diff --git a/src/Psl/Encoding/Base64/decode.php b/src/Psl/Encoding/Base64/decode.php
new file mode 100644
index 0000000..35d4c3e
--- /dev/null
+++ b/src/Psl/Encoding/Base64/decode.php
@@ -0,0 +1,33 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Psl\Encoding\Base64;
+
+use Psl\Encoding\Exception;
+
+use function base64_decode;
+use function preg_match;
+
+/**
+ * Decode a base64-encoded string into raw binary.
+ *
+ * Base64 character set:
+ *  [A-Z]      [a-z]      [0-9]      +     /
+ *  0x41-0x5a, 0x61-0x7a, 0x30-0x39, 0x2b, 0x2f
+ *
+ * @psalm-pure
+ *
+ * @throws Exception\RangeException             If the encoded string contains characters outside
+ *                                              the base64 characters range.
+ */
+function decode(string $base64): string
+{
+    if (!preg_match('%^[a-zA-Z0-9/+]*={0,2}$%', $base64)) {
+        throw new Exception\RangeException(
+            'The given base64 string contains characters outside the base64 range.'
+        );
+    }
+
+    return (string) @base64_decode($base64, true);
+}
diff --git a/src/Psl/Encoding/Base64/encode.php b/src/Psl/Encoding/Base64/encode.php
new file mode 100644
index 0000000..517a83a
--- /dev/null
+++ b/src/Psl/Encoding/Base64/encode.php
@@ -0,0 +1,25 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Psl\Encoding\Base64;
+
+use Psl\Str;
+
+use function base64_encode;
+use function pack;
+use function unpack;
+
+/**
+ * Convert a binary string into a base64-encoded string.
+ *
+ * Base64 character set:
+ *  [A-Z]      [a-z]      [0-9]      +     /
+ *  0x41-0x5a, 0x61-0x7a, 0x30-0x39, 0x2b, 0x2f
+ *
+ * @psalm-pure
+ */
+function encode(string $binary): string
+{
+    return base64_encode($binary);
+}
diff --git a/src/Psl/Encoding/Exception/ExceptionInterface.php b/src/Psl/Encoding/Exception/ExceptionInterface.php
new file mode 100644
index 0000000..91cd66d
--- /dev/null
+++ b/src/Psl/Encoding/Exception/ExceptionInterface.php
@@ -0,0 +1,11 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Psl\Encoding\Exception;
+
+use Psl\Exception;
+
+interface ExceptionInterface extends Exception\ExceptionInterface
+{
+}
diff --git a/src/Psl/Encoding/Exception/IncorrectPaddingException.php b/src/Psl/Encoding/Exception/IncorrectPaddingException.php
new file mode 100644
index 0000000..002af6f
--- /dev/null
+++ b/src/Psl/Encoding/Exception/IncorrectPaddingException.php
@@ -0,0 +1,9 @@
+<?php
+
+namespace Psl\Encoding\Exception;
+
+use Psl\Exception;
+
+final class IncorrectPaddingException extends Exception\InvalidArgumentException implements ExceptionInterface
+{
+}
diff --git a/src/Psl/Encoding/Exception/RangeException.php b/src/Psl/Encoding/Exception/RangeException.php
new file mode 100644
index 0000000..01b2675
--- /dev/null
+++ b/src/Psl/Encoding/Exception/RangeException.php
@@ -0,0 +1,9 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Psl\Encoding\Exception;
+
+final class RangeException extends \RangeException implements ExceptionInterface
+{
+}
diff --git a/src/Psl/Encoding/Hex/decode.php b/src/Psl/Encoding/Hex/decode.php
new file mode 100644
index 0000000..f126061
--- /dev/null
+++ b/src/Psl/Encoding/Hex/decode.php
@@ -0,0 +1,39 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Psl\Encoding\Hex;
+
+use Psl\Encoding\Exception;
+use Psl\Str;
+
+/**
+ * Convert a hexadecimal string into a binary string.
+ *
+ * Hex ( Base16 ) character set:
+ *  [0-9]      [a-f]      [A-F]
+ *  0x30-0x39, 0x61-0x66, 0x41-0x46
+ *
+ * @psalm-pure
+ *
+ * @throws Exception\RangeException If the hexadecimal string contains characters outside the base16 range,
+ *                                  or an odd number of characters.
+ */
+function decode(string $hexadecimal): string
+{
+    if (!ctype_xdigit($hexadecimal)) {
+        throw new Exception\RangeException(
+            'The given hexadecimal string contains characters outside the base16 range.'
+        );
+    }
+
+    /** @psalm-suppress MissingThrowsDocblock */
+    $hex_len = Str\length($hexadecimal, '8bit');
+    if (($hex_len & 1) !== 0) {
+        throw new Exception\RangeException(
+            'Expected an even number of hexadecimal characters.',
+        );
+    }
+
+    return hex2bin($hexadecimal);
+}
diff --git a/src/Psl/Encoding/Hex/encode.php b/src/Psl/Encoding/Hex/encode.php
new file mode 100644
index 0000000..bd3d48d
--- /dev/null
+++ b/src/Psl/Encoding/Hex/encode.php
@@ -0,0 +1,19 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Psl\Encoding\Hex;
+
+/**
+ * Convert a binary string into a hexadecimal string.
+ *
+ * Hex ( Base16 ) character set:
+ *  [0-9]      [a-f]
+ *  0x30-0x39, 0x61-0x66
+ *
+ * @psalm-pure
+ */
+function encode(string $binary): string
+{
+    return bin2hex($binary);
+}
diff --git a/src/Psl/Internal/Loader.php b/src/Psl/Internal/Loader.php
index 26df4b9..5c23c39 100644
--- a/src/Psl/Internal/Loader.php
+++ b/src/Psl/Internal/Loader.php
@@ -323,6 +323,10 @@ final class Loader
         'Psl\Password\hash',
         'Psl\Password\needs_rehash',
         'Psl\Password\verify',
+        'Psl\Encoding\Base64\encode',
+        'Psl\Encoding\Base64\decode',
+        'Psl\Encoding\Hex\encode',
+        'Psl\Encoding\Hex\decode',
     ];
 
