diff --git a/lib/std/crypto/crypto.c3 b/lib/std/crypto/crypto.c3
new file mode 100644
index 000000000..a684ce5b1
--- /dev/null
+++ b/lib/std/crypto/crypto.c3
@@ -0,0 +1,2 @@
+module std::crypto;
+
diff --git a/lib/std/crypto/rc4.c3 b/lib/std/crypto/rc4.c3
new file mode 100644
index 000000000..f05b3d39e
--- /dev/null
+++ b/lib/std/crypto/rc4.c3
@@ -0,0 +1,65 @@
+module std::crypto::rc4;
+// Copyright (c) 2021 Christoffer Lerno. All rights reserved.
+// Use of this source code is governed by the MIT license
+// a copy of which can be found in the LICENSE_STDLIB file.
+
+struct Rc4
+{
+	uint i, j;
+	char[256] state;
+}
+
+/**
+ * Initialize the RC4 state.
+ *
+ * @param [inout] this "The RC4 state"
+ * @param [in] key "The key to use"
+ * @require key.len > 0 "The key must be at least 1 byte long"
+ **/
+fn void Rc4.init(Rc4* this, char[] key)
+{
+	// Init the state matrix
+	foreach (char i, &c : this.state) *c = i;
+	for (int i = 0, int j = 0; i < 256; i++)
+	{
+		j = (j + this.state[i] + key[i % key.len]) & 0xFF;
+		@swap(this.state[i], this.state[j]);
+	}
+	this.i = 0;
+	this.j = 0;
+}
+
+/**
+ * Encrypt or decrypt a sequence of bytes.
+ *
+ * @param [inout] this "The RC4 State"
+ * @param [in] in "The input"
+ * @param [out] out "The output"
+ * @require in.len <= out.len "Output would overflow"
+ **/
+fn void Rc4.crypt(Rc4* this, char[] in, char[] out)
+{
+	uint i = this.i;
+	uint j = this.j;
+	char* state = &this.state;
+	isz len = in.len;
+	foreach (idx, c : in)
+	{
+		i = (i + 1) & 0xFF;
+		j = (j + state[i]) & 0xFF;
+		@swap(state[i], state[j]);
+		out[idx] = in[idx] ^ state[(state[i] + state[j]) & 0xFF];
+	}
+	this.i = i;
+	this.j = j;
+}
+
+/**
+ * Clear the rc4 state.
+ *
+ * @param [out] this "The RC4 State"
+ **/
+fn void Rc4.destroy(Rc4* this)
+{
+	*this = {};
+}
diff --git a/test/unit/stdlib/crypto/rc4.c3 b/test/unit/stdlib/crypto/rc4.c3
new file mode 100644
index 000000000..084ce816c
--- /dev/null
+++ b/test/unit/stdlib/crypto/rc4.c3
@@ -0,0 +1,16 @@
+import std::crypto;
+import std::io;
+
+fn void! rc_crypt() @test
+{
+	Rc4 rc;
+	rc.init(&&x"63727970746969");
+	char[200] x;
+	char[] text = "The quick brown fox jumps over the lazy dog.";
+	rc.crypt(text, &x);
+	char[*] res = x"2ac2fecdd8fbb84638e3a4
+	                820eb205cc8e29c28b9d5d
+	                6b2ef974f311964971c90e
+	                8b9ca16467ef2dc6fc3520";
+	assert(res[:text.len] == x[:text.len]);
+}
\ No newline at end of file