1,7 @@
 module jwt4d.model;
 
 import std.json;
-import std.typecons : Nullable;
 import std.base64;
-import secured;
 
 /**
  * Enum defining the names of registered JWT claims, as per RFC 7519.
@@ -97,6 +95,11 @@ struct JwtClaims {
         return this;
     }
 
+    long expiration() const {
+        if (JwtClaim.Expiration !in this.obj.object) return -1;
+        return this.obj.object[JwtClaim.Expiration].integer;
+    }
+
     ref notBefore(long notBefore) {
         if (notBefore < 0) {
             this.obj.object.remove(JwtClaim.NotBefore);
@@ -106,6 +109,11 @@ struct JwtClaims {
         return this;
     }
 
+    long notBefore() const {
+        if (JwtClaim.NotBefore !in this.obj.object) return -1;
+        return this.obj.object[JwtClaim.NotBefore].integer;
+    }
+
     ref issuedAt(long issuedAt) {
         if (issuedAt < 0) {
             this.obj.object.remove(JwtClaim.IssuedAt);
@@ -128,66 +136,3 @@ struct JwtClaims {
         return this.obj.toJSON();
     }
 }
-
-string writeJwt(in JwtClaims claims, string secret) {
-    JSONValue headerObj = JSONValue.emptyObject;
-    headerObj.object["typ"] = "JWT";
-    headerObj.object["alg"] = "HS256";
-    string headerBase64 = Base64URLNoPadding.encode(cast(ubyte[]) headerObj.toJSON());
-
-    string claimsBase64 = Base64URLNoPadding.encode(cast(ubyte[]) claims.toJson());
-
-    string prefix = headerBase64 ~ "." ~ claimsBase64;
-
-    ubyte[] signatureBytes = hmac_ex(
-        cast(ubyte[]) secret,
-        cast(ubyte[]) (prefix),
-        HashAlgorithm.SHA2_256
-    );
-    string signatureBase64 = Base64URLNoPadding.encode(signatureBytes);
-
-    return prefix ~ "." ~ signatureBase64;
-}
-
-unittest {
-    JwtClaims claims;
-    claims.issuer = "example.com";
-    claims.subject = "user123";
-    claims.expiration = 123;
-
-    string token = writeJwt(claims, "test");
-    import std.stdio;
-    writeln(token);
-}
-
-JwtClaims readJwt(string token, string secret) {
-    import std.algorithm : splitter;
-    import std.array : array;
-
-    auto parts = token.splitter('.').array;
-    JSONValue headerObj = parseJSON(cast(string) Base64URLNoPadding.decode(parts[0]));
-    // TODO: Verify header object and algorithm.
-    JSONValue claimsObj = parseJSON(cast(string) Base64URLNoPadding.decode(parts[1]));
-    // TODO: Verify claims object structure.
-    ubyte[] signatureBytes = Base64URLNoPadding.decode(parts[2]);
-    bool verified = hmac_verify_ex(
-        signatureBytes,
-        cast(ubyte[]) secret,
-        cast(ubyte[]) (parts[0] ~ "." ~ parts[1]),
-        HashAlgorithm.SHA2_256
-    );
-    if (!verified) {
-        throw new Exception("Verification failed!");
-        // TODO: Custom error handling.
-    }
-    return JwtClaims(claimsObj);
-}
-
-unittest {
-    JwtClaims claims;
-    claims.issuer = "example.com";
-    string token = writeJwt(claims, "test");
-    JwtClaims readClaims = readJwt(token, "test");
-    import std.stdio;
-    writeln(readClaims.toJson());
-}
diff --git a/source/jwt4d/package.d b/source/jwt4d/package.d
index 569b836..d83f996 100644
--- a/source/jwt4d/package.d
+++ b/source/jwt4d/package.d
@@ -1,2 +1,5 @@
 module jwt4d;
 
