{"id":586,"date":"2025-03-28T22:21:39","date_gmt":"2025-03-28T21:21:39","guid":{"rendered":"http:\/\/sfisqlu.cluster100.hosting.ovh.net\/?page_id=586"},"modified":"2025-04-17T17:25:24","modified_gmt":"2025-04-17T15:25:24","slug":"presentation","status":"publish","type":"page","link":"http:\/\/sfisqlu.cluster100.hosting.ovh.net\/index.php\/m-138\/presentation\/","title":{"rendered":"Presentation"},"content":{"rendered":"\n<p class=\"wp-block-paragraph\">The M-138 and M-138A were cipher devices used by the U.S. Army between 1934 and 1944 as an evolution and flat version of the M-94.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">They consisted of a frame, usually made of aluminium (see History) with a ruler and channels in which paper or plastic strips, each bearing a different alphabet repeated twice, could slide.<\/p>\n\n\n\n<div style=\"height:20px\" aria-hidden=\"true\" class=\"wp-block-spacer\"><\/div>\n\n\n\n<div class=\"wp-block-group is-content-justification-center is-nowrap is-layout-flex wp-container-core-group-is-layout-82936891 wp-block-group-is-layout-flex\">\n<p class=\"wp-block-paragraph\">The M-138 had a flat, one-piece aluminium frame with 25 channels.<\/p>\n\n\n\n<figure class=\"wp-block-image size-large is-resized\"><img loading=\"lazy\" decoding=\"async\" width=\"1024\" height=\"706\" src=\"http:\/\/sfisqlu.cluster100.hosting.ovh.net\/wp-content\/uploads\/2025\/03\/m-138-e1743195192198-1024x706.png\" alt=\"\" class=\"wp-image-600\" style=\"width:300px\" srcset=\"http:\/\/sfisqlu.cluster100.hosting.ovh.net\/wp-content\/uploads\/2025\/03\/m-138-e1743195192198-1024x706.png 1024w, http:\/\/sfisqlu.cluster100.hosting.ovh.net\/wp-content\/uploads\/2025\/03\/m-138-e1743195192198-300x207.png 300w, http:\/\/sfisqlu.cluster100.hosting.ovh.net\/wp-content\/uploads\/2025\/03\/m-138-e1743195192198-768x529.png 768w, http:\/\/sfisqlu.cluster100.hosting.ovh.net\/wp-content\/uploads\/2025\/03\/m-138-e1743195192198-1536x1059.png 1536w, http:\/\/sfisqlu.cluster100.hosting.ovh.net\/wp-content\/uploads\/2025\/03\/m-138-e1743195192198.png 1676w\" sizes=\"auto, (max-width: 1024px) 100vw, 1024px\" \/><figcaption class=\"wp-element-caption\"><em>M-138 &#8211; National Cryptologic Museum<\/em><\/figcaption><\/figure>\n<\/div>\n\n\n\n<div class=\"wp-block-group is-nowrap is-layout-flex wp-container-core-group-is-layout-3a88641f wp-block-group-is-layout-flex\">\n<p class=\"wp-block-paragraph\">The M138-A had an hinged aluminium frame, with 30 channels.<\/p>\n\n\n\n<figure class=\"wp-block-image size-large is-resized\"><img loading=\"lazy\" decoding=\"async\" width=\"1024\" height=\"768\" src=\"http:\/\/sfisqlu.cluster100.hosting.ovh.net\/wp-content\/uploads\/2025\/03\/m138a-3-1024x768.png\" alt=\"\" class=\"wp-image-591\" style=\"width:300px\" srcset=\"http:\/\/sfisqlu.cluster100.hosting.ovh.net\/wp-content\/uploads\/2025\/03\/m138a-3-1024x768.png 1024w, http:\/\/sfisqlu.cluster100.hosting.ovh.net\/wp-content\/uploads\/2025\/03\/m138a-3-300x225.png 300w, http:\/\/sfisqlu.cluster100.hosting.ovh.net\/wp-content\/uploads\/2025\/03\/m138a-3-768x576.png 768w, http:\/\/sfisqlu.cluster100.hosting.ovh.net\/wp-content\/uploads\/2025\/03\/m138a-3.png 1280w\" sizes=\"auto, (max-width: 1024px) 100vw, 1024px\" \/><figcaption class=\"wp-element-caption\"><em>M-138-A &#8211; National Cryptologic Museum<\/em><\/figcaption><\/figure>\n<\/div>\n\n\n\n<div style=\"height:100px\" aria-hidden=\"true\" class=\"wp-block-spacer\"><\/div>\n\n\n\n<h2 class=\"wp-block-heading has-text-align-right\"><strong>Use<\/strong><\/h2>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<p class=\"wp-block-paragraph\">Both devices are used the same way.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">If the text is longer than the number of channels of the device, it must first be broken in 25 or 30-letter sections (for M-138 and M-138-A, respectively) then the enciphering or deciphering process is performed for each section.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Each section may be enciphered with a different key, or with the same, the latter being much less secure.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><span style=\"text-decoration: underline;\">To encipher<\/span><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The strips are inserted into the channels in the order given by the key.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The strips are slid to make the plaintext appear either at the first or last column (on the M-138-A, both are marked &#8220;CLEAR TEXT&#8221;).<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The ciphertext is read off any other column except the center one (which reads &#8220;DO NOT COPY&#8221; on the M-138-A) as it also bears the clear text.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><span style=\"text-decoration: underline;\">To decipher<\/span><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The procedure is the same; the strips are inserted according to the key and aligned so the cipher text appears at the leftmost or rightmost column.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The columns are scanned to fin the one with readable text.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>The M-138 and M-138A were cipher devices used by the U.S. Army between 1934 and 1944 as an evolution and flat version of the M-94. They consisted of a frame, usually made of aluminium (see History) with a ruler and channels in which paper or plastic strips, each bearing a different alphabet repeated twice, could [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":582,"menu_order":1,"comment_status":"closed","ping_status":"closed","template":"wp-custom-template-m-138","meta":{"footnotes":""},"class_list":["post-586","page","type-page","status-publish","hentry"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v28.1 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>M-138 | Presentation<\/title>\n<meta name=\"description\" content=\"The M-138 and M-138A were cipher devices used by the U.S. Army between 1934 and 1944 as an evolution and flat version of the M-94.\" \/>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" 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