diff --git a/.github/workflows/php.yml b/.github/workflows/php.yml
index a66f991..fe398de 100644
--- a/.github/workflows/php.yml
+++ b/.github/workflows/php.yml
@@ -1,10 +1,6 @@
 name: Test coverage
 
-on:
-  push:
-    branches: [ "master" ]
-  pull_request:
-    branches: [ "master" ]
+on: [push, pull_request]
 
 permissions:
   contents: read
@@ -14,7 +10,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        php-version: [8.0, 8.1, 8.2, 8.3]
+        php-version: [8.2, 8.3]
 
     runs-on: ubuntu-latest
 
diff --git a/README.md b/README.md
index 66329a3..1310b5c 100644
--- a/README.md
+++ b/README.md
@@ -76,6 +76,7 @@ $token = new Token([
     "iss" => "customer-data-service",
     "sub" => $user->id,
     "custom_claim_foo" => "bar",
+	"iat" => \time(),
     "exp" => \strtotime("+1 hour")
 ])
 ```
@@ -86,6 +87,19 @@ Or you can set a claim on a `Token` by calling the `setClaim` method.
 $token->setClaim("nbf", \strtotime("+1 week"));
 ```
 
+**NOTE:** The `exp` and `nbf` public claims that represent a timestamp *must* be formatted as a `NumericDate` (an integer Unix timestamp.) `Proof` will automatically attempt to validate the token against the given expiration claim (exp) and, if present, the not before claim (nbf). As such, it expects those values to be integers.
+
+Please see the official [RFC](https://datatracker.ietf.org/doc/html/rfc7519#section-2), specifcally the definition of **NumericDate**.
+
+> A JSON numeric value representing the number of seconds from
+	1970-01-01T00:00:00Z UTC until the specified UTC date/time,
+	ignoring leap seconds.  This is equivalent to the IEEE Std 1003.1,
+	2013 Edition [POSIX.1] definition "Seconds Since the Epoch", in
+	which each day is accounted for by exactly 86400 seconds, other
+	than that non-integer values can be represented.  See RFC 3339
+	[RFC3339] for details regarding date/times in general and UTC in
+	particular.
+
 ### Encode a Token into a JWT
 
 With a `Token` instance, you can encode it into a signed JWT by passing it into the `encode` method. You will be returned a signed JWT string.
@@ -228,7 +242,8 @@ If the JWT is invalid, an exception will be thrown. This exception will need to
 * `InvalidTokenException` is thrown if the JWT cannot be decoded due to being malformed or containing invalid JSON.
 * `SignatureMismatchException` is thrown if the signature does not match.
 * `ExpiredTokenException` is thrown if the token's `exp` claim is expired.
-* `TokenNotReadyException` is thrown if the token's `nbf` claim is not ready (i.e. the timestamp is still in the future.).
+* `TokenNotReadyException` is thrown if the token's `nbf` claim is not ready (i.e. the timestamp is still in the future.)
+* `SignerNotFoundException` is thrown if a `kid` (key ID) was passed in the JWT header that does not exist in the key map.
 
 The middleware defaults to looking for the JWT in the `Authorization` HTTP header with a `Bearer` scheme. For example:
 
diff --git a/composer.json b/composer.json
index 67ad6c2..2233d01 100644
--- a/composer.json
+++ b/composer.json
@@ -11,7 +11,7 @@
         }
     ],
 	"require": {
-		"php": "^8.0",
+		"php": "^8.2",
 		"ext-openssl": "*",
         "paragonie/hidden-string": "^2.0",
         "psr/http-message": "^1.0|^2.0",
@@ -26,6 +26,6 @@
         "phpunit/phpunit": "^9.0",
         "vimeo/psalm": "^5.0",
         "symfony/var-dumper": "^5.1",
-        "nimbly/capsule": "^2.0"
+        "nimbly/capsule": "^3.0"
 	}
 }
diff --git a/phpunit.xml b/phpunit.xml
index 27d112c..49c73ba 100644
--- a/phpunit.xml
+++ b/phpunit.xml
@@ -2,7 +2,7 @@
 <phpunit
 	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 	xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/9.3/phpunit.xsd"
-	bootstrap="vendor/autoload.php"
+	bootstrap="tests/bootstrap.php"
 	executionOrder="depends,defects"
 	forceCoversAnnotation="true"
 	beStrictAboutCoversAnnotation="false"
diff --git a/src/ExpiredTokenException.php b/src/ExpiredTokenException.php
index 1ac0c22..764bf94 100644
--- a/src/ExpiredTokenException.php
+++ b/src/ExpiredTokenException.php
@@ -2,6 +2,9 @@
 
 namespace Nimbly\Proof;
 
