Add crypto_stream_salsa20_xor_ic() to set the initial counter value.

src/libsodium/crypto_stream/salsa20/amd64_xmm6/stream_salsa20_amd64_xmm6.S

@@ -33,19 +33,20 @@ mov  $0,%rax
 mov  %r9,%rcx
 rep stosb
 sub  %r9,%rdi
+movq $0,472(%rsp)
 jmp ._start
 
 .text
 .p2align 5
 
-.globl  crypto_stream_salsa20_xor
-.globl _crypto_stream_salsa20_xor
+.globl  crypto_stream_salsa20_xor_ic
+.globl _crypto_stream_salsa20_xor_ic
 #ifdef __ELF__
-.type  crypto_stream_salsa20_xor, @function
-.type _crypto_stream_salsa20_xor, @function
+.type  crypto_stream_salsa20_xor_ic, @function
+.type _crypto_stream_salsa20_xor_ic, @function
 #endif
-crypto_stream_salsa20_xor:
-_crypto_stream_salsa20_xor:
+crypto_stream_salsa20_xor_ic:
+_crypto_stream_salsa20_xor_ic:
 
 mov %rsp,%r11
 and $31,%r11
@@ -60,9 +61,10 @@ movq %rbx,456(%rsp)
 movq %rbp,464(%rsp)
 mov  %rdi,%rdi
 mov  %rsi,%rsi
+mov  %r9,%r10
+movq %r8,472(%rsp)
 mov  %rdx,%r9
 mov  %rcx,%rdx
-mov  %r8,%r10
 cmp  $0,%r9
 jbe ._done
 
@@ -75,17 +77,16 @@ movl %ecx,64(%rsp)
 movl %r8d,4+64(%rsp)
 movl %eax,8+64(%rsp)
 movl %r11d,12+64(%rsp)
-mov  $0,%rcx
 movl   24(%r10),%r8d
 movl   4(%r10),%eax
 movl   4(%rdx),%edx
-movq %rcx,472(%rsp)
+movq 472(%rsp),%rcx
 movl %ecx,80(%rsp)
 movl %r8d,4+80(%rsp)
 movl %eax,8+80(%rsp)
 movl %edx,12+80(%rsp)
 movl   12(%r10),%edx
-mov  $0,%rcx
+shr  $32,%rcx
 movl   28(%r10),%r8d
 movl   8(%r10),%eax
 movl %edx,96(%rsp)

src/libsodium/crypto_stream/salsa20/ref/xor_salsa20_ref.c

@@ -4,6 +4,8 @@ D. J. Bernstein
 Public domain.
 */
 
+#include <stdint.h>
+
 #include "api.h"
 #include "crypto_core_salsa20.h"
 #include "utils.h"
@@ -16,10 +18,10 @@ static const unsigned char sigma[16] = {
     'e', 'x', 'p', 'a', 'n', 'd', ' ', '3', '2', '-', 'b', 'y', 't', 'e', ' ', 'k'
 };
 
-int crypto_stream_xor(
+int crypto_stream_salsa20_xor_ic(
         unsigned char *c,
   const unsigned char *m,unsigned long long mlen,
-  const unsigned char *n,
+  const unsigned char *n, uint64_t ic,
   const unsigned char *k
 )
 {
@@ -33,7 +35,10 @@ int crypto_stream_xor(
 
   for (i = 0;i < 32;++i) kcopy[i] = k[i];
   for (i = 0;i < 8;++i) in[i] = n[i];
-  for (i = 8;i < 16;++i) in[i] = 0;
+  for (i = 8;i < 16;++i) {
+      in[i] = (unsigned char) (ic & 0xff);
+      ic >>= 8;
+  }
 
   while (mlen >= 64) {
     crypto_core_salsa20(block,in,kcopy,sigma);

src/libsodium/crypto_stream/salsa20/stream_salsa20_api.c

@@ -9,3 +9,11 @@ size_t
 crypto_stream_salsa20_noncebytes(void) {
     return crypto_stream_salsa20_NONCEBYTES;
 }
+
+int
+crypto_stream_salsa20_xor(unsigned char *c, const unsigned char *m,
+                          unsigned long long mlen, const unsigned char *n,
+                          const unsigned char *k)
+{
+    return crypto_stream_salsa20_xor_ic(c, m, mlen, n, 0U, k);
+}

src/libsodium/include/sodium/crypto_stream_salsa20.h

@@ -10,6 +10,7 @@
  */
 
 #include <stddef.h>
+#include <stdint.h>
 #include "export.h"
 
 #ifdef __cplusplus
@@ -36,6 +37,11 @@ int crypto_stream_salsa20_xor(unsigned char *c, const unsigned char *m,
                               unsigned long long mlen, const unsigned char *n,
                               const unsigned char *k);
 
+SODIUM_EXPORT
+int crypto_stream_salsa20_xor_ic(unsigned char *c, const unsigned char *m,
+                                 unsigned long long mlen,
+                                 const unsigned char *n, uint64_t ic,
+                                 const unsigned char *k);
 #ifdef __cplusplus
 }
 #endif