ChaCha20 implementation for encryption

Add an implementation in 8051 assembler for the generation of a
ChaCha20 keystream and an encryption function using the keystream
in C.

A test function using the example from RFC7539 is provided in
chacha20_test.

Keys, nonce, and block counter and the pointers to the input plaintext,
its length and the output pointer are provided via a struct chacha20,
which also holds the working state of the ChaCha20 algorithm

crypto/chacha.h

@@ -0,0 +1,28 @@
+#ifndef _CHACHA_H_
+#define _CHACHA_H_
+
+#include <stdint.h>
+
+// In order to test, CHACHA20_TEST needs to be defined
+// #define CHACHA20_TEST
+
+struct chacha20_t {
+	uint8_t wstate[64];		// 64 bytes written here
+	uint8_t constant[16];		// 128 bit constant (4 Big-Endian u32)
+        uint8_t key[32];		// 256-bit secret key (8 Big-Endian u32)
+        uint8_t cnt[4];			// 32-bit block counter 1, 2.. Big Endian!
+        uint8_t nonce[12];		// 96-bit nonce (3 Big-Endian u32)
+        __xdata uint8_t *plaintext;
+        uint16_t length;
+        __xdata uint8_t *cyphertext;
+};
+
+// Encrypt a plaintext with ChaCha20 as per RFC7539
+void chacha20_encrypt(void) __banked;
+
+#ifdef CHACHA20_TEST
+// Test ChaCha20 using the example from RFC7539
+void chacha20_test(void) __banked;
+#endif
+
+#endif