diff --git a/Makefile b/Makefile
index 9cc73ad..bce0cd4 100644
--- a/Makefile
+++ b/Makefile
@@ -26,6 +26,7 @@ cleanprograms:
 	$(RM) bin/multiply
 	$(RM) bin/nonzero
 	$(RM) bin/pscat
+	$(RM) bin/rot13
 	$(RM) bin/simexec
 	$(RM) bin/sleep
 	$(RM) bin/streq
@@ -35,6 +36,7 @@ cleanprograms:
 	$(RM) bin/rldecode
 	$(RM) bin/rlencode
 	$(RM) bin/roll
+	$(RM) bin/roll_stdio
 	$(RM) bin/tail
 	$(RM) bin/true
 
@@ -109,6 +111,12 @@ roll_stdio.o: lib/libio.h src/roll.c sysexits
 roll_stdio: libio sysexits roll_stdio.o
 	$(CC) $(CFLAGS) -o bin/roll_stdio build/libio.o build/roll_stdio.o
 
+rot13.o: libio sysexits src/rot13.c
+	$(CC) $(CFLAGS) -c -o build/rot13.o src/rot13.c
+
+rot13: libio rot13.o
+	$(CC) $(CFLAGS) -o bin/rot13 build/libio.o build/rot13.o
+
 rldecode.o: sysexits src/runlength.c
 	$(CC) $(CFLAGS) -Df=decode -c -o build/rldecode.o src/runlength.c
 
diff --git a/src/rot13.c b/src/rot13.c
new file mode 100644
index 0000000..cc99dc1
--- /dev/null
+++ b/src/rot13.c
@@ -0,0 +1,41 @@
+#include <ctype.h> /* isalpha(3), isupper(3) */
+#include <stdio.h> /* getc(3), putc(3) */
+#include <stddef.h> /* stdin, stdout, EOF */
+#include <string.h> /* strchr(3), strlen(3) */
+#include <sysexits.h> /* EX_OK, EX_USAGE */
+#include <unistd.h> /* write(2) */
+#include "libio.h" /* fdputs(3) */
+#include "usefulmacros.h" /* ARRAYLEN */
+
+static char *program_name = "rot13";
+
+/* Assumes this is a character in the provided glyph system.
+ * Probably not Unicode-capable but doesn't assume Alphabet.
+ * Returns `c` rotated around `a` by `r` characters. */
+static int
+caesar(char c, int r, const char *a){
+	return a[(strchr(a, c) - a + r) % strlen(a)];
+}
+
+/* It's faster to assume Alphabet. */
+static char alpha_upper[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLM";
+static char alpha_lower[] = "abcdefghijklmnopqrstuvwxyzabcdefghijklm";
+
+int main(int argc, char *argv[]){
+	char *a;
+	int c;
+
+	if(argc > 1){
+		write(2, "Usage: ", 7);
+		fdputs(2, argv[0] == NULL ? program_name : argv[0]);
+		return EX_USAGE;
+	}
+
+	while((c = getc(stdin)) != EOF){
+		if(isalpha(c)){
+			a = isupper(c) ? alpha_upper : alpha_lower;
+			putc(a[c - a[0] + 13], stdout);
+		}else
+			putc(c, stdout);
+	}
+}