+/**
+ * This expception is thrown when a JWT has expired based on its `exp` (expiration) claim.
+ */
 class ExpiredTokenException extends TokenDecodingException
 {
 }
\ No newline at end of file
diff --git a/src/InvalidTokenException.php b/src/InvalidTokenException.php
index 17fe624..b3f2eb1 100644
--- a/src/InvalidTokenException.php
+++ b/src/InvalidTokenException.php
@@ -2,6 +2,14 @@
 
 namespace Nimbly\Proof;
 
+/**
+ * This exception is thrown when something is fundamentally wrong with the JWT being decoded:
+ *
+ * - Token is not a JWT
+ * - Token does not contain a signature
+ * - Token (header or payload) does not contain valid JSON
+ * - Expiration (exp) or Not Before (nbf) claims are not in Unix timestamp format
+ */
 class InvalidTokenException extends TokenDecodingException
 {
 }
\ No newline at end of file
diff --git a/src/Proof.php b/src/Proof.php
index f5daa67..8b8931a 100644
--- a/src/Proof.php
+++ b/src/Proof.php
@@ -11,7 +11,7 @@ class Proof
 	/**
 	 * @param SignerInterface $signer The default signing key used for encoding and decoding tokens.
 	 * @param integer $leeway Time in seconds to add to expiration and "not-before" date calculations to account for drift. The leeway can be negative if you wish.
-	 * @param array<string,SignerInterface> $keyMap If you're using multiple different signing keys, you can map them here as key/value pairs. If no kid is present in header or no kid provided when signing, then the default signing key will be used.
+	 * @param array<string,SignerInterface> $keyMap If you're using multiple different signing keys, you can map them here as key/value pairs. If no `kid` is present in header or no `kid` provided when signing, then the default signing key will be used. If the default key should also be mapped to a key ID, be sure to add it here as well.
 	 */
 	public function __construct(
 		protected SignerInterface $signer,
@@ -23,24 +23,15 @@ public function __construct(
 	/**
 	 * Encode a Token instance into a JWT.
 	 *
-	 * @param Token $token
-	 * @param string|null $kid Key ID to include in header
+	 * @param Token $token Token instance with JWT claims.
+	 * @param string|null $kid Key ID to include in header. The given key ID *must* map to a key in the key map.
 	 * @throws SignerNotFoundException
 	 * @throws TokenEncodingException
 	 * @return string
 	 */
 	public function encode(Token $token, ?string $kid = null): string
 	{
-		if( $kid !== null ) {
-			$signer = $this->getSignerByKeyId($kid);
-
-			if( empty($signer) ){
-				throw new SignerNotFoundException("No signer found for key ID.");
-			}
-		}
-		else {
-			$signer = $this->signer;
-		}
+		$signer = $this->getSigner($kid);
 
 		$header = [
 			"alg" => $signer->getAlgorithm(),
@@ -71,7 +62,7 @@ public function encode(Token $token, ?string $kid = null): string
 	/**
 	 * Decode a JWT string into a Token instance.
 	 *
-	 * @param string $jwt
+	 * @param string $jwt The encoded JWT string.
 	 * @throws InvalidTokenException
 	 * @throws SignerNotFoundException
 	 * @throws SignatureMismatchException
@@ -83,29 +74,20 @@ public function decode(string $jwt): Token
 	{
 		$parts = \explode(".", $jwt);
 
-		if( \count($parts) < 3 ){
+		if( \count($parts) !== 3 ){
 			throw new InvalidTokenException("Invalid number of token parts.");
 		}
 
 		[$header, $payload, $signature] = $parts;
 
