Fix ABI break (revert to original behaviour)

Signed-off-by: Dave Rodgman <[email protected]>

Author: daverodgman

tf-psa-crypto/drivers/builtin/src/chacha20.c

@@ -366,6 +366,9 @@ static void chacha20_block(const uint32_t initial_state[16],
 void mbedtls_chacha20_init(mbedtls_chacha20_context *ctx)
 {
     mbedtls_platform_zeroize(ctx, sizeof(mbedtls_chacha20_context));
+
+    /* Initially, there's no keystream bytes available */
+    ctx->keystream_bytes_used = CHACHA20_BLOCK_SIZE_BYTES;
 }
 
 void mbedtls_chacha20_free(mbedtls_chacha20_context *ctx)
@@ -418,7 +421,7 @@ int mbedtls_chacha20_starts(mbedtls_chacha20_context *ctx,
     }
 
     /* Initially, there's no keystream bytes available */
-    ctx->keystream_bytes_used = 0U;
+    ctx->keystream_bytes_used = CHACHA20_BLOCK_SIZE_BYTES;
 
     return 0;
 }
@@ -431,11 +434,10 @@ int mbedtls_chacha20_update(mbedtls_chacha20_context *ctx,
     size_t offset = 0U;
 
     /* Use leftover keystream bytes, if available */
-    while (size > 0U && ctx->keystream_bytes_used > 0U &&
-           ctx->keystream_bytes_used < CHACHA20_BLOCK_SIZE_BYTES) {
+    while (size > 0U && ctx->keystream_bytes_used < CHACHA20_BLOCK_SIZE_BYTES) {
         output[offset] = input[offset] ^ ctx->keystream8[ctx->keystream_bytes_used];
 
-        ctx->keystream_bytes_used = (ctx->keystream_bytes_used + 1) % CHACHA20_BLOCK_SIZE_BYTES;
+        ctx->keystream_bytes_used++;
         offset++;
         size--;
     }