The cipher passage in Charles I's letter to Prince Charles, 1 August 1648 (British Library, Harley MS 6988, f. 208)

This study identifies the key to the 88-group cipher passage in Charles I's letter to Prince Charles of 1 August 1648 and presents its decipherment. The letter survives as British Library, Harley MS 6988, f.208r (DECODE R8342); the key is catalogued as Egerton MS 2550, f.5 (DECODE R3055).

The decipherment reveals a request for help in freeing the king: "I would haue you looke this way to free me." It also refers to instructions conveyed through Mr Lysle to the prince's brother. Every retained group, including six nulls, has an entry in the key. The manuscript's insertions complete since, brother and whose under that same key, while three replacements also fit the reading. These corrections support the identification alongside the passage's continuity with the surrounding clear text.

Version 2 retains the decipherment published in the initial release (https://doi.org/10.5281/zenodo.23043134) and adds the complete key transcription: 647 rows covering all 646 numeric values, with a duplicated number preserved. The PDF and ZIP include the full letter, autograph and printed-input ciphertexts, an 88-token alignment, correction notes, and a verification script requiring only the Python standard library.

The edition retains the literal form informexion, marks 41 key readings as tentative, and treats the docket's August 1647 date as provisional. This is a preprint; the complete table was transcribed after the target reading was known and has not undergone independent palaeographic review.

Editorial text, transcriptions and data are CC BY 4.0; code is MIT. Restricted manuscript photographs and crops are not included.