aes: Implement AES-128-CBC decryption function

Implement a compatible AES-128-CBC decryption function as a counterpart
of the encryption function pulled from tegra20-common/crypto.c .

Signed-off-by: Marek Vasut <marex@denx.de>

include/aes.h

@@ -63,4 +63,14 @@ void aes_decrypt(u8 *in, u8 *expkey, u8 *out);
  */
 void aes_cbc_encrypt_blocks(u8 *key_exp, u8 *src, u8 *dst, u32 num_aes_blocks);
 
+/**
+ * Decrypt multiple blocks of data with AES CBC.
+ *
+ * @key_exp		Expanded key to use
+ * @src			Source data to decrypt
+ * @dst			Destination buffer
+ * @num_aes_blocks	Number of AES blocks to decrypt
+ */
+void aes_cbc_decrypt_blocks(u8 *key_exp, u8 *src, u8 *dst, u32 num_aes_blocks);
+
 #endif /* _AES_REF_H_ */

lib/aes.c

@@ -632,3 +632,31 @@ void aes_cbc_encrypt_blocks(u8 *key_exp, u8 *src, u8 *dst, u32 num_aes_blocks)
 		dst += AES_KEY_LENGTH;
 	}
 }
+
+void aes_cbc_decrypt_blocks(u8 *key_exp, u8 *src, u8 *dst, u32 num_aes_blocks)
+{
+	u8 tmp_data[AES_KEY_LENGTH], tmp_block[AES_KEY_LENGTH];
+	/* Convenient array of 0's for IV */
+	u8 cbc_chain_data[AES_KEY_LENGTH] = { 0 };
+	u32 i;
+
+	for (i = 0; i < num_aes_blocks; i++) {
+		debug("encrypt_object: block %d of %d\n", i, num_aes_blocks);
+		debug_print_vector("AES Src", AES_KEY_LENGTH, src);
+
+		memcpy(tmp_block, src, AES_KEY_LENGTH);
+
+		/* Decrypt the AES block */
+		aes_decrypt(src, key_exp, tmp_data);
+		debug_print_vector("AES Xor", AES_KEY_LENGTH, tmp_data);
+
+		/* Apply the chain data */
+		apply_cbc_chain_data(cbc_chain_data, tmp_data, dst);
+		debug_print_vector("AES Dst", AES_KEY_LENGTH, dst);
+
+		/* Update pointers for next loop. */
+		memcpy(cbc_chain_data, tmp_block, AES_KEY_LENGTH);
+		src += AES_KEY_LENGTH;
+		dst += AES_KEY_LENGTH;
+	}
+}