Trithemius Progressive Key Cipher

Explicit Trithemius arithmetic progression per ASCII letter, bounded modular shift/step and decode. Classical educational encoding only; not secure encryption, no hidden normalization or inferred key.

Description

Trithemius-style substitution advances a shift with character position.

Trithemius-style substitution advances a shift with character position.^[1]

Explicit start and progression values define this implementation's historical cipher variant. Use Trithemius Progressive Key Cipher to evaluate the supplied inputs under the specific model below.

When to use Trithemius Progressive Key Cipher

Use Trithemius Progressive Key Cipher 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 Trithemius Progressive Key Cipher and understand its results

Choose the intended encoding, alphabet or check and keep the exact character, key and measurement conventions described below.
Enter Text, Initial shift, Progression step, Decode using the field table and validation rules below, then run the tool.
Read Transformed 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
initialShift	Initial shift	number	Required; constraints below
progressionStep	Progression step	number	Required; constraints below
decode	Decode	boolean	Required; constraints below

Enter uppercase A–Z text of at most 4096 characters, integer initial and step shifts from 0 to 25 and a decode boolean. At zero-based position i use initial+i×step modulo 26, adding it for encoding and subtracting it for decoding. Empty text is accepted and punctuation is rejected. This is a historical educational transformation.

shift[i] = (initial+i×step) mod 26; output = text ± shift mod 26

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

Output field	Meaning	Value type
transformed	Transformed	string

json

{
  "input": {
    "text": "AAAA",
    "initialShift": 0,
    "progressionStep": 1,
    "decode": false
  },
  "output": {
    "transformed": "ABCD"
  }
}

Limitations and assumptions

Explicit start and progression values define this implementation's historical cipher variant. 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.

Alternative or Complementary methods

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

Use Autokey Cipher Tool for a related task: Plaintext autokey extends an initial key with earlier message letters.

References

Trithemius Progressive Key Cipher — tool.io

Similar or alternative tools

Variant Beaufort Cipher Tool
Variant Beaufort subtracts repeated key letters from plaintext letter indices.
Autokey Cipher Tool
Plaintext autokey extends an initial key with earlier message letters.