QR code resolution

The Cool Maze QR code is a dynamically generated grid of 25x25 “modules”.

A QR code consisting of 25x25 modules

It contains a secret key that needs to be sufficiently long to guarantee privacy, but we also want the encoded data to be as short as possible, to improve the scanning experience. Let’s examine the different possible QR code configurations.

For each possible resolution and each error correction level, we find the maximum number of characters we can encode (hover for the details):

Error correction level
L M Q H
(low) (medium) (quartile) (high)
Dimensions Version 1 21x21 17 14 ✖ ✖
Version 2 25x25 32 26 20 14
Version 3 29x29 53 42 32 24
Version 4 33x33 78 62 46 34
Select an entry in the table to view the details
This QR code contains the 17 characters cmaz.io/#a/Ck5E6Y, including a secret key of length 6 having 6*6 = 36 bits of entropy. It fits in a grid of 21x21 modules, with the error correction level L.
However, 36 bits of entropy is insufficient to guarantee privacy in our end-to-end encryption system.
QR code with error correction level L, dimensions 21x21, encoding 17 characters
This QR code contains the 14 characters cmaz.io/#a/1FY, including a secret key of length 3 having 3*6 = 18 bits of entropy. It fits in a grid of 21x21 modules, with the error correction level M.
However, 18 bits of entropy is insufficient to guarantee privacy in our end-to-end encryption system.
QR code with error correction level M, dimensions 21x21, encoding 14 characters
This QR code contains the 32 characters cmaz.io/#a/0gSAQU782WmGkzo9z1-zk, including a secret key of length 21 having 21*6 = 126 bits of entropy. It fits in a grid of 25x25 modules, with the error correction level L.QR code with error correction level L, dimensions 25x25, encoding 32 characters
This QR code contains the 26 characters cmaz.io/#a/qqLhdWY6Ui-rV4j, including a secret key of length 15 having 15*6 = 90 bits of entropy. It fits in a grid of 25x25 modules, with the error correction level M.QR code with error correction level M, dimensions 25x25, encoding 26 characters
This QR code contains the 20 characters cmaz.io/#a/1guowGZar, including a secret key of length 9 having 9*6 = 54 bits of entropy. It fits in a grid of 25x25 modules, with the error correction level Q.
However, 54 bits of entropy is insufficient to guarantee privacy in our end-to-end encryption system.
QR code with error correction level Q, dimensions 25x25, encoding 20 characters
This QR code contains the 14 characters cmaz.io/#a/0sn, including a secret key of length 3 having 3*6 = 18 bits of entropy. It fits in a grid of 25x25 modules, with the error correction level H.
However, 18 bits of entropy is insufficient to guarantee privacy in our end-to-end encryption system.
QR code with error correction level H, dimensions 25x25, encoding 14 characters
This QR code contains the 53 characters cmaz.io/#a/mqtiqwrfl71FLPjgbBnlf9FPSGPX.ClNUzy9PM9x4L, including a secret key of length 42 having 42*6 = 252 bits of entropy. It fits in a grid of 29x29 modules, with the error correction level L.
However, a grid of 29x29 modules (Version 3) is a bit slower to scan than a grid of 25x25 (Version 2).
QR code with error correction level L, dimensions 29x29, encoding 53 characters
This QR code contains the 42 characters cmaz.io/#a/suI4gDVCLZq3Nqqq.gyOG7lPc3BL1ae, including a secret key of length 31 having 31*6 = 186 bits of entropy. It fits in a grid of 29x29 modules, with the error correction level M.
However, a grid of 29x29 modules (Version 3) is a bit slower to scan than a grid of 25x25 (Version 2).
QR code with error correction level M, dimensions 29x29, encoding 42 characters
This QR code contains the 32 characters cmaz.io/#a/p8DA-7tSjlpxFRWcEqidQ, including a secret key of length 21 having 21*6 = 126 bits of entropy. It fits in a grid of 29x29 modules, with the error correction level Q.
However, a grid of 29x29 modules (Version 3) is a bit slower to scan than a grid of 25x25 (Version 2).
QR code with error correction level Q, dimensions 29x29, encoding 32 characters
This QR code contains the 24 characters cmaz.io/#a/y.kDtd6mtJYHh, including a secret key of length 13 having 13*6 = 78 bits of entropy. It fits in a grid of 29x29 modules, with the error correction level H.
However, a grid of 29x29 modules (Version 3) is a bit slower to scan than a grid of 25x25 (Version 2).
QR code with error correction level H, dimensions 29x29, encoding 24 characters
This QR code contains the 78 characters cmaz.io/#a/qH4I6WTZHcxQvEHoud6.nH9BBNOIDo33Ayd-5wzHtQrBDhtJ-FupxbMuGxeAuFJcbHl, including a secret key of length 67 having 67*6 = 402 bits of entropy. It fits in a grid of 33x33 modules, with the error correction level L.
However, a grid of 33x33 modules (Version 4) is slower to scan than a grid of 25x25 (Version 2).
QR code with error correction level L, dimensions 33x33, encoding 78 characters
This QR code contains the 62 characters cmaz.io/#a/sUlpZO1CglZMeFqdoGhhQgbLnX9zB8hwcdnmJh6tbVBzKZDoR.3, including a secret key of length 51 having 51*6 = 306 bits of entropy. It fits in a grid of 33x33 modules, with the error correction level M.
However, a grid of 33x33 modules (Version 4) is slower to scan than a grid of 25x25 (Version 2).
QR code with error correction level M, dimensions 33x33, encoding 62 characters
This QR code contains the 46 characters cmaz.io/#a/SM1kVy7b1VwK1obRjip7llF1n8sK3442799, including a secret key of length 35 having 35*6 = 210 bits of entropy. It fits in a grid of 33x33 modules, with the error correction level Q.
However, a grid of 33x33 modules (Version 4) is slower to scan than a grid of 25x25 (Version 2).
QR code with error correction level Q, dimensions 33x33, encoding 46 characters
This QR code contains the 34 characters cmaz.io/#a/spgwo6XTZZKNKcrx1L8onVH, including a secret key of length 23 having 23*6 = 138 bits of entropy. It fits in a grid of 33x33 modules, with the error correction level H.
However, a grid of 33x33 modules (Version 4) is slower to scan than a grid of 25x25 (Version 2).
QR code with error correction level H, dimensions 33x33, encoding 34 characters
✖ With the error correction level Q, it is not possible to fit the prefix cmaz.io/#a/ plus a secret key, in a grid of 21x21 modules.
✖ With the error correction level H, it is not possible to fit the prefix cmaz.io/#a/ plus a secret key, in a grid of 21x21 modules.

Our sweet spot is the 25x25 grid with the low level of error correction (L). The QR code displayed is never “damaged”, except for the small red icon in the center, which covers 4% of the grid area. The level L offers a correction rate of 7%, which is sufficient in this case. We encode a string of 32 characters, including 21 characters for the secret key. The secret key alphabet has 64 letters, which amounts to an entropy of 6 bits per character. Our secret key has a total entropy of 126 bits and fits nicely in a convenient 25x25 “Version 2” QR code.