des: add possibility to calculate DES-CBC-MAC with small buffer

This patch adds the possibility to calculate the DES-CBC-MAC of a
source buffer (i.e. the last block of the buffer encrypted in CBC
mode) without having to allocate a destination buffer that is as
long as the complete source buffer, but instead only 8 bytes
for the MAC.
Signed-off-by: David Goldwich <david.goldwich@gmail.com>
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavutil/des.c

@@ -298,7 +298,7 @@ int av_des_init(AVDES *d, const uint8_t *key, int key_bits, int decrypt) {
     return 0;
 }
 
-void av_des_crypt(AVDES *d, uint8_t *dst, const uint8_t *src, int count, uint8_t *iv, int decrypt) {
+static void av_des_crypt_mac(AVDES *d, uint8_t *dst, const uint8_t *src, int count, uint8_t *iv, int decrypt, int mac) {
     uint64_t iv_val = iv ? AV_RB64(iv) : 0;
     while (count-- > 0) {
         uint64_t dst_val;
@@ -321,12 +321,21 @@ void av_des_crypt(AVDES *d, uint8_t *dst, const uint8_t *src, int count, uint8_t
         }
         AV_WB64(dst, dst_val);
         src += 8;
-        dst += 8;
+        if (!mac)
+            dst += 8;
     }
     if (iv)
         AV_WB64(iv, iv_val);
 }
 
+void av_des_crypt(AVDES *d, uint8_t *dst, const uint8_t *src, int count, uint8_t *iv, int decrypt) {
+    av_des_crypt_mac(d, dst, src, count, iv, decrypt, 0);
+}
+
+void av_des_mac(AVDES *d, uint8_t *dst, const uint8_t *src, int count) {
+    av_des_crypt_mac(d, dst, src, count, (uint8_t[8]){0}, 0, 1);
+}
+
 #ifdef TEST
 #undef printf
 #undef rand

libavutil/des.h

@@ -33,7 +33,7 @@ struct AVDES {
  * @brief Initializes an AVDES context.
  *
  * @param key_bits must be 64 or 192
- * @param decrypt 0 for encryption, 1 for decryption
+ * @param decrypt 0 for encryption/CBC-MAC, 1 for decryption
  */
 int av_des_init(struct AVDES *d, const uint8_t *key, int key_bits, int decrypt);
 
@@ -49,4 +49,13 @@ int av_des_init(struct AVDES *d, const uint8_t *key, int key_bits, int decrypt);
  */
 void av_des_crypt(struct AVDES *d, uint8_t *dst, const uint8_t *src, int count, uint8_t *iv, int decrypt);
 
+/**
+ * @brief Calculates CBC-MAC using the DES algorithm.
+ *
+ * @param count number of 8 byte blocks
+ * @param dst destination array, can be equal to src, must be 8-byte aligned
+ * @param src source array, can be equal to dst, must be 8-byte aligned, may be NULL
+ */
+void av_des_mac(struct AVDES *d, uint8_t *dst, const uint8_t *src, int count);
+
 #endif /* AVUTIL_DES_H */