crypto.cipher: fix decryption in CBC mode, add test (#25584)

Author: tankf33der

vlib/crypto/cipher/aes_cbc_test.v

@@ -4,6 +4,30 @@
 import crypto.aes
 import crypto.cipher
 
+fn test_aes_cbc_double() {
+	orig1 := []u8{len: 64, init: index}
+	orig2 := []u8{len: 16, init: index}
+	key := []u8{len: 16}
+	iv := []u8{len: 16}
+
+	mut block := aes.new_cipher(key)
+	mut en_cbc := cipher.new_cbc(block, iv)
+	mut cip1 := []u8{len: orig1.len}
+	mut cip2 := []u8{len: orig2.len}
+	en_cbc.encrypt_blocks(mut cip1, orig1)
+	en_cbc.encrypt_blocks(mut cip2, orig2)
+
+	mut block2 := aes.new_cipher(key)
+	mut dec_cbc := cipher.new_cbc(block2, iv)
+	mut plain1 := []u8{len: orig1.len}
+	mut plain2 := []u8{len: orig2.len}
+	dec_cbc.decrypt_blocks(mut plain1, cip1)
+	dec_cbc.decrypt_blocks(mut plain2, cip2)
+
+	assert plain1 == orig1
+	assert plain2 == orig2
+}
+
 fn test_aes_cbc() {
 	key := '6368616e676520746869732070617373'.bytes()
 	iv := '1234567890123456'.bytes()

vlib/crypto/cipher/cbc.v

@@ -122,8 +122,7 @@ pub fn (mut x Cbc) decrypt_blocks(mut dst []u8, src []u8) {
 	x.b.decrypt(mut (*dst)[start..end], src_chunk)
 	xor_bytes(mut (*dst)[start..end], (*dst)[start..end], x.iv)
 	// Set the new iv to the first block we copied earlier.
-	x.iv = x.tmp
-	x.tmp = x.iv
+	x.iv, x.tmp = x.tmp, x.iv
 }
 
 fn (mut x Cbc) set_iv(iv []u8) {