diff --git a/cipher/affine.c b/cipher/affine.c
new file mode 100644
index 00000000..49a91cd7
--- /dev/null
+++ b/cipher/affine.c
@@ -0,0 +1,207 @@
+/**
+ * @file
+ * @brief An [affine cipher](https://en.wikipedia.org/wiki/Affine_cipher) is a
+ * letter substitution cipher that uses a linear transformation to substitute
+ * letters in a message.
+ * @details Given an alphabet of length M with characters with numeric values
+ * 0-(M-1), an arbitrary character x can be transformed with the expression (ax
+ * + b) % M into our ciphertext character. The only caveat is that a must be
+ * relatively prime with M in order for this transformation to be invertible,
+ * i.e., gcd(a, M) = 1.
+ * @author [Daniel Murrow](https://github.com/dsmurrow)
+ */
+
+#include <assert.h>  /// for assertions
+#include <stdio.h>   /// for IO
+#include <stdlib.h>  /// for div function and div_t struct as well as malloc and free
+#include <string.h>  /// for strlen, strcpy, and strcmp
+
+/**
+ * @brief number of characters in our alphabet (printable ASCII characters)
+ */
+#define ALPHABET_SIZE 95
+
+/**
+ * @brief used to convert a printable byte (32 to 126) to an element of the
+ * group Z_95 (0 to 94)
+ */
+#define Z95_CONVERSION_CONSTANT 32
+
+/**
+ * @brief a structure representing an affine cipher key
+ */
+typedef struct
+{
+    int a;  ///< what the character is being multiplied by
+    int b;  ///< what is being added after the multiplication with `a`
+} affine_key_t;
+
+/**
+ * @brief finds the value x such that (a * x) % m = 1
+ *
+ * @param a number we are finding the inverse for
+ * @param m the modulus the inversion is based on
+ *
+ * @returns the modular multiplicative inverse of `a` mod `m`
+ */
+int modular_multiplicative_inverse(unsigned int a, unsigned int m)
+{
+    int x[2] = {1, 0};
+    div_t div_result;
+
+    if (m == 0) {
+        return 0;
+    }
+    a %= m;
+    if (a == 0) {
+        return 0;
+    }
+
+    div_result.rem = a;
+
+    while (div_result.rem > 0)
+    {
+        div_result = div(m, a);
+
+        m = a;
+        a = div_result.rem;
+
+        // Calculate value of x for this iteration
+        int next = x[1] - (x[0] * div_result.quot);
+
+        x[1] = x[0];
+        x[0] = next;
+    }
+
+    return x[1];
+}
+
+/**
+ * @brief Given a valid affine cipher key, this function will produce the
+ * inverse key.
+ *
+ * @param key They key to be inverted
+ *
+ * @returns inverse of key
+ */
+affine_key_t inverse_key(affine_key_t key)
+{
+    affine_key_t inverse;
+
+    inverse.a = modular_multiplicative_inverse(key.a, ALPHABET_SIZE);
+
+    // Turn negative results positive
+    inverse.a += ALPHABET_SIZE;
+    inverse.a %= ALPHABET_SIZE;
+
+    inverse.b = -(key.b % ALPHABET_SIZE) + ALPHABET_SIZE;
+
+    return inverse;
+}
+
+/**
+ * @brief Encrypts character string `s` with key
+ *
+ * @param s string to be encrypted
+ * @param key affine key used for encryption
+ *
+ * @returns void
+ */
+void affine_encrypt(char *s, affine_key_t key)
+{
+    for (int i = 0; s[i] != '\0'; i++)
+    {
+        int c = (int)s[i] - Z95_CONVERSION_CONSTANT;
+
+        c *= key.a;
+        c += key.b;
+        c %= ALPHABET_SIZE;
+
+        s[i] = (char)(c + Z95_CONVERSION_CONSTANT);
+    }
+}
+
+/**
+ * @brief Decrypts an affine ciphertext
+ *
+ * @param s string to be decrypted
+ * @param key Key used when s was encrypted
+ *
+ * @returns void
+ */
+void affine_decrypt(char *s, affine_key_t key)
+{
+    affine_key_t inverse = inverse_key(key);
+
+    for (int i = 0; s[i] != '\0'; i++)
+    {
+        int c = (int)s[i] - Z95_CONVERSION_CONSTANT;
+
+        c += inverse.b;
+        c *= inverse.a;
+        c %= ALPHABET_SIZE;
+
+        s[i] = (char)(c + Z95_CONVERSION_CONSTANT);
+    }
+}
+
+/**
+ * @brief Tests a given string
+ *
+ * @param s string to be tested
+ * @param a value of key.a
+ * @param b value of key.b
+ *
+ * @returns void
+ */
+void test_string(const char *s, const char *ciphertext, int a, int b)
+{
+    char *copy = malloc((strlen(s) + 1) * sizeof(char));
+    strcpy(copy, s);
+
+    affine_key_t key = {a, b};
+
+    affine_encrypt(copy, key);
+    assert(strcmp(copy, ciphertext) == 0);  // assert that the encryption worked
+
+    affine_decrypt(copy, key);
+    assert(strcmp(copy, s) ==
+           0);  // assert that we got the same string we started with
+
+    free(copy);
+}
+
+/**
+ * @brief Test multiple strings
+ *
+ * @returns void
+ */
+static void tests()
+{
+    test_string("Hello!", "&3ddy2", 7, 11);
+    test_string("TheAlgorithms/C", "DNC}=jHS2zN!7;E", 67, 67);
+    test_string("0123456789", "840,($ {ws", 91, 88);
+    test_string("7W@;cdeRT9uL", "JDfa*we?z&bL", 77, 76);
+    test_string("~Qr%^-+++$leM", "r'qC0$sss;Ahf", 8, 90);
+    test_string("The quick brown fox jumps over the lazy dog",
+                "K7: .*6<4 =-0(1 90' 5*2/, 0):- +7: 3>%& ;08", 94, 0);
+    test_string(
+        "One-1, Two-2, Three-3, Four-4, Five-5, Six-6, Seven-7, Eight-8, "
+        "Nine-9, Ten-10",
+        "H&60>\\2*uY0q\\2*p4660E\\2XYn40x\\2XDB60L\\2VDI0 "
+        "\\2V6B6&0S\\2%D=p;0'\\2tD&60Z\\2*6&0>j",
+        51, 18);
+
+    printf("All tests have successfully passed!\n");
+}
+
+/**
+ * @brief main function
+ *
+ * @returns 0 upon successful program exit
+ */
+int main()
+{
+    tests();
+    return 0;
+}