     public const INTERFACES = [
@@ -340,6 +344,7 @@ final class Loader
         'Psl\Observer\SubjectInterface',
         'Psl\Observer\ObserverInterface',
         'Psl\Result\ResultInterface',
+        'Psl\Encoding\Exception\ExceptionInterface',
     ];
 
     public const TRAITS = [
@@ -379,6 +384,8 @@ final class Loader
         'Psl\Type\Type',
         'Psl\Json\Exception\DecodeException',
         'Psl\Json\Exception\EncodeException',
+        'Psl\Encoding\Exception\IncorrectPaddingException',
+        'Psl\Encoding\Exception\RangeException',
     ];
 
     private const TYPE_CONSTANTS = 1;
diff --git a/src/Psl/Internal/lazy_iterator.php b/src/Psl/Internal/lazy_iterator.php
index ef85f56..66d3731 100644
--- a/src/Psl/Internal/lazy_iterator.php
+++ b/src/Psl/Internal/lazy_iterator.php
@@ -13,6 +13,8 @@ use Psl\Iter;
  * @psalm-param (callable(): \Generator<Tk, Tv, mixed, mixed>) $fun
  *
  * @psalm-return Iter\Iterator<Tk, Tv>
+ *
+ * @internal
  */
 function lazy_iterator(callable $fun): Iter\Iterator
 {
diff --git a/src/Psl/Internal/validate_offset.php b/src/Psl/Internal/validate_offset.php
index 7082c43..689167d 100644
--- a/src/Psl/Internal/validate_offset.php
+++ b/src/Psl/Internal/validate_offset.php
@@ -15,6 +15,8 @@ use Psl;
  * @codeCoverageIgnore
  *
  * @throws Psl\Exception\InvariantViolationException If the offset is out-of-bounds.
+ *
+ * @internal
  */
 function validate_offset(int $offset, int $length): int
 {
diff --git a/src/Psl/Internal/validate_offset_lower_bound.php b/src/Psl/Internal/validate_offset_lower_bound.php
index 9c6adf4..75691f9 100644
--- a/src/Psl/Internal/validate_offset_lower_bound.php
+++ b/src/Psl/Internal/validate_offset_lower_bound.php
@@ -13,6 +13,8 @@ use Psl;
  * @psalm-pure
  *
  * @throws Psl\Exception\InvariantViolationException If $offset is out-of-bounds.
+ *
+ * @internal
  */
 function validate_offset_lower_bound(int $offset, int $length): int
 {
diff --git a/tests/Psl/Encoding/Base64Test.php b/tests/Psl/Encoding/Base64Test.php
new file mode 100644
index 0000000..06c02d6
--- /dev/null
+++ b/tests/Psl/Encoding/Base64Test.php
@@ -0,0 +1,36 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Psl\Tests\Encoding;
+
+use PHPUnit\Framework\TestCase;
+use Psl\Encoding\Base64;
+use Psl\Encoding\Exception;
+use Psl\SecureRandom;
+
+final class Base64Test extends TestCase
+{
+    /**
+     * @dataProvider provideRandomBytes
+     */
+    public function testEncodeAndDecode(string $random): void
+    {
+        $encoded = Base64\encode($random);
+        self::assertSame($random, Base64\decode($encoded));
+    }
+
+    public function testDecodeThrowsForCharactersOutsideTheBase64Range(): void
+    {
+        $this->expectException(Exception\RangeException::class);
+
+        Base64\decode('@~==');
+    }
+
+    public function provideRandomBytes(): iterable
+    {
+        for ($i = 1; $i < 128; ++$i) {
+                yield [SecureRandom\bytes($i)];
+        }
+    }
+}
diff --git a/tests/Psl/Encoding/HexTest.php b/tests/Psl/Encoding/HexTest.php
new file mode 100644
index 0000000..7f20012
--- /dev/null
+++ b/tests/Psl/Encoding/HexTest.php
@@ -0,0 +1,49 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Psl\Tests\Encoding;
+
+use PHPUnit\Framework\TestCase;
+use Psl\Encoding\Hex;
+use Psl\Encoding\Exception;
+use Psl\SecureRandom;
+use Psl\Str;
+
+use function bin2hex;
+
+final class HexTest extends TestCase
+{
+    /**
+     * @dataProvider provideRandomBytes
+     */
+    public function testRandom(string $random): void
+    {
+        $enc = Hex\encode($random);
+        self::assertSame($random, Hex\decode($enc));
+        self::assertSame(bin2hex($random), $enc);
+        $enc = Hex\encode($random);
+        self::assertSame($random, Hex\decode($enc));
+    }
+
+    public function testDecodeThrowsForCharactersOutsideTheHexRange(): void
+    {
+        $this->expectException(Exception\RangeException::class);
+
+        Hex\decode('gf');
+    }
+
+    public function testDecodeThrowsForAnOddNumberOfCharacters(): void
+    {
+        $this->expectException(Exception\RangeException::class);
+
+        Hex\decode('f');
+    }
+
+    public function provideRandomBytes(): iterable
+    {
+        for ($i = 1; $i < 128; ++$i) {
+                yield [SecureRandom\bytes($i)];
+        }
+    }
+}