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How do transposition ciphers differ from substitution ciphers?

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A Vigenere cipher table displays the alphabet in both the top row and left column, with rows shifted to create a grid for encryption and decryption. Highlighted letters illustrate how to use the table. An explanation and example using the words cipher, key, and plaintext appears to the right. — cryptology In encrypting plaintext, the cipher letter is found at the intersection of the column headed by the plaintext letter and the row indexed by the key letter. To decrypt ciphertext, the plaintext letter is found at the head of the column determined by the intersection of the diagonal containing the cipher letter and the row containing the key letter.

In the field of cryptology, transposition and substitution ciphers represent the two fundamental mathematical operations used in data encryption.1

The primary difference lies in how they manipulate plaintext:

Transposition Ciphers: These systems rearrange the order of the existing symbols in the plaintext without changing the symbols themselves. Because the original letters are simply moved to new positions, the frequency distribution of the characters in the ciphertext remains identical to that of the original plaintext.21
Substitution Ciphers: These systems replace original plaintext elements—such as single letters, pairs, or groups of symbols—with entirely different symbols or groups of symbols. While the symbols change, the original sequence or order of the message remains intact.31

In modern practice, these methods are often combined to create product ciphers, where transpositions serve as one of several steps to increase security by obscuring patterns that might otherwise be vulnerable to cryptanalysis.23