International Teleprinter Code (also known as Baudot-Murray Code) enables messages to be sent as a series of electrical impulses.  Each letter of the alphabet is represented as a 5 bit code comprised of impulses or the absence of impulses (written at Bletchley Park as X and , respectively).

### Figure 1. Teleprinter’s produced messages on punched paper tape (as shown in Figure 1) where an impulse caused a hole to be made in the paper and the absence of an impulse left the paper intact. A teleprinter uses the entire alphabet, but it can also be put into “figure shift” so that figures such as numbers, dashes and colons can be produced (see full Teleprinter Alphabet below).

When adding two characters of teleprinter code, the following rules apply:

1. If the symbols are the same then it is “•”
2. If the symbols are different then it is “x”
 x + x = Same = • x + • = Different = x • + x = Different = x • + • = Same = •

Example 1 shows that if you have a character (e.g. M) and add a character(e.g. Q), this will give you a new character (e.g. J).

### Example 1.

 M Q J • + x = x • + x = x x + x = • x + • = x x + x = •

If you then take the new character (e.g. J) and add the same character (e.g. Q ) again, this cancels out the first addition leaving the original character (e.g. M); as shown in Example 2.

### Example 2.

 M Q J Q M • + x = x + x = • • + x = x + x = • x + x = • + x = x x + • = x + • = x x + x = • + x = x

See the Lorenz / Tunny Addition Square below for further information.

## Lorenz

The Lorenz is an additive cipher.  When a message is sent using Lorenz, the sending Lorenz adds a key to the plaintext of the message to produce the cipher text.

When the message is received, the receiving Lorenz adds the same key to the cipher text, which cancels out the first key leaving the original plain text message.

 Plain Text + Key = Cipher Text + Key = Plain Text

As shown in example 2:

 M + Q = J + Q = M

See Enciphering with Lorenz for further detail.

## Teleprinter Alphabet

 CONVENTIONAL IMPULSE MEANING NAME 1 2 3 4 5 IN LETTER SHIFT IN FIGURE SHIFT / • • • • • (no meaning) 9 • • x • • space space H • • x • x H £ T • • • • x T 5 O • • • x x O 9 M • • x x x M full stop N • • x x x N comma 3 • • • x • carriage return carriage return R • x • x • R 4 C • x x x • C colon V • x x x x V equals G • x • x x G @ L • x • • x L close bracket P • x x • x P 0 (zero) I • x x • x I 8 4 • x • • • line feed line feed A x x • • • A dash U x x x • • U 7 Q x x x • x Q 1 W x x • • x W 2 5 or + x x • x x move to FIG shift (none) 8 or – x x x x x (none) move to LET shift K x x x x • K open bracket J x x • x • J ring bell D x • • x • D who are you? F x • x x • F per cent X x • x x x X / B x • • x x B ? Z x • • • x Z + Y x • x • x Y 6 S x • x • • S apostrophe E x • • • • E 3

The original of the teleprinter alphabet can be seen on Page 3 of the General Report on Tunny, 