-		/** @var object{algo:string,typ:string,kid:mixed} $decoded_header */
+		/** @var object{alg:string,typ:string,kid:mixed} $decoded_header */
 		$decoded_header = \json_decode($this->base64UrlDecode($header));
 
 		if( \json_last_error() !== JSON_ERROR_NONE ){
 			throw new InvalidTokenException("Token header could not be JSON decoded.");
 		}
 
-		if( isset($decoded_header->kid) ){
-			$signer = $this->getSignerByKeyId((string) $decoded_header->kid);
-
-			if( empty($signer) ){
-				throw new SignerNotFoundException("No signer found for decoding.");
-			}
-		}
-		else {
-			$signer = $this->signer;
-		}
+		$signer = $this->getSigner($decoded_header->kid ?? null);
 
 		$signature_verified = $signer->verify(
 			"{$header}.{$payload}",
@@ -127,28 +109,52 @@ public function decode(string $jwt): Token
 
 		$timestamp = \time();
 
-		if( isset($decoded_payload->exp) &&
-			$decoded_payload->exp < ($timestamp + $this->leeway) ){
-			throw new ExpiredTokenException("The token has expired.");
+		if( isset($decoded_payload->exp) ){
+
+			if( !\is_int($decoded_payload->exp) ){
+				throw new InvalidTokenException("Expiration (exp) claim is not correctly formatted.");
+			}
+
+			if( $decoded_payload->exp < ($timestamp + $this->leeway) ){
+				throw new ExpiredTokenException("The token has expired.");
+			}
 		}
 
-		if( isset($decoded_payload->nbf) &&
-			$decoded_payload->nbf > ($timestamp - $this->leeway) ){
-			throw new TokenNotReadyException("The token is not ready to be accepted yet.");
+		if( isset($decoded_payload->nbf) ){
+
+			if( !\is_int($decoded_payload->nbf) ){
+				throw new InvalidTokenException("Not before (nbf) claim is not correctly formatted.");
+			}
+
+			if( $decoded_payload->nbf > ($timestamp - $this->leeway) ){
+				throw new TokenNotReadyException("The token is not ready to be accepted yet.");
+			}
 		}
 
 		return new Token((array) $decoded_payload);
 	}
 
 	/**
-	 * Get a SignerInterface instance by its Key ID (kid) from the key map.
+	 * Get the SignerInterface instance to use.
 	 *
-	 * @param string $kid
-	 * @return SignerInterface|null
+	 * @param string|null $kid Get a specific signer by its key from the KeyMap as defined in the constructor. If null, default signer will be returned.
+	 * @throws SignerNotFoundException
+	 * @return SignerInterface
 	 */
-	private function getSignerByKeyId(string $kid): ?SignerInterface
+	private function getSigner(?string $kid = null): SignerInterface
 	{
-		return $this->keyMap[$kid] ?? null;
+		if( $kid !== null ) {
+			if( !isset($this->keyMap[$kid]) ){
+				throw new SignerNotFoundException("No signer found for key ID.");
+			}
+
+			$signer = $this->keyMap[$kid];
+		}
+		else {
+			$signer = $this->signer;
+		}
+
+		return $signer;
 	}
 
 	/**
diff --git a/src/SignatureMismatchException.php b/src/SignatureMismatchException.php
index 329886d..45736f6 100644
--- a/src/SignatureMismatchException.php
+++ b/src/SignatureMismatchException.php
@@ -2,6 +2,13 @@
 
 namespace Nimbly\Proof;
 
+/**
+ * This exception is thrown when the signature of the JWT does not match with the computed signature.
+ * If this exception is thrown, it is *very* likely that the JWT was either tampered with or the incorrect
+ * signer is being used.
+ *
+ * In either case, DO NOT trust the JWT being sent.
+ */
 class SignatureMismatchException extends TokenDecodingException
 {
 }
\ No newline at end of file
diff --git a/src/Signer/HmacSigner.php b/src/Signer/HmacSigner.php
index 4eb5e1d..22dd9f1 100644
--- a/src/Signer/HmacSigner.php
+++ b/src/Signer/HmacSigner.php
@@ -5,6 +5,7 @@
 use Nimbly\Proof\SignerInterface;
 use Nimbly\Proof\SigningException;
 use ParagonIE\HiddenString\HiddenString;
+use SensitiveParameter;
 