+public import jwt4d.model;
+public import jwt4d.reader;
+public import jwt4d.writer;
diff --git a/source/jwt4d/reader.d b/source/jwt4d/reader.d
new file mode 100644
index 0000000..4d77e54
--- /dev/null
+++ b/source/jwt4d/reader.d
@@ -0,0 +1,119 @@
+module jwt4d.reader;
+
+import std.json;
+import std.base64;
+import std.datetime;
+import secured;
+
+import jwt4d.model;
+
+class JwtException : Exception {
+    this(string message) {
+        super(message);
+    }
+}
+
+class JwtFormatException : JwtException {
+    this(string message) {
+        super(message);
+    }
+}
+
+class JwtVerificationException : JwtException {
+    this() {
+        super("JWT could not be verified.");
+    }
+}
+
+class JwtExpiredException : JwtException {
+    this() {
+        super("JWT has expired.");
+    }
+}
+
+class JwtNotBeforeException : JwtException {
+    this() {
+        super("JWT is not valid yet (not before time).");
+    }
+}
+
+/**
+ * Reads a token and parses the claims. If the token is valid and its signature
+ * is verified, this method will also evaluate the "exp" (expiration) and "nbf"
+ * (not before) claims, and throw an exception if the token is not valid.
+ * Params:
+ *   token = The token to read.
+ *   secret = The secret used to sign the token, to verify it.
+ * Returns: The set of claims.
+ */
+JwtClaims readJwt(string token, string secret) {
+    import std.algorithm : splitter;
+    import std.array : array;
+
+    auto parts = token.splitter('.').array;
+    JSONValue headerObj = parseJSON(cast(string) Base64URLNoPadding.decode(parts[0]));
+    verifyHeaderFormat(headerObj);
+    string algorithm = headerObj.object["alg"].str;
+    if (algorithm != "HS256") {
+        throw new JwtFormatException("Unsupported algorithm: " ~ algorithm);
+    }
+    
+    JSONValue claimsObj = parseJSON(cast(string) Base64URLNoPadding.decode(parts[1]));
+    verifyClaimsFormat(claimsObj);
+
+    ubyte[] signatureBytes = Base64URLNoPadding.decode(parts[2]);
+    bool verified = hmac_verify_ex(
+        signatureBytes,
+        cast(ubyte[]) secret,
+        cast(ubyte[]) (parts[0] ~ "." ~ parts[1]),
+        HashAlgorithm.SHA2_256
+    );
+    if (!verified) {
+        throw new JwtVerificationException();
+    }
+
+    JwtClaims claims = JwtClaims(claimsObj);
+    verifyStandardTimeClaims(claims);
+    return claims;
+}
+
+private void verifyHeaderFormat(in JSONValue j) {
+    if (j.type != JSONType.OBJECT) {
+        throw new JwtFormatException("Header must be a JSON object.");
+    }
+    if ("typ" !in j.object || j.object["typ"].type != JSONType.STRING) {
+        throw new JwtFormatException("Header is missing the required 'typ' string property.");
+    }
+    if (j.object["typ"].str != "JWT") {
+        throw new JwtFormatException("Header 'typ' property must be 'JWT'.");
+    }
+    if ("alg" !in j.object || j.object["alg"].type != JSONType.STRING) {
+        throw new JwtFormatException("Header is missing the required 'alg' string property.");
+    }
+}
+
+private void verifyClaimsFormat(in JSONValue j) {
+    if (j.type != JSONType.OBJECT) {
+        throw new JwtFormatException("Claims must be a JSON object.");
+    }
+}
+
+private void verifyStandardTimeClaims(in JwtClaims claims) {
+    long currentTimestamp = Clock.currTime(UTC()).toUnixTime!long;
+    if (claims.expiration > 0 && claims.expiration <= currentTimestamp) {
+        throw new JwtExpiredException();
+    }
+    if (claims.notBefore > 0 && claims.notBefore > currentTimestamp) {
+        throw new JwtNotBeforeException();
+    }
+}
+
+unittest {
+    import jwt4d.writer;
+    JwtClaims claims;
+    claims.issuer = "example.com";
+    string token = writeJwt(claims, "test");
+    JwtClaims readClaims = readJwt(token, "test");
+    import std.stdio;
+    writeln(readClaims.toJson());
+}
\ No newline at end of file
diff --git a/source/jwt4d/writer.d b/source/jwt4d/writer.d
new file mode 100644
index 0000000..3e83c77
--- /dev/null
+++ b/source/jwt4d/writer.d
@@ -0,0 +1,45 @@
+module jwt4d.writer;
+
+import std.json;
+import std.base64;
+import secured;
+
+import jwt4d.model;
+
+/**
+ * Writes the given claims to a JWT, signed with the given secret.
+ * Params:
+ *   claims = The claims to encode.
+ *   secret = The secret to use when signing the JWT.
+ * Returns: The token.
+ */
+string writeJwt(in JwtClaims claims, string secret) {
+    JSONValue headerObj = JSONValue.emptyObject;
+    headerObj.object["typ"] = "JWT";
+    headerObj.object["alg"] = "HS256";
+    string headerBase64 = Base64URLNoPadding.encode(cast(ubyte[]) headerObj.toJSON());
+
+    string claimsBase64 = Base64URLNoPadding.encode(cast(ubyte[]) claims.toJson());
+
+    string prefix = headerBase64 ~ "." ~ claimsBase64;
+
+    ubyte[] signatureBytes = hmac_ex(
+        cast(ubyte[]) secret,
+        cast(ubyte[]) (prefix),
+        HashAlgorithm.SHA2_256
+    );
+    string signatureBase64 = Base64URLNoPadding.encode(signatureBytes);
+
+    return prefix ~ "." ~ signatureBase64;
+}
+
+unittest {
+    JwtClaims claims;
+    claims.issuer = "example.com";
+    claims.subject = "user123";
+    claims.expiration = 123;
+
+    string token = writeJwt(claims, "test");
+    import std.stdio;
+    writeln(token);
+}
\ No newline at end of file