des_undes/validation.md

Validation

The reconstructed sources were checked in two ways.

1. Header/API syntax check

des.c, undes.c and the common support compile syntactically against the actual crypt.h from the supplied cryptl99.zip. There are no remaining placeholder CRYPTLIB declarations.

2. Full CRYPTLIB round-trip on a modern compiler

For a behavioural sanity check, the supplied CRYPTLIB 0.99 source and libdes 3.14 source were compiled with GCC 14.2 together with the reconstruction.

A binary test file was then processed as:

plain.bin -> des_v2 -> encrypted.bin -> undes_v2 -> roundtrip.bin

plain.bin and roundtrip.bin compare byte-for-byte equal.

The generated single-block encrypted file has the expected size:

4       version
4       IV
256     password-check block
16384   encrypted data block
-----
16648 bytes total

The prefix begins with 75 27 00 00, i.e. file version 0x00002775 in the little-endian format used by the DOS executable.

A second test with an incorrect password exits with status 1 and prints the same executable string:

Password is not correct, no decryption possible

Scope of this validation

This confirms that the reconstructed control flow is internally consistent with the supplied CRYPTLIB source. It does not prove that a modern GCC build produces ciphertext byte-for-byte identical to the original Turbo C build. The strongest compatibility test would be to decrypt an actual file created by the 1998 des.exe, or run the original executable in a DOS environment and compare outputs.