An armoured block whose markers say only MESSAGE, which is the vaguest label the format offers.
The shape
The container is the standard one. What sets this member apart is subtraction: the marker line carries no qualifying word, so there is nothing in it to describe the payload.
- Opening marker
-----BEGIN PGP MESSAGE-----, five hyphens either side- Qualifier
- none; no
KEY, noSIGNED, noPRIVATE - Fields
- often none at all, so the empty line can be the second line of the block
- Body
- base64, fixed line width, no spaces
- Length
- anything from a few lines to thousands
- Closing marker
-----END PGP MESSAGE-----
-----BEGIN PGP MESSAGE-----
[body: 18 lines of base64 characters, removed here]
=xxxx
-----END PGP MESSAGE-----One label, several payloads
The reason this card exists is that a single marker name is reused for contents that have almost nothing to do with each other. The table below is the point of the page.
| What the body may hold | What the outside shows |
|---|---|
| Data encrypted to one or more keys | nothing in the marker line changes |
| Data signed in binary form instead of clear signed | nothing in the marker line changes |
| Data compressed and not protected at all | nothing in the marker line changes |
| Several of those wrapped around one another | nothing in the marker line changes |
So the word MESSAGE is a statement about the fence, not about the field inside it. A reader who treats the marker as a promise of encryption has read a label that was never making that promise.
One consequence follows immediately. If somebody says they are sending you an encrypted note and what arrives opens with this marker, the marker agrees with them only in the weakest sense. It agrees that they sent an armoured block.
Compare this with the rest of the family and the design becomes clear. A key block says which half of a key it holds. A signed message puts its readable part on display. This block declines to describe itself, and that is not an oversight in the format; the container is deliberately incurious about what it is carrying, because the same wrapper had to serve every case.
Length, in lines
Because the label withholds so much, people reach for the next available signal, which is size. It is worth setting out what size can and cannot indicate here.
Every block of this kind carries structural material as well as content, so even a very short message produces a block of several lines. That fixed cost sets a floor: there is a minimum size below which these blocks do not go, whatever they hold. Above the floor, length grows with the payload, but not in any proportion a reader can work backwards from, because compression may have shrunk the content before it was encoded and because encoding to more than one recipient adds material of its own.
The practical reading is narrow. A block of five or six lines is carrying something small. A block of two thousand lines is carrying something large. Between those two statements there is nothing further to extract, and two blocks of similar length are not thereby similar in any other way.
How to check it by eye
A short pass over a MESSAGE block
- Read the marker word and confirm it stands alone.
MESSAGE, with nothing in front of it. - Read the closing marker for the same word. Mixed markers mean text from two places.
- Look at where the empty line falls. With no fields present it sits directly under the opening marker, which is normal here and unusual elsewhere.
- Sight down the right hand edge of the body for the even column that a fixed line width produces.
- Notice the length, then set that impression aside. Size is not a category.
What a wrong shape means
The failures are the family failures, and they arrive here more often than elsewhere because these blocks are pasted into conversations rather than saved to files. Line breaks disappear and the body becomes one enormous line. A reply adds a symbol to the front of every line. A message box truncates the middle and joins the halves without comment.
A distinct case: a block that opens with BEGIN PGP MESSAGE when you were expecting a key. That is not damage, it is a different object, and the reasonable reading is that the wrong thing was copied rather than that the sender was trying something.
Where the block was cut short, the giveaway is usually the tail. The checksum line and the footer are the last things to arrive, so their absence is the first sign of a length limit.
What a correct shape does not prove
The list here is longer than on the other cards, because the label withholds so much by design.
- Not that the body is encrypted. That is exactly the ambiguity the shared marker introduces.
- Not that it was encrypted to a key you hold, or to any key you have ever seen.
- Not that it can be opened at all, by you or by anybody.
- Not that it is whole. A body missing lines from the middle keeps its markers and its column.
- Not who assembled it, and not when.
- Not that the surrounding message describes it accurately. Text above a block is not part of the block.
Of all the shapes on this site, this one has the widest gap between what the label says and what the label rules out.
What this card is not
The form that keeps its message readable is at a clear signed message, and the form that carries a signature and nothing else is at a detached signature block. The way a block arrives flattened into one line is catalogued at a block that lost its line breaks.