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
2026-08-30 14:52:51 +08:00 · 2026-04-26 10:30:55 +02:00
parent e280ccd2f9
commit 85244d85f9
6 changed files with 619 additions and 6 deletions
@@ -0,0 +1,98 @@
+#include "rtl837x_common.h"
+#include "chacha.h"
+
+void chacha_20(void);
+void chacha_update(void);
+void chacha_count(void);
+
+// generate a block of ChaCha20 keystream as per RFC7539
+
+
+__xdata struct chacha20_t __at(0x7000) chacha20;
+__xdata uint8_t plaintext[256];
+__xdata uint8_t cyphertext[256];
+
+void chacha20_print_block(void)
+{
+        for (uint8_t i=0; i < 64; i++) {
+            print_byte(*(uint8_t * __xdata)(chacha20.wstate + i));
+	    if (i%4 == 3)
+		    write_char(' ');
+        }
+}
+
+
+void chacha20_encrypt(void)
+{
+	__xdata uint8_t *p = chacha20.plaintext;
+
+	while (chacha20.length) {
+		register uint8_t i;
+		chacha_count();
+		memcpy(chacha20.wstate, chacha20.wstate + 64, 64);
+#ifdef DEBUG
+		chacha20_print_block(); write_char('\n');
+#endif
+		chacha_20();
+		chacha_update();
+#ifdef DEBUG
+		chacha20_print_block(); write_char('\n');
+#endif
+		for (i = 0; i < ((chacha20.length > 64) ? 64 : chacha20.length); i++)
+			*chacha20.cyphertext++ = *p++ ^ chacha20.wstate[i];
+		chacha20.length -= chacha20.length > 64 ? 64 : chacha20.length;
+#ifdef DEBUG
+		print_string("\n  round done\n");
+#endif
+	};
+}
+
+
+// Test the example of RFC 7539 Section 2.4.2
+void chacha20_test(void)
+{
+	__code uint8_t chacha_c[16] =
+		{ 0x61, 0x70, 0x78, 0x65,  0x33, 0x20, 0x64, 0x6e,
+		  0x79, 0x62, 0x2d, 0x32,  0x6b, 0x20, 0x65, 0x74 };
+
+	__code uint8_t key[32] =
+		{ 0x03, 0x02, 0x01, 0x00,  0x07, 0x06, 0x05, 0x04,
+		  0x0b, 0x0a, 0x09, 0x08,  0x0f, 0x0e, 0x0d, 0x0c,
+		  0x13, 0x12, 0x11, 0x10,  0x17, 0x16, 0x15, 0x14,
+		  0x1b, 0x1a, 0x19, 0x18,  0x1f, 0x1e, 0x1d, 0x1c };
+	
+	__code uint8_t nonce[12] = {
+		  0x00, 0x00, 0x00, 0x00,  0x4a, 0x00, 0x00, 0x00,
+		  0x00, 0x00, 0x00, 0x00 };
+
+	__code uint8_t sunscreen[] = "Ladies and Gentlemen of the class of '99: If I could offer you only one tip for " \
+				     "the future, sunscreen would be it.";
+
+/*
+	Ciphertext Sunscreen (from RFC 7539 Section 2.4.2):
+	000  6e 2e 35 9a 25 68 f9 80 41 ba 07 28 dd 0d 69 81  n.5.%h..A..(..i.
+	016  e9 7e 7a ec 1d 43 60 c2 0a 27 af cc fd 9f ae 0b  .~z..C`..'......
+	032  f9 1b 65 c5 52 47 33 ab 8f 59 3d ab cd 62 b3 57  ..e.RG3..Y=..b.W
+	048  16 39 d6 24 e6 51 52 ab 8f 53 0c 35 9f 08 61 d8  .9.$.QR..S.5..a.
+	064  07 ca 0d bf 50 0d 6a 61 56 a3 8e 08 8a 22 b6 5e  ....P.jaV....".^
+	080  52 bc 51 4d 16 cc f8 06 81 8c e9 1a b7 79 37 36  R.QM.........y76
+	096  5a f9 0b bf 74 a3 5b e6 b4 0b 8e ed f2 78 5e 42  Z...t.[......x^B
+	112  87 4d 
+*/
+	strcpy(plaintext, sunscreen);
+	memcpyc(chacha20.constant, chacha_c, 16);
+	memcpyc(chacha20.key, key, 32);
+	chacha20.cnt = 0;
+	memcpyc(chacha20.nonce, nonce, 12);
+	chacha20.plaintext = plaintext;
+	chacha20.length = strlen_x(plaintext);
+	chacha20.cyphertext = cyphertext;
+
+	print_string("Encrypting...\n");
+	chacha20_encrypt();
+	print_string("Cypertext:\n");
+	uint16_t len = strlen_x(plaintext);
+	for (uint8_t i = 0; i < len; i++) {
+		print_byte(cyphertext[i]); write_char(' ');
+	}
+}