Gronsfeld Cipher Tool

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

Description

Gronsfeld substitution shifts letters using a repeated decimal-digit key.

Gronsfeld substitution shifts letters using a repeated decimal-digit key.^[1]

Its limited historical key alphabet does not provide modern confidentiality. Use Gronsfeld Cipher Tool to evaluate the supplied inputs under the specific model below.

When to use Gronsfeld Cipher Tool

Use Gronsfeld 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 Gronsfeld 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 text and a nonempty ASCII digit key, each no longer than 4096 characters. Repeat key digits over text positions and add each digit shift modulo 26. Empty text is accepted; the tool only encodes and supplies no automatic punctuation filtering.

cipher[i] = (letterValue[i]+digitKey[i mod keyLength]) 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": "123"
  },
  "output": {
    "encoded": "BDF"
  }
}

Limitations and assumptions

Its limited historical key alphabet does not provide modern confidentiality. 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 Porta Cipher Tool for a related task: Porta substitution pairs key letters and swaps between alphabet halves.

References

Gronsfeld Cipher Tool — tool.io

Similar or alternative tools

Variant Beaufort Cipher Tool
Variant Beaufort subtracts repeated key letters from plaintext letter indices.
Porta Cipher Tool
Porta substitution pairs key letters and swaps between alphabet halves.