There are a number of different types of substitution ciphers. If the cipher operates on single letters, it is termed a simple substitution cipher; a cipher that operates on larger groups of letters is termed polygraphic. A monoalphabetic cipher uses fixed substitution over the entire message, whereas a polyalphabetic cipher uses a number of substitutions at different positions in the message, where a unit from the plaintext is mapped to one of several possibilities in the ciphertext and vice versa.
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SRAGX WTVFU MIIZX XWIAU NZDRC VWQIQ NPSFU WVMTQ URSED SEYYS
GMDRZ EPNRW QPXGZ DKTQD YYGPB LFSZZ PQJGG IJURI XWCVW BXZIR
QSXRD TFZSY UPYVI VDDMZ VGWIT PASUT GUVQY EYYFB TBQPX PWCTE
XCTUF VVMRD PZQYR XSWTU
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