docs, obfuscation

ndk/obfuscate/obfuscate.h

@@ -0,0 +1,30 @@
+#ifndef OBFUSCATE_H
+#define OBFUSCATE_H
+
+#include <stdint.h>
+
+/* Payload uses lower 40 bits; upper 24 bits hold verification tag. */
+#define OBF_PAYLOAD_BITS 40u
+#define OBF_PAYLOAD_MASK ((1ULL << OBF_PAYLOAD_BITS) - 1ULL)
+
+/* Return codes */
+#define OBF_OK           0
+#define OBF_ERR_NULL    -1  /* out pointer is NULL */
+#define OBF_ERR_RANGE   -2  /* val exceeds encodable payload width */
+#define OBF_ERR_VERIFY  -3  /* tag mismatch (wrong seed or corrupted data) */
+
+/*
+ * Encode uint64_t payload (e.g. flag bits) using seed-derived keystream.
+ * seed is typically unix time in whole seconds shared by encoder and decoder.
+ *
+ * On success writes single uint64_t to *out and returns OBF_OK.
+ */
+int32_t encode(uint64_t val, uint64_t seed, uint64_t *out);
+
+/*
+ * Decode and verify. seed must match the value used at encode time.
+ * On success writes recovered payload to *out and returns OBF_OK.
+ */
+int32_t decode(uint64_t encoded_val, uint64_t seed, uint64_t *out);
+
+#endif /* OBFUSCATE_H */