When reviewing a prosecutor’s evidence file, there are moments when a lawyer comes across a piece of evidence so powerful that the immediate reaction is simple: This case is over.

A mobile phone message that captures the crime itself. CCTV footage clearly recording the suspect’s movements. A handwritten note containing what appears to be a confession.

Yet any lawyer who has handled criminal cases long enough has witnessed something strikingly different — that very same “decisive evidence” disappearing entirely inside the courtroom, rendered incapable of supporting guilt by even a single word.

Because in criminal proceedings, the outcome is often determined not by what actually happened, but by a far more technical question:

Can what happened be proven through evidence that is legally admissible?

No matter how obvious the truth may appear outside the courtroom, inside the courtroom only facts proven beyond reasonable doubt through evidence lawfully obtained and sufficiently reliable can serve as the foundation for conviction.

This is precisely where one of criminal law’s oldest principles originates:

When doubt exists, it must benefit the defendant.

For this reason, the mere existence of evidence is never enough.

The first question is always not whether evidence exists, but how that evidence was obtained.

Nearly twenty years ago, in its landmark 2007 en banc decision (Supreme Court Case No. 2007Do3061), the Supreme Court of Korea declared that evidence collected in violation of procedures mandated by the Constitution and the Criminal Procedure Act cannot, in principle, be used as evidence of guilt. The same rule applies equally to secondary evidence subsequently derived from that unlawfully obtained evidence.

Even physical evidence — such as seized property whose form itself remains unchanged — may lose admissibility if procedural violations are serious enough to fundamentally undermine the constitutional principle requiring judicial warrants.

That is precisely why Article 308-2 of the Korean Criminal Procedure Act expressly provides:

“Evidence obtained through procedures not conducted in accordance with due process shall not be admissible as evidence.”

The rise of digital evidence has introduced an entirely new layer of complexity.

Mobile phone forensic data, messenger conversations, and documents extracted from computers are now among the most powerful forms of evidence — yet paradoxically, they are also among the most legally fragile.

A single altered word can entirely change meaning. Throughout the copying, extraction, and conversion process, the constant risk remains that the output may no longer perfectly match the original source.

Recognizing this, in its July 26, 2013 Supreme Court decision (2013Do2511), commonly known as the Wangjaesan Case, the Court ruled that documents extracted from digital storage devices may only be admitted as evidence where the identity between the original data and the extracted output can be established.

To satisfy this standard, the prosecution must prove integrity — that the original data remained unchanged from the moment of seizure through final extraction — while also demonstrating the reliability of the tools and procedures used during duplication and forensic analysis.

The Supreme Court expanded this principle even further in its November 18, 2021 en banc decision (2016Do348).

The Court held that even when investigators lawfully receive voluntary submission of digital storage devices such as mobile phones containing vast amounts of personal information, authorities must selectively seize only electronic information directly related to the alleged offense.

Moreover, throughout the process of searching, copying, and extracting data, the individual subject to seizure must be guaranteed the right to participate.

In other words, before digital evidence can even be evaluated for whether its contents are truthful, it must first prove that it was lawfully secured in its original and unaltered state.

And this principle continues to evolve.

Most recently, in its February 26, 2026 decision (2025Do4422), the Supreme Court ruled that seizing items not specifically identified within the scope of a search and seizure warrant constitutes an unlawful seizure.

The Court further held that such illegality cannot later be cured simply because investigators subsequently obtain another warrant, or because the defendant and defense counsel later consent to the evidence being used in court.

Of particular significance, the Court held that seizure of recorded telephone conversations exchanged between a suspect and defense counsel directly implicates the constitutional right to counsel — one of the most fundamental components of the right to defense — and is therefore unlawful in principle.

The Court also concluded that investigative reports subsequently created based upon that unlawfully obtained material, as secondary derivative evidence, were likewise inadmissible and therefore incapable of supporting conviction.

Evidence that initially appeared utterly decisive was excluded from court in its entirety because of procedural illegality.

For anyone facing criminal proceedings, the first question should never be merely:

What did I do?

The more important question is this:

By what legal procedure was the truth collected, and how can that truth ultimately be proven?

Truth exists at the scene of an incident.

But guilt and innocence are determined somewhere else entirely —

by whether that truth survives as admissible evidence inside the courtroom.

Reducing the number of moments when decisive evidence disappears in court.

Or conversely, restoring overlooked truth and bringing it back to life through admissible evidence.

I believe that somewhere in the space between those two tasks lies the true role of a criminal defense lawyer.

– Kim Min-hoo
Attorney, Suhn Law Group