Jean Champhuon, sieur du Ruisseau, an advocate who married Nau's sister Claire in 1563, also joined Mary's service.
On 29 March 1575, Elizabeth gave him a letter of introduction to the Earl of Shrewsbury the Scottish Queen's keeper at Sheffield Castle.
In January 1577, Nau sent cipher code keys to his brother-in-law the treasurer Jean de Champhuon, sieur du Ruisseau, to Mr Douglas, to John Lesley, Bishop of Ross, and to Ralph Lygon, for use in their correspondence with Mary.
[9][10] At this time, Mary was contemplating marriage with John of Austria, a brother of Philip II of Spain, and the Archbishop was her negotiator.
Nau also asked him to buy some jewellery; a locket with a catch or a sealed box (une petite boite fermee et cachetee), a pair of bracelets made in the latest fashion, and a diamond or emerald shaped like a heart or triangle.
[16] Nau advised that the precious stone would cost less from a specialist lapidary than from a goldsmith, and prices were cheaper because of the wars in France.
However, the Scottish court at Stirling Castle would not allow him an audience, apparently because Mary's letter was addressed to her son, not the King.
Fontenay wrote to Claude Nau about his good reception, James had met him in his cabinet at Holyroodhouse, and lent him a horse to join the hunting at Falkland Palace.
Nau also hoped to put forward the idea of the "association", a scheme to return Mary to Scotland as joint ruler with her son.
[26] While Fontenay was still in Edinburgh, in March 1585, he warned Nau that a rumour was circulating at the Scottish court that Mary made him sleep with her (que sa majeste vous faisoit coucher avec elle), and so they should modify their familiar behaviour when the Master of Gray visited.
[39] Nau was accused of deciphering a letter from Anthony Babington and composing a reply from Mary (by discussion and dictation) which Gilbert Curle translated into English.
[40] Francis Walsingham sent news to the Scottish Court in September 1586 that Mary was to be moved to Fotheringhay, and that "the matters whereof she is guilty are already so plain and manifest (being also confessed by her two secretaries), as it is thought, they shall required no long debating".
[42] Nau said he deciphered one or more incoming letters from Babington and had advised Mary not to reply and thus incriminate herself, and then reluctantly made a French draft which Curle translated into English and ciphered.
[46] Nau wrote a history of the years 1542 to 1545 which describes Regent Arran taking power in Scotland, possession of Holyroodhouse and Falkland Palace, and the exchequer.