How long must the key be?

Provide at least one key letter for each message letter. The key never repeats; a short key is refused. Case is retained in the message, while nonletters pass through without consuming a key letter.

Is a typed key a secure one-time pad?

Security requires a truly random secret key as long as the message, used only once and shared securely. This calculator checks arithmetic and length, not those conditions.

Can I decode without the key or use binary XOR?

No. The workflow needs your letter key and uses modulo-26 arithmetic. Missing-key cryptanalysis, binary XOR and automated pad generation are not implemented.

One-Time Pad Letter Calculator with Supplied Key

This calculator opens EQNVZ with the supplied key XMCKL, decoding to HELLO. Letters map to A=0 through Z=25: encoding adds the matching key letter modulo 26 and decoding subtracts it. Every message letter consumes one supplied key letter, with no repeating key. This is an arithmetic demonstration; it does not generate or verify randomness, manage secure keys, recover a missing key, or implement binary XOR.

Cipher text5 characters

Plain textOne-time pad, 5 of 5 key letters used

HELLO

decode A = 0 to Z = 25; each message letter plus one key letter, mod 26, and no key letter is used twice; case kept; everything else passes through and uses no key letter

letter	A	B	C	D	E	F	G	H	I
value	0	1	2	3	4	5	6	7	8
letter	J	K	L	M	N	O	P	Q	R
value	9	10	11	12	13	14	15	16	17
letter	S	T	U	V	W	X	Y	Z
value	18	19	20	21	22	23	24	25

E4 - X 23 = -19, -19 + 26 = 7, H
Q16 - M 12 = 4, E
N13 - C 2 = 11, L
V21 - K 10 = 11, L
Z25 - L 11 = 14, O

encode add the same key letters to encode

Key

Method: One-time pad, 5 of 5 key letters used

Letters shifted by the key: 5

Passed through: 0

Encode: add the same key letters to encode

Each letter is shifted by its own letter of a key you supply, and no key letter is used twice. This shows the arithmetic only: the tool does not make keys.

Textbook checks, one per method

Each row is worked by the same code as the stage above. Load puts it in the workbench with its keys.

Caesar, shift 3ABCXYZ
DEFABC
Keyword KRYPTOSKNOWLEDGE IS POWER
DGHVETPST BM IHVTL
Polybius 5 x 5, I and J share cell 24JAM
24 11 32, which decodes as IAM
A1Z26, numbers separated by spaces1 26
AZ
Nihilist, square ZEBRAS, key RUSSIANDYNAMITE WINTER PALACE
37 106 62 36 67 47 86 26 / 104 53 62 77 27 55 / 57 66 55 36 54 27
Book cipher, a 40-word reference text3 17 52
error: word 52 is outside the 40-word reference, and nothing is guessed
One-time pad, key XMCKLHELLO
EQNVZ

Puzzles and teaching, not secrets. These are historical and puzzle ciphers. Caesar has only 25 shifts on A to Z, so the ranked list that Decode shows reads most messages without the key, and keyword, Polybius and A1Z26 text gives way to letter counting. The one-time pad mode shows the arithmetic of a key you type and nothing more: this tool does not make keys and makes no claim that a message stays secret.

Accuracy. Each method follows the convention shown beside it: Caesar shifts count from 0 at the first letter of the alphabet and wrap after the last, the 5 x 5 Polybius and Nihilist squares give I and J one cell, A1Z26 reads 1 26 as AZ, and a book number outside your reference is an error, never a guess. The ranked list of Caesar shifts is an estimate from English letter frequency. These are historical and puzzle ciphers, not modern secure encryption, and the one-time pad mode only shows the arithmetic of the key you type, with no claim that it keeps a message secret.

Common questions

How long must the key be?: Provide at least one key letter for each message letter. The key never repeats; a short key is refused. Case is retained in the message, while nonletters pass through without consuming a key letter.
Is a typed key a secure one-time pad?: Security requires a truly random secret key as long as the message, used only once and shared securely. This calculator checks arithmetic and length, not those conditions.
Can I decode without the key or use binary XOR?: No. The workflow needs your letter key and uses modulo-26 arithmetic. Missing-key cryptanalysis, binary XOR and automated pad generation are not implemented.

Each method follows the convention shown beside it: Caesar shifts count from 0 at the first letter of the alphabet and wrap after the last, the 5 x 5 Polybius and Nihilist squares give I and J one cell, A1Z26 reads 1 26 as AZ, and a book number outside your reference is an error, never a guess. The ranked list of Caesar shifts is an estimate from English letter frequency. These are historical and puzzle ciphers, not modern secure encryption, and the one-time pad mode only shows the arithmetic of the key you type, with no claim that it keeps a message secret.