Autokey Cipher Tool

Apply plaintext Autokey explicit uppercase key convention. Classical educational encoding only; not secure encryption, no hidden normalization or inferred key.

Description

Plaintext autokey extends an initial key with earlier message letters.

Plaintext autokey extends an initial key with earlier message letters.^[1]

The initial key and message convention determine the historical substitution stream. Use Autokey Cipher Tool to evaluate the supplied inputs under the specific model below.

When to use Autokey Cipher Tool

Use Autokey Cipher Tool to evaluate the supplied inputs under the specific model below. Choose the intended encoding, alphabet or check and keep the exact character, key and measurement conventions described below.

How to use Autokey Cipher Tool and understand its results

Choose the intended encoding, alphabet or check and keep the exact character, key and measurement conventions described below.
Enter Text, Key using the field table and validation rules below, then run the tool.
Read Encoded and interpret units, signs, ordering and placeholders as explained below.
If the tool reports an input error, correct that field and rerun. A successful false result, empty match list or unknown-evidence placeholder has the specific meaning described here; check the limitations before applying the result.

Input field	Meaning	Value type	Requirement
text	Text	string	Required; constraints below
key	Key	string	Required; constraints below

Enter uppercase A–Z plaintext and a nonempty uppercase A–Z seed key, each at most 4096 characters. Extend the key with earlier plaintext after the seed and add letter values modulo 26. The tool encodes plaintext autokey only, accepts empty text and neither decodes nor uses ciphertext to extend the stream.

keyStream[i] = seed[i] if i<seedLength, otherwise plaintext[i−seedLength]; cipher=(plain+keyStream) mod 26

Field order, units and branch conditions follow the implementation explanation above.

Output field	Meaning	Value type
encoded	Encoded	string

json

{
  "input": {
    "text": "ABC",
    "key": "BCD"
  },
  "output": {
    "encoded": "BDF"
  }
}

Limitations and assumptions

The initial key and message convention determine the historical substitution stream. Apply the exact scope and assumptions described above.

All listed fields require explicit values. Unexpected fields, incorrect types, nonfinite numbers and sparse arrays fail validation. Domain-specific bounds, alignments and supported tokens are stated above; extremely large inputs or outputs can also be rejected. These errors differ from successful results such as false, no matches or unknown evidence.

This educational transformation or format check does not establish modern cryptographic confidentiality, authentication or key security.

Alternative or Complementary methods

Use Variant Beaufort Cipher Tool for a related task: Variant Beaufort subtracts repeated key letters from plaintext letter indices.

Use Trithemius Progressive Key Cipher for a related task: Trithemius-style substitution advances a shift with character position.

References

Autokey Cipher Tool — tool.io

Similar or alternative tools

Variant Beaufort Cipher Tool
Variant Beaufort subtracts repeated key letters from plaintext letter indices.
Trithemius Progressive Key Cipher
Trithemius-style substitution advances a shift with character position.