#SHA-256 Checksums

sha256.c / sha256.h provide a pure C, streaming SHA-256 implementation conforming to FIPS 180-4. The module is used for post-capture integrity verification and for the hash / verify CLI commands.

#Design Constraints

• Pure C89, no C++ and no MFC dependency – the file must be compiled without the precompiled header (StdAfx.h).
• No heap allocation: the SHA256_CTX context is caller-allocated.
• Compatible with both MSVC 6.0 and MSVC 2017+.
• All symbols are wrapped in extern "C" to allow calling from C++ files.

#SHA256_CTX Structure

typedef struct {
    unsigned char data[64];      /* current 64-byte input block */
    unsigned int  datalen;       /* number of bytes in current block */
    unsigned int  bitlen[2];     /* total message length in bits (64-bit as two 32-bit) */
    unsigned int  state[8];      /* running hash state (H0..H7) */
} SHA256_CTX;

The context is designed to be stack-allocated or embedded as a member variable. No dynamic memory is used.

#API

void sha256_init  (SHA256_CTX *ctx);
void sha256_update(SHA256_CTX *ctx, const unsigned char *data,
                   unsigned int len);
void sha256_final (SHA256_CTX *ctx, unsigned char hash[32]);
void sha256_hex   (const unsigned char hash[32], char out[65]);

The typical usage pattern is to stream file data in 64 KB blocks via repeated sha256_update() calls.

#Post-capture Integration

After every successful capture file is finalized, CDV::CapturingThread() calls ComputeFileSHA256() (in DShow.cpp) to hash the completed AVI file. ComputeFileSHA256() reads the file in 64 KB blocks and feeds them through the streaming SHA-256 API. On success it calls WriteSHA256Sidecar() to write a sha256sum-compatible sidecar file (.sha256) next to the AVI:

<64-hex-hash> *<bare-filename>

The computed hash is also stored in CaptureStats::szSHA256 and written to the CSV capture log.

SHA-256 computation is controlled by CDV::m_enableSHA256 (default: true), toggled via the IDC_CHK_SHA256 checkbox in the Capture Config dialog (IDD_CAPTURE_CONFIG).

#Build System Integration

#CMake

The CMake project file declares LANGUAGES C CXX to support sha256.c:

project(WinDV VERSION 1.6.0 LANGUAGES C CXX)

sha256.c is explicitly excluded from the precompiled header:

set_source_files_properties(sha256.c PROPERTIES SKIP_PRECOMPILE_HEADERS ON)

#VC6

In the VC6 .dsp file, sha256.c has the /Y- flag (# SUBTRACT CPP /YX /Yc /Yu) to disable precompiled header processing.

#CLI Commands

The hash and verify CLI commands expose the same functionality for off-line use:

#hash

WinDV hash [-q] <file> [file2 ...]

For each file, ComputeFileSHA256() hashes the file and prints <hash> <filename> to stdout in sha256sum format. Unless -q (quiet) is given, a .sha256 sidecar file is written next to each input file.

Exit code: 0 if all files were hashed successfully, non-zero if any file could not be read.

#verify

WinDV verify <file> [file2 ...]

For each file, VerifyFileSHA256() reads the .sha256 sidecar, hashes the file, and compares the results. Prints OK or FAILED per file.

Return codes per file (accumulated into the process exit code):

#Sidecar File Format

The .sha256 sidecar file uses the standard sha256sum format:

a1b2c3d4...  *filename.avi

• 64 lowercase hexadecimal characters (the SHA-256 hash)
• Two spaces
• * prefix (binary mode indicator, per sha256sum convention)
• Bare filename (no path components)

This format is compatible with the sha256sum -c verification command available on Linux/macOS and via Git for Windows.