 class HmacSigner implements SignerInterface
 {
@@ -28,7 +29,7 @@ class HmacSigner implements SignerInterface
 	 */
 	public function __construct(
 		protected string $algorithm,
-		string $key
+		#[SensitiveParameter] string $key
 	)
 	{
 		if( \array_key_exists($algorithm, $this->supported_algorithms) === false ){
diff --git a/src/Signer/KeypairSigner.php b/src/Signer/KeypairSigner.php
index 79054af..29bd907 100644
--- a/src/Signer/KeypairSigner.php
+++ b/src/Signer/KeypairSigner.php
@@ -92,7 +92,7 @@ public function verify(string $message, string $signature): bool
 			$this->algorithm
 		);
 
-		if( $status < 0 ){
+		if( $status === false || $status < 0 ){
 			throw new SigningException("An error occured when trying to verify signature of message.");
 		}
 
diff --git a/src/SignerNotFoundException.php b/src/SignerNotFoundException.php
index 6be19ef..bb2e0e5 100644
--- a/src/SignerNotFoundException.php
+++ b/src/SignerNotFoundException.php
@@ -4,6 +4,10 @@
 
 use Exception;
 
+/**
+ * This exception is thrown when a `kid` was passed in the JWT header but no such key exists in
+ * the key map.
+ */
 class SignerNotFoundException extends Exception
 {
 }
\ No newline at end of file
diff --git a/src/SigningException.php b/src/SigningException.php
index f333987..ea6fa29 100644
--- a/src/SigningException.php
+++ b/src/SigningException.php
@@ -4,6 +4,9 @@
 
 use Exception;
 
+/**
+ * This exception is thrown when a signature could not be generated.
+ */
 class SigningException extends Exception
 {
 }
\ No newline at end of file
diff --git a/src/Token.php b/src/Token.php
index 9beac83..ca18333 100644
--- a/src/Token.php
+++ b/src/Token.php
@@ -7,9 +7,13 @@
 class Token implements JsonSerializable
 {
 	/**
-	 * Token constructor.
+	 * @param array<array-key,mixed> $claims Array of key/value pairs of JWT claims for token.
 	 *
-	 * @param array<array-key,mixed> $claims JWT claims for token.
+	 * For timestamp based public claims, be sure to use a Unix timestamp.
+	 *
+	 * * `exp` expiration date, in Unix timestamp format
+	 * * `nbf` token not valid before before given date, in Unix timestamp format
+	 * * `iat` date token issued at, in Unix timestamp format
 	 */
 	public function __construct(protected array $claims = [])
 	{
diff --git a/src/TokenDecodingException.php b/src/TokenDecodingException.php
index b0182e8..26d69d8 100644
--- a/src/TokenDecodingException.php
+++ b/src/TokenDecodingException.php
@@ -4,6 +4,15 @@
 
 use Exception;
 
+/**
+ * This exception isn't thrown directly, but encompasses a class of exceptions.
+ *
+ * - InvalidTokenException
+ * - ExpiredTokenException
+ * - SignatureMismatchException
+ * - TokenNotReadyException
+ *
+ */
 class TokenDecodingException extends Exception
 {
 }
\ No newline at end of file
diff --git a/src/TokenEncodingException.php b/src/TokenEncodingException.php
index 81b6b07..68b5b0f 100644
--- a/src/TokenEncodingException.php
+++ b/src/TokenEncodingException.php
@@ -4,6 +4,9 @@
 
 use Exception;
 
+/**
+ * This exception is thrown when encoding a token into a string JWT has failed.
+ */
 class TokenEncodingException extends Exception
 {
 }
\ No newline at end of file
diff --git a/src/TokenNotReadyException.php b/src/TokenNotReadyException.php
index 2b922e4..f447f74 100644
--- a/src/TokenNotReadyException.php
+++ b/src/TokenNotReadyException.php
@@ -2,6 +2,10 @@
 
 namespace Nimbly\Proof;
 
+/**
+ * This exception is thrown when a JWT has an `nbf` (not before) claim and represents a date
+ * at which a token becomes active or usable however that date is still in the future.
+ */
 class TokenNotReadyException extends TokenDecodingException
 {
 }
\ No newline at end of file
diff --git a/tests/KeypairSignerTest.php b/tests/KeypairSignerTest.php
index 036a79d..7ff91da 100644
--- a/tests/KeypairSignerTest.php
+++ b/tests/KeypairSignerTest.php
@@ -1,9 +1,9 @@
 <?php
 
-use PHPUnit\Framework\TestCase;
 use Nimbly\Proof\Proof;
-use Nimbly\Proof\Signer\KeypairSigner;
+use PHPUnit\Framework\TestCase;
 use Nimbly\Proof\SigningException;
+use Nimbly\Proof\Signer\KeypairSigner;
 
 /**
  * @covers Nimbly\Proof\Signer\KeypairSigner
@@ -159,6 +159,19 @@ public function test_signing_with_no_private_key_throws_signing_exception(): voi
 		$keypairSigner->sign("Message");
 	}
 
+	public function test_signing_failure_throws_signing_exception(): void
+	{
+		$keypairSigner = new KeypairSigner(
+			Proof::ALGO_SHA256,
+			null,
+			\openssl_get_publickey(\file_get_contents(__DIR__ . "/keys/public.pem"))
+		);
+
+		$this->expectException(SigningException::class);
+
+		$keypairSigner->sign("Message");
+	}
+
 	public function test_verify_with_no_public_key_throws_signing_exception(): void
 	{
 		$keypairSigner = new KeypairSigner(
@@ -168,4 +181,16 @@ public function test_verify_with_no_public_key_throws_signing_exception(): void
 		$this->expectException(SigningException::class);
 		$keypairSigner->verify("Message", "signature");
 	}
+
+	public function test_verify_failure_throws_signing_exception(): void
+	{
+		$keypairSigner = new KeypairSigner(
+			Proof::ALGO_SHA256,
+			\openssl_get_privatekey(\file_get_contents(__DIR__ . "/keys/private.pem"))
+		);
+
+		$this->expectException(SigningException::class);
+
+		$keypairSigner->verify("Message", "Signature");
+	}
 }
\ No newline at end of file
diff --git a/tests/ProofTest.php b/tests/ProofTest.php
index 182e029..bb80773 100644
--- a/tests/ProofTest.php
+++ b/tests/ProofTest.php
@@ -178,6 +178,23 @@ public function test_malformed_json_payload_throws_invalid_token_exception(): vo
 		$proof->decode($jwt);
 	}
 
+	public function test_invalid_exp_claim_throws_invalid_token_exception(): void
+	{
+		$token = new Token([
+			"exp" => (string) \date("c")
+		]);
+
+		$proof = new Proof(
+			new HmacSigner(Proof::ALGO_SHA256, "supersecret")
+		);
+
+		$jwt = $proof->encode($token);
+
+		$this->expectException(InvalidTokenException::class);
+
+		$proof->decode($jwt);
+	}
+
 	public function test_expired_token_throws_expired_token_exception(): void
 	{
 		$token = new Token([
@@ -195,6 +212,23 @@ public function test_expired_token_throws_expired_token_exception(): void
 		$proof->decode($jwt);
 	}
 
+	public function test_invalid_nbf_claim_throws_invalid_token_exception(): void
+	{
+		$token = new Token([
+			"nbf" => \date("c")
+		]);
+
+		$proof = new Proof(
+			new HmacSigner(Proof::ALGO_SHA256, "supersecret")
+		);
+
+		$jwt = $proof->encode($token);
+
+		$this->expectException(InvalidTokenException::class);
+
+		$proof->decode($jwt);
+	}
+
 	public function test_forthcoming_token_throws_token_not_ready_exception(): void
 	{
 		$token = new Token([
diff --git a/tests/bootstrap.php b/tests/bootstrap.php
new file mode 100644
index 0000000..80fdf79
--- /dev/null
+++ b/tests/bootstrap.php
@@ -0,0 +1,5 @@
+<?php
+
+require __DIR__ . "/../vendor/autoload.php";
+
+\error_reporting(E_ALL & ~E_WARNING);
\ No newline at end of file