Words / Pair 37 of 41

You cannot check an image, however clear it is

It reads the same at a glance. Header line, wall of characters, footer line, all present and correct. One of them can be handed to your software and one of them can only be looked at.

The screen shows you the same thing

Signed text is a run of characters that stands or falls as a unit. Alter one letter, drop a line break, add a stray space inside the block, and the check fails. That strictness is what makes it useful. It is also why an image of that text is worthless for checking.

A picture keeps the appearance and discards the thing appearance was standing in for. What reaches you is a grid of coloured dots that resembles the characters. Your eye is satisfied and your software has nothing to work with.

What verification needs from you

The exchange only works if the exact characters survive the trip. Retyping a block out of a picture will not get you there, and even a perfect transcription would only hand you the same claim in a checkable wrapper. You would be verifying that whoever made the picture could copy characters, which was never in doubt.

Why a flat image is so easy to make

• No key is needed. The characters only have to look like the right sort of characters.
• Length hides everything. Nobody reads a wall of hexadecimal closely, and nobody can check it by eye anyway.
• A crop can remove the part that would have failed, or the date, or the line naming what is being agreed to.
• Images travel well in chats and forums, which is exactly where an unverifiable claim wants to be.

None of that makes every picture dishonest. Plenty are posted by people who screenshot everything out of habit. It makes every picture unverified, which is a different statement and the one you have to act on. If the sender holds the real block, sending it costs them a few seconds.

Cropping deserves a line of its own. A block has two boundary lines, a header and a footer, and an image can show one without the other. It can also show a block in full while leaving out the paragraph underneath which spelled out what was being agreed to. You cannot tell from the picture what was outside the frame, and the frame was chosen by the person sending it.

The near miss that goes the other way

There is a case worth holding in mind before you conclude anything harsh. Real signed text is fragile in transit. Chat clients wrap long lines, some strip trailing spaces, some replace straight quotes with curly ones, and any of that will break a check on text that was entirely genuine when it left the sender. A failed check is not the same as a forgery, and it is not evidence of bad intent.

Which is one of the reasons people fall back on pictures. Somebody has a block that keeps failing after being pasted around, so they photograph it instead, and the fragility that made the text awkward has now been solved by removing the only thing that made it useful. If a check fails, ask for the text again as a file or from another route before deciding anything about the sender.

What to do with each

If you treat a real block as a picture

You ask for something you already had and wait a little. The sender may be mildly put out. Nothing is lost that a message cannot recover.

If you treat a picture as a real block

You have accepted an unchecked claim while feeling you checked it, which is worse than not checking. Whatever address or instruction sat in the image now has your confidence behind it.

If it is a picture

Ask for the text and do nothing else until it arrives. Do not transcribe an address out of the image in the meantime, and do not treat the image as partial evidence.

If it is a block

Run the check, then go on to the harder question of which key signed it. A block that verifies against an unknown key is covered on the wrong key page.

The same reasoning applies to anything that reaches you as an image and asks for a decision. Pictures of pages, pictures of lists, pictures of confirmations. Related pairs sit on the full pair index, and the shape this site uses for all of them is explained under how the pairs work.

Questions readers send about this pair

Can I just type the block out of the screenshot?

You can, and the result is not evidence. Either it fails on a mistyped character, or it passes and tells you only that the characters in the image were consistent. Neither outcome tells you who wrote them.

The image is very high quality. Does that help?

Not at all. Quality is about how easily you can read the shapes. Verification is about the characters themselves, and an image